Industrial Fluid Power ( Pdfdrive.com ).pdf 1p4z34

Dhanvantari College of Engineering, Nasik

Fluid Power System B.E. Mechanical SEM - II •

ADVANTAGES OF FLUID POWER

1. Fluid power provides highly accurate and precise movement of the actuator with relative ease. This is particularly important in applications such as machine tool movement control where tolerances are often specified in microns and must be repeatable during several million cycles. 2. Fluid power is not hindered by the geometry of the machine and it can be used to actuate devices that are located away from power source. This is a decided advantage over mechanical systems that are dependent upon the machine geometry. 3. The power capacity of a fluid system is extremely large and is limited only by the strength capacity of the material. 4. Fluid power provides flexible and easy control of variable force, distance and speed. 5. Fluid power provides an efficient method of multiplying forces. 6. Fluid power can be varied from a delicate touch of a few ounces to a gigantic force of several hundred tons [@36000 tons or more]. 7. In fluid power, small forces can be amplified to control large forces thereby providing leverage. 8. Only fluid power systems are capable of providing constant force or torque regardless of speed changes. 9. Fluid power systems provide instant and smooth reversible motion. 10. Fluid power systems also provide infinitely variable speed control. 11. Fluid power systems provide fast response to controls. 12. Fluid power systems provide automatic protection against overload. 13. No harm is done to a fluid power system should it stall. 14. Torque output continues even if hydraulic motor is stalled. 15. Fluid power systems have the highest power to weight ratio of any known power source. 16. Fluid power systems use fewer moving parts than comparable mechanical or electrical systems. Hence, they are simpler to maintain and operate. 17. Fluid power systems are safe, economical, efficient and reliable. 18. Fluid power systems are compatible with either electrical, electronic or mechanical means of control. 19. Fluid power is readily available. 20. Pneumatic power is free from fire hazards and hence preferred to electrical systems; hydraulic power is self lubricating thereby reducing wear of moving parts and hence preferred to mechanical systems. •

APPLICATION OF FLUID POWER Fluid power systems are widely used in industry to perform hundreds of important tasks. Some of these important applications are studied in this article.

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Fluid power application in machine tool Over 90% of all machine tools are controlled or operated with fluid power. Due to this it has been possible to summaries the application n in tabular form (Table 1.3) for this industry.


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Fluid power applications in material handling Fluid power controls the telescopic mast and grabbing jaws of the forklift trucks used in industry. Conveyor hois hoists, cranes, dampers, tilting ramps and levelling docks are a few other examples of fluid powered tools for modern material handling. A classic example showing fluid power application in material handling is the hoist used by automobile service stations tto raise a car for servicing. This application of fluid power generally uses both a liquid and a gas. Tilting a container for the purpose of emptying or discharging its contents is carried out as a pivoting motion performed by cylinders acting on levers. Fluid power operated tilting units are used as accessory devices for handling of bulk materials. erials. (Ref. Fig. 1.1)

The feeder shown in Fig. 1.2, is used to pick up a part from an overhead conveyor and locate it in a machining station. The verti vertical cylinder stroke controls the removal of the part from the conveyor carrier and locating it in the production station. Angular motion around the vertical axis results from a second horizontally mounted cylinder whose piston rod is attached to a rack engaging aging with pinion. • Fluid power for automation Parts supplied by vibratory cup conveyor normally need to be selected and advanced at the end of the magazine chute. A set is to be up designed to take the parts singly from the chute and load them on the work holders of a rotary index table. Fluid power is more advantageously used for automation.


• Fluid Power For Forming Operation Force is a major criterion in motion taking place to form metals. Fluid powered actuators (cylinders) in varied types and sizes are the dominating elements in the motions required for metal forming. • Fluid power in agriculture The need for food and fibre production has caused unprecedented leadership in agricultural equipment development. They are extensively used in forage harvesters, backhoes, chemical sprayers and organic fertilizer spreaders. They are used for controlled apportioning and supply of feed for animals, collection and removal of manure in mass raising of animals, wool shearing and slaughtering. They are mainly used for tilting, lifting and swivelling gear equipment for fieldwork, crop protection and weed control. • Fluid Power in Construction Another sector of our economy that has benefited from the brute power of hydraulics and pneumatics is the construction industry. Crawler tractors, road graders, bucket loader, trenchers, backhoes, hydraulic shovels, pan scrapers, bull dozers, vibrator screens, crushers, rollers and asphalt mixers are just a few of the many applications. Even the smaller tools of construction such as rock drills used for breaking of concrete are powered by fluid. Bin gate controls used in large concrete mixing plant, weight batching mixer controls, forming presses for concrete products, brick and block conveying and handling equipment, spray painting equipment are some of the fluid powered equipment used in the building industry. • Fluid Power in aviation Aerospace and aviation applications of fluid power include controlling of landing of landing gear, elevens, rudders, elevators, payload bays and trim tabs. • Fluid power in marine industry Fluid power in marine shipping is basically used for automatic helmsman ship cargo handling, actuation of hatch covers and a vast bulk of dock and shipyard machinery. Under sea application of hydraulics include submersibles that are used for exploration and development of the ocean resources. • Fluid power in Transportation system Transportation systems provide examples of the most varied used of fluid power.

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Power brakes (both high pressure pressur and vacuum assisted), power steering systems, (Ref. Fig. 1.3 and Fig. 1.4) power windows, and powered seat adjustments are all typical fluid power devices. Hydrostatic transmissions are very common in all types of vehicles. uspension systems are dampened with hydraulic shock observers, and some combine pneumatics by using the compressible nature of gases as the basis for air air-oil oil suspension systems. Fig. 1.5 shows a hydraulic wheel motor which is a recent addition to the ransportation line. They give almost ost unlimited flexibility to the design which can mount the power plant in a convenient location to power individually driven wheels as they are required to the load and provide traction to propel it in almost any direction with a variety of step lless speeds.


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Fluid power in Food – Processing Industry The food processing industries include bakery product, dairy products, meat and fist product and beverage industry. Extreme cleanliness and accurate weighing and vacuum seal packing is of utmost importance. Hence, pneumatic power is used extensively in various food processing equipment such as grinders, pulverizes, shifters, screeners, presses, cutters, dehydrators, conveyors, oven, agitators, wrappers and cartooning machinery. But the most important application is in canning – from the moment the can is formed, until it is finally filled and labelled.

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Fluid power in Mining Industry Fluid power has wide application in continuous coal and mineral mining and quarrying. The coal-mining machine which can dig and load coal at the rate of 2 tons/min, is equipped with several hydraulic cylinders, jacks and motors. Others mining application using fluid power pneumatic include rock drills, hammers, chippers, hydraulic track laying machines, shuttle cars, roof bottling machines and conventional hydraulic cranes, hoist and jacks.

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Fluid power in Utilities and Communication Utilities and communication are two industries where the use of flued power is vital. Line utility vehicles are used to persons working above the ground. Other applications include hydraulic trenchers, cable boring machines, earth augers, pipe laying machines and tempers.

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Fluid power of Numerical Control The dimensions from drawing and the machining data are coded and put on control tape. When this control tape is fed to the machine, the machining data is translated into motion and force for hydraulic control operations hydraulic actuation provides a very suitable link between the control tape signals and the actual machining motions because of ease of control, rapid response, variable speed and amplification force.

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Some interesting fluid power applications (a) The actuators that blink the eyes and move the fingers on the almost human mannequins at Disney world are hydraulically operated. (b) Hinged doors can be actuated pneumatically.

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Application with scope for technological advances Three applications given recent attention in fluid power industry are miniature pneumatic, moving part logic and fluidics. Miniature pneumatics makes use of small air powered components such as cylinders and valves to carry out small assigned tasks as well as to control large components. Moving part logic and fluidics make use of logic elements with function similar to several electronic counterparts such as capacitors, resistors and amplifiers to control other hydraulic and pneumatic systems.

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STATIC AND DYNAMIC POWER TRANSMISSIONS According to general dictionary definition, hydraulics is that branch of physics, which deals with the utilization of energy (either kinetic or pressure) of a liquid to do work. Hence, a hydraulic device can perform work by Page | 6


(a) utilizing the momentum of a moving liquid, or (b) utilizing the pressure energy of a confined fluid. Hence, a hydraulic power transmission is classified into two general types as shown in Fig. 1.6 Hydraulic Power Transmission

Hydrodynamic / Hydrokinetic It use the kinetic energy of a high Velocity flow of fluid.

Hydrostatic It uses high pressure and relatively Low velocities of fluid.

Fig. 1.6: Classification of Hydraulic Power Transmission. •

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Hydrodynamic / Hydrokinetic Power Transmission: The branch of hydraulic, which uses the impact or momentum, or kinetic energy of a moving liquid to transmit power is called hydrodynamics. e.g.: Centrifugal pumps, and Water turbines etc. Hydrostatic Power Transmission: The branch of hydraulics which uses the pressure force obtained by pushing a confined fluid to transmit power is called as hydrostatics. e.g. Hydraulic jacks, hydraulic rams, hydraulic presses, hydraulic elevators etc. The term hydrostatic transmission refers to the use of hydraulic pumps and motors for converting fluid power into mechanical rotary motion. A brief comparison of hydrodynamic and hydrostatic transmission is given in the Table 1.4. Table 1.4 Hydrostatic Transmission Hydrodynamic Transmission 1. A change in fluid pressure 1. A change in fluid velocity yields output energy. yields output energy. 2. Positive displacement units. 2. Non-Positive displacement 3. Output torques available at all units. speeds. 3. Output torque is not available at low input speeds. 4. A speed ratio of 60 : 1 is 4. Input speed ratio 6 : 1 output practical. speed maximum. 5. Tends to creep. 5. No creeping tendency. 6. Non-reversible. 6. Fully reversible. 7. Input and output directly 7. Input and output can be remote coupled. or directly coupled. 8. Torque remains practically 8. Torque varies over a wide constant over full speed range range. of hydraulic motor. 9. Direct braking available. 9. Practically no internal 10. Substantially constant speed braking. with load variations. 10. Speed drops off with load. 11. Positive control available. 12. Both open circuit and closed 11. Only limited control. Page | 7


circuit systems are available.

12. Only open circuit systems are available.

As seen from the comparison above, since hydrostatic transmissions utilize positive displacement pumps and motors and because they are flexible, this system offers a number of advantages over hydrodynamic transmission system. During the last few decades, improvements in design and operational performance have opened new fields of usage which hold unlimited possibilities for machines and equipment of the future. Int the field of fluid power, almost all the systems in the industry are hydrostatic devices. Hence, the various equipments and systems covered in this text book will be hydrostatic in nature. It should be noted that although the term hydraulic is used frequently in the text book, it mainly refers to hydrostatic. Hence, the term hydraulics as treated in this text book pertains to power transmitted and controlled thorough the use of pressurized liquids.

Questions 1. 2. 3. 4. 5. 6. 7.

Define fluid. Define fluid power. What do you mean by hydraulic and pneumatics? How did fluid power develop in the 19-20the century? What are the various methods by which power can be transmitted? Compare the various methods of power transmission. What are the advantages of using fluid power method of transmission? List 10 fields of applications where hydraulics and pneumatics can be used more effectively than the other power sources. 8. What effect has fluid power had on automation? 9. Why is hydraulic power especially useful with heavy work? 10. Explain in brief the areas of application of fluid power. 11. What is hydrostatic and hydrodynamic power transmission? Explain in brief. 12. Compare hydrostatic and hydrodynamic power transmission.

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Chapter – 3 Hydraulic Fluids •

INTRODUCTION A fluid has been defined in Chapter 1 as any liquid or gas. However, in hydraulics the term fluid is generally used to refer to hydraulic liquids used for power transmission. Hence, in this section on hydraulics, the term ‘fluid’ will mean ‘hydraulic fluid’. The primary purpose of the hydraulic fluid is to transmit power. Fluid is picked up by the pump from the reservoir, fed through the control valves to cylinders and motors where the power is expended, and then returned to the reservoir where it is cooled and settled before starting the cycle again. How well a fluid transmits power is determined how easily it is pumped, how stiff it is and a number of other service related properties that determine how suitable a certain fluid is to a particular application system and environment. Proper selection and care of hydraulic fluid is to a particular application system and environment. Proper selection and care of hydraulic fluid for a system will have an important effect on the efficiency of the hydraulic system, on the cost of maintenance and on the service life of hydraulic components.

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FUNCTIONS OF HYDRAULIC FLUID The hydraulic fluid performs the following functions:

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Primary Function: Power Transmission The primary function of the hydraulic fluid is to transmit power to perform useful work. The hydraulic fluid must transmit an applied force from one part of the system to another and must respond quickly to reproduce any change in magnitude and direction of the applied force. Hence, it should have good flow ability and it should be incompressible to make the pump start instantaneously.

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Secondary Functions In addition to transmission of power, the hydraulic fluid should perform the following secondary functions. 1. Lubrication: In an hydraulic system, the internal lubrication is provided by the fluid. The hydraulic fluid minimizes the wear due to friction by providing adequate lubrication for bearing, sliding surfaces in pumps, valves, cylinders motors and other components of the system. Hence, for long service life the fluid must contain anti wear and antitrust additives. 2. Sealing: In an hydraulic system, there are many instances where due to the close mechanical tolerances between the moving parts, providing a mechanical seal is impractical. Here, the hydraulic fluid itself provides the film strength to seal the close clearances against leakage. 3. Cooling: In an hydraulic system, while circulating the hydraulic fluid carries away the heat generated and dissipates it in the reservoir. Thus it cools the system.

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SERVICE RELATED FLUID PROPERTIES An hydraulic fluid has some inherent properties, while some service required properties can be added by the addition of suitable additives. These properties which affect the performance of an hydraulic system include specific gravity, viscosity, bulk module, pour Page | 9


point, neutralization number, flash point, fire point, auto ignition temperature, anti wear properties, anti-rust properties, defoaming and detergent dispersing properties. The object is to select a fluid that has the properties for a specific application and then have them remain stable during continuous use for the recommended time between changes. This could be as long as 1500 hours (or 3 years) in some cases. •

Specific Gravity (s) Specific weight of a liquid indicates weight per unit volume Speciic weight w

=

ρg

…(3.1) Specific weight of water is 9810 N/m3 Specific gravity of a liquid is the ratio of specific weight of that liquid to the specific weight of water. Specific Gravity (s) =

౩

ൌ

ρ౩ ρ

S.I Unit: Specific gravity is a dimensionless quantity. Specific gravity is also known as ‘relative density’. Specific gravity of water is 1. For commercially available hydraulic fluids, the specific gravity may range from 0.80 to 1.45. •

1) 2) 3) 4)

Viscosity The ability of a fluid to be pumped and transmitted through the system is most important. This ability to flow is determined by the fluid viscosity. Viscosity is a measure of the internal resistance of a fluid to shear and is related to the internal friction of the fluid itself. Thick fluids flow more slowly than thin fluids because they have more internal friction. The term fluidity is the reciprocal of viscosity. Thus a fluid having a high fluidity has low viscosity and a fluid having a low fluidity has a high viscosity. Viscosity can be defined in the following manner: Absolute (dynamic) viscosity Kinematic viscosity Relative viscosity in Say bolt Second Universal (SSU) and SAE numbers (for automotive oils). 1. Absolute or Dynamic Viscosity (H): Absolute viscosity or dynamic viscosity is defined as the force required to move a flat surface with an area of one unit at a velocity of one unit, when it is separated from a parallel stationary flat surface by an oil film one unit thick. S.I. unit; Pascal second (Pa - s) However, the commonly used unit is Poise and centipoise 1 Poise = 0.1 Pa – s 1 Centi Poise () = 0.01 Poise = 0.001 Pa – s = 1 MPa - s Page | 10


2. Kinematic Viscosity (v): Kinematic viscosity is defined as the ratio of absolute viscosity and mass density of fluid. Kinematic viscosity (v) =

S.I. unit:

ౡౝ ౣయ

ൌ

ొ ౣమ ౡౝ ౣయ

µ

… (3.3)

ρ

ౣ ౩

మ

ൌ

ൌ

మ

ൌ

However, the commonly used unit for kinematic viscosity is stoke and centistokes. 1 Stoke = 1 cm2/s 1 Centistoke (cSt) = 1 mm2/s

3. Relative Viscosity in SSU units: During the selection of fluids, it is very convenient to known the relative viscosity of the fluid. It is measured by using the Say bolt Viscometer. Hence, it is measured in Say bolt Second Universal, abbreviated as SSU. Here, the resistance of the fluid to flow is measured as the number of seconds it takes for a fixed quantity of 60 ml sample of oil to drain through a small orifice of standard length and diameter at a constant temperature of 100o F (37.7o) or 210oF (98.9 oC). The elapsed time is the SSU viscosity for the fluid at the given temperature. For thicker fluids, the same test is carried out using a larger orifice to derive the say bolt Seconds Furol (SSF) viscosity. For most applications, the viscosity is in the range of 100 SSU to 200 SSU. However, it is a general rule that viscosity should never go below 45 SSU and above 4000 SSU, regardless of temperature. 4. SAE number: The Society of Automotive Engineers has established standard SAE numbers to specify the range of viscosities of engine oils at specific test temperatures. Winter number (0W, 5W, 10W, 15W, etc…) are determined by tests at cold temperatures. Summer number (20, 30, 40, etc…) designate the SSU range at 100o C. Table 3.1: SAE Viscosity Grades for oils. SAE

Viscosity Grade 0W 5W

Absolute Viscosity () at Temperature oC (oF) Max 3250 at – 30 (22) 3500 at – 25 (13)

Kinematics Viscosity (cSt) at 100o C (212o F) Min Max 3.8 3.8 Page | 11


10W 15W 20W 25W 20

3500 at – 20 (-4) 3500 at – 15 (5) 4500 at – 10 (14) 6000 at – 5 (23)

30 40 50

4.1 5.6 5.6 9.3 5.6 Less than 9.3 9.3 Less than 12.5 12.5 Less than 16.3 16.3 Less than 21.9

Viscosity is generally considered to be the most important physical property of an hydraulic fluid an is the starting point in the selection of a fluid. If the fluid does not have the proper viscosity, it cannot perform regardless of other superior characteristics. If the viscosity is too low (lightweight oil), although the transmission efficiency will be high, the following drawbacks may be encountered. 1. Less film strength causes more wear and tear of moving parts. The oil film may also break down causing seizure. 2. Increase in internal leakage causes more pressure loss. 3. Leakage losses may result in increased temperatures. 4. Lower volumetric efficiencies in pumps and motors. 5. Slower response of actuator and hence less precision control. If the viscosity is too high (heavy weight oils), although the self sealing obtained between the mating surfaces is excellent, the following drawbacks may be encountered: 1. 2. 3. 4. 5. 6. 7.

High resistance to flow results in sluggish operation. High pressure drop due to friction. Excessive heat generation. Increased power consumption due to frictional losses. Low mechanical efficiency. Starvation of the pump inlet, causing cavitation. Difficulty in separating air from oil in reservoir. 5. Viscosity Index (VI): The viscosity index (VI) is a measure of the relative change in viscosity for a given change in temperature. An oil with a high viscosity index shows less change in viscosity for a given change in temperature than does an oil with a low viscosity index. The viscosity index measures the stability between two temperature extremes. Viscosity index is computed by using SSU designation for the reference oils (Pennsylvania crude paraffin base fraction with a VI of 100 and Coastal crude naptha base fraction with a VI of 0) and for the oil for which the VI is to be determined. The viscosity index is calculated as follows

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Dhanvantari College of Engineering, Nasik VI

LU 100 LH

L = SSU viscosity of a reference oil at 100oF with a VI of 0 that has the same viscosity at 210o F as the oil to be calculated. H = SSU viscosity of a reference oil at 100oF with a VI of 100 that has the same viscosity at 210o F as the oil to be calculated. U = SSU viscosity at 100oF of the oil whose VI is to be calculated. The proper viscosity index of a fluid for a specific application is determined from the fluid temperature change requirements of the system. For example. (a) In production machinery, the operating temperature range of the oil is small and hence a low VI is suitable. (b) In mobile hydraulic equipments which may be required to operate in extreme temperature conditions like below freezing to near 160o F, a fluid with a high VI above 100 is required. •

Bulk Modulus of Elasticity (K) The bulk modulus of elasticity is a measure of the rigidity of the fluid. It gives an indication of how much the fluid compresses under pressure. Mathematically, the bulk modulus is the reciprocal of compressibility i.e. it is the ratio of change in pressure to the change in volume. Bulk Modulus (K) ൌ

∆ ∆!/!

…(3.5)

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S.I. Units: Pascal (Pa): The negative sign s for the fact that as the pressure increases, the volume decreases from the eqn (3.5). Hence, we can say that higher the bulk modulus, the less compressible or stiffer the liquid. Air entrained in hydraulic oils reduces its bulk modulus making it spongy. This particularly affects the positioning circuits where the fluid is required to be incompressible to maintain accuracy even with changes in load. Hydraulic fluids have a bulk modulus in the range of 2068 MPa to 2758 MPa at room temperatures in the pressure range of 6.8 to 41.5 MPa.

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Pour Point Pour point is the lowest temperature at which an oil will flow. Low temperature hydraulic applications, particularly those involving mobile equipment, use pour point as an indication of the ability of the oil to be pumped as the temperature drops. As a general rule, the pour point should be 15o to 20o F below the lowest temperature of the system during start up to be sure that the pump will not cavitate and become damaged. Chemical additives may be used to lower the pour point. The addition of these pour point depressant does not vary the viscosity over the selected temperature range.

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Neutralization Number Neutralization number is a measure of the acidity or alkalinity of a hydraulic fluid. This is referred to as the pH factor of a fluid. Petroleum base fluids have a tendency to become acidic with time high acidity causes oxidation rate in an oil to increase rapidly. Hydraulic fluids are fortified with additives to reduce the tendency to become acidic and keep the neutralisation number below 0.1 during normal service. Fluids with a low neutralization number are recommended to prevent harmful chemical reaction.

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Flash Point, Fire Point and Auto- Ignition Point The flammability of a hydraulic fluid is described from the flash point, fire point and auto ignition temperature. 1. Flash Point: The flash point is a temperature at which a liquid gives off sufficient quantity so as to ignite momentarily or flash when a test flame is ed over the surface. A high flash point is desirable because it indicates good resistance to combustion and a low degree of evaporation at normal or working condition. 2. Fire Point: The fire point is a temperature at which the fluid will ignite and remains ignited for five seconds when a test flame is ed over the surface. 3. Auto Ignition Temperature: The auto ignition temperature is reached when the sample will self ignite and combustion continues. All the three temperatures indicate how hazardous the fluid will be in the presence of metal open flames or elevated temperatures. Typical application include coal mines, ships, aircraft and space craft.

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Antiwar Properties A good hydraulic fluid must be able to provide full lubrication for all integral moving parts of the system. However under extreme speed and pressure condition, the fluid film thickness depletes and a condition called as boundary lubrication occurs. Here, due to metal to metal , wear of the moving parts occurs. Hence, antiwar additives are added to the hydraulic fluid to reduce this wear caused by friction between moving parts. Petroleum based fluids provide excellent lubricating qualities.

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Oxidation Oxidation is a chemical reaction in which the oxygen combines with the fluid to result in the formation of acid and sludge. Air provides the oxygen necessary to promote oxidation. Petroleum base hydraulic oils are particularly susceptible to oxidation, since oxygen readily are soluble in the oil and additional reactions take place in the products to from gum, sludge and varnish. The first stage products which stay in the oil are acidic in nature and can cause corrosion throughout the system, in addition to increasing the viscosity of oil. The insoluble gum, sludge and varnish plug orifices, increase wear and cause valves to stick. The two main accelerators of oxidation are: 1. High Temperature and High Pressure: Oxidation rate increases at elevated temperatures. The rate approximately doubles for every 18o F (10o C), and it is estimated that the life of the oil is halved for each 15o F rise in temperature. When the pressure is increased, the viscosity increases. This will result in more frictional heat, thus raising the operating temperature of the system which in turn increases Page | 14


the rate of oxidation. Also as pressure increases, the amount of air that can be held in solution by an oil increases rapidly. More air results in more oxidation. 2. Effect of contamination on oxidation: Contaminats like cutting oil, grease, dirt, moisture paint, and insoluble oxidation products themselves act as catalysts and accelerate oxidation. Also metals such as copper, iron, aluminium which are used aid oxidation, especially in the presence of water. Additives are added to hydraulic fluids to resist oxidation. They may be of the type that breaks the chain of reactions thus preventing oxidation, or they may reduce the effect of oxidation catalyst (metal deactivator type). Advantages: 1. They have excellent lubricating and antiwear qualities, and for practical purposes are equal to petroleum base fluids. 2. Since they are available in the low range of viscosity (VI from 80 to - 40), they are used in plants and outside, from high speed precision machine tools which require low viscosity fluids, to sub-zero aircraft and mobile equipment application. Since they do not contain any water or other volatile material, they operate well at 3. higher temperature than water containing fluids. 4. They are suitable for high pressure system than water containing fluids. 5. Replenishing fluid can be added directly without regard for changing the viscosity or chemical composition of the fluid. 6. Here there is neither the separation of the continuous phase from the emulsion, nor periodic replenishment of additives which evaporate with water. Disadvantages: 1. It is the costliest hydraulic fluid being used. It is about 7 times more costly than petroleum base fluid. 2. They can be used only where the operating temperature is relatively constant. 3. They do not operate well in low temperature systems. Auxiliary heating may be required in cold environment. 4. These fluids have a high specific gravity and may cause pump cavitation. 5. Seals which are normally used for petroleum base fluids are not suitable for use with synthetic fluids. Seals should be changed when the system is being converted to this fluid. Suitable sealing materials are butyl, rubber, Teflon, viton. 6. Avoidance of continued skin is advised. When this fluid comes in with hot surface, irritating fumes are developed. • 1. 2. 3. 4. •

SELECTION OF FLUIDS Fluid selected for a particular application is governed by following factors Operating pressure of fluid in the system Operating temperature and variation in the system Environmental conditions Component material for compatibility with the selected fluid EFFECT OF TEMPERATURE AND PRESSURE ON HYDRAULIC FLUID For all oils, viscosity decreases as the temperature increases. Viscosity index is a measure of change in viscosity with the change in temperature Viscosity index can be improved by some additives. Fig. 3.1 shows variation in Kinematic viscosity with temperature for oils with different viscosity index. With the increase in the pressure of hydraulic oils, the viscosity is found to increase. Modern hydraulic systems employ very high operating pressures, often exceeding 1000 bar. At Page | 15


extremely high pressure, viscosity of oil increases considerably and eventually may turn solid. Fig. 3.1 shows the temperature versus viscosity curves with operating pressure as parameter. • 1. 2. 3. 4. 5. 6. •

i. ii. iii.

ADDITIVES: By using additives, the performance of the hydraulic fluid can be improved. Following additives are used, for this purpose. Oxidation inhibitors Viscosity index improvers Corrosion inhibitors De foamers Anti-wear agents Pour point depressants. Rusting Rusting is the corrosion of ferrous parts of the machinery in the presence of water in the hydraulic fluid. Rusting occurs at the metal surfaces and produce flaking. Rusting contaminates the system and promotes wear. It also allows excessive leakage past the affected parts and may cause components to seize. Rust occurs due to the presence of water in the hydraulic system. The source of this water may be: Condensation of the moisture in the air present in the system. Leakage from oil cooler. Through the use of coolants during machine operations. Rust can be prevented by adding “rust inhibitors” to the hydraulic fluid. This inhibitors “plate” the ferrous surfaces, forming a thin protective coating on the metal that prevents it from rusting.

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Foaming Foaming is the result of entrainment of air in oil. Most oil normally contain air in solution, some as much as 10% by volume. Air in the solution is not usually harmful, although it promotes oxidation. Air entertainment is caused by improper oil levels in the reservoir, at the pump inlet due to partial vacuum or due to server agitation in the hydraulic system. It is important to bleed the air out of all lines and components before starting the system. Air can be separated out by ing the fluid through a sieve screen in the reservoir. Forming depressant can be added to promote the “breaking out” of the air from the fluid rather than preventing entrapment initially.

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Demulsibility Demulsibility is the ability of the fluid to separate out water. Addition of antirust additives causes the water present in the hydraulic fluid to emulsify. This prevents the water from settling and breaking through the antirust film. However, too much of water in the oil will promote the collection of contaminants which accelerate the wear and tear. Hence, the fluid is refined to have a high degree of Demulsibility.

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QUALITY REQUIREMENTS OF A GOOD HYDRAULIC FLUID In addition to performing both primary and secondary functions, a satisfactory hydraulic liquid must have the following properties: 1. It should be practically incompressible i.e. it should have high bulk modulus of elasticity. Page | 16


2. It must have sufficient lubricating properties to maintain the established metal clearances and prevent galling and scoring. Film strength must be sufficient to prevent the fluid from being wiped or squeezed from between the surfaces when spread out in a thin layer. 3. It must have satisfactory viscosity and viscosity index so that it will perform adequately within the normal operating temperature ranges of the machinery. 4. It should be chemically stable to prevent formation of sludges, gums and varnishes. 5. It should remain physically stable over a period of time with changes in temperature and operating condition, thereby reducing the fluid replacement cost. 6. The pour point of the fluid should be about 15o F to 20o F below the minimum temperature expected in normal operation. 7. It should have a low neutralization number to be non-corrosive to the metals in the system. 8. It should have a high flash point, to prevent possible evaporation. 9. It must be compatible with seals and gaskets. 10. It should not have a high level of toxicity. 11. It should have good anti-wear properties. 12. It should have a high oxidation resistance. 13. It should have good anti-rust properties. 14. It should have good resistance to forming 15. It should have good water separating ability. • 1. 2. 3. 4. 5. 6.

TYPE OF HYDRAULIC FLUIDS Following are the types of hydraulic fluids: Pete oleum base fluids Fire resistance fluids High water content fluids (HWCF) Water in oil emulsions Water Gycol fluids Synthetic fluids

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Petroleum Base Fluids: Almost 80% oil sold each year are petroleum base fluids. Three basic types of mineral oils are used: 1. Pennsylvania or paraffin base oils. 2. Gulf Coast or naphthenic and asphaltic base oils. 3. Mixed base oils containing both naphthenic and paraffin compounds. Petroleum based fluids are refined from selected crude oil and formulated with additives to prevent rust, wear, oxidation and foaming. The characteristic or properties of petroleum oil fluids depend on three factors: • The type of crude oil used • The degree and method of refining • The additives used. Advantages: 1. Petroleum oil has natural ability to transmit fluid power efficiently. 2. It has good lubricating and anti wear properties. 3. Good heat dissipation under normal operating condition. 4. Compatible with most sealing materials. 5. It is long lasting and stable when operating temperatures are below 70o C. 6. It is easy to keep clean by filtration and gravity separation of contaminants.

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Disadvantages: 1. The main disadvantage is that it can burn. Hence, it cannot be used in applications like heat treatment, hydroelectric welding, die casting, etc. 2. Petroleum oils are naturally susceptible to oxidation since oxygen readily combines with both carbon and hydrogen in the oils make up. •

1. 2. 3. 4.

Fire Resistant Fluids A fire resistant fluid is one that is not only difficult to ignite, but will not combustion and propagate the flame when the ignition source is removed. Since, the fire resistant fluids have poor flammability, they reduce fire hazards. Fire resistant fluids are of four basis types: High Water Content Fluids – HWCF (90-95% water) Water-in-oil emulsions (40% water) Water Gycols (35-45% water) Synthetic Fluids Typically a hydraulic fire starts when a hose bursts and sprays fluid against a hot surface. The various mechanisms used to resist fire are analysed below:

•

1.

2.

3.

1. HWCF fluids: They do not burn because the water content (90-95% water) to additive ratio is too high for the additive to ignite. 2. Water glycol and water-in-oil emulsions: These are not totally fire resistant but they burn when the temperature is sufficiently high over an extended period of time. The water content in these fluids turn to steam and snuffs out the fire. 3. Synthetic fluids: They prevent ignition of fluid because of their fire resisting qualities. Fire resistant fluids should be used where any hazards exist that may danger human life or destroy valuable property. Hence, they are commonly used in coal mines, hot metal working processes, in foundries and in fluid power systems for air craft and marine. High Water Content Fluids (HWCF) Typically HWCF fluids are 95% water and 5% additive or 90o water and 10% additive. The additive portion consists of viscosity improver (water is too thin), anti friction additives, rust and corrosion inhibitors, defoaming agents and biocides and fungicides (to control bacterial growth). HWCF are not offered adversely by fine filtration and the additives do not separate out. There are a number of HWCF fluids available, each with its own chemical composition and performance characteristics. Micro emulsions: These are formulations with emulsified particles less than one micron dispersed in a solution which is nearly clear in concentrate form, but turns opaque when used in a hydraulic system. They have high film strength and good wear resistance properties. Synthetic soulables: They contain non petroleum water soluble lubricants in an aqueous solution. The lubrication molecules are polarised and thus attracted to the metal parts in the system where they form a plating that serves as a boundary lubrication layer between moving parts in the system. Thickened fluids: The additives form mechanical matrix which gives the water solution the viscosity and lubricity of a petroleum based fluid, under high shear. Advantages: 1. HWCF fluids are highly fire resistant. 2. They have high specific heat and thermal conducting characteristics. Also since 90% of the fluid is water, they provide excellent cooling. 3. They are least expensive of all hydraulic fluids. The cost of HWCF fluids is 20% that of petroleum base fluids. Page | 18


Disadvantages: 1. The temperature must be kept within the range of 5-50oC to prevent freezing and evaporation, both of which can cause the fluid condition and concentration to change. 2. Since the viscosity of most HWCF fluids is lower than petroleum oil, they tend to scour the system and carry foreign particles in suspension. So fine filtration is required. 3. The high water content requires close monitoring of the added oil additives to keep the chemical balance correct and control the microbe level. 4. Central reservoirs, mounted above the pump inlet is preferred since, then the fluid will be added, conditioned and monitored at one location. It will also prevent pump cavitation. 5. Neo-synthetic or Macro-emulsion: Here synthetic lubrication droplets are in the 1-10 micron range. Due to relatively high density and low viscosity of the fluid, inlet conditions and fluid conductor sizing should be carefully controlled to keep the fluid at a relatively low velocity. Excessive turbulence can cause cavitation. 6. Petroleum compatible paints cannot be used because of solvent effects of the fluid. Temperature Considerations: Operating temperature should be limited to a maximum of 50o C in order to minimize evaporation and deterioration of the fluid. Temperature below freezing (0oC) may cause separation of the phases or otherwise affect the fluid additives. •

Water in oil emulsions: They are called as invert emulsions. They are mixture of oil and water with the water percentage as 35-40%. Here each droplet of water is covered by oil. Thus the oil is the continuous phase. Advantages: 1.Due to the presence of water, they are fire resistant. 2.Dispersed water gives the emulsion fluids better cooling ability. 3.Because oil is the continuous phase in the emulsion, they mostly have better lubricating properties than HWCF fluids. 4.They are not corrosive to metals normally encountered in Industrial hydraulic systems. They are easily compatible with seals normally used for petroleum base fluids. Disadvantages: 1. Low operating temperatures of not more than 50o C to avoid evaporation and oxidation. 2. Evaporation causes loss of certain additives which is turn reduce the life of fluids. So, the water content must be maintained. Also the relevant additives should be added after consultation with manufactures. 3. Phase separation can occur due to emulsions getting trapped in stagnant areas or due to repeated freezing or thawing. Re-emulsification can be effected by circulating through pump. 4. Viscosity decreases as water content is reduced. Also viscosity decreased due to high shear in the hydraulic pump. Hence, to compensate this, a viscosity level higher than petroleum oils is initially manufactured into the fluids purposely. 5. Higher specific gravity of these fluids can affect the pump inlet conditions. 6. Emulsion fluids tend to hold dirt and fine metallic particles in suspension more readily than petroleum oils. Hence, filtration requires magnetic plugs to remove ferrous material. • • • •

Water Gycol Fluids Water Gycol fluids are a mixture of 35-45% water Gycol (usually ethylene gycol), and A high viscosity, lubricating and thickening agent. Page | 19


Additives are added to prevent foaming, rusting, and corrosion. Water glycol fluids are designed for use in hydraulic systems working in areas with a source of heat, ignition or where there is a potential fire hazard.

1. 2. 3. 4. 1. 2. 3. 4. 5. 6.

7.

Advantages: Due to presence of water they are fine resistant and have good cooling ability. At low speeds and loads, good wear resistance and lubricating properties. They have low toxicity and do not cause irritation to the skin. They are compatible with seals, hoses and packing material used for petroleum oils. Disadvantages: Water gycol fluids are limited to use in low to medium pressure and non critical application because their water content limits their lubricating and wear resisting qualities. Low operating temperatures of not more than 50o C to avoid evaporation and oxidation. Evaporation may cause loss of certain additives thereby reducing the life of the fluids, since they can absorb water. Water content must be continuously checked and maintained to keep the viscosity at a suitable level. Asbestcs, leather and cork impregnated materials should be avoided in rotary seals, since they can absorb water. Water gycol fluids attack zinc, cimum and magnesium forming a sticky or gummy substance which can plug orifices and filters and cause valve spool to stick. Hence parts which are galvanised should be avoided. The cost of water gycol fluids is greater than that of petroleum base fluids. • Synthetic Fluids Synthetic fluids are artificially synthesized chemicals which are less flammable than petroleum oils. Typically these are • Straight phosphates esters • Oil synthetic blends which have a phosphate ester base • Halogenated (chlorinated and/or fluorinated) hydracarbons. Synthetic fluids prevent ignition because of their fire resistant qualities. They are suitable for all applications.

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Chapter – 4 Sealing Devices and Pipes SECTION TION I: SEALING DEVICES •

INTRODUCTION The success of applying fluid power to any application depends largely on the ability of the sealing device to prevent both internal and external leakages in the system. Oil leakages anywhere in a hydraulic system, re reduces duces efficiency and power losses. Internal leakages does not result in loss of fluid from the system because the fluid returns to the reservoir. Most hydraulic components possess clearances that permit a small amount of internal leakage. This internal le leakage akage increases as the component clearances between mating parts increase due to wear. If the system leakage becomes large enough, system will not operate properly. External leakage represents the loss of fluid from the system. Improperly assembled pipe fitting itting is the most common cause of external leakage. Seals are used in hydraulic systems to prevent excessive internal and external leakage and to keep out contamination.

•

TYPES OF SEALS: a) Positive seals: These seals do not allow any leakage whatsoever (external external or internal) b) No positive seals: Permit a small amount of internal leakage (e.g. clearance used to provide a lubricating film between a valve spool and its housing bore) c) Static seals: Static seals are used between mating parts that do not move rrelative to each other fig. 4.1 shows some typical examples which includes flange gaskets and seals. (Ref. Fig. 4.1) these seals are compressed between two mating parts. d) Dynamic seals: These sales are assembled between mating parts that move relative to each h other. Hence, dynamic seals are subject to wear because one of the mating parts rubs against the seal. The following represents the most widely used types of seal configuration (Ref. fig. 4.2) Washer Cup packing Flange Packing U – packing V – packing O – Ring

1. 2. 3. 4. 5. 6.


Washer

Cup Packing

Flange Packing

U – Packing

V – Packing

O – Ring Fig. 4.2: U – V, Cup and Flange shaped seals. •

SELECTION OF SEALING DEVICES Following are some conditions which affects the selection of sealing device for a particular application: 1) Speed: Speed is an important factor in determining frictional temperature buildup, which results in wear. Seals may also be affected by frequency depending upon the revolutions per minute. This is important when a seal must operate satisfactory under eccentricity, wobble etc. 2) Pressure: Pressure in the system in with a sealing device increases the force. This increases friction, causing heat build up and more wear of the components. 3) Temperature: Temperature at the point of seal depends on the materials in , the shaft speed, pressure, amount of lubrication, heat from bearings, heat from fluid etc. The sealing device selected must be unaffected by frictional temperature. 4) Compatibility between sealing devices, the fluid and other materials that make up the complete system is extremely important. Foreign materials, corrosion, excessive heat and many other conditions can affect sealing operations. Page | 22


•

SEALING MATERIALS Sealing materials may be classed into three general categories (see table 4.1) 1. Leather 2. Fabricated rubber 3. Homogeneous 1. Leather: Leather is the oldest material used for sealing devices, and still very popular for many applications. Leather seals have low frictional properties as and relatively high tensile strength. Leader is used for applications. Where higher system operating pressures are used. These are manufactured in u, v, cup and flange shapes. Fabricated rubber sealing devices: These are composed of synthetic rubber 2. compounds and fabrics. The fabrics reinforce the synthetic rubber to give sealing devices resistance to extrusion. Cotton duck, asbestos and nylon are three common kinds of fabrics in use. Duck is use for normal operating temperature, asbestos for high temperature operation, and nylon for greater strength and flexibility, Fabricated seals have a wider operating temperature range than leather. 3. Homogeneous sealing devices: these are compounded from many different base polymers of synthetic rubber. They are made in many hardness, depending on the shape, application, and intended operating pressure. Homogeneous seals operate over a wide temperature range, similar to that of fabricated seals. Natural rubber is rarely used as a seal material because it deteriorates with time in the presence of oil. In contrast, synthetic rubber materials are compatible with most oil.

•

COMPATIBILITY OF SEALS WITH FLUIDS Synthetic seals because of their great diversity, will vary in their compatibility with Seals. The phosphate easters are not compatible with the commonly use nitrile (Buna) And neoprene seals. Therefore, a changeover from petroleum, SECTION II : PIPES

•

FLUID CONDUCTORS: Fluid conductors is a general term which includes various kinds of conduction lines that carry hydraulic fluid between components plus the fitting or connectors used between the conductor. Hydraulic system today use principally four types of conducting lines – 1. Steel pipes 2. Steel tubing, 3. Flexible hoses. 4. Plastic tubing. 1. Steel pipes: Pipes and pipe fittings are classified by nominal size. Pipes have tapered threads, as opposed to type and hose fittings, which have straight threads. Hydraulic pipe threads are the dryseal type. Ref. fig. 4.3

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They differ from standard pipe threads because they engage the roots and crests before the flanks. In this way, spiral clearance is avoided. Pipes can have only m male threads, and they can not be bent around obstacles. The large number of pipe fittings gives rise to leakages, especially as the pressure increases. 2. Steel Tubing: Seamless steel tubing is the most widely used type of conductor for hydraulic systems, as it provides significant advantages over pies. The tubing can be bent into almost any shape, thereby reducing the number of required fittings. Tubing is easier to handle and can be reused without any seating problems. For low volume systems, tubing can han handle the pressure and flow requirements with less bulk and weight. However, tubing and its fittings are more expensive. Tubing is not sealed by threads but special kind of fittings eg. Compression fittings. Flare fitting etc. 3. Plastic tubing: Plastic tubing gained rapid acceptance in the fluid power industry because it is relatively inexpensive. Also, it can be readily bent to fit around obstacles, obstacles it is easy to handle, and it can be stored on reels. Another advantage is that it can be colour-coded colour to represent different parts of the circuit because it is available in many colours.. Since plastic tubing is flexible, it is less susceptible to vibration damage than steel tubing. Fittings for plastic tubing are almost identical to those designed for steel tubing tubing. In fact many steel tube fittings can be sued on plastic tubing. Plastic tubing is used universally in pneumatic systems because air pressures are low, normally less than 100 psi. Of course, plastic tubing is compatible with most hydraulic fluids and hence is used in low--pressure hydraulic applications. Materials for plastic tubing include polyethylene, polyvinyl chloride, polypropylene, and nylon. Each material has special properties that are desirable for specific applications. 4. Flexible Hoses: The fourth urth major type of hydraulic conductor is the flexible hose, which is used when hydraulic components such as actuators are subjected to movement. Examples of this are found in portable power units, mobile equipment, and hydraulically powered machine tools. Hose is fabricated in layers of elastomer (synthetic rubber) and braided fabric or braided wire, which permits operation at higher pressures.

Fig. 4.4: Construction of flexible hose


As illustrated in Fig. 4.4, the outer layer is normally synthetic rubber and serves to protect the braid layer. The hose can have as few as three layers (one being braid) or can have multiple layers to handle elevated pressures. When multiple wire layers are used, they may alternate with synthetic rubber layers, or the wire layers may be placed directly over one another. •

QUICK DISCONNECT COUPLINGS One additional type of fitting is the quick disconnect coupling used for both plastic tubing and flexible hose. It is used mainly where a conductor must be disconnected frequently from a component. This type of fitting permits assembly and disassembly in a matter of a second or two. 1. Straight through: This type offers minimum restriction to flow but does not prevent fluid loss from the system when the coupling is disconnected. 2. One –way shutoff: This design locates the shutoff at the fluid source connection but leaves the actuator component unblocked. Leakage from the system is not excessive in short runs, but system contamination due to the entrance of dirt in the open end of the fitting can be a problem, especially with mobile equipment located at the work site. 3. Two-way shutoff: This design provides positive shutoff of both ends of pressurized lines when disconnected. Such a coupling puts an end to the loss of fluids. As soon as you connecting, the plug s an O-ring in the socket, creating a positive seal. There is no chance of premature flow or waste due to a partial connection. The plug must be fully seated the socked before the valves will open. •

SELECTION OF PIPES FOR HYDRAULIC SYSTEM The choice of which type of conductor to use depends primarily on the system’s operating pressures and flow rates. In addition, the selection depends on environmental conditions such as the type of fluid, operating temperatures, vibration etc. Conducting lines are available for handing working pressures up to 10,000 psi or greater. In general, Steel tubing provides greater pluming flexibility and neater appearance and requires fewer fittings than piping. However, Steel piping is less expensive than steel tubing. Plastic tubing is finding increased industrial usage because it is not costly and circuits can be very easily hooked up due to its flexibility. Flexible hoses are used primarily to connect components that experience relative motion. They are made from a large number of elastomeric (rubber like) compounds and are capable of handling pressures exceeding 10,000 psi. Stainless steel conductors and fittings are used if extremely corrosive environments are expected. However, they are very expensive and should be used only if necessary. Copper cium conductor should not be used in hydraulic systems because the copper promotes the oxidation of petroleum oils. Zinc, magnesium, and cium conductors should not be used either because they are rapidly corroded by water – glycol fluids. Galvanmed conductors should also be avoided because the galvanized surface has a tendency to flake off into the hydraulic fluid. When using steel piping or steel tuning, hydraulic fittings should be made of steel except for inlet, return, and drain lines where malleable iron may be used. Conductors and fittings must be designed with human safety in mind. They must be strong enough not only to withstand the steady-state system pressures but also the instantaneous pressure spikes resulting from hydraulic shock. Whenever control valves are Page | 25


closed suddenly, this stops the fluid, which possesses large amounts of kinetic energy. This produces shock waves whose pressure levels can be two to four times the steady-state system design values. Pressure spikes can also be caused by sudden stopping or starting of heavy loads. These high-pressure pulses are taken into by the application of an appropriate factor of safety. A conductor must have a large enough cross-sectional area to handle the flow-rate requirements without producing excessive fluid velocity. • 1) 2) 3) 4) 5) 6) 7)

PRESSURE DROP IN HOSES/PIPES Pressure drop in hoses/pipes depends upon the following factors: Flow rate Pipe diameter, pipe length, pipe geometry A small change in bore size can have a marked effect on pressure drop. Doubling the pipe length results in doubling the pressure drop. Pressure drop is essentially a result of change in the energy form as some of the pressure energy is exchanged for velocity energy and heat. Some of the energy is lost along the way because of resistance and friction. Sudden contractions, sudden enlargements, number and kinds of bends, size and smoothness of pipes as well as the temperature and properties of fluid effects the pressure in the system.

Questions 1. 2. 3. 4. 5. 6. 7. 8. 9.

What are different types of seals? State the criteria for selection of sealing devices What are the various sealing materials available? Write a short note on ‘Compatibility of seals with fluids”. What is meant by “Fluid conductors”? What are the various types of piping available? Write in brief about “Selection of pipes for hydraulic system. Explain “Quick disconnect couplings” with application. List the factors affecting the pressure in piping.

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Chapter – 5 Fluid Conditioning •

INTRODUCTION Conditioning the hydraulic fluid means to provide ample s for the fluid in a system to maintain its proper operating temperature, and to keep it clear and free of contamination. Different fields of applying fluid power requires varying degrees of fluid conditioning. Reservoirs, heat exchangers, filters and strainers, are all important components of a fluid power system to assure smooth and trouble free operation of machines and equipment. 40 to 60% of all the trouble in hydraulic circuits results from improper care of the hydraulic fluid.

•

HYDRAULIC RESERVOIR CAPACITY The hydraulic reservoir should contain enough fluid that its working level is always maintained during the systems operation. It should also have additional capacity to hold all the fluid in a system during shut downs. The reservoir capacity is generally between two and three times the capacity of Hydraulic pump.

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HEAT EXCHANGERS Coolers and heaters are called heat exchangers and are used to control fluid operating temperatures in a hydraulic system. Heat exchangers are used to control and maintain proper operating temperatures of the system fluid. The steady temperature reached by the fluid in a hydraulic system depends on both the amount of heat generated and heat dissipating ability of the system. If heat energy is allowed to accumulate in a hydraulic system, high temperatures result with possible damage to seals, fluids and moving parts. High temperature also affects the viscosity of fluid, which may change the operating performance characteristics of machine. The most popular types of heat exchangers used with hydraulic system use air or water as cooling medium. The round core with plate fins is generally used for fluids having a high rate of heat transfer. If can be used also for systems with high operating pressures. This design operates more efficiently with forced air circulation when used for cooling viscous fluids. Water cooled heat exchangers of the shell and tube design are generally used for hydraulic system when ample water supply is available. A heat exchanger of this type consists of an assembled bundle of tubes inserted into a shell. The tubes are baffled to direct the hydraulic fluid through the shell side of the unit at right angles to the tube bundle. The cooling medium generally flows through the tube.

The main factors for selecting the proper heat exchangers are as follows. 1. Determine the actual heat generated during system operations. 2. Select the hydraulic fluid with careful consideration of type of fluid, viscosity, density, specific heat, flow rate and inlet temperature. 3. For the cooling fluid, determine. a) Flow and inlet temperature for water cooled exchangers. b) Ambient temperature for air cooled units. Page | 27


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SOURCES OF CONTAMINATION IN A HYDRAULIC SYSTEM: A contaminant nant is any material foreign to a hydraulic fluid that has a deterious effect on the fluid’s performance in a system. Hydraulic fluids get contaminated by gathering impurities. In a hydraulic systems, fluid contamination is prominent due to substances. Excessive Excessive heat energy causes severe degradation of fluid leading to system failure. Electromagnetic radiations contaminate hydraulic systems often generate noise thereby polluting the environments. The sources of contamination in hydraulic systems can be ddivided ivided into three general categories. 1. Built in contamination. 2. Ingressed contamination 3. Self-generated generated contamination. • Build – in Contamination: Hydraulic draulic system manufacturers generally are careful to provide internally clean products but, in spite of the these efforts, new equipment usually contains some built-in Contamination. These contaminants might include burrs, chips, flash, dirt, dust, fibers, sand, moisture, pipe sealants, weld splatter, paints and flushing solutions. New components within a system syste may also become sources of contamination due to improper storage, handling, and installation practices. New directional valves, and pumps may contain contaminants that appear in the system fluid after a very short period of operation. As the machine is assembled, the reservoir may accumulate rust, paint chips, dust, cigarette butts, and even paper cups (see Fig. 5.1). Although the reservoir is cleaned prior to use, many contaminants are invisible to the human eye and are not removed by wiping with a rag ag or blowing off with an air hose. Contaminants such as weld scales may not break off and enter the fluid stream until they are loosened by high-pressure high pressure fluid forced between them and the parent metal or by vibration of the machine while it is running. • Ingressed ngressed Contamination: Ingressed or environmental contamination contamination that is added to the hydraulic system during servicing or maintenance (or from lack of maintenance) or is introduced to the system from the environment surrounding the equipment. One common way in which contamination may be ingressed occurs when the system is filled with new oil (Fig. 5.2). New oil is refined and blended under fairly clean conditions but when it is delivered and pumped through filling lines, metal and rubber particles from the lines may enter the storage tanks along with the new oil. The storage tanks also may contain rust generated by the condensation of moisture. If new oil is stored under reasonably clean conditions, the most common contaminants in makeup flu fluid id are metal, silica, and fibers.

Dhanvantari College of Engineering, Nasik Drum of clean Oil

1.5 billion particles

Cylinder

(a) Oil Storage Drum

(b) Cylinder rod Fig. 5.2

Dirt and other particulates can enter the system during servicing and maintenance. Components are usually replaced or repaired on site in an unclean environment. Contamination from the area around the equipment can enter the system from any disconnected line or port. Another source of increased contamination is the air breather cap on the reservoir. Air enters the reservoir through the air breather cap every time a pump cycles or an actuator is filled and discharged with fluid. The breather is actually a coarse screen that allows unfiltered dirt into the system. Many times, a breather cap that has become clogged due to lack of maintenance is removed and never replaced. The clogged breather also may be totally overlooked so the air required to assist fluid in reaching the pump finds another path. This exposes the reservoir to the entry of further contamination. Contamination from the environment also can enter the system through power unit access plates that have been removed and not replaced. If access to strainers or other components depends on the removal of power unit covers, good resealing may not be possible. Another main source of environmental contamination occurs when cylinder rods remain extended in a heavily contaminated atmosphere for long periods of time. Fine particles may settle on the rod and then be pulled into the system when the rods are retraced (Fig. 5.2 b). As seals and wipers on these rods wear, the contamination ingression rate can increase considerable.

• Self – Generated Contamination: This type of contamination is created internally within the system by the moving parts of hydraulic components. These contaminants are produced by wear, corrosion, cavitation, and decomposition and oxidation of the system fluid. Every internal moving part within the system can be considered a source of self generated contamination for the entire system. Component housings that are often subjected to flexing and other stresses also can contribute to contamination in the form of metal particles and casting sand. Water vapour that enters the system through the reservoir can condense on the walls of components and conductors during equipment shutdowns. Eventually, rust can form and be washed into the system.

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Dhanvantari College of Engineering, Nasik Table 5.1: Guidelines for controlling contamination in hydraulic systems.

Contamination Source Inbuilt in components, pipes, manifolds, etc. Plus Present in initial charge of fluid

Plus Ingressed through air breather Plus Ingressed during fluid replenishment Plus Ingressed during maintenance Plus Further generated contamination produced as a result of the above and the severity of the duty cycle.

•

1. 2. 3. 4. 5. 6. 7. 8.

Controller Good flushing procedures, system not operated on load until acceptable contamination level obtained. Integrity of supplier. Fluid stored under correct conditions (exclusion of dirt, condensation, etc.). Fluid filtered during filling. An effective air breather with rating compatible with degree of fluid filtration. Suitable filling points which ensure some filtration of fluid before entering reservoir. This task undertaken by responsible personnel. Design should minimize the effects. Correct fluid selection and properties (viscosity and additives) maintained. Good system design minimizing effects of contamination present of system components.

CONTAMINATION CONTROL Controlling contamination in any hydraulic system is an on going process that can greatly improve system performance. The effect to keep contamination to a minimum begins with the system design process and continues throughout the useful life of the equipment. Although built in and Ingressed contamination is a continuous problem in any hydraulic system, steps can be taken during assembly and serving to minimize their effects. Procedure exists for tubing and conductor cleanliness, for component washing, drying and storing, for various flushing methods, and for filling a system with new oil. Practicing these cleanliness techniques results in a cleaner system. In operation care of Hydraulic fluid includes: Preventing contamination by keeping the system right and using proper air and fluid filteration devices and procedures. Establishing fluid change intervals so the fluid will be replaced before it breaks down. Keeping the reservoir filled properly to take advantage of its heat dissipating characteristics. Repairing all leaks immediately. Inspect and clean or replace oil filters in the system. It is also important that the right fluid be used and that its temperature be properly maintained. Clean equipment and clean workstations are essential when servicing or maintaining hydraulic equipment. Good maintenance and service practices can prevent contamination and expensive down time.

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•

• •

STRAINERS AND FILTERS Strainers and filters are used to remove a contaminating particles from the hydraulic system. The term strainer and filter is used interchangeable, because they have a common function. Nation Fluid Power ower Association defines these as – Filter – a device whose primary function is the retention, by some porous medium, of insoluble contaminants from a fluid. Strainer – a coarse filter. To put it simply, whether the device is a filter or strainer, iits ts function is to trap contaminants from fluid flowing through it. “Porous medium” simply refers to a screen or filtering material that allows fluid to flow through it, but stops other materials. •

FILTER LOCATION There are three general areas in the syste system m for locating a filter the inlet line the pressure line, or a return line. Both filters and strainers are available for inlet lines. Filters alone are generally used in other lines. Off – line filtration systems are also available available, 1. Inlet Strainers and Fil Filters.. Figure 5.3 shows a typical strainer of the type installed on pump inlet lines inside the reservoir. It is relatively coarse as filters go, being constructed of fine mesh wire. A 100-mesh 100 mesh strainer, suitable for thin oil, protects the pump from particles particl above about 150 microns in size.

There also are inlet line filters (see Fig. 5.4). These are usually mounted outside the reservoir near the pump inlet. They too, must be relatively co coarse. arse. A fine filter (unless it’s very large) creates more pressure drop than can tolerated in an inlet line of a pump.


The level of contamination entering the pump is a critical factor. Intel filters should be used only to prevent large particles from entering the pump and causing catastrophic failure. Pressure Line Filters Filters.. A number of filters are designed for installation right in the 2. pressure line (Fig. 5.5) and can trap much smaller particles than inlet line filters. Such a filter might be used where system components such as valves are less dirt dirt-tolerant tolerant than the pump. The filter traps fine contamination form the fluid as it leaves the pump and also protects the system in the event of a catastrophic fai failure of the pump.


Downstream of the pump, the ability of a pressure filter to trap particles is influenced by flow and pressure transients which ten tend d to drive particles through the filter media. Pressure line filters must be able to withstand the operating pressure of the system as well as any pump pulsations. Changing a pressure line filter element requires shutting down the hydraulic system. 3.Return Line Filters: Fig. 5.6 shows Return line filter also can trap very small particles before the fluid returns to the reservoir. They are particularly useful in systems which do not have a large reservoir which allow contaminants to settle out of the fluid. A return line filter is nearly a must in a system with a high – performance pump, which has very close clearances and usually cannot be sufficiently protected by an inlet line filter. Full-flow flow return filters. Should have enough capacity to handle maximu maximum return flow without opening the by valve. The performance of any return line filter depends on the magnitude of flow and pressure changes. 4.Off-Line Line Filter Systems.


The effectiveness of pressure and return line ffilters ilters line filters is reduced by shock, surges, pulsation, and vibration, depending on media types and how well they are ed. Steady flow, relatively free of pressure fluctuations, provides optimum filter performance. The simplest way to achieve this this is to remove the filter from the main system and place it in an independently powered circulating system where filter performance is more predictable. Ref. Fig. 5.7 Off – line filter systems in which reservoir fluid is circulated through a filter at a constant nstant rate are sometimes used when operating system conditions are severe and the needed quality of filtration is difficult to obtain within the operating system. These systems may be an integral part of the machinery or may be a portable unit for use whe whenn and where necessary. With off-line line filtration, flow rate or filter type can be altered readily without affecting the design of the main system. Furthermore, the off off-line line filter system can be run before starting the main system to clean the fluid in the reservoir and reduce the contamination level the pump is subjected to at start-up. start Being independent of the main hydraulic system, off off-line filters lters can be placed where they are most convenient to service. •

MINIMUM SIZE OF PARTICLES TO BE TRAPPED: A simple screen or a wire strainer is rated for filtering fineness by a mesh number or its near equivalent standard sieve number. The higher the mesh or sieve number, the finer the screen.


Filters used to be described by nominal and absolute ratings in microns. A filter nominally rated at 10 microns, for example, would trap most particles 10 microns in size or larger. The filter’s absolute rating, however, would be a somewhat higher size, perhaps 25 microns. The absolute rating was, in effect, the size of the largest opening or pore in the filter. Absolute rating was an important factor only when it was mandatory that no particles above a given size be allowed to circulate in the system. Filter rating today is far more sophisticated than in the days of the old nominal and absolute ratings. Manufacturers still use the “10 or 25 micron filter” name, but also use Beta ratios to more closely identify how well a filter does its job. Beta Ratio: The Beta ratio or rating is used for fine filters and is determined under laboratory testing using a procedure developed in the early 1970s. Although not a true measure of how well a filter will do in an operating system, the Beta rating is a good indicator of the filtration performance. The Beta ratio of an operating filter during steady state flow test is simple the count upstream divided by the count downstream of fine test dust, based on any selected particle size: Number of upstream particles Beta X Number of downstream particles In the formula, X is the particle size in microns. A ratio of 1.0 means that no particles are stopped. A ratio of 75 to 1 means that 75 particles are stopped for ever one that gets through, an efficiency of 100 – (100 75) or 98.7 percent. Most filters with absolute ratings have Beta ratios of over 75 to 1. For a filter with a Beta ratio greater than 1 the downstream concentration of particles above a given size will stabilize to provide an almost constant contamination level. For example, to select a filter for silt control, specify a filter with a Beta ratio of B3-5 = 75. For partial silt control, filter with a B10-5 = 75 might be chosen. For chip removal only, the Beta ratio might be N25-40 = 75. Beta ratios are plotted and manipulated in many ways to show separation efficiency, apparent capacity, actual capacity, and filter life profile all for any chosen particle sizes within the capability of the instruments. • SELECTION OF FILTERS The following information is important in order to select the proper filter for any application: 1. The size and physical nature of the detrimental contaminates to be removed 2. The viscosity, density, operating temperature, and corrosive properties of the hydraulic fluid being used. 3. The materials used for fabricating the system before the filter and after the filter, seals, fittings, tubing, hose, instruments, and other parts should be analysed. 4. The operating characteristics of the system, such as shock load, fluid velocity, and direction. 5. Pressures and pressure drops that the filter media must encounter during the system’s operation 6. The component in the system that requires the closest tolerances and hence the least contamination.

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•

FILTERING MATERIALS There are two basic classifications of filtering materials: absorbent or adsorbent. Absorbent filter medium traps particles by mechanical means. Absorbent media are divided into two basic types: surface and depth. Surface media may be woven screen, disc, or etched types and are most commonly used for finer filtration. Depth media are generally used for finer filtration and are made of a wide range of materials. The most common hydraulic filter media are combinations of cellulosic, synthetic, and glass fibers. These materials are blended for specific performance characteristics and durability and are usually resin impregnated to provide added strength. Adsorbent or active, filters such as charcoal and Fuller’s earth should be avoided in hydraulic systems since they may remove essential additives from the hydraulic fluid. •

1.

2. 3. 4. 5. 6. 7. 8. 9. •

FILTRATION SYSTEM REQUIREMENTS In General, the practical and performance requirements of a filtration system can be summarized as follows: The system must be capable of reducing the initial contamination to the desired level within an acceptable period of time, without causing premature wear or damage to the hydraulic components. The system must be capable of achieving and maintaining the desired level and must allow a suitable safety factor to provide for a concentrated ingress which could occur. The quality of maintenance available at the end location should be considered. Filters must be easily accessible for maintenance. Indication of filter condition to tell the when to replace the unit must be provided. In continuous process plants, facilities must be provided to allow changing of elements without interfering with plant operation. The filters must provide sufficient dirt holding capacity for an acceptable interval between element changes. The inclusion of a filter in the system must not produce undesirable effects on the operation of components, for example, high back pressures on seal drains. Sampling points must be provided to monitor initial and subsequent levels of contamination. BENEFITS FROM PROPER CONDITIONING OF THE HYDRAULIC FLUID: Using the correct hydraulic fluid, selected especially for the equipment used as well as for the operating conditions involved, is most important. Proper conditioning of the fluid selected must be maintained continually to keep the fluid clean and at proper operating temperature if the hydraulic system is to function properly and given long life. Proper conditioning provides the following benefits: 1. Reduction of power losses: The use of properly sized reservoirs and adequate coolers reduces system leakage by controlling the viscosity of the fluid with temperature control. This also helps to minimize friction by maintaining adequate lubrication between moving parts in pumps, fluid motors, and cylinders. When these properties are maintained, power losses are minimized. 2. More continuous production: When the hydraulic fluid is properly conditioned, less pressure drop occurs, which maintains good control, fast response, and accurate timing of machine operations. Strainers, filters, and magnets play an important part in keeping the system clean and preventing costly interruptions in production. Page | 36


3. Reduction of cost of maintenance: The life expectancy of hydraulic equipment under normal operation conditions can be many years. Hydraulic equipment has been known to operate 15 to 20 years or move without trouble. The main reason for this kind of service life is the fact that the hydraulic fluid was kept clean and properly conditioned. Detrimental contamination and high temperatures can create excessive maintenance costs, if not controlled. When system fluids are properly conditioned, it requires a minimum amount of maintenance to keep equipment in good operating condition. 4. Reduction of the cost of hydraulic fluid: Good tight connection are, of course, needed to prevent external leakage. Temperature control is also important, because hot oil leaks more readily than oil at its proper operating temperature. When hydraulic fluid is overheated, it oxidizes rapidly and loses its important physical properties. A good hydraulic fluid does not wear out or lose its desirable properties unless it is overheated or contaminated with proper conditioning equipment increases the life of the fluid and the system’s components.

Questions 1. 2. 3. 4. 5. 6. 7. 8.

What do you mean by fluid conditioning? What are the various sources of contamination? Explain in brief. Write a short note on : Contamination and its control. Define filter and strainer. Write in brief about ‘Filter Location.’ Explain Beta Ratio. What are the materials available for filter? Given guidelines for selecting a filter. What are the benefits from conditioning of the hydraulic fluid?

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Chapter – 6 Hydraulic pumps •

INTRODUCTION In a hydraulic system, the pump converts mechanical energy into hydraulic energy. This mechanical energy is delivered to the pump via a prime mover. The prime mover used depends on the type of application as indicated below: (i) A.C. induction motor rotating at a constant speed of 1500 rpm or at 1200 or 1800 rpm (at 50 Hz supply), are generally used. Often pump and electric motor are supplied as one unit. (ii) Mobile hydraulic application such as power steering units, backhoe pumps, farm tractors and utility vehicles are driven at varying speeds upto 3600 rpm. Depending on the size of the equipment by power take off arrangements directly from the I.C. engine or the transmission. (iii) Air and space craft hydraulic systems, where weight considerations are critical and substantial power must be generated from small self contained units, high speed pumps driven by D.C. electric motor to speeds of 12,000 rpm are used. Due to the mechanical action, the pump takes the hydraulic fluid from the reservoir and delivers it to the hydraulic circuit at system pressure. In doing so, it raises the energy level of the fluid. This high energy fluid is then used to do work like actuating an hydraulic cylinder or rotating an hydraulic motor. The smooth and effective working of the hydraulic system depends on the matching of the pump selected with the required fluid power actuator regarding its power requirements, pressure and flow characteristics, the speed range required and operating characteristic. Hence, a pump is known as the heart of a hydraulic system. Fig. 6.1: Pump Symbol The symbol of the pump is as shown in Fig. 6.1.

•

PUMPING THEORY A fluid is said to be pumped when its volume is displaced and transferred from one place to another. This pumping action is achieved by using a pump. All pumps operate on the principle whereby a partial vacuum is created at the pump inlet due to the internal operation of the pump. This partial vacuum causes the fluid to be sucked into the pump inlet from the oil reservoir which is vented to the atmosphere. The pump then mechanically pushes the fluid out into the discharge line.

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Outlet value 2

Motion provided by Prime - Mover

To system

Pumping Chamber

Atmospheric Pressure

Reservoir

Fig. 6.2: Pumping Action of a Simple Piston Pump The pumping action can be visualised by considering a simple piston pump as shown in Fig. 6.2. Essentially, a pump consists of a inlet port connected to the hydraulic fluid source (i.e. and oil reservoir), a pumping chamber attached to a drive mechanism (prime mover) and an outlet port connected to the hydraulic system. The pump consists of two ball check valves: Check Valve 1: It is connected to the pump inlet line and allows the fluid to enter the pump only. It is the outlet valve. Check Valve 2: It is connected to the pump outlet line and allows the fluid to leave the pump only. It is the outlet valve. Now, when the piston is pulled to the right, a partial vacuum is created in the pumping chamber. This vacuum keeps the outlet valve 2 in a closed position and allows the atmospheric pressure to push the fluid from the reservoir into the pump via the inlet valve 1, which is open. When the piston is pushed to the right, the fluid movement closes the inlet valve 1 and forcibly ejects out the fluid via the outlet valve 2 which opens into the discharge line. Thus, the fluid is pumped. Note: 1) A pump is not a source of power. The source of power is the prime mover which drives the pump. 2) A pump does not pump pressure. It produces flow. The pressure is developed due to the resistance afford by the hydraulic system to this flow. • PARAMETERS FOR ANALYSIS OF PUMP PERFORMANCE 1. Work done by pump: Energy or work equivalent added to the fluid by the pump is given by, work down W = Force F displacement S = pA S = p (AS) •

PUMP CLASSIFICATION The classification of pumps is as shown in the pump classification tree in Fig. 6.4

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There are re two basic types of pumps 1. Hydrodynamic pump or Non – positive displacement pump. 2. Hydrostatic Pump or Positive displacement pump. Table 6.1 Positive Displacement Pum Pump

1. When the pumping action displaces a constant amount of fluid per revolution

Non – Positive Displacement Pump

1. When the pumping action transfers the fluid using inertia principle, it is referred to as non-positive positive displacement pump.


of pump shaft, it is referred to as positive displacement pump. 2. In a positive displacement pump, the high and low pressure sections of the pumping chamber are separated so that the fluid cannot leak back and return to the low pressure side. Hence, positive displacement is assured. 3. Positive displacement pumps cause the fluid to move by varying (alternately increasing and decreasing) the physical size of the pumping chamber.

4. For a given pump size, since the volume pumped per cycle is fixed by the positive displacement characteristic of the pumping chamber, the volume of fluid pumped is dependent only on the number of cycles made by pump. Hence, the pump output flow, neglecting the small internal leakage is constant and not dependent on the resistance of the external system. 5. Due to very small clearances, these pumps are self priming. 6. When the valve is completely closed and there is no place for the fluid to go, the resistance of the system becomes infinite. The pressure rises continuous with each pump stroke until either the piping or the pump itself fails. Hence, these pumps must be protected against over pressure by providing pressure relief valves. 7. High volumetric efficiency (at 80-95%). Also the efficiency remains almost constant throughout the design pressure range. 8. High pressure (upto 700 bars) capabilities. 9. Because of high pressure capability., they are small and compact in design. 10. Close tolerances and specials designs result in costly installation and maintenance. 11. They are used where the primary consideration is pressure and power output. Hence, they are the heart of hydraulics. e.g. gear pump, piston pump, vane pump, etc.

2. In an non-positive displacement pump, both the inlet and outlet sections are connected. Hence, as the resistance in the system increases, fluid circulates within the pumping chamber. Hence, positive displacement is not assured. 3. The action of the mechanical drive in the pumping chamber (either centrifugal or spring force) speeds up the fluid so that its velocity s for its ability to move against the resistance of the system. 4. For a given size, the volume of fluid pumped is dependent on the speed of rotating member and the resistance the pump must overcome. When the resistance of the external system starts to increase, some of the fluid slips back into the pumping chamber, causing a reduction in pump output flow.

5. Due to large clearances, these pumps are not self priming. 6. When the resistance of the external system becomes infinitely large (for e.g. a close valve blocks the outlet line), then due to the internal circulation of the fluid in the pumping chamber, the pump will produce no flow. So, pressure regulation/relief is not required.

7. Volumetric efficiency varies from zero (infinite load condition) to at 60%.

8. Low pressure (17 – 20 bars), high volume capabilities. 9. Because of handing large volume of fluid, they are bulky and robust in design. 10. Because of their simple design and fewer number of moving parts, they cost less to install and maintain. 11. They are used where the primary consideration is the transfer of fluid from one location to another. Hence, they are used in pumping stations and factories. e.g. centrifugal (impeller) pump, axial (propeller) pump, etc.

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Note : Hydraulic pumps are invariable positive displacement type i.e. Hydrostatic in nature. • Type of positive Displacement Pump Positive displacement pumps are further classified as Unidirectional Bidirectional fixed or variable displacement type. 1. Fixed xed displacement Pump: Fixed displacement pump is the one in which the amount of fluid ejected per revolution (displacement) is constant. Here, the internal pump volume cannot be adjusted without replacing certain components, and hence the pump is considered sidered to have a fixed displacement. A unidirectional fixed displacement pump capable of delivering pressurised fluid from either port is indicated by the symbol of Fig. 6.6 (b). Fig. 6.6: Fixed Delivery, Positive Displacement Pump Symbol 2. Variable Displacement Pump: A variable displacement pump is the one in which the amount of fluid ejected per revolution (displacement) can be varied even though the pump speed remains constant. This displacement is varied by varying the size of the pumping chamber, using sing external controls. Note: The volume flow rate can also be varied by varying the drive speed. However, this is a less desirable alternative than varying the internal displacement to change the pump output. A slash arrow across the fixed displacement pump symbol indicates that the pump displacement can be varied. Fig. 6.7 (a) shows the simplified symbol, while Fig. 6.7 (b) shows the complete symbol indicating manual control by the parallel lines at the left with the vertical terminating line. The drai drain, n, if included is always shown in the complete symbol. Direction of rotation is also included, if pertinent. Bi-directional, directional, variable displacement pumps are illustrated as in Fig. 6.7 (c) & Fig. 6.7 (d)


• •

GEAR PUMPS External Gear Pump Fig 6.8 shows a typical external gear pump. It consists of a drive gear and a driven gear enclosed within a precision machined housing. The close fit between the meshing gears and the housing maintains a sea seall between inlet and outlet sides wear plates. The suction side is where the teeth come out of mesh and the discharge side is where the teeth go into mesh. As the teeth remesh on the suction side, they increase the volume of the inlet chamber causing a partial tial vacuum. This partial vacuum sucks the fluid into the port inlet from the reservoir which is vented to the atmosphere. This fluid now gets trapped between the gear teeth and the pump housing. When the gears rotate, the trapped fluid is transferred arou around the periphery of both gears and finally gets ejected into the discharge side. As the teeth remesh on the discharge side, they decrease the volume of the outlet chamber by an amount equal to volume increased on the suction side as the teeth mesh. This forces rces the fluid from the outlet port at system pressure.


Most gear pumps are spur, helical or herringbone designs i. ii. iii.

Spur type gear pumps are generally less expensive but aare re noisy at relatively high speeds. Helical type gear pumps are relatively less noisy, but they are limited to low pressure applications (below 15 bar) because they develop excessive end thrust. Herringbone gear pumps are the quietest; provide greater flow rates with much less pulsating action. They eliminate the thrust action and used to develop a much higher pressure (above 50 bar) Spur gears are an unbalanced design. As shown in Fig. 6.8, the output pressure against the teeth cause heavy side loads to act on the gears and the shaft. This limits both the pressure and the speed at which the pump can be operated. These external spur gear can be balanced by drilling ages either through the gears themselves or in the end plates to equalize the pressure in n opposite directions on the gears (see Fig. 6.10), Most gear pumps do not provide means for balancing the out of balance forces on the gears and ing shafts.

Pump Performance and characteristics: charact Let

Do = Outside diameter of gear teeth [m] Di = Inside diameter of gear teeth [m] L = Width of gear teeth [m] Vp = Displacement volume of pump [m3 / rev] N = rpm of pump QT = Theoretical Pump flow rate [m3 / s]

From the gear geometry, ometry, the volumetric displacement is given by, VD = Theoretical Flow Rate,

L


This eqn (6.12) shows that the pump flow rate varies directly with speed. Hence, theoretical flow rate is constant at a given speed. However, there are small clearnces (less than 25 micron) between the teeth tip and the pump housing. As a result some of the oil from the discharge port can leak directly back towards the suction side. This is tremble as slip. Due to this slip, the actual flow rrate QA is less than the theoretical flow rate QT. For spur gears, volumetric efficiency

is about 90%

Note: A gear pump is a constant (fixed) displacement pump i.e. its discharge is constant at a given shaft speed. The only way the discharge rate can be regulated is by varying the shaft speed.

Fig. 6.11 shows the typical characteristic curves of a spur gear pump. It consists of two parts; 1. 2.

Head capacity (HQ) curve : It shows the relation between pump discharge pressure and pump capacity Power capacity ty (PQ) curve: It shows the relation between the power input and pump capacity. The characteristic curves drawn depict the capacity and power input at various speeds. Note: (1) At any given speed, the capacity is assumed to be constant (QT QT). Slip increases with the rise in pump discharge pressure. (2) The power curve increases with both the operating speed and discharge pressure.


2.

Internal Gear Pump

Fig. 6.12 shows a typical internal gear pump. This design consists of a regular spur gear G and an outside ring gear with internal teeth called as the rotor R which is set off center. The spur gear drives the rotor. Between the two gears on one side is a crescent shaped spacer C around which oil is carried. The inlet port is located where the teeth unmesh and one end plate where the teeth remesh and the other end of the crescent shaped spacer is located. The outlet port is also located in the end of the crescent shaped spacer is located. This crescent scent acts to ensure a seal between the suction and discharge side. In operation the oil is directed from the inlet to the outlet port in the following manner. The drive gear G drives the rotor R and makes a fluid tight seal at the place where the teeth mesh. esh. Rotation causes the teeth to unmesh near the inlet port thereby increasing the cavity volume. This increased cavity volume produces a vacuum at the inlet which in turn sucks the fluid into the inlet port. As the rotor R continues to turn, this sucked-in sucked fluid is trapped between the internal and external gear teeth on both sides of the crescent shaped spacer C. From here it is carried around and forced out into the outlet port of the pump. At the outlet port, the teeth will remesh thereby reducing the ccavity avity volume. This decreased cavity volume forces out the fluid from the outlet port. Characteristics of the internal gear pump resemble those of the spur gear pump. Like the external type, internal gear drives are fixed displacement units and are available availab in single and multiple configurations. Wear on internal gear pumps has a tendency to reduce the volumetric efficiency more quickly than on external gear pumps. They are used mostly as lubrication and charge pumps at pressures under 70 bar.


Gerotor (generated rotor) pumps are a version of the internal gear pump. This pump consists of two elements: an inner gerotor and an outer gerotor, both mounted on fixed centers but eccentric to each other. The inner gerotor is power driven and draws the outer gerotor around as they mesh together. The inner gerotor always has one teeth less than the outer gerotor. Each meshing pair to teeth of the two gerotors engage at just one place in the pump i.e. at X. At the right hand side

of thee point of mesh, as the gerotors rotate, pockets of increasing size are formed. These are the suction pockets of inlet port. On the left side of the point, pockets of decreasing size are formed. These are the discharge pockets of outlet port. The tips ooff the t inner and outer gerotors make as shown to seal the pumping chamber from each other. The operation of the gerotor pump is as shown in Fig. 6.14. During the initial half of the cycle, the gradual enlarging chamber is exposed to the inlet port, creating a partial vacuum into which the hydraulic fluid flows. During the next 180o of the revolution, rev the chamber gradually decreases in size as the teeth mesh, and the fluid is forced out th through the outlet port into the system.


3.

Lobe Pump

The lobe pump illustrated in Fig. 6.15 also comes from the family of gear pump. The lobe pump operates on the same principal as the external gear pump. But unlike the external gear pump, both lobes are driven externally so that they do not actually conta each other. Hence, they are quieter than the other types of gear pumps. Each lobe has only three mating “teeth’s” that are mush wider and more rounded than those found on a regular externa external gear pump. mp. Due to the smaller number of mating elements, the lope pump output will have a somewhat greater amount of pulsation, although its volumetric displacement is generally greater thann that for other types of gear pumps. They have a relatively low pressure capability. 4.

Screw Pump

A typical screw pump s as shown in Fig. 6.16 (a). The screw pump is an axial flow positive displacement unit. It consists of three precision ground screws, meshing with within a


close fitting housing. One screw transmits power and is known as power rotor. The remaining two screws act as idlers. The two symmetrically ally opposed idler rotors turn because of the action of the fluid transferred through the pump. They act as rotating sealing elements and perform m no work themselves. Pumping action occurs when the meshing of the rotors seal and enfold the fluid and then transfer it to the enclosures continuously in an uniform manner. The idlers are in rolling with the central power rotor and are free to ffloat loat in their respective housing bores on an hydrodynamic oil film. There are no radial bending loads. Axial unbalanced forces on the rotor set are ed by balance piston and thrust cage as shown in Fig. 6.16(a). Fig. 6.16(b) explains the characterist characteristic curves of a screw pump Desirable characteristics inherent in the design of screw pumps: 1. They are quite because the rolling action of the rotors eliminate the hydraulic noise and vibration traditionally associated with many positive displacement pumps. 2. Nearly early all fluids are compatible with the pump because only minimum lubrication properties are necessary. 3. They produce non pulsating flow. 4. High speed operations of 3500 rpm and more are available. 5. Because the internal parts are few and rugged, these pumps aare re highly reliable. 6. High pressure designs are available for 200 bar operation with output flow rates upto 300 lpm. 5.

6.

VANE PUMPS The major source of leakage in a gear pump arises from the small gaps between the gear teeths, and also between the gear teeth aand nd pump housing. The vane pump reduces this leakage by replacing the gears with vanes. Vane pumps are classified as fixed or variable displacement types and unbalanced or balanced design. Three pump combinations are available: displacement Vane pump 1. Unbalanced, Fixed displacemen 2. Unbalanced, Variable displacement Vane pump 3. Balanced, Fixed displacement Vane pump Unbalanced, Fixed displacement Vane Pump:


A typical unbalanced, fixed displacement vane pump is as shown in Fig. 6.17. The essential components of the vane pump include the inlet and outlet ports, the driven rotor, sliding vanes, and a stationary cam ring. The rotor, vanes and can ring, and sometimes the end wear plates, are replaceable as a cartridge unit. The rotor is eccentrically mounted on the stationary cam-ring. The rotor is splined to the pump shaft and rotates within the cam ring.The vanes are located in the radial rotor slots and follow the contour of the cam-ring. During startup only the centrifugal force and during operation both the centrifugal and the force due to system pressure are responsible to move the vanes against the hardened and ground contour of the cam ring. The wear plates are against both sides of the cam-ring thus making up the pumping element. While ing over the inlet port, the sliding vanes are extended out of the rotor slots, thereby increasing the volume of the suction side. The increasing volume creates suction at the inlet port and causes the fluid to enter the low pressure inlet cavity. The inlet and outlet ports are isolated from each other by the spacing of the vanes. As the rotor turns, this fluid gets trapped between the rotating vanes and is transferred to the outlet port. While ing over the outlet port, the sliding vanes are retracted back into the rotor slots, thereby decreasing the crescent shaped space between the rotor and cam-ring. This decreasing volume causes the fluid to be ejected out of the outlet port into the system. This pump permits the pumping action on one side of the rotor only. Now, there is a large difference in pressure between the inlet and outlet ports. This pressure difference creates a severe load on the vanes and a large side load on the bearings of the rotor shaft. Hence, it is said to be ‘unbalanced design’. Note: This same undesirable side load exist for the gear pump also. Hence, they are also unbalanced design type. Analysis Vane Pump Volumetric Displacement: Let

Dc = diameter of cam ring [m] DR = diameter of rotor [m] L = width of rotor [m] N = Speed of rotor [rpm] e

= eccentricity [m]

emax = maximum possible eccentricity [m] =

ి ౎

The maximum volumetric displacement is given by,

V D మ L

౎

V m

D D D D L

π D D e L 4

…(6.13) Page | 50


The actual volumetric displacement occurs when emax = e. Hence, the actual volumetric displacement is given by, VD[m3] = D D e L π

…(6.14) From equation (6.14), we see that, (i) When electricity is zero, there is no flow. (ii) When this eccentricity is changed, then the pump displacement changes. Pumps having provision to vary this eccentricity are said to be of “variable displacement typer”. 7.

Unbalanced, Variable Displacement Vane Pump A variable displacement vane pump is the one which can change its displacement by varying the eccentricity between the drive rotor unit and the cam-ring. This can be achieved by mechanically setting the cam-ring position with the help of an external hand wheel as shown is Fig. 6.18. It should be noted that when the eccentricity is maximum, the displacement of the pump is also maximum. As the hand wheel is turned and the eccentricity is reduced, the flow reduces and when the rotor and cam ring are concentric, the displacement is zero. One main disadvantage of a balanced vane pump is that it cannot be designed as a variable displacement unit. If at all the displacement has to be varied, then the entire unit consisting of the elliptical cam ring has to be changed.

8. PISTON PUMPS Piston pumps are the oldest form of hydraulic pump, being in critical applications such as naval gun turrets, steering control systems, etc. A piston pump works on the principle that a reciprocating piston can draw in a fluid when it retracts in a cylinder bore and discharge it when it extends. There are two basic types of piston pumps: 1. Radial Piston Pump: Here the pistons are arranged radially in a cylinder block. It operates by converting the rotary shaft motion to radial reciprocating piston motion. 2. Axial Piston Pump: Here the pistons are arranged parallel to each other and to the axis of the cylinder block. This pump operates on the principle of converting rotary shaft motion into axial reciprocating piston motion. Piston pumps are available in both fixed displacement design and variable displacement design. 9. Radial piston pumps 1.Fixed Displacement, Radial Piston Pump: The operation and construction of a fixed displacement radial piston pump is as shown in Fig 6.25. It consists of a stationary pintle to direct the fluid in and out of the cylinder, a rotating cylinder barrel with pistons and a rotor containing a reaction ring. The pistons are reciprocating in the slots of the rotating cylinder. The pistons always remain in constant with the reaction ring due to both the centrifugal force and the back pressure on the pistons. The cylinder block along with the pintle is mouthed eccentricity in the reaction ring of the pump housing. Fig. 6.25: Fixed displacement Radial Piston Pump

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As the cylinder barrel rotates, the pistons pis on one side travel outwards thereby increasing the volume in the pintle opening. This is the inlet port. Due to increase in volume, a partial pressure in created which sucks in the fluid from the reservoir as each cylinder es the suction port. Whe When the piston es the point of maximum eccentricity, it is forced in wards by the reaction ring. This decrease in the volume, forces the fluid to enter the discharge port of the pintle. This fluid is then ejected into the system. Pump displacement is de determined termined by the size and the number of pistons (as there may be more than one bank in a single cylinder block) and the length of their stroke. 2.

Variable Displacement Radial Piston Pump:

The piston displacement and volume flow rate in radial pi piston ston pump design is varied by changing the position of the reaction ring w.r.t. the center line of the ing pintle. This is accomplished either mechanically, electrically or hydr hydraulically aulically to cause varying conditions of flow. If the cam ring is moved over center, the action of the pump rreverses the direction of fluid flow, even though the pump continues to rotate at constant speed in the same direction.


3.

Performance characteristics characteristics:

The performance curves for radial piston pump are as shown in Fig. 6.27. The 3 curves shown in the p-Q Q and Q Q-N charts represent three pumps of differe fferent sizes. Observe the linear requite either er pressure / flow / load compensation to limit system ppressure and volume flow rate simultaneously. The system pressure along with flow rate capacity decide the power capacity of pump.

Solved Problems Problem 6.1 A gear pump has x 75 mm outside diamet diameter, x 50 mm inside diameter, and a 25 mm width. Calculate the volumetric efficiency, if the pump has an actual flow of 100 lpm at 1800 rpm and rated pressures. Solution: The volumetric displacement of a gear pump is given by

…(1) The theoretical flow rate of the gear pump is given by, QT = VD [m3]

N [rpm] = 6.1359

QT = 010,11045

10-5

= 110.45 lpm

The volumetric efficiency is then found by,

1800 m3/min …(2)


η 90.5%

…Ans.

Problem 6.2 A vane pump has a rotor diameter of 50 mm, a cam ring diameter of 75 mm and a vane width of 50 mm. The eccentricity is 8 mm. Calculate the volumetric efficiency if the pump has an actual flow of 110 lpm at 1500 rpm and rated pressure. Solution: The volumetric displacement of a vane pump is given by, VD = D D e L

0.075 0.05 0.008 0.05

VD = 7.854 10-5 m3

The theoretical flow rate of vane pump is given by, QT = VD [m3] N [rpm] = 7.845 105 1500

QT = 0.1178 m3/min = 117.8 lpm

The volumetric efficiency is then found by,

η ఽ 100 . 100

౐

η 93.38%

Problem 6.3 An axial piston pump with swash plate angle of 15o is used. The pump has nine, 12 mm pistons arranged on a 125 mm piston circle diameter. The operating speed is 5000 rpm. Calculate the theoretical volume flow rate. Solution: The volumetric displacement of the swash plate piston pump is given by, VD = n A Dp tan θ 9 0.012 0.125 tan 15

VD = 3.4092 10-5 m3

…(1)

The theoretical flow rate is given by QT [m3 N pm] = 3.4092 10-5 5000

QT = 0.17046 m3/min.

Page | 54


QT = 170.46 lpm

…Ans.

Questions 1. 2. 3. 4. 5.

Which parameters are considered for the analysis of pump performance? Give classification of pumps. Differentiate between Positive displacement and Non-positive displacement pumps. Write in brief about Performance characteristic of gear pump. Write short note on (i) Internal gear pump (ii) Lobe pump (iii) Screw pump (iv) Gerotor pump. 6. What are various types of vane pumps? 7. Draw and explain, Pressure compensated unbalance variable displacement vane pump 8. Explain Balanced fixed displacement vane pump. 9. Write a short note on Radial piston pump and explain its performance characteristics. 10. What do you mean by Characteristic curves of a pump? 11. Write a short note on Axial piston pump. 12. Compare the pumps on the basis of following parameters (a) Pressure ring (b) Speed rating (c) Flow capacity (d) Overall efficiency (e) Cost 13. Give criteria for selection of pumps.

Page | 55


Chapter 7 – Pressure Control Valves •

NECESSITY FLUID CONTROL THROUGH VALVES Fluid power is the power transmitted and controlled through use of a pressurized fluid. In the definition of fluid power, the transmission of fluid power is controlled by ‘hydraulic valves’. These hydraulic valves controls the pressure, rate of flow, and direction of fluids in accordance with the basic principles of flow. Hydraulic valves provide the interface between the hydraulic fluid, the control signal and the hydraulic actuators. The control signal may be mechanical, manual, hydraulic, pneumatic or electrical. The action of the control valve may be:

a) Digital: Here the valve changes from one set position to another e.g.: A Two position lever operated D.C. valve. b) Analog: Here the movement of the value control element is dependent upon the strength or value. Valves can be categorized as: i. Direct Acting Valves: Valves are direct acting because of the arrangement of the variety of elements including poppers, diaphragms, flat slides, balls and rotating and sliding spools. ii. Pilot operated Valves: Valves can be actuated from a remote location by using pilot force. This pilot actuating force can be supplied manually by the human operator directly by the fluid under pressure or form a pilot circuit, or by electrical devices such as solenoids or servo electrical drives. In general, a hydraulic valve influences just one of these functions: 1) Pressure Control: These valves limit, reduce and / or regulate the maximum pressure in a circuit or part of a circuit. 2) Flow Control: These valves very the fluid flow rate using restrictions in fluid ages which may be fixed, variable, or flow and pressure compensated. By changing the fluid flow rate to or from the actuator, it can alter the speed of an actuator. 3) Direction Control: These valves are used to check, divert, shuttle, and / or proportion the flow of fluid in one, two, three, four, or more ways. Pressure and flow compensation are commonly included in these valves. •

a) b) c) d) a)

PRESSRE CONTROL VALVES Pressure control valves are used in hydraulic circuits to maintain desired pressure levels in various parts of the circuits. A pressure-control valve maintains the desired pressure by: (1) Diverting high pressure fluid to low pressure area, thereby limiting the pressure in the high pressure area. Pressure control valves based on this principle are categorized as: Pressure relief valve Sequence valve Unloading valve Counterbalance valve (2) Restricting the flow into another area. Pressure control valve based on this principle is Pressure reducing valve\ Page | 56


i. ii. iii. iv. v.

•

The five popular pressure control valves are: Relief Valve: It limits the maximum pressure that can be applied in that portion of the circuit to which it is connected. Counterbalance Valve: It maintains the resistance against flow in one direction but permits free in the other direction. Sequence Valve: It directs flow flow to more than one portion of the fluid circuit, in sequence. Unloading Value: An unloading valve is used to permit a pump to operate at minimum load. Pressure reducing Valve: It maintains a reduced pressure at its outlet regardless of the high inlet pressure. PRINCIPLE OF PRESSURE CONTROL VALVE The principal feature of most pressure control valves is that the hydraulic forces are resisted by a spring. The action of a simple pressure control valve is as shown in Fig. 7.1. When the force arising from the pilot pressure is greater than the spring force, the valve spool will move towards the spring until an equilibrium position is obtained where the pilot pressure is just equivalent to the spring force. As the pilot pressure varies, the spool position will after after so as to try and maintain the force equilibrium. The valve spool shown is in the normally closed position. A pressure control valve may be either a normally closed or normally closed or normally open two way valve. Relief, counterbalance, sequence and unloading valves are two way, normally closed valves that are partially or fully opened while performing their design function. A reducing valve is a normally open valve that restricts and finally blocks fluid fflow into a secondary area.

•

RELIFE VALVES Relief valves are the most common of the pressure control valves. It is a normally closed valve which partially opens permitting flow to tank port when the pressure at the inlet port overcomes the spring force. T The he function of a relief valve is to set the maximum pressure in a hydraulic system. They are located near the pump outlet, in-between in the pressure line and the reservoir so as to protect the pump and other system components from pressure overload. Circuits using positive fixed displacement pumps must have pressure relief valves. Safety valves operate only when there is a circuit malfunction. A relief valve may function in dual capacity as both relief and safety device. Relief valves are of two basic types 1) Direct Operating Relief Valve 2) Pilot Operating Relief Valve •

Direct Operating Relief Valve When the pressure control valve shown in Fig. 7.1 is closed, the inlet and outlet posts are isolated by the valve spool. An adequate hydraulic seal is obtained owing owin to the minute


clearance between the spool and the housing. However, as the working pressure increases, this seal becomes less efficient. So, we use a direct acting relief valve that uses either a conical poppet or a ball to seal against a mating valve se seat. at. Being a type seal, this is more effective at high pressure.

In these direct acting relief valve, the pressure at post P acts on the exposed surface of the poppet/ball to apply a force which is rresisted esisted by the spring force. When the pressure at the port P is insufficient to overcome the force of spring, the valve remains closed. When the pressure at post P has risen sufficiently to overcome the sprig force, the poppet / ball is lifted off its seatt permitting the fluid to flow to the tank port T, relieving the pressure in the system. When the system pressure drops to or below the spring set value, the valve automatically reseats. The symbol for a direct acting relief valve is as shown in Fig. 7.2 (c). If there is no arrow through the spring, the valve is pre pre-set i.e. non-adjustable. adjustable. The arrow shown through the spring, indicates that we can manually adjust the tension in the spring, so as to set the maximum pressure permissible in the system. Thee pressure at which the valve opens is called as the cracking pressure. As the flow through the valve increases, the poppet / ball is forced further off its seat causing increased compression of the spring. When the valve is by by-ing ing its full rated flow, the pressure is called ‘full flow pressure’ and it is considerably higher than the cracking pressure. The pressure at which the valve ceases to fluid after being opened is called the ‘closing pressure’. Adjustments within the pressure range of the va valve lve is made with the adjustment screw which acts to compress the valve spring. Relief valves of the ball or poppet type have a rapid response to pressure surges, typically 25 ms, but the pressure flow characteristic is not constant. The poppet or ball te tends to hammer on the seat giving rise to “relief valve whine”; seat damage can occur are best suited for infrequent duties.


•

Guided Piston Relief Valve

A variation of the direct acting relief valve is the guided piston relief valve which has all the advantage of a direct acting poppet valve but is more suitable for continuous duty. It is as shown in Fig. 7.3. It is much quieter in operation but is best suited for low pressure applications (up to 100 bars bars) under constant floe conditions. The response time is still fast although slightly slower than the direct poppet type relief valve. It also has a high pressure overside characteristic. •

PILOT OPERATED RELIEF VALVE

The pilot operated relief valve is as shown in Fig. 7.4. It is two stage valve and is sometimes referred to as a compound pilot drained relief valve. It gives good regulation of pressure over a wide range of flow. It has low pressure over-ride over ride and hence hen gives increased pressure sensitivity. The valve consists of a main main-spool spool connecting the pressure and tank ports. The mainspool is controlled by a small built in direct acting relief valve consisting of a poppet held against the seat by an adjustable spring. Through a small hole or jet in the main spool, causing it to be pressure balanced. Because the main spool on both sides of the spool, and the main spool remain seated with a relatively light bias spring. The fluid on op of the main spool caused the pilot relief valve to sense the system pressure.


When the system pressure. Increases sufficiently to move the pilot poppet valve from its seat, the fluid above the spool goes to drain. This caused a pressure drop across the jet thereby throwing the main spool in hydraulic imbalance. This resulting pressure imbalance on the than spool causes it to move in the direction of the lower pressure. Thus the spool lifts against the spring, thereby relieving the major flow from the pressure port to the tank port ort and preventing any further rise in pressure. The small amount of flow which es through the pilot direct acting relief valve is also returned to the tank port (i.e. it is internally drained drained). Alternatively the pilot section may have an external drai drain n connection to avoid the effect of back pressure in the tank line. A separate pilot or vent port V which is plugged for operation is fitted so that can be remotely operated. This port is o the pilot side of the main spool and connected to the tank causes the main spool to imbalance at a very low pressure. This venting features is a useful method of unloading a pump or circuit. Alternatively the main valve can be remote controlled by connecting another relief valve to the vent port V. This will regulate regulate the pressure from its minimum valve upto the limit set by the main valve pilot section. Both these features are demonstrated in Fig. 7.5 in which the 4/3 way solenoid actuated D.C. valve enables the relief valve to be remotely operated by an electrical signal to give three different pressure settings, one of which is nominally zero, the relief valve than being vented. When solenoid a energized, internal pressure control is achieved; with solenoid b energized, remote pressure control a and b de-energized energized,, the valve is vented. The D.C. value may be integral with the relief valve or a separate valve connected to the vent port. •

Relief Valve selection and Pressure Setting Most direct acting valves have rapid response time. Poppet types are the most tolerant to fluid contamination and also tend to have less internal leakage than the spool valves, which make them suited to high pressure working. However, the direct acting valves have high pressure override characteristics which make them unsuitable for systems with widely varying flows. Pilot operated relief valves have good pressure regulation over a wide range of flows with low pressure over ride characteristics and close tolerance between the cracking and resetting pressure. They are popular in machine tool fields, where exact pressure levels are required to be maintained. They are also used in circuits where rapid operating can create shock waves. Also a pilot relief valve can usually open much faster than an equivalent size direct acting spring loaded relieff valve with a heavy spring and a mass to get into motion. A frequently used thumb rule is for the main relief valve in a circuit to be set at 10 10-20 above the maximum required working pressure, taking into the type of valve, its position relative to the actuator and the pressure losses in the system. When there is more than one pressure losses in the system. When there is more than one pressure valve compensated


pumps, the controls must not be set at pressures which are too close together as intera interaction or hunting may result. It is usual to set secondary relief valve such as port or cross line relief valves at a pressure higher than the main relief valve. •

SEQUENCE VALVES A sequence valves primary function is to direct flow in a predetermined sequen sequence. It is a pressure actuated tuated valve senses a change in pressure in the set pressure has been reached. It thus causes the actions in the system to take place in a definite predetermined order, and helps to maintain the requisite minimum pressure in the prim primary ary line while the secondary operations occur. The sequence valve may be normally open or normally closed changing its initial state when the system reaches the set pressure. The sequence valve is always externally drained from the separate drain connection on from the spring chamber. This is because, unlike a conventional relief valve, a high pressure can occur in the output port during the normal course of operation. Should it be internally drained, any pressure is the output port will be reflected back into o the spring chamber causing a malfunction. In fact a sequence valve may be used as a relief valve is any circuit where excessive back pressures are encountered in the return line. The independently drained pilot makes sequence valves insensitive to down sstream back pressure. Note: (1) A good rule to with almost all pressure control valves is: “when a pressure control valve operates, if the flow from the secondary port performs work or is pressurized, then the valve must be externally drained.” (2) In circuits, where pressure sensing is used to control cylinders movements, it must be borne in mind that sequence valve operate when a specific pressure has been achieved and do not guarantee that the cylinders have completed or reached a particularr point in their stroke.

•

Direct Operating Sequence Valve

The operating principle of a direct acting normally closed sequence valve is as shown in Fig. 7.6. In the closed position (a), fluid es throu through gh the valve from the inlet port P to primary outlet port A at (low) system pressure when the first step in the sequence has been completed, the system pressure increases to act against the in indicated dicated area of the piston. Continued increase in pressure caus causes the piston. Continued increase in pressure causes the piston to compress the spring and unseat the valve, thereby directing the flow of fluid at high


pressure through secondary outlet port B. Fluid pressure is maintained in both branches of the circuit at high pressure so long as the sequence valve in open. Adjustment of the sequence valve is accomplished by compressing or extending the piston spring with the cap screw. •

Pilot / Remote

Remote operation: peration: In some system, it is desirable to provide an interlock so that the sequence does not occur until the primary actuator reaches a definite position. In these applications, the bottom cover on the sequence valve is assembled for remote operation. For remote operation, the age for direct operation is plugged while the age for remote operation is connected to a separate pressure source as required for operation off the spool. Pilot operation: Sequence valve may also be pilot operated. The dr drain from the pocket housing spring in Fig. 7.7 can be used to control the poppet by selectively adding pressurized pilot fluid to assist the spring is holding the poppet closed. •

UNILOADING VALVE A relief valve can be unloaded in two ways a) Pressure Release (i.e. venting): It was seen that the two-stage stage relief valve in Fig. 7.5 could be unloaded by connecting the vent port V to tank (see Fig. 7.6). Venting causes the main spool to be unbalanced and open at a very low pressure dumping the pump flow form port P to port T. The main flown may be quite large but the flow through the vent port will be very small. b) Pilot Pressure: The valve in Fig. 7.1 will function as a direct acting unloader when subject to a remote pilot pressure. As long as the force resulting fro from m the pilot pressure is greater than the forces set by the control spring, the relief valve will open fully, allowing the main flow to go back to the tank at low pressure. Difference Between Venting and Pilot pressure Unloading


In Fig. 7.8 (a), opening the vent port V release pressure and causes the main spool to open. This is independent of the setting of the control spring. In Fig. 7.8(b), the pressure signal at x from a remote source pilots the valve open again against st the spring setting.

An unloading valve is used to permit a pump to operate at minimum load (see Fig. 7.9). The unloading valve operates on the principle that the pump delivery is diverted back to the reservoir when n sufficient pilot pressure is applied to move the spool against the spring force. Pilot pressure acts against the lower end of spool (i.e. on differential area) which is help in the normally closed position by the action of the spring. As the pilot pressu pressure increases, the spool moves upward, discharging the pump to the reservoir. It should be noted that the pilot fluid applied to move the spool upward becomes a static system. In other words, if merely pushes the spool upward and maintains a static


pressure to hold it open. When the pilot pressure is relaxed, the spool is moved down by the spring, and flow in directed through the valve and into the circuit. The unloading valve is useful in systems having one or more fixed delivery pumps to control the amount of flow at any given time, by discharging the fluid to the reservoir when they are not is use in the circuit. They are especially used in feed and traverse circuits where rapid approach, feed and return strokes are needed. Unloading valve, reduces power requirement and helps to prevent heat build up in a system, which is caused by fluid being discharging over the relief valve at its pressure setting. The power saving gained by unloading the volume pump rather than discharging it over a direct acting relief valve is given by P=pQ

…(7.1)

p = Discharge pressure across the direct acting relief valve (N/m2) Q = Flow rate of pump (m3/s)

Where

Problem 7.1 What size electric motor drive would be needed in the “unloading circuit” if the rapid advance operating pressure is 20 bar and the feed operating pressure is 140 bar? Rapid Return operating pressure is also 20 bar. Assume pumps to be 100% efficient. Solution: The power required for rapid advance stage is given by, P advance = p Q = (20 10 )5

షయ

P advance = 2500 w = P retract

…(1) The power required for feed stage is given by, P Feed = p Q = (140 105)

షయ

P Feed = 3500 W

…(2) This sample problem shows that using an unloading valve to eliminate the large pump during the high pressure feed portion of the cycle reduces the input horsepower. From (1) and (2) we can conclude that the maximum power requirement in this unloading circuit is, P max = 3500 W …(3) Without the unloading valve, the power requirement would be, Page | 64


P’ max

p max

Q max = (140

105)

P’ max = 17,500 W …(4) Thus, we can say that the power saved is P saved = P’ max - P max = 17500 – 3500 P saved = 14,000 W •

…Ans

COUNTERBALANCE VALVE (Pressure Control Valve)

A counterbalance valve is basically a relief valve and is used to set up a back pressure in a circuit to prevent a load from falling. They are frequently employed in vertical presses, loaders, lift trucks, and other machines that mustt position or hold suspended load. In such applications, the counterbalance valve creates a back pressure to prevent the load running away when the cylinder is retracting. They are not used, typically to varying loads that would require frequent va variance riance of pilot pressure. For this purpose a pilot operated check valve should be substituted for the counterbalance valve. The counterbalance valve operates on the principle that fluid is trapped under pressure until pilot pressure either direct or emote emote,, overcomes the spring force setting in the valve. Fluid is then allowed to escape, letting the load descend under control (see Fig 7.10)


The counter valve might be operated either directly or remote 7.7.1 Direct Operation: If a direct pilot operation is used as shown in Fig. 7.12(a), pressure on the rod end of the piston must exceed the pressure setting of the valve to for inertia ia and friction. This is usually about 30% greater than the pressure required to sustain vertical load. Thus, the usual pressure setting is 1.3 times the load induced pressure. 7.7.2 Remote Operation: If the counterbalance valve were to be operated by a remote re pilot line rate than a direct pilot, the pilot pressure setting could be much lower. If as seen in Fig. 7.12(b), the remote pilot pressure is taken from the pressure line at the top of the cylinder, a choice of the operating pressure can be made for the valve. A counterbalance valve is a normally closed valve until acted upon by the remote pilot pressure source. Therefore a much lower spring force can be selected to allow the valve to operate at a lesser pilot pressure.


Problem 7.2 Consider a 100 kN press where the tool weight 5 kN. The cylinder bore is 80 mm while the piston rod diameter is 60 mm. Determine for both direct and remote pilot. (1) The counterbalance valve setting (2) The pressure required to achieve 100 kN pressing force. Assume a 2:1 pilot input ratio for remote pilot Solution: (I) Direct Pilot Pressure Applied: Cylinder bore

D = 80 mm = 0.08 m

Piston rod diameter

d. = 60 mm = 0.06 m

Full bore area

A = " 0.08 5.0265 10 $

a = " % 0.08 0.06 2.199 10 $

Annulus area

Pressure at annulus side to balance tools is given by,

P

౗౤౤౫ౢ౫౩

య

.షయ 22.736 10 Pa 22.74 bar

Suggested counterbalance valve setting is given by, P direct = p annulus 1.3 = 22.74 1.3 P dorect = 29/562 bar = 30 bar …Ans. Pressure at full bore side to overcome counterbalance is given by, .షయ

p full-bore = p direct 30 .షయ

p full-bore = 13.125 bar

Pressure to achieve 100 kN on bore side p top =

್೚ೝ೐

!

య

.షయ 198.95 10 Pa = 198.95 bar

Total pressure required to achieve the 100 kN pressing force is p = p top + p full bore = 198.95 + 13.125 p = 21.075 bar …Ans. Page | 67


(II) Remote Pilot Pressure Applied: Given that there is a 2:1 pilot input ratio. Now the counterbalance valve setting for direct pressure is 30 bar. Pressure on the remote pilot required to open the Valve =

= 15 bar

Pressure at full bore side to drive down to tooling = 15 bar

…Ans.

Pressure require to achieve 100 kN pressing for is "# య షఱ . షయ

189 bar

…Ans.

This is greater than the 15 bar pressure required to pilot the remote pilot operated counterbalance valve open. Therefore there will be not back pressure set up on the annulus side of the piston during the pressing operation. •

PRESSURE REDUCING VALVES A pressure reducing valve is a normally open type pressure control valve which throttles or closed to maintain constant pressure in regulated line. They are used to limit the pressure in certain portion of the circuit to a valve lower than that required in the rest of the circuit. Their applications are important where limited pressure must be controlled for operations such as clamping light metal objects. They, work on the principle that when the branch circuit by restricting the fluid flow. •

Direct Operating Pressure Reducing Valve A direct acting pressure reducing valve of non-relieving type is as shown in Fig. 7.13. Fluid es unobstructed from C to D in illustration Fig. 7.13 (a). The main spool is held open by spring, and the leakage around the free floating spool es at low pressure to the drain. As the pressure of the system at the outlet of the valve increases, it acts through age E against the lower end of spool. An imbalance exists because the opposing end of the spool is open to drain. Further increase in pressure at the outlet causes the spool to move upwards by compressing the positioning spring and thus restrict the flow of fluid at the outlet until the pressure drops to the specified level. In this position, pressure at outlet will remain constant

Page | 68


Chapter 8 – Flow Control Valves •

NECESSITY OF FLOW CONTROL VALVE Flow control valves are used to regulate the fluid volume flow rate from displacement pumps to or from branch actuator circuits. They provide velocity control of cylinders, or speed control of hydraulic motors. By controlling the rate of flow in a hydraulic circuit, it is possible to control the speed of hydraulic cylinders and motors. The speed of a hydraulic actuator is determined by its size and the oil flow rate at its inlet and outlet. 1) A large diameter cylinder will have a larger capacity and hence the time required to drain the oil will be more thereby slowing the cylinder stroke. Similarly, a smaller diameter cylinder will have a faster stroke. 2) Alternatively, changing the flow rate from the pump would also change the extension time of the cylinder. However, changing the flow rate from the pump would mean, changing the pump size. Thus, from (1) and (2), we see that changing either the cylinder or pump size to regulate speed is impractical, especially if the speed change is desired during the stroke. Hence, we use a flow control device. Flow control valves, in its simplest from, is nothing more than an orifice, and could be as basic as a needle valve. By varying the size of the opening, one can vary the amount of oil entering the cylinder and thus control its speed. Flow control valves are typically used in regulating cutting tool speeds, spindle speeds, surface grinder speeds, and the travel rate of vertically ed loads moved upward and downward by fork lifts and dump lifts. Flow control valves are also used to allow one fixed displacement pump to supply fluid to two or more branch circuits at different flow rates. •

PRINCIPLE OF FLOW CONTROL Flow control valves achieve their primary function of regulating the fluid flow by varying the area of an orifice. The flow characteristics of orifices play a major part in the design of hydraulic control devices. Flow through the control orifice is usually considered to be turbulent and the quantity of the fluid flowing can be given by, q = K A0 ,∆-

…(8.1)

Where q = flow rate through the valve A0 = orifice area

∆ p = pressure drop over the orifice K = constant which takes into the orifice characteristics, fluid viscosity and Reynold’s number.

Page | 69


An orifice is a sudden restriction in the flo flow w path and may be fixed but is generally variable. Ideally it should be of zero length and sharp edged in which case it will be insensitive to temperature (i.e. viscosity) changes in the fluid flowing. The flow through the orifice shown in Fig. 8.1 will vary ary as the square root of the pressure drop and will be sensitive to viscosity changes. This type of orifice can be used to control flow rates if the pressure drop and fluid temperature remain reasonably constant and minor variations in flow rate are acceptable. When precise speed control control is required under varying load conditions it is necessary to maintain a constant pressure drop over the orifice. The relationship between the flow and the position of the adjusting device can be linear, logarithmic or specially contoured to follow a particular curve. •

RESTRICTORS / NON NON-COMPENSATED COMPENSATED FLOW CONTROL VALVES Restricting devices may be an orifice plate or a check valve with a small hole drilled through it. These devices are classified as non non-compensated compensated flow control valve. (see Fig. 8.2) These se restrictors are not compensated for changes in the fluid temperature or pressure. Without compensation the flow through these valves can vary at a fixed setting if either the pressure or temperature of fluid changes. Viscosity changes, which often accompany pany temperature changes of the fluid, can also cause flow variations through a valve. Compensation automatically changes the valve adjustment or pressure drop across the orifice to provide a constant flow at a given setting. NonNon compensated flow control vvalves alves create an orifice in the pipe to restrict the flow. The control element of the valve may be a needle type valve or a check valve or their modifications. •

Simple On – off Valve


The simplest of flow control devices device is a simple on-off off valve with a small hole drilled in it to provide orifice control (see Fig 8.3). They are basically used to create a fixed rate of pressure drop to operate pilot valves, to stabilize a system with a slight back pressure, etc. •

Needle Valve

A needle valve is a variable restrictor device which allo allows ws the orifice size to vary by adjustments. A needle valve has a pointed stem that can be adjusted manually to control accurately the rate of fluid flow through the valve. The needle valve is also used as a stop valve in hydraulic circuits to shut off the flow of fluid from one part of a circuit to another part. The characteristics of a simple needle valve are as shown in Fig 8.4 (c). Generally, a needle valve is coupled with a non-return return valve / check valve enabling regulated flow in one direction and free flow in the reverse direction (see Fig. 8.4(d)). • PRESSURE COMPENSATED FLOW CONTROL VALVE The flow through the valve varies as the square root of the pressure drop drop across it. Hence, any change in pressure at the control inlet or outlet changes the flow through the valve. A pressure compensated flow control valve automatically compensates for pressure fluctuations and thus maintains a constant pressure drop from iits ts inlet to outlet, thereby providing a constant flow regardless of changes in workload.


•

Restrictor Type / Two port Pressure Compensated Flow Control Valve:

Fig 8.7 shows diagrammatically diagrammatically a two port, pressure compensated flow control valve. Flow rate is set by an adjustable metering orifice (1), which may also be viscosity compensated. In the un operated condition the compensating spool (2) is biased fully open by the compensatory spring (3). As soon as the flow occurs, there will be a pressure drop across valve. The pressure upstream of the metering orifice tends to close the valve but this is opposed by the spring assisted by the pressure from downstream of the metering orifice. T The compensatory spool adopts a balance position with a consequential pressure drop over the compensation orifice (4) formed by the partially closed spool. A rise in supply pressure tends to close the spool and the increased pressure drop across the compensating nsating orifice balances the increase in supply pressure. If the load pressure rises, the compensating orifice opens, again maintaining the pressure drop over the metering orifice at a set value. This pressure drop is usually 3 to 6 bar, dependent upon the he size of the metering orifice. The total pressure drop across the valve is dependent upon the difference between supply and load pressure, but a minimum total pressure across the valve of 5 -12 bar is normally required for the valve to function correctly. correc The damping orifice (5) stabilizes the compensator and prevents hunting as pressure fluctuates. A stroke limiter or anti – lunge device is sometimes fitted to the compensatory spool to eliminate a flow surge which occurs when the circuit starts up.. When there is no flow through the metering orifice, the pressure compensating spool will be fully open and as soon as the flow commences, there will be a pressure drop through the valve causing the compensator to lunge or jump. The stroke limiter is a movable movable end stop, stop which limits the travel of the compensating spool. This device which has to be adjusted every time the setting of the flow control valve is changed is used to position the compensation spool, somewhere


near its expected final location. However, ever, large variations in pressure can no longer be corrected. This arrangement of providing a stroke limiter is known as “No Jump Feed Adjustment” •

By Type / Three port Pressure Compensated Flow Control Valve

Fig. 8.6 shows diagrammatically a by type / three port pressure compensated flow control valve. It is basically a pressure compensated flow control valve with a built in relief valve, so that any excess flow is by by-ed to the tank at a pressure just above the load pressure. It can only be used as a “meter in” control. Let ∆ be the relief valve spring setting. Hence, p will also be the pressure drop across the control orifice. Let p Load be the load pressure. Then, the system pressure is given by, p system = p Load + ∆p An accepted value of ∆p p is about 7 bar. Therefore, the system pressure pressur p system would be at 7 bar above the load induced pressure p load. The spring-loaded loaded spool sets up a constant pressure drop across the control orifice, independent dent of load or supply pressure. Once the regulated flow circuit is supplied, the excess flow is byed to tank. In this design, the tank line must go directly to the reservoir and not to a line which may be pressurized. By flow control can accurately regulate the speed of an actuator which operates against a wide range of loads and reduce reduce the heat generated in the circuit. •

Characteristics and Uses of Pressure Compensated Flow Control Valve Pressure compensated flow control must used when accurate speed control at varying supply or load pressure is required. The minimum regulated stable flow from a good quality flow control valve will be in the region of 0.1 lpm. In any precision flow control valve application it is essential to have well filtered fluid (better than 10µm) to promote efficient


control and long life of valve. The smaller the flow to be controlled, the finer the filtration necessary. Various types of valve adjusting mechanism are available: hand knob, lockable hand knob, lever, DC motor control, etc. It must be ed that whenever a flow control valve is used in the system, tem, there will always be some pressure drop and associated heat generation. •

TEMPERATURE COMPENSATED FLOW CONTROL VALVE The viscosity of a hydraulic oil is dependent on the oil temperature. Hence, some valve manufacturers refer to temperature tem compensation on and others to viscosity compensation. The simplest way to eliminate the effect of viscosity is to use a sharp edged orifice, the flow through which is independent of viscosity. In some designs of viscosity / temperature compensated throttle valve the orifice aperture, over which the throttling of flow takes place consists of two adjacent flat plates: one fixed and one movable. A “V” shaped notch in which one of the plates is masked or unmasked as the movable plate is rotated relative to the fixed plate plate.. The design of the throttle gives a sharp edged orifice which makes the flow ppractically ractically independent of viscosity and hence the temperature, practically at higher flow rates. Problems can still occur at low flow rates (< 0.5 lpm) in which case a valve wil willl function better with a low viscosity oil. Flow through these valves is load dependent but this can be remedied by the addition of a pressure pressurecompensating spool. A check valve is frequently incorporated to allow relatively unrestricted reverse flow. An alternative lternative method of temperature compensation favored by some manufacturers is to have part of the orifice adjusting mechanism made of a material with a high coefficient of thermal expansion. When the temperature of the fluid increases, a tapered metal rod / spindle in the mechanism lengthens thus reducing the control orifice opening. • PRESSURE AND TEMPERATURE COMPENSATED FLOW CONTROL VALVE

Fig. 8.7 shows a pressure and temperature com compensated pensated flow control system. Its operation is essentially the same as the restrictor type, pressure compensated flow control


valve. Note that in the compensatory spool, the pressure is sensed to the bottom of the spool through a age drilled in its bo body, instead of having the sensing age in the valve body. Also instead of using the usual throttling arrangement, a cup shaped device with “V” notches is used for better metering. This cup is help by a small spring against the shoulder of an Aluminium alloy rod which extends through the cup into the oil flow. If it was set for a particular flow rate and the temperature went up, the oil would become a little thinner and tend to flow faster through the throttle. However, the in increased creased temperature also cau causes the Aluminium rod to expand and close the throttle opening to compensate for the change in oil viscosity. Thus, even with an thinner oil, the flow rate stays essentially the same. •

SPEED CONTROL OF A CYLINDER In a simple hydraulic cylinder circuit th there ere are three positions in which the flow control valve can be placed relative to the cylinder namely: (1) Meter-In (2) Meter Meter-Out (3) Bleed-off These three positions help in the speed control of cylinder as per the requirement of the application •

Meter In Circuit A typical meter in circuit is as shown in Fig. 8.8. Here, the flow control valve is placed between the pump and actuator. It controls the quantity of oil entering the actuator. The pump must deliver more oil than is required to drive the actuator actu at the selected speed, with the excess oil ing to the tank at the relief valve setting. The circuit pressure has to be at a higher value than that required to overcome the load owing to the requirements of the flow control valve (a drop of approxi approximately 10 bar as previously stated). When the circuit is initially started, the compensatory spool will be fully open causing a flow surge before the compensatory adjusts to take correct control. In many machine tool applications, an initial flow surge would cause the tool to dig into the work piece. In these situations, flow control valves with “No Jump Feed Adjustment” must be used. An alternative is to design the circuit so that there is always flow through the flow control valve. This keeps the compensating nsating spool “active”, preventing flow surges or kicks. The fluid in the cylinder has to be pressurized before the piston begins to move; this requires a flow of fluid to cause compression. The force or pressure needed to start the cylinder moving is ge generally greater than the pressure needed to maintain movement (owing to static friction and load inertia). Once the load has started to move, the resistance to movement reduces and the pressure on the piston falls with an expansion of the


fluid causing a sudden acceleration. Some degree of instability exists, initially caused by the action of the pressure compensatory in the flow control valve. Meter-in circuits are generally used when the load characteristics are constant and positive. If the load is erratic or negative, the actuator will have a jerky motion. Meter-in circuits provide accurate control only when the load is opposing the actuator movement. If there is a tendency for the direction of the load to reverse, i.e. is act in the direction of motion, or to over run, then the meter-in system looses control. To overcome this problem, a back pressure has to be introduced by using a counterbalance valve or valve or over centre valve in the tank line, which in turn means increasing the system pressure. If a fixed displacement pump is used over a wide range of piston speeds, a large percentage of the fluid flows over the relief valve resulting in a “hot” system. The by type of flow control is used in meter-in circuits and has an integral relief valve which provides protection between the actuator and the control. This type of flow control is much more effective than the restrictor type for meter-in systems, because the by feature allows the oil to be exhausted to the reservoir at just slightly higher pressure than that necessary to do the work. With the restrictor type, the pump delivery not used would discharge over the main relief valve at maximum pressure. Meter-in circuits are used on surface grinders, welders, milling machines and other machine tools where fine speed control is essential. •

Meter – Out Circuit A typical meter-in circuit is as shown in fig 8.9. Here, the flow control valve is installed in the return line metering the fluid discharge. As in the case of “meter-in”, the pump must deliver more oil than is required by the cylinder. The circuit pressure has to overcome the cylinder load resistance and the pressure drop across the flow control valve. However, as the flow control valve is on the rod side of the piston, a somewhat reduced pressure is required at the bore side end (owing to differential areas) to overcome the pressure drop across the flow control valve. This makes it marginally more efficient on the extend stroke. Initially, the compensatory spool is fully open, and full pump flow is ed into the cylinder until the piston moves forward building up pressure at the flow control valve. The compensatory spool will now come into operation and restricts the flow to its correct value. There is an initial flow surge before the compensatory spool adjusts as in the case of “meter in”. When using a meter-out system, the pressure in the rod-end of the cylinder must be carefully considered. For e.g.: if the ratio of piston area to the piston rod area is 2:1 and the system pressure in 150 bar, then with no external load on the piston, the pressure in the rodend will be 300 bar. If this condition is likely to occur a separate relief valve may be fitted to the rod end side of the cylinder to prevent over-pressurization, as shown in Fig. 8.10 (Note: if the secondary relief valve “blows”, speed control will be lost.)

Page | 76


P P

P

M

Fig. 8.9: Meter Out Circuit With meter-out speed control, the quantity of oil leaving the cylinder is controlled. When the cylinder is extending, the oil from the rod-end is metered which si a smaller quantity than that flowing into the full bore end. Consequently, under extend conditions, meter-out flow control is not as sensitive as meter-in control. When the cylinder is retracting, the revers is true. Muter-out circuits are best where negative loads may occur, because back pressure is maintained on the exhaust side of the actuator preventing erratic motion. Meter-out circuits provide accurate speed control even with reversing loads. However, as with the meter-in system, considerable heat will be generated when used with a fixed delivery pump and a wide range of piston speeds.

Page | 77


Meter-out out type of speed control circuits operate satisfactorily for drilling, boring, reaming and tapping operations. •

Bleed-Off Off Circuit

Two typical bleed off circuits are shown in Fig 8.11. Here, the flow control valve is arranged ged to by a part of the pump output directly to the tank when the flow control valve is completely closed, the full flow from the pump would go into the cylinder. However, the moment the flow control valve is opened, some portion of the pump outlet will wil be bleed off any amount necessary to control how fast the cylinder moves. ((Note: Note: Only when the piston is stationary, the system pressure will reach the setting of the primary relief valve. Until then, the bleed off circuit will slow down the cylinder spe speed.)


Unlike the meter-in and meter-out circuit there is no excess flow going over the relief valve. The excess oil bleed off over the flow control valve is at a pressure induced by the cylinder load. Hence, the bleed-off circuits are more efficient in energy saving and work in a cooler environment. However, bleed off circuits provide less accuracy is speed control, because they don’t compensate for any change in fluid losses due to pressure change. Here the measured flow goes to the tank rather than the cylinder. This makes the cylinder speed subject to changes in the pump delivery and hydraulic system leakage which occur as the work load pressure changes. To minimize these effects, it is recommended to bleed-off no more than half the pump delivery and to avoid using a bleed-off circuit completely where there is a wide fluctuation is the work load pressure. In general, bleed-off speed control is best employed when the vast majority of the pump outlet is utilized by the cylinder and only a small percentage is byed. Also it is employed in systems where the pressure is reasonably constant and precise speed control is not the criteria. Problem 8.1 A cylinder has to exert a forward thrust of 100 kN and a reverse thrust of 10 kN. The effects of using various methods of regulating the extend speed will be considered. In all cases the retract speed should be approximately 5 m / min utilizing full pump flow. Assume that the maximum pump pressure is 160 bar and the pressure drops over the following components and their associated pipe work (where they are used): Filter

=

3 bar

Directional valve (each flow path)

=

2 bar

Flow control valve (controlled flow)

=

10 bar

Flow control valve (check valve)

=

3 bar

Determine: (a) the cylinder size (assume 2:1 ratio piston area to piston rod area), (b) pump size, and (c) circuit efficiency When using: Case 1: No flow controls (Figure (I)) (calculate extend speed) Case 2: ‘Meter-in’ flow control for extend speed 0.5 m / min Case 3: ‘Meter-out’ flow control for extend speed 0.05 m / min. Solution: Case 1: No flow controls (Figure (I)) (a) Maximum available pressure at full bore end of cylinder is given by 160 – 3 – 2 = 155 bar Page | 79


Back-pressure pressure at annulus side of cylinder = 2 bbar. ar. This is equivalent to 1 bar at the full bore end because of the 2:1 area ratio. Therefore, maximum available pressure to overcome load at full-bore end is 155 – 1 = 154 bar Full bore area = Load / Pressure 0.00649 m2

= Piston diameter =

0.5

= 0.0909 m = 90.9 mm

Select a standard cylinder say with 100 mm bore Full bore area = 7.85

10-3 m2

Annulus area = 4. 4.00

10-3 m2

70 mm rod diameter.

This is approximately a 2:1 ratio. (b) Flow rate required for a retract speed of 5 m/min is Area

velocity = 4.00 Extend speed =

10-3

5 m3/min = 20 lpm


Pressure to overcome load on extend =

య .షయ

12.7 10 / 127 bar

య

Pressure to overcome load on retract = షయ 2.5 10 N/m2 = 25 bar

Working back from directional control valve tank port: (i) Pressure at pump on extend Pressure drop over directional control valve B to T is 2 bar (Piston area ratio)

=1

Load – induced pressure

= 127

Pressure drop over directional control valve P to A

=2

Pressure drop over filter

=3

Therefore pressure required at pump during extend stroke = 133 bar Relief valve setting = 133 + 10% = 146 bar. (iii) Pressure required at pump on retract (22) + 25 + 2 + 3 = 34 bar Note: The relief will not be working other than at the extremities of the cylinder stroke. (2) Also when movement is not required, pump flow can be discharged to tank at low pressure through the center condition of the directional control valve. (c) System efficiency: System Efficiency =

!! " ## !! ##

Efficiency on extend stroke = $$ 100 = 95.5%

Efficiency on retract stroke = $ 100 = 73.5%

Page | 81


Chapter – 9 Direction Control Valve •

INTRODUCTION The primary function of a Direction control valve (DCV) is to direct the fluid flow through the desired ages whenever required. These flow of the fluid is then used to actuate the hydraulic cylinder of the position or reciprocate the other components in hydraulic system. In case of rotary motion, the fluid flowing in specific direction is used to reverse the direction of rotating element smoothly. Sometimes, a DCV is used to operated other controlling devices and thus works as pilot directional valve. •

VALVE PROTS AND POSITIONS A DCV has number of openings known as ports. It different permutations of flow between its various ports. Thus these ports give different ways for the flow of fluid through the vale. Accordingly the DCV are classified as one way valve (check valve), two way valve, three way valve and four way valve. Number of permutations of flow offered by DCV are termed as ‘Positions of valve’. A typical direction control valve has one normal position and two or here working positions. 1. Normal position: It is also known as zero position or neutral position. The neutral position is defined as the position to which the valve returns after the actuating force has been withdrawn. In all fluid power control systems, the neutral position is indicated as “0”. 2. Working position: These are the position obtained with help of he actuating force. These positions are designed as per the requirement of the application. •

Valve symbol

(a) Valve position

(b) Flow directions

(c) Cut off

Fig. 9.1: Symbol of direction control valve Typically, a direction control valve is specified by two numbers. One, for number of ports and other, for number of positions. e.g. A 4/2 DCV means four way two position valve. These valves are symbolically represented in a circuit diagrams. The symbols show only the functional aspect of the valve and not its principle of design or constructional details. 1. Each valve position is represented by a square (Fig. 9.1(a). Hence, number of squares in the symbol is equal to the number of positions that the valve offers. 2. Each square has number of connections, which is equal to number of ports. 3. Inside a square, the lines indicate the direction flow using arrows (Fig. 9.1(b)) Page | 82


4. 5. 6. 7.

Flow ‘Cut-off’ off’ are shown by short perpendicular line inside the square (Fig. 9.1(c)) Connections ions to inlet and outlet ports are drawn to normal position. The working line connections are indicated by A, B, C. The compressor line connection (the pressure source) is indicated by P, and return line by R, S, T. (Fig. 9.1 (d)) 8. The pilot lines are indicated ated by X, Y, Z. Thus the symbol for 4/2 DCV becomes A

P

B

R

Fig. 9.1 (d) : A 4/2 DCV • VALVE ACTUATION The direction control valve can be actuated by four different methods, accordingly they are i. Manually operated ii. Mechanical operate operated iii. Pilot operated iv. Solenoid operated As per the requirement the two way valve can be designed to give the state of ‘normally open’ or ‘normally closed’. •

FOUR WAY TWO POSITION VALVES The 3 way valves is suitable for a single acting cylinder or a hydraulic motor rotating in a specific direction. But double acting cylinder or reversible motor needs a four way valve. A four way 2 position valve can be symbolically represented as shown in fig 9.7 position A pushes the piston in one direction while position B pushes pu it in other direction. The actual working of this valve can be visualised with the help of fig 9.8


•

FOUR WAY THREE POSITION VALVE When there is a need of one more state in the hydraulic circui circuit using hydraulic cylinder then it is needed to add one more position in a four way valve. Now it become four way three position valve. The 3rd position introduced is added in between is known as Center Position. (Fig 9.9) The center position can be design designed so as to give the required state. The commonly required states are the respective. Position are shown in fig A typical application using 4 way 3 position valve are discussed below. (a) open center (b) closed center (c) Tandem center •

Solenoid operated ed DCV A solenoid is a electro electro-mechanical mechanical device that converts electrical energy into liner mechanical motion. In hydraulics the solenoids are preferably used to actuated the DCV (Fig. 9.5)

A solenoid is made of two parts. One is coil and second is armature. Applying electricity to the coil creates a magnetic field that attracts armature in to it. The armature pushes the spool to obtain desired position. •

TWO WAY VALVE It is also known as on-off on valve and is primarily rily used for simple shut-off shut operation. These valves make use of a Gate (96 (a) or a disc (b) or a ball (c) to obstruct the flow age.


The Gate valve is normally used for low pressures and operated at two positions namely fully on and fully off. The disc & ball valves are used for high pressure application and can have analogs control over the flow. •

Open centre position:


Fig. 9.10 (c): Tandem center In an open center enter circuit all ports are open to each other in the center position. When a three position open center type valve is used in hydraulic circuit pump flow is directed to the reservoir when the valve is in center. The other two positions are used to recipro reciprocate the cylinder. This is shown in figure 9.10 (a) Open center valve helps to prevent that build up in the system by allowing the pump flow to go back to the reservoir at a min. pressure during idle time of a machine. When the valve is in it’s center position sition no work is done by any part of the system. So in order to actuate pilot valves or other devices which uses pressure energy separate pilot sources must be used with open center systems. •

Closed Center Position: In closed center position, all ports aare re closed to each other in center position. When closed center type four way valve is used in a hydraulic circuit then, the pump flow must through the relief valve (Fig 9.10 (b)). Closed center versions are used when multiple circuits or functions must mu be accomplished from one hydraulic power source. To prevent heat build up in this type of center position, venting or unloading through pressure control methods are used. •

Tandem center position: Tandem center valve directs the pump flow directly to th thee reservoir port with the other two working port closed when in center position. These are used to hold a cylinder or motor under load or to permit the pump flow to be connected to a series of valves for multiple circuitry. This is explained in figure 9.10 9.10(c).


• CHECK VALVE Theoretically it is but a one way valve and is used as a non return valve. Thus it’s primary function is to allow the flow in one direction & to restrict it in in-other direction. The check valve is symbolically shown as in fig 9.11. The check eck valve is designed to allow the flow in one direction conditionally or unconditionally. Some check Valves allow the flow in one direction unconditionally while other allow the flow on the basis of some condition. Mostly, this condition is in term of a certain pressure. It is the pressure at which the valve just opens and is known as Cracking pressure. A spring is shown in the symbol of this type of valve to vary the cracking pressure. Simple application of check valve in shown in fig 9.12. The check valve, va allow the inflow to the cylinder unconditionally While, the return flow must through the flow restrictor and thus it controls the down falling speed of the load.

•

Pilot operated check valve:


A pilot lot operated check valve is a two way valve which allows free flow in one direction but prevents the reverse flow until it is actuated by pilot pressure or other means. So these valves allow conditional flow in reverse direction and are called as pilot operated check valves. These valves are used as a inter locking device. These valves can also be used for controlling the sequence of a machining cycle and to prevent the load from dropping. This application is shown in Fig. 9.13. •

CARTRIDGE DGE VALVE: These consist of a valve shell which can be mounted in a standard recess in a valve block or manifold. This from of construction has been used for many years particularly for pressure controls, flow controls & check valves. A cartridge valve iiss designed to be assembled into a cavity of a ported manifold block (alone or along with other cartridge valves a hydraulic components) in order to perform the valve’s intended function. The cartridge valve is assembled into the manifold block either by screw rew threads (threaded design) or by a bolted cover (Slip in design). Cartridge valve provides several advantages over conventional line or sub sub-plate mounted spool-type type directional, pressure, & flow control valves. In may applications, the advantages include 1. 2. 3. 4. 5.

Greater system design flexibility Lower installed cost Smaller package size Better performance & control Improved reliability


6. Higher pressure capability 7. More efficient operation 8. Greater contamination tolerance 9. Faster cycle times 10. Lower noise levels 11. Elimination of external leakage & reduction of internal leakage. Cartridge valve offers a design alternative rather than a replacement for conventional sliding spool valves. Often the most economical system employs combinations of threaded design (Screw in design) a slip in design cartridge valves with conventional sliding spool valves, all mounted on a common manifold.

Chapter – 10 Hydraulic Actuators •

INTRODUCTION Pumps perform the function of adding energy to a hydraulic system for transmission. Actuators, on the other hand extract energy from a fluid and convert it to a mechanical output to perform useful work. Fluid power can be transmitted through either linear or rotary motion by using actuators. Depending on the way the fluid power is transmitted, the actuators are classified as shown in Fig. 10.1

Part I: Hydraulic Cylinder •

LINER ACTUATORS Actuators

Liner Motion Transmission

Hydraulic Cylinder

Rotary Motion Transmission

Rotary Actuators

Fig. 10.1: Types of Hydraulic actuators A hydraulic cylinder is a device that converts fluid power into linear mechanical force and motion. Hydraulic cylinders extend and retract to perform a complete cycle of operation. Hence, they are also called as linear motors or reciprocating motors. It usually consists of a movable element (either piston or ram) operating within a cylindrical bore.

Page | 89


Depending g upon the type of movable element used and also on its motion type, hydraulic cylinders are classified as given in Fig. 10.2

Hydraulic Cylinder

Double Action

Single Action

Spring Return

Telescopic

Single End Red

Piston Type Gravity Return Double le End Red Ram Type

Gravity Return Kicker Cylinder Return

Tandam

Fig. 10.2: Classification of Hydraulic cylinders • •

SINGLE ACTING CYLINDER Single acting cylinders, piston type

A single acting cylinder is shown schematically in Fig. 10.3 (a). It consists of a piston inside a cylindrical housing called a barrel. Attached to one end of the pistonis a rod, which extends outside one endd of the cylinder (rod end). At the other end (blank end or head end) is a port for the entrance and exit of oil. A single acting cylinder can exert a force in only the extending direction as fluid from the pump enters the blank end of the cylinder. Single acting cylinders don’t retract hydraulically. Retraction is accomplished by using gravity load or by the inclusion of a compression spring.


Single acting cylinder with spring return is used for clamping operations, small presses and other such applications where little or no load is subjected on the return stroke (see Fig 10.4) Load

Fluid In/Out (a) Gravity return S.A. cylinder c

(b) Used to lift load

Fig. 10.5: Gravity Return Single Acting Cylinder Gravity return cylinders are used on mobile equipment’s, farm implements and other machine applications controlling vertically suspended loads. The high energy fluid on entering tering the blank end of the cylinder causes it to rise. The fluid metered out through the flow control valve allows the cylinder to retract slowly. The flow control valve prevents sudden retraction of cylinders on encountering heavy loads (see Fig 10.5) •

Single ngle Acting Cylinders, Ram Type


A single acting cylinder refereed to as ram use large cylinder rod approaching the size of the cylinder bore to give maximum to the load end of the rod. They are uused extensively in single acting applications such as car hoists, dump cylinders and hydraulic presses. The retracting force is usually provided by either gravity or by a small diameter auxiliary piston cylinder called as kicker cylinder (see Fig 10.6) • •

DOUBLE ACTING CYLINDER Double Acting, Single End Rod Cylinder

A double acting cylinder with single rod end is shown schematically in Fig. 10.7 (a). It consists of a piston inside a cylindrical barrel. barrel. Attached to one end of the piston is the piston rod, while the other face of the piston is blank. Hence, it is known as single rod end. A double acting cylinder can be extended and retracted hydraulically by using fluid power in both directions. For or a double acting cylinder of the single rod type there are two areas: Large area A1 piston area on bland side = (D2) Small area A2 piston area on the rod side = (D2 – d2)


Because of these unequal areas, the cylinder is classified as diff differential erential cylilder. Note: (1) It is apparent that the blank piston side havling a larger area will produce an effectively greater force, during extension stroke. (2) The piston side with large piston rods (i.e. smaller area) will exhaust large quantities of fluid and hence retract rapidly. (3) Since the piston has two effective areas which are different in size, the forces acting on either side of the piston are different. This gives rise to unbalanced side loads. Hence this type is considered to be a hydraulically unbalanced design. • Double Acting, Double End Rod Cylinder

A cylinder with a single piston and a piston rod extending from each end is a double acting double end rod, “DER” type (see Fig 10.8). For such a cylinder the word “extend” and “retract” have no meaning. Note: (1) Since each end contains the same size rod, both sides of the piston have same area. Hence, this type is a non non-differential type cylinder. (2) Since area of each end iiss same, the velocity of the piston is the same for both strokes. (3) Since area of each side of piston is same, the force acting on each face of the piston is same. Hence, this type is considered to be hydraulically balanced design. Since, the force and speed are the same for either ends, this type of cylinder is typically used when the same task is to be performed at either end. They are also used as a metering cylinder where the fluid is directed to another actuator for controlling speed or position.


• Tandem Cylinder

As shown is Fig 10.9, tandem cylinders consist of two or more cylinders mounted in inline and have a common piston rod driving the pistons in their respective cylinders. Fluid entering, leaving, or holdi holding ng at any port or combination of parts can actuate piston assemblies to various positions. The main advantage of this design is that greater force can be extracted from this single piston cylinders in tandem mounting when both the piston assemblies are moving ing in with the work. • Special designs (I) Cylinder with cables:

When long strokes are needed, the cylinder design shown in Fig. 10.10 used. It consists of a cable which provides a long pull with a relatively shore stroke. Fluid entering port B, extends one or both piston assemblies, which stroke the cable a distance determined by the pulley arrangement. Fluid entering the ports A and C, retract the piston assemblies.


(II) Nested cylinders:

A nested-design design using three cylinders is as shown in Fig 10.11. It provides greater strength with a long stroke. Fluid entering the cylinder ports labelled A extend all three piston assemblies, while the fluid entering the ports labelled labelled B retract the piston assemblies. (III) Positioning cylinder:

The positioning type cylinders shown in Fig 10.12 use a moving cylinder barrel, one moving piston assembly and one stationary piston assembly. This design sign allows a full stroke which might be used to insert the work piece into a machine and then half a stroke for repeating a series of work strokes. It is typically used in welding machines. •

TELESCOPIC CYLINDER Fig 10.13 (a) shows a schematic sketch of a telescopic cylinder. It consists of nested multiple cylinders (or tubular rod segments) called sleeves, which slide inside each other.


These sleeves work together to provide a long working stroke than is possible with a standard cylinder. Upto 4 to 5 sleeves eves can be used. It operates on the principle that the rod with the largest area gives the greatest force at the least pressure and moves out first; the next largest moves at a slightly higher pressure; and so on for as many stages as the unit may contain. in. Hence, it can side that the maximum load is exerted when the cylinder is collapsed. In the extended position, the load is a function of the diameter of the smallest sleeve. Fig. 10.13: Telescopic Cylinder Note: Telescopic cylinder rare usually single acting. Telescopic cylinders are used where long work strokes are required, but the full retraction length must be minimized. One application for a telescopic cylinder is the high lift fork truck. • CONSTRUCTIONAL FEATURES OF DOUBLE ACTING CYLINDER DER

Fig. 10.14 illustrates the constructional features of a double acting, single rod end cylinder. (1) Barrel: It is made of samples steal tubing honed to a fine finish on o the inside. (2) Piston: The piston is made up of ductile iron or steel. It contains U U-cup packing to seal against leakage between the piston and barrel. Sometimes cast cast-iron iron piston rings and / or ruber seals are also used as piston seals. (3) Piston Rod: A piston rod is made up of high tensile steel, and it is super finished and hardened by chrome chrome-plating. plating. One end of the piston rod is attached to the piston face, while the other end is attached to the load resistance.


(4) End-caps: End caps are provided on either ends of the barrel. They are generally cast (from or aluminum) and incorporate the ports. Base end cap closes the barrel on the bare end side of the piston, while the rod rod-end end cap closes the barrel on the rod-end rod side of piston. The rod-end cap houses ouses the bearing and sealing components of the piston rod. (5) Ports: A port is an internal or external opening in a cylinder which allows the age of fluid into or out of the cylinder. The extend port is located in the rod end cap.’s the bearing and sealing components of the piston rod. (6) Tie-Rods: Tie-rods Tie rods are used to secure the end caps to the barrel. These tie rods are generally made up of high tensile steel. During operation, a large thrust acts upon the end plats. The tie rods help the end plate plate to withstand this thrust. The number of tie-rods tie used varies from 4-8, 8, depending on the thrust force experienced. (7) Seal: As seen, seals are required to prevent leakage at various locations. •

•

• • •

A wiper or scraper seal is fitted to the end cap where the rod enters the cylinder to remove dust particles. In very dusty atmosphere external rubber bellows may be used to prevent the dust particles from entering the cylinder. However, these bellows are vulnerable to puncture and need splitting and regular ins inspection. pection. A dual purpose O-ring O ring is fitted behind the bearing. It prevents the high pressure fluid from leaking out along the rod. The wiper seal, bearing and this internal O-ring O seal are sometimes combined as a cartage assembly to simply manufacture. The rod is generally attached to the piston via a threaded end. Leakage can occur along the rod. So seals are again needed. These can be a static O O-ring ring around the rod. Similarly seals in the from of O O-ring or Y-seals seals are provided at the points of fitting off the end caps inside the barrel. Also modified U-cap U cap piston packing are provided on the piston surface. They prevent the leakge of oil from the high pressure side to the low pressure side.

(8) Bearing: A long wearing, cartridge type brome bushing provid provides es the required bearing action for the extension and retraction of the piston rod. It is located in the rod end cap, just behind the wiper seal. •

CYLINDER MOUNTINGS


Cylinder mounting is determined by the aapplication. pplication. The application of hydraulic cylinder is so vast that it can be said that it is limited only by the ingenuity of the fluid power designer. Some of these applications of hydraulic cylinders are as shown in fig 10.15 Depending on the application various types of cylinder mountings are in existence. Cushioning devices are installed in either or both ends of the cylinder.

As shown in Fig. 10.21 (a), when the cylinder is retracting, the exhaust fluid f flow is unrestricted until The plunger enters the cap. When the plunger enters the cap (see Fig 10.21(b)), the normal exhaust fluid flow from the barrel to the port gets booked and deceleration starts. The exhaust flow is then rerouted through the byp by ass port and metering valve at a controlled rate, decelerating the piston. The metering valve can be adjusted to set the required decelerating rate. A check valve in also included in the rod rod-end end cap. It is used to by the metering valve and direct a full ll flow of fluid at system pressure to the full area of piston during acceleration of the extension stroke. Cushioning shown in Fig 10.21, is used for the base end cap. A similar arrangement can be used in the rod-end end cap as well. However, in the rod end cap cushioning arrangement, the check valve design is not incorporated. • •

HYDRAULIC CYLINDER CALCULATIONS Calculation of force, Velocity and power during extension and retraction strokes Consider a double acting, single end rod cylinder. This cylinder has two differential piston areas. (I) Effective area during extension Blank – end Piston Area A1 = (D2) (II) Effective area during retraction Rod - end Piston Area = (D2 – d2) Due to these differential areas, the force output and piston velocity vary during extension and retraction strokes.


Force [N] = Pressure [Pa] Piston Area [m2]

(I) Force:

F extension – stroke = p (D2)

…(10.1) Fretraction – stroke = p

D2 – d2

(II) Piston Velocity: Piston Velocity [m/s] = v

v

Extension =

/

Retraction = π

…(10.2)

–

(III) Power: The power developed by the hydraulic cylinder is found out by Power [W] = piston velocity [m/s] Force [N] = Input Flow [m3/s Pressure [Pa]]

…(10.3)

Problem 10.1: A pump supplies oil at 75 lpm to a 50 mm diameter, double acting cylinder. The rod diameter is 25 mm and the load acting on the cylinder during extension and retraction is 4.5 kN. Calculate the hydraulic pressure, piston velocity and the cylinder horsepower both during extension stroke and also during retraction stroke. Solution: (I) Extension Stroke: 1) Effective Area A =

D2 = (0.05)2 = 1.963 10-3 m2

2) Hydraulic Pressurepext.

.!"#$

= #.%&'"#$ 22.92 10! 22.92 bar (

3) Piston Velocityu vext. = ) 4) Power

#.%&'"#$

…Ans.

0.634 m/s

P ext. = Fv = (4.5 103) 0.634 = 2853 W = 2.853 kW

…Ans. …Ans.

(II) Retraction Stroke:

(1) Effective Area a = (D2 – d2) = (0.052 – 0.0252) = 1.473 10-3 m2

.!"#$

(2) Hydraulic Pressure pret. = #.*'"#$ = 30.55 105 Pa = 30.55 bar

…Ans.

Page | 99


Thus, more pressure is required to retract than extend the same load due to the effect of the rod. (3) Piston velocity vret. =

= 0.8486 m/s

…Ans.

Thus, for the same pump flow Q, the piston retraction velocity is greater than that for extension due to the effect of the rod. (4) Power P ret. = F v = (4.5 103) 0.8486 = 3819 W = 3.819 kW …Ans. Thus, more horsepower is supplied by the cylinder during retraction stroke because the piston velocity is greater during retraction while the load force remained the same during both strokes. •

Calculation of Piston-rod size and cylinder length:

The procedure to compute the piston rod size and cylinder length under end thrust condition is as follows: (1) Determine the column strength factor from the mounting table arrangement as shown in Table 10.1

(2) Calculate the corrected length of the rod by using the formula Corrected Length = Actual Stroke

Column Strength Factor

…(10.4)


• • •

Increasing the swash plate angle increases the motor displacement and hence increases the torque capability but reduces the drive shaft speed. Reducing the swash plate angle reduces the motor displacement and hence reduces the torque capability but increases the drive shaft speed. Reversal of the motor is accomplished by titting the yoke over centre (i.e. – ve angles). The variable displacement unit shown is of the internally pressure compensated type. • Bent axis Piston Motor This type of motor also develops torque due to the pressure acting on reciprocating pistons. This design however has the cylinder block and the driveshaft mounted at an angle to each other so that the force is exerted on the drive shaft flange. Speed and torque depend on the angle between the cylinder block and drive shaft. The larger the angle, the greater the displacement and the torque, but the smaller the speed. This angle varries from a minimum 7

#o

to a maximum of 30o.

In a variable displacement unit, the torque is varied by varying the angle between the cylinder block and the driver shaft either by using a hand wheel or by pressure compensation method. • HYDRAULIC MOTOR PERFORMANCE CALVULATIONS (I) Work done by motor (W): W = fluid pressure p [N/m2] Volumetric Displacement VD [m3]

…(10.15)

(II) Theoretical Torque Capacity of motor (TT): TT [Nm] =

ీ

…(10.16) (III) Actual torque delivered by motor (TA):

TA [N - m] = …(10.17)

(IV) Theoretical flow rate or motor (QT) QT [m3/s =

ీ య /

…(10.18)

(V) Theoretical power required to operate the motor(PT): PT [Watt] = Pressure p [N/m2] Theoretical discharge QT [m3/s]

…(10.19) (VI) Actual power required to operate the motor (PA):

Page | 101

Dhanvantari College of Engineering, Nasik PA [Watt] = Pressure p [N/m2] Actual discharge QA [m3/s]

…(10.20) (VII) Actual power delivered by motor (P output): P output [Watt] =

ఽ

…(10.21)

(VIII) Volumetric Efficiency (nv): The volumetric efficiency of a motor is defined as,

η

Theoretical Flow rate the motor should consume 100 Q 100 …(10.22) Actual Flow rate consumed by motor Q

Observe that the Volumetric efficiency of a motor is the inverse of that for a pump, because a pump does not produce as much flow as it theoretically should, where as a motor uses more flow than it theoretically should due to slippage. (IX) Mechanical Efficiency (m): The mechanical efficiency of a motor is defined as, η

Actual Torque delievered by motor T 100 100 Torque motor should theoretically deliver T

…(10.23)

(X) Overall Efficiency (o): The overall efficiency of a motor is defined as, η

Actual power delivered by motor P ! η η 100 100 Torque motor should ddelivered to motor P !

…(10.24)

+,-

= &$" 100

The performance of any hydraulic motor depends on the seal between the inlet and outlet sides. Internal leakage (slippage) between the inlet and outlet reduces efficiency. The overall efficiency of motors is dependent on the motor type as indicated in the table below: Table 11.4: Sr. No. 1

Motor Type Gear motors

2

Vane motors

3

Piston motors

Overall Efficiency 70% 75% 75% 85% 85% 95% Page | 102


Problems on Hydraulic Motors Problem 10.4: A hydraulic motor has an 100 cm3 volumetric displacement. If it has a pressure rating of 75 be and it receives oil from a 50 lmp pump, find the speed, torque capacity and power capacity of the motor. Solution: (I) Flow Rate of Motor: Q[m3/s] =

. / 0",

!$"#$

&$

&$

1#$$"#$ 2", &$

Motor Speed N = 500 rpm

…Ans. (II) Torque Capacity of motor: TT [N - m] =

,/ ". +

1*!"#$ 2"1#$$"#$ 2 +

…Ans.

TT = 119.37 N -

(III) Power Capacity of motor: Power [Watt] =

+ , - &$

+"!$$"##%.'* &$

…Ans.

Power Capacity = 6250 W = 6.25 kW

Problem 10.5 A hydraulic motor has a displacement of 150 cm3 and operates with a pressure of 75 bar and a speed of 1800 rpm. If the actual flow rate consumed by the motor is 0.005 m3/s and the actual torque delivered by the motor is 165 N – m, find all the three efficiencies and the actual kW delivered by the motor. Solution: (I) Volumetteric Efficiency (" ) : Theoretical flow rate of motor is given by QT =

ವ య /!"#$%!&

'()(షల *(+

4.5 10,- &- /(

Page | 103


Now, η

/౐ /ఽ

100

0.)(షయ .)

100 90%

…Ans.

(II) Mechanical Efficiency (η ): Theoretical torque delivered by motor is given by, TT =

,/ ". /3 +

1*!"#$ 2"1#!$"#$ 2 +

= 179.05 Nm η

T 165 100 92.15% T 179.05 100

…Ans. (III) Overall Efficiency (". ) η

η η 90 92.15 82.935% 100 100

…Ans.

(IV) Actual power delievered by motor (P output): η

82.935 =

P output /ఽ

9"(

100

100

P output 31.1 10' Watt 31.1 k W

…Ans. or

P output Ans.

+ ,"- &$

+"#:$$"#&! &$

…

P 31.1 10- Watt 31.1 kW

Problem 10.6 A hydrostatic transmission operating at 70 bar has the following characteristics: Pump VD = 82 cm3

Motor VD =?

η = 82% η = 88% N = 500 rpm

η = 92% η = 90% N = 400 rpm

Find: (1) Displacement of torque (2) Motor output torque Page | 104


Solution: (I) Displacement of motor (VD): (a) Pump QT [m3/s] = •

Diaphragm type Accumulator

A typical diaphragms type accumulator is as shown in Fig. 11.6. It consists of a flexible rubber diaphragm, secured in a steel shell. The diaphragm acts as an elastic barrier between the oil and gas. A shut off button, which is secured to the diaphragm diaphra base, covers the inlet of the line connection when the diaphragm is fully stretched. This prevents the diaphragm from being pressed into the opening during precharge period. On the gas side, the screw plug allows the charging of the accumulator by means of a charging device. Fig. 11.7 illustrates the operation of a diaphragm type accumulator. The hydraulic pump delivers oil into the accumulator and deforms the diaphragm. As the system pressure increase, the gas gets compressed thus storing potential energy. energy. Now, when the system pressure decreases and an additional oil is required to be pumped into the system, this stored potential energy in the accumulator forces the additional required oil into the system. Advantages: (1) It has a small weight to volume volume ratio. Hence, it is exclusively used for airborne applications. (2) The inertia of the device is very small since there are no pistons,, ram or spring.


•

Bladder type accumulator:


Chapter 11 – Hydraulic System Components Part I : Accumulator and Pressure Intensifiers •

ACCUMULATOR INTRODUCTION An hydraulic accumulator is a device that stores the potential energy of an incompressible fluid held unde underr pressure by an external source against some dynamic force. This dynamic force can come from three different sources: gravity, mechanical springs and compressed gases. The stored potential energy in the accumulator is a quick secondary source of fluid power er capable of doing useful work as required by the system. The are three basic types of accumulators used in the hydraulic system as shown in the accumulator classification tree in Fig. 11.1

Pressure accumulators umulators are primarily used in many hydraulic circuits to (1) Store fluid under pressure, and / or (2) Cushion shock waves in the circuit piping •

WEIGHT LOADED ACCUMULATOR The weight loaded accumulate shown in Fig. 11.2 is historically the oldest typ type. It consists of a vertical heavy wall steal cylinder, which incorporates a piston with packing to prevent leakage. Dead weight in the from of large ballast is attached on top of the piston. The force of gravity of the dead weight provides the potential eenergy nergy in the accumulator which provides a constant fluid pressure, regardless of whether the chamber in full or near empty and the rate and quantity of output. In the other types of accumulators, the fluids output pressure decreases as a function of volume output of the accumulator. They are usually found on heavy presses where constant pressure is required, or in applications where usually large volumes are necessary. The main disadvantage of this type is that they are extremely large is size and heavy weight, ght, which makes it unsuitable for mobile equipment.


Let, F = Force exerted by ballast weight (N) A = Cross sectional area of accumulator (m2) S = Stroke of accumulator piston (m) The output pressure available from weighted accumulat accumulator or is given by, area a m2

Stroke S(m)

…(11.2) •

SPRING LOADED ACCUMULATOR Fig. 11.3 shows a typical spring loaded accumulator. A spring loaded accumulator is similar to the weight loaded accumulator except that the piston is preloaded preloaded with a spring. The spring is the source of energy that acts against the piston, forcing the fluid into the hydraulic piston. The load characteristic of a spring are such that the energy storage depends on the force required to compress the spring spring.. The uncompressed length of spring represents zero energy storage. As the spring is compressed to the maximum installed length, the minimum pressure value of fluid in the cylinder is established. As fluid under pressure enters the cylinder, the spring is further compressed, thereby increasing the spring force at that instant and hence pressure increases. Similarly as the fluid under pressure leaves the cylinder, the spring decompresses. Fig. 11.8 shows a bladder type accumulator which consist of a elastic bladder which provides the barrier between the fluid and gas. The bladder is integrally moulded with a gas valve. Which is fitted in the accumulator. The bladder along with gas valve can be installed or removed through the shell opening at the poppet valve valve.. This poppet valve closes the inlet when the accumulator bladder from being pressed into the opening. Thus it prevents the bladder from damage and rupture and increases the volumetric efficiency. An drain plug is provided near the poppet valve to bleed the th air from the system. Fig. 11.9 illustrates the operation of bladder type accumulator. The hydraulic pump delivers oil into the accumulator and deforms the bladder. As the system pressure increases, the gas gets compressed thus storing hydraulic energy. W When hen the pressure in the external circuit falls below that in the accumulator the gas expands and force the fluid into the circuit.


Advantages: (1) It provides a positive sealing between the the gas and oil chambers. (2) It has a small weight to volume ratio (3) It has a small inertia and hence provides a quick pressure response for pressure Regulating,, pump pulsation and shock dampening applications. •

ACCUMULATOR APPLICATION / FUNCTION CIRCUI CIRCUITS Since hydraulic accumulators store pressurized fluid for system use on demand, they can be used to serve a variety of system functions. Typical of these functions are maintain system pressure, absorbing hydraulic shocks, supplementing pump delivery, pro providing an emergency source of power, balancing loads and acting as a barrier between dissimilar fluids. Some of these functions are discussed in detail in the following articles: •

Accumulator as an auxillary power source: Accumulator are used as an auxi auxillary llary power source in a system where intermittent operations are preformed. These accumulators store the oil delivered by the pump during a portion of work cycle, and then release this stored oil an demand, thereby serving as a


secondary power source to assist as the pump. Thus, the accumulator reduces the input horse-power power by storing energy during idle times of the machine and also facilitates the use of a smaller sized pump. This application is depicted in Fig. 11.10 in which a 4/2 manually operated, spring return D.C valve is used in conjuction with an accumulator. When the 4/2 D.C valve is manually actuated to position I, oil flows from the accumulator to the blank end of the cylinder. This extends the piston unit it reaches the end of its stroke. When th the desired operation is occurring (when the cylinder is in fully extended position), the accumulator is being charged by the pump. When the accumulator is fully charged, the pump. When the accumulator is fully charged, the pilot operated relief or unloading valve opens and allows the pump delivery to return to the reservoir at very low pressure. The accumulator maintains its charge since it is isolated by check valve. • PRESSURE INTERNSIFIERS A pressure intensifier is a device that is used to increase the pressure pressure in a hydraulic system to a valve several times above the pump discharge pressure. It accepts a high volume flow at relatively low pump pressure and converts a portion of this flow to required valve of high pressure. due to this pressure boosting cap capacity, acity, it is also known as pressure booster. Intensifiers are basically used in applications such as hydraulic presses rivelting machines, sport welding machines where a great force is required to be applied through a relative short distance. • Single Actingg Intensifier A Single acting pressure intensifier is as shown in Fig 11.14. The intensifier unit has a high pressure piston with an effectively larger area. The direction control valve directs the low pressure fluid into the left hand side of larger pis piston. ton. The low pressure acting on the larger piston generates a force F. Now, when this force is transmitted to the smaller piston it generates a considerably high pressure on the fluid located on the right side of smaller piston. Although high pressure flui fluid d is one available to do work, it should be noted that the volume of fluid discharged at the high pressure end will be proportionately less than that required a large end.


The increase in press pressure ure is in direct proportion to the ration of the larger piston area and the smaller piston area. The volume output is inversely. •

Double Acting Intensifiers A Double acting intensifier is as shown in Fig 11.15. It consists of an automatically reciprocating g large piston that has two small rod ends. The piston has its large area exposed to the low pressure oil. The force of this low pressure oil moves the piston and causes the small area of piston rod to force the oil out at intensified high pressure. This ddevice is symmetrical about a vertical center line. Thus, as the large piston reciprocates each other during each strake of the unit duplicate each other during each stroke of the large piston. Thus in effect the double acting intensifier stimulates the operation operation of two single acting intensifiers.


Double acting intensifiers are used for applications needing longer work strokes or for maintaining high pressures for a longer period of time. The working king of a double acting intensifier is as shown in Fig. The D.A. intensifier is actuated by a 4/2 solenoid operated D.C. valve. Valve E check valves A and B are installed an either side on the high pressure outlet line, while check valves C and D are insta installed on the low pressure outlet lines. When valve E is actuated and position I is attained then the low pressure oil from the pump is directed to the left intensifier cylinder. It pushes the major piston to the right discharging fluid in the right end of intensifier cylinder to the tank. As the ram moves to the right fluid is intensifier in Booster II. This intensified fluid closed the check valve D and check valve A. Thus, the high pressure is delivered I for intensification in next cycle when the main piston ston reaches the end of its stroke on the right, the solenoid gets energized in the opposite direction and valve E now attains position II. The major piston starts moving towards left. The fluid in the Booster cylinder I gets intensified. This high pressur pressure fluid closes check valves V and C and is then delivered to the output through check valve A.

A doub •

Air Oil Intensifier In an air oil intensifier, there are two cylinders. cylinders. The air cylinder with large piston and the booster hydraulic cylinder with a small piston. Both eh piston are connected by a piston rod. Air is directed into the top end of intensifier (i.e. the head end of air cylinder). This forces the hydraulic pisto piston n (ram) through the lower seal and into the lower booster cylinder. In the bottom end of the intensifier (i.e. the head end of booster cylinder), the hydraulic fluid gets trapped by the retreating hydraulic piston and generates high pressure. the degree if the pressure boost is determined by the area ratios of the air piston to the hydraulic ram. This type of air oil intensifier are capable of producing output hydraulic pressure upto 200 bar. In applications such as punch press, it is necessary to extend a hydraulic cylinder


rapidly using little pressure to get the ram near the sheet metal strip as possible. The cylinder must exert a large force is needed to punch the work piece from the sheet metal strip. Since the strip is thin, only a small flow rate is required. required. After the completion of punching operation, the fast retraction of cylinder is required. This application requires an air air-over hydraulic intensifier as explained below.


Fig. 11.17 shown an air over oil intensifier circuit. Shop air at 6 bar pressure is used to extend and retract the cylinder during the low pressure portion of the cycle. The air oil intensifier is used to provide the required high pressure oil during the critical operation cycle. The system operation is as follows Valve A extends and retracts the working cylinder using shop air. Valve B appres shop air pressure to the top end of intensifier (i.e. head end of air cylinder). This produces a high hydraulic pressure at the bottom end of the intensifier (i.e. head end of booster cylinder). Actuation of valve A directs the air to the approach tank. This forces oil at 6 bar pressure through the booster cylinder to the head end of the working cylinder. When the working cylinder s the work piece, it experiences load and the high pressure portion of the cycle is initiated. Valve B is actuated directing the shop air to the top end of intensifier, which inturn intensifier. This high pressure oil cannot return to the approach tank because, it part is blocked off by the downward motion of the booster cylinder piston. Thus, the working cylinder receives high pressure oil at the head end to overcome the load. When valve B is released, the shop air is blocked and the top end of the intensifier is vented to the atmosphere. This terminates the high pressure portion of the cycle. When valve A is released, the air in the approach tank is vented and the shop air is directed to the return tank. This delivers oil at shop pressure to the rod end of the working cylinder causing it to retract. At the same time shop air enters the rod end of air cylinder, thereby causing the intensifier piston to move up. The oil from the head end of the retracing working cylinder enters the bottom end of intensifier and flows back to the approach tank. This competes the entire cycle. • Advantages of Intensifier There are reasons other than economic, that ditals the use of an intensifier rather than high pressure pumps for certain applications. The reasons which justify the use of an intensifier are 1) Decompression of fluid under pressure can be at lower pressures values, permitting softer action with less expensive components. 2) High pressure fluid is localized in what can normally be a low pressure machine. So there are minimum number of high pressure seals to maintain and also high stresses are developed only in these high-pressure piping. Hence maintenance is simple. 3) Pressure can be maintained over a period of time with low horsepower input. 4) Input fluid to the intensifier or booster can be different from the working fluid in the circuit such as shop air steam, raw-water, etc…. 5) Fluid pressure can be quickly dissipated in the event of a line breakage. 6) Shock loading from punch and shear operations can be more easily controlled. 7) High pressure can be varied with more sensitive low pressure devices, according to size ratio.

Page | 114


It has a dia--seal piston directly acting on a snap action switch as shown in Fig 11.21. here, the fluid input pressure is balanced against the spring. When the fluid pressure exceeds the spring setting, the piston moves to actuate the electric switch. It can operate with pressure varying form 0--800 bars. The electrical switching element in a pressure switch opens ns or closed an electric circuit in response to the actuating force it receives from the pressure sensing element. The design shown uses single pole, double throw snap action switches for maximum reliability. There are two types of switching element: nor normally mally open (N.O.) and normally closed (N.C.) – A N.O. switch is the one in which no current can flow through the switching element until the pump fails to operate.

Part III: Power Units and Hydrostatic Transmission •

HYDRAULIC POWER UNITS: Power package kage units consist of an oil reservoir, pump, valves and various require controls, all assembled into one unit of supply pressurized fluid. They are not only compact in size but also provide the function of direct pressure and flow control within the basic package. They have been developed from extensive experience by manufacturers of fluid power components to supply a need. They result in substantial cost savings to the consumer. Power packages are available as stock units or can be assembled to meet cust customer specifications incorporating features peculiar to a particular application. These power packages are also equipped with pressure gauge, monitoring system, pressure relief heat exchanger and sight level gauge to facilitate the consumer. These power units uni conform to JIC and other fluid power standards.


Chapter – 12 Hydraulic Circuit Design, Analysis and Maintenance Part I: Hydraulic Circuit Design and Analysis •

INTRODUCTION In the preceding chapters on hydraulics, we have studied the basic principles on which hydraulic power transmission functions and also briefly seen the various system components. In this chapter, we will discuss the basis hydraulic circuits. It is important to note that a hydraulic circuit and a hydraulic system are different. A circuit is an arrangement of components such as pumps, actuators, valves and conductors which are interconnect so as to perform one or more specific tasks, but not a complete work cycle. On the other hand, a system is composed of several circuits, and it refers to the complete assembly of component parts that transmit and control the fluid power. A system is capable of competing one or several operations that constitute a work cycle. Both system and circuits are designed to accomplish output objectives. The design phase includes sizing the components and plumbing to meet the output requirements, as well as establishing work cycles using time, flow, pressure and power calculations. The analysis phase includes assembly of the system components, installation of the hydraulic machine and finally the diagnosis of the performance of the machine. When analysing or deg a hydraulic circuit and a turn a hydraulic system, the following three important considerations must be taken into . (A) Safety of operation: It includes 1) Pressure rating, temperature rating and operating speed of system components. 2) Compatibility between system components. 3) Interlock for sequential operations. 4) Power failure locks and emergency shutdown features. 5) Environmental conditions and fire hazards. (B) Performance of desired function: It includes 1) System components must meet the required performance specifications and perform the duty cycle for which they are designed. These power units are available from 2-20 kW size with reservoirs to 150 litre capacity. Single or double pumps of the gear and vane type are most common, mounted directly to a motor through a flexible coupling. Fig. 11.23 shows a basic compact power unit incorporating direction, pressure and flow control functions. It employs a constant displacement pump to pressurizes the fluid. The drive motor in reversed to change the direction of flow from the pump. Check valves in pump inlet line provide the correct suction characteristics regardless of rotation direction of the pump. The unit consists of a spool type D.C. valve. In the Fig 11.23 (a), the D.C. valve is in position I. The pressurized fluid from pump is being directed through the right port. The fluid entering the left port is returned back to the tank. The fluid is also directed to the left hand of spool D.C. valve. When the pump drive motor is reversed, the pressurised fluid from pump then causes the spool to move to the right Page | 116


so that position II of D.C. valve is attained through the left port. The fluid entering the right port is returned back to tank. Also fluid is directed to the right hand of spool D.C. valve. The check valves stop the flow of fluid from reservoir to system, when the pump motor stops. An external relief valve or pressure switch can be installed at point 1. An internal relief valve or pressure switch can be installed at point 2. The relief vale serves as a safety device for thermal protection or an operational malfunction.


.


2. System components must withstand the hydraulic shocks. 3. The life expectancy of the system components should be the same as that of the machine. (C) Efficiency of operation: It includes 1) System should be simple, safe and functional. 2) Systems components must be standardized and should be easily available for repaint and replacement during maintenance. 3) System should have minimum operation cost.


It is very important for the designer to have a working knowledge of components and how they operate in a circuit. Circuit diagrams are used to design and analyse both hydraulic circuits and systems. They are constructed from standard graphic symbols representing repr components. Hence, it is necessary to known there hydraulic fluid power symbols. Table 12.1 gives a list of symbols that conform to the American National Standards Institute (ANSI) specifications. The design and analysis of hydraulic circuits and systems is systematic and is carried out in the following steps. 1. Size the actuator form output objective. 2. Establish the work cycles using time, flow pressure and power calculations. 3. Design the circuit. 4. Size and select the components. 5. Assemble the circuit oor system. 6. Monitor the performance of the machine. 7. Check the machine for safe operation and compliance with OSHA standards. LINEAR CIRCUITS • A linear circuit is a simple reciprocating circuit. Here the actuator can be a single cylinder or a double acting cylinder. • Control of a Single Acting Hydraulic Cylinder Fig 12.1 gives the circuit used to control a S.A. hydraulic cylinder. The operation is described as follows: 1) When the 3/2 way D.C. valve is manually actuated to position (I) By operating the hand lever, the full pump flow is directed to the blank end of the cylinder, thereby extending the piston rod. At full extension, pump flow goes through the relief valve. 2) Deactivation of the hand lever, causes the 3/2 way D.C. valve to occupy position

Hydraulic Cylinder D.C. valve is shifted to position (I), instead of having a free fall under gravity, the counterbalance valve permits the cylinder to be forced down words when pressure is applied at thee top. When the D.C. valve is shifted to position (II), the fluid enters the rod-end rod of the cylinder by ing the counterbalance valve through the check valve. •

SEQUENCING CIRCUIT Sequencing circuits order cyclic events, such as the operation of two cyli cylinders one after the other. For this purpose a sequence circuits may be a clamping and drilling op operation as shown in Fig. 12.9. Here re the sequence by a pressure step increase across sequence valve S1 and S2.


When the 4/3way way solenoid actuated D.C. way is shifted ifted to position (I), the fluid flows to the blank end of both cylinders. The clamp cylinder extends first because the fluid flow is un obstructed. When the clamp cylinder s the work piece, the pressure rises shifting sequence valve S1. then the fluid uid flows into the blank end of the drill cylinder, thereby starting the drilling operation by extending the drill cylinder. When the solenoid is actuated so the 4/3 way D.C. valve is in position (II), then the fluid flow is directed to the rod end of bothh cylinders. Initially the drill cylinders retracts completely since its fluid flow is unobstructed when the drill cylinder has completely withdrawn, then the pressure starts rising causing the sequence valve S2 to shifted. Then the fluid flows into the rodd end of the clamp cylinder, thereby de-clamping clamping the work pieve by withdrawing the clamp-cylinder cylinder. Note: 1) Sequence valve S1 is in the blank end pilot line of drill cylinder causing it to extend after the clamp cylinder extension, where as the sequence val valve S2 is in the rod end pilot line of clamp cylinder causing it to retract after the retraction of drill cylinder. •

TO SPEED CONTRO OF A HYDRAULIC MOTOR Fig. 12.20 illustrates the speed control of a hydraulic motor using a pressure compensated flow control ol valve when the 4/3 way FD.C. valve is actuated to position (I), the motor rotates in one direction. Its speed can be varied by adjusting the setting of the thrttle of the flow control valve. In this way the speed can be infinitely varied as the excess oil il goes through the pressure relief valve. When the 4/3 way D.C. valve is actuated to position (II), the direction of the motor rotation is reversed. The pressure relief valve prodives overload protection if for example, the motor experiences an


excessive torque load. When the 4/3 way D.C. valve is deactivated, it returns to its epring centered neutral position. In the tendemcentre position, the motor stop suddenly and becomes hydraulically locked. •

HYDRAULIC MOTOR BRAKING SYSTE In a fluid power system, system if the hydraulic motor is driving a machine having a large inertia, then it will experience a flywheel effect, and stopping the flow of fluid to the motor would cause it to act as a pump. In a situation such as this, the circuit should be designed to provide vide fluid to the motor while it is pumping to prevent it from pulling in air. In addition, provision should be made for the discharged fluid from the motor to be returned to the tank either unrestricted or through a rapidly but without damage to the syste system. Fig. 12.21 is a motor circuit that possesses these desirable characteristics for either direction of motor rotation.

Part II: Troubleshooting, Maintenance and Safety Considerations in Hydraulic Circuits •

1) 2) 3) 4) 5)

TROUBLESHOOTING AND MAINTENANCE Troubleshooting ing means finding the problem. In fluid power systems, problems are first determined to be one or a combination of the five general types and then through a logical proceure, problem statements are written and tested to determine which specific component orr part of the system is at fault. The five general types of problems are: Pressure Flow Leakage Heat Noise and Vibration The procedure by which specific problems are identified cosists of organsing and writing problem statements and then proving or rejecti rejecting ng them through inspection, making simple calculations, conduction tests on system, and finally doing the teardown and disaccembly for visual verification. Some repairs may have to be made to find the problem, but troubleshooting does not means reparing the machine. chine. Neither does it mean performing routine maintenance, replacing components or redeg the circuit. Rather, troubleshooting means performing the


diagnosis and then outlining a course of action that will bring the machine to satisfactory operation. •

(I) Pressure Related Problem Statements (A) NO PRESSURE (1) Faulty pressure gauge. (2) Complete pump failture. (3) Circuit open to the reservoir. (3) Motor or pump coupling failure. (B) LOW PRESSURE (1) Inaccurate guage. (2) Load resistance has than expected. (3) Pressure relief valve set low or leaking. (4) Unloading valve set too low or leaking. (5) Pressure reducing valve set too low or bying. (6) Worn pump. (7) Variable displacement pump compensator yoke mechanism inoperative. (C) HIGH PRESSURE (1) Strained pressure guge. (2) Load resistance higher than expected. (3) Load resistance higher than expected. (4) Undersized actuators. (5) Pressure relief valve set too high. (6) Unloading valve set too high. (7) Internal leak in the system. (8) High low valve inoperative. (9) Pressure compensator setting on variable displacement pump set too high. (10) Variable displacement pump compensator yoke mechanism inoperative. (11) Restriction in the between the pump and pressure relief valve.

•

Flow Related Problem Statements (A) NO FLOW (FROM THE PUMP) (1) Low fluid level. (2) Partially clogged inlet filter. (3) Clogged inlet vent. (4) Inlet line leaking. (5) Low flow control valve setting. (6) Relief valve not fully closing. (7) Internal leak in the system. (8) Low setting on variable displacement pump. (9) Variable displacement pump yoke not shifting. (10) Pilot pressure to variable displacement pump missing or low. (11) Low pump speed. (12) Worn pump. (13) Control valve not fully shifting. (14) Restriction in the line between pump and actuator. (C) EXCESSIVE FLOW (1) Overized pump. (2) Undersized actuators. (3) High setting pm flow control valve. (4) Overstroking of the yoke mechanism on variable displacement pump. (5) High pump speed.

•

Leakage Problem Statements (A) LINES AND FITTINGS (1) Broken line.

(2) Cracked port or fittings Page | 123


(3) Fatigue from hydraulic shock or vibration. (4) SAE/ISO straight O-ring fditting parts improperly assembled or damaged. (5) Fitting too loose or over tightened. (6) Improperly paired fittings (7) Tapered fittings don’t have sealing tape or compound. (8) O = ring seal punched, rolled, cut or nibbled.

1) 2) 3) 4) 5)

(B) STATIC SEALS O– ring seal pinched, rolled, cut or nibbled. Sealing surface damaged. Gland misaligned or orvertightened. Seal has hardened and set, losing resilience. Misaligned flange does not contain the seal.

(C) DYNAMIC SEALS 1)Seal fails from normal wear. 2) V-ring (gland) need adjusted. 3) Pressure activated seals hardened and set. 4) Incorrect seal for application 5) Garter ring broken on lip seal. 6) Seal undersized and extrude. 7) Seal in backwards (not facing pressure). 8) Seal improperly installed without thimble or driving tool. 9) Shaft wobbing from wear, runout, or bend. 10) Shaft worn, notched, or has rough finish. 11) External drain plugged on pump / motor, blows seal. • Excessive Heat Problem Statements (A) FLUID 1) Pressure relief or unloading valve pressure set too high. 2) Fixed displacement pump too large for the application. 3) Circuit dumps excessive fluid over the pressure relief valve. 4) Pump or motor overloaded. 5) Excessive pump, motor, or cylinder slippage. 6) Low fluid level. 7) Air in the fluid. 8) Fluid viscosity to high. 9) Transmission lines undersized. 10) Heat exchanger undersized or restricted. (B) COMPONENTS 1) High viscosity fluid. 2) Excessive pressure. 3) Excessive slippage caused by wear. 4) Cavitation (fluid starvation). 5) Excessive speed. 7) Mechanical interference (metal to metal ). 8) Misalignment. 9) Warn or damaged bearing. •

Noise and Vibration Problem Statements

Page | 124


(A)HYDRAULIC NOISES 1) Pump starved for fluid. 2) Cavitation. 3) Air leak in the suction side of the pump. 4) Pressure relief valve or component sticking. 5) Fluid noise across broken or notched valve seat. 6) Air in the fluid. 7) Fluid noise acriss restriction. 8) Fluid to viscous (or cold). (B) SYSTEM NOISES 1) Pump or motor is failing. 2) Hydraulic transmission lines rattle. 3) Pump / motor coupling slaps from or being loose. 4) Electric motor chatters or whines. 5) Fan housing chatters. 6) Power supply mounting transmits noise. 7) Broken valve spring allows chatter. (C) SYSTEM VIBRATION 1) Hydraulic transmission lines vibrate or pound. 2) Electric motor coupling vibrates. 3) Electric motor vibrates. 4) Cooling fan out for balance. 5) Pump / motor coupling out of alignment. 6) Electric motor / pump shaft bent. •

Problem Statements for Components In addition to problem statements that describe problems associated with pressure, flow, heat, leakage, noise, and vibration, there are a number of problems that can be associated with specific components. These failures are peculiar to the describe the problem cause of the problem. (I) Pumps (A) PUMP WON’T TURN 1) Bearing seized. 2) Drive motor sized. 3) Internal motor seized. 4) Internal parts seized or broken. 5) Varnish build up between parts.

(B) NO PRESSURE (ALSO SEE PRESSURE SECTION) 1) Improper assembly. 2) Pump shaft sheared inside case. 3) Broken internal parts. 4) Vanes stuck in slots. 5) Fluid supply obstructed. 6) Pump turning dry. 7) Yoke mechanism in wrong position. 8) Pump turning in wrong direction. (C) LOW PRESSURE (ALSO SEE PRESSURE SECTION) 1) No load resistance. 2) Improper assembly. 3) Internal parts damaged from running dry. 4) Excessive internal wear. (D) NO FLOW (ALSO SEE FLOE SECTION) Page | 125


1) Pump shaft sheared inside case. 3) Pump loses prime. 5) Pump turning dry. 7) High inlet head.

2) Complete pump failure. 4) Pump turning in wrong direction. 6) Improper assembly. 8) Broken internal parts.

(E) LOW FLOW (ALSO SEE FLOW SECTION) 1) Exerssive internal wear. 2) Pressure port connected to inlet or drain. 3) Inlet or outlet restrictions. 4) Variable displacement pump setting low. 5) Faulty pilot operator or strokling mechanism. (F) LEAXK (ALSO SEE LEAK SECTION) 1) Loose case bolts. 2) In Static seals, set, dissolved (incompatible with fluid) 3) Normal seal wear. 4) Bearing failure causes seal wear. 5) Seal installed backward or incorrectly (does’t fact pressure). 6) Shaft damaged or subject to abrasion. 7) Case drain obstructed. 8) Cracked case. 9) Misaligned flange. (G) BEARING FALS 1) Normal wear. 3) Coupling misalignment. 5) Overhung load. (II) Motors

2) Installed incorrectly. 4) Applied end force through couping.

(A) MOTOR WON’T TURN 1) Load resistance seized. 2) Internal parts brocken. 3) Return line restricted (quick disconnected uncoupled). (B) LOW TORQUE FROM THE MOTOR 1) Pressure setting low. 2) Load resistance too high for motor size. 3) Misalignment causes binging. 4) Undersized plumbing causing pressure drops. 5) Exercessive wear allows internal leakage.

(C) LOW SPEED FROM THE MOTOR 1) Flow control setting too low. 2) Fixed displacement pump too smail for application. 3) Motor displacement too large for application. 4) Variable displacement mechanism setting too low or blocked. 5) Undersized plumbing reduces flow from pump. 6) Excessive wear allows internal leakage. 7) Pump speed too slow. (III) Valves (A) PRESSURE RELIEF / PRESSURE UNLOADING / PRESSURE REDUCING 1) Spool or poppet sticks open or closed on debris. Page | 126


2) Pressure gauge giving inaccurate information. 3) Pilot or return spring brocken or bent. 4) Valve set incorrectly. 5) Valve assembled incorrectly. 6) Valve body or parts damaged. (B) CHECK VALVES 1) Valve check reversed. 2) Valve check stuck open. 3) Incorrect valve size or spring for application. 4) Hydraulic shock has damaged internal parts. 5) Valve leaks from normal wear. 6) Worn plug, ball, spool, or poppet prevents leak tight seal due to misalignment. (C) DIRECTIONAL CONTROL VALVES (FALLES TO SHIFT, SHIFT SLOW) 1) Burr or debris blocking valve spool. 2) Valve spool is silted. 3) Valve solenoid malfunctions or is burned out. 4) Oil viscosity is too high. 5) Ports connected incorrectly. 6) Valve assembled incorrectly. 7) Valve spool reversed. 8) Pilot pressure is low. 9) Pilot drain is blocked. 10) Faulty electric solenoid control. 11) Solenoid overheats (high spool force or internal short). (IV) Cylinders (A) WN’T EXTEND OR RETURN LOAD RESISTANCE 1) Load resistance too great. 2) Load resistance binding. 3) Cylinder undersized. 4) Cylinder rod overextended (cocked). 5) Cylinder barrel bent, binding piston. 6) Blown piston seal. (B) MOVEMENT TOO SLOW 1) Dirty filter / reservoir vent. 3) Undersize pump. 5) Restriction in the line.

2) Oversize cylinder bore. 4) Incorrect flow control setting. 6) Directional control valve not shifting completely. 7) Fluid viscosity too low (from selection or overheating). 8) Inappropriate circuit. (C) MOVEMENT TOO FAST 1) Undersize cylinder bore. 2) Oversize pump. 3) Incorrect flow control settings. 4) Overrunning load resistance. 5) Inappropriate circuit. (D) ERRATIC MOVEMENT 1) Dirty filter / reservoir vent. 2) Air in the oil causing sponginess. 3) Load resistance binding / releasing. 4) Directional control valve chatters. 5) Low pressure relief valve setting. 6) Rod seal ingests air on return stroke (load returned system) (E) DRIFT 1) Internal leak past piston.

2) External leak at rod seal, fittings, or lines. Page | 127


3) Leak past directional control valve spool. 4) Directional control valve not centering. (5) Relief valve leaking across seat. •

Tee Test for Hydraulic Circuits Test connection (see Figures 12.22) can be made at A, B, C, or D. when the connection is made at C or D the tester is in series with the control valve and cylinder to check individual components. First determine low much fluid should be circulating through the circuit. Then by controlling the pressure with the load valve on the tester, the amount of fluid available through

Page | 128


Chapter – 14 Air Compressors •

INTRODUCTION Most of the pneumatic systems use compressed air as their operating medium. Hence, we require an air pump to generate this compressed air. Air compressors act as these air pumps. An air compressor compresses air from a low inlet pressure (usually atmospheric) to a higher desired pressure level. This is accomplished by reducing the volume of air. The symbol for an air compressor is as sh shown below: • CLASSFICATION OF AIR COMPRESSORS Compressors are classified as shown in the family tree of air compressors. (Ref Fig 14.1)

The principle of working of both the Positive Displacement type and Dynami Dynamic type of air compressors are discussed below: 1. Positive Displacement Air Compressor: In this type, a fixed volume of air is delivered during each rotation of the compressor shaft. The pressure of this fixed volume of air is increased by reducing its volume in an enclosed chamber. 2. Dynamic (Turbo) Air Compressor: In this type, rotating vanes or impellers are used to impart velocity / force to the flow of air being handled. The pressure comes from this dynamic effect of the imparted velocity / force. •

Lobe Compressor pressor / Roots Blower It consists of two intermeshing lobes driven by external gears and placed in a casing. The air is forced from the suction side to the delivery side with continuous rotation of the two lobes. Since no internal compression takes plac place, e, there is practically no volume change. Operating pressure is mainly limited by leakage between lobes and the housing. To operate efficiently, clearances must be very small. These small clearances are fixed by timing gears, thus eliminating the need for internal lubrication. Were leads to a rapid fall in efficiency.


It is basically used when a positive displacement compressor is needed with high delivery volume (of about 1500 m3 / min) but low pressure (1 – 2 bar)

•

Screw Compressor A screw compressor consists of two intermeshing rotating screws a screw compressor operates on a similar principle as that of a hydraulic screw pump male screw and female screw. These male and female screws are synchronized by external timing ing gears. As th these screws rotate, air is drawn into the housing and gets trapped between the screws and is then carried along to the discharge port where it is delievered. The helix of the male and female screws compression is accomplished by rolling th the trapped air into propressively smaller volume as the screws rrotate tate are designed to permit complete charging of the inter lobe space before they remesh. On completion of the filling operation, the inlet end of the male and female male screws begin to re-engage re engage each other and the volume of this space is reduced and compression beings; air is discharged at the end of the other side. This is virtually a continuous process are lobe following the other very closely so that almost pulsation ion free compressed air is obtained. The air movement during the compression stroke is as shown in Fig. 14.11. Since there is no between the screws and the housing, hence no lubrication is required. However, if the female screw is driven by the ma male le screw, then oil is required to be sprayed to the inlet air to reduce friction between the screws and is known as “wet screw compressors”. The oil in the compressed oil is then removed by using oil separation units. Pressure valves, actuators and other operating o devices. Initially, the operating pressure of the designed system should be determined. Depending upon the operating pressure required, the air compressor type suitable for the system is to be selected. Given below is a table indicating the compressor compr type and its pressure ratio


Compressor Type Piston Vane Liquid Ring Diaphragm Screw Lobe

Maximum Pressure Ratio 10 8 5.5 5 3 2

1. Most pneumatic systems operates at 6 bar pressure. So, a compressor which can assure a pressure of about 6 -7 bar in the distribution line is selected. 2. When the distribution lines are long, then we select a compressor having 7 – 8 bar capacity, so as to compensate for the line or leakage losses. 3. When two or more operations require air at higher pressure, it is economical to install separate small compressors to supply air for these operations. 4. When small amount of air is required at pressure lower than that carried in the main distribution lines, they are obtained by installing a reducing valve in the branch line leading to the area requiring the low pressure air. 5. When large amount of low pressure air (less than 2 bar) is required, it is economical to install a dynamic compressor. •

Capacity The total volume of compressed air required to be delivered by the compressor is known as “capacity ” of the compressor. య

The capacity of the compressor is expressed in of of compressed in delievered at NTP (Normal Temperature and Pressure) conditione. The NTP condition is p = 1.01 bar absolute T = 0o C or 273K. A compressor delivery volume can be specified in of: 1. Theoretical Volume: It is the product of swept volume and the rotational speed in rpm. 2. Effective Volume: When loses are included in the theoretical volume, we get effective volume.

Page | 131


Chapter – 15 Air Treatment •

INTRODUCTION Air in a pneumatic system must be clean and dry to reduce wear and tear of the various pneumatic devices and to extend their maintenance period. Clean, pure and dry compressed air is especially required in chemical and medicine industry. Hence, before consumption, this compressed air requires further treatment such as 1. Atmospheric air that is sucked in by the air compressor is contaminated with smoke, dust and water. More over, the air that is supplied to the system from the compressor is further contaminated by virtue of generation of contaminants downstream. These contaminants may have highly damaging effect on the finely finished mating surfaces of pneumatic devices. Hence, it requires to be filtered. 2. Operating pressure does not remain constant due to fluctuating consumption of compressed air. Now, the performance and accuracy of a pneumatic system depends on the pressure stability of air-supply. Hence, the pressure requires to be regulated. 3. The compressed air supplied to various pneumatic devices also require to lubricats the mating components of these devices. Hence, the compressed air required to be lubricated. The air which leaves the compressor should be dry since humid air can cause damage 4. in the following manner: (a) Due to rust formation on unprotected steel surfaces, (b) Condensed water may mix with oil to form a sticky white emulsion which values to jam and also blocks the pneumatic piping. Hence, the compressed air required to be dried. This treatment falls into three distinct stages. Stage 1: Inlet filtering removes particles which can damage the air compressor. Stage 2: The air from the compressor is cooled and dried to reduce its humidity level before it is stored in the receiver. This stage is called as ‘primary air treatment’. Stage 3: The compressed air before being used by the various pneumatic devices is filtered, dried, lubricated and regulated. This stage is called as ‘secondary air treatment’.

• RELATIVE HUMIDITY AND DEW POINT • Relative humidity Atmospheric air contains moisture in the form of water vapour. The content of water vapour in the air varied depending on the prevailing atmospheric conditions at a particular place or time. The amount of water vapour in the atmospheric air depends on the ‘relative humidity of air (RH). Relative humidity is defined as, RH = 100

…(15.1)

Relative humidity is always expressed in of percentage. Relative humidity is dependent on both the temperature and pressure of the air. Relative humidity rises quickly with increasing pressure. Page | 132


• Dew Point The temperature at which air becomes saturated is refereed to ‘d ‘dew ew point’. When dew point is reached, the moisture begins to condense from the air. So, if the dew point is high, higher will be the temperature necessary to condense and separates the water and vice-versa. •

Water Trap As water condensation is a major handicap dicap to pneumatic lines and systems, it is essential to place appropriate water traps at each end of the pipeline or at places where branching off pneumatic lines is to take place. One such water trap or water collector in a pneumatic line is shown in Fig Fig. 15.1.

• AIR FILTERS In a pneumatic system, the air filter performs the following functions. 1. Its primary function is to filters the dirt and smoke particles in the air before they can damage to the system devices. 2. It also performs the secondary function of condensing and removing the water vapour that is present in the air ing through it. • Dry filter and Wet Filter Dry Filter are similar to those found in motor car oil filters. Their filter cartridge is detachable and is required quired to be replaced when servicing. In wet filters, the in coming air is bubbled through an oil bath during which the dirt particles get attached to the oil droplets. This bubbled air is then made to through an wire mesh filter, where the oil droplets droplets with the dirt particles are consequently removed. During servicing, it is required to be cleaned with proper detergent. • Coarse Filters and Micro Filters: Filters are classified according to the size of particles they filter. Dust partices are generally y larger then 10 um, whereas smoke and oil particles are about 1 mm in size. A filter having nominal rating will black 98% of particles of specified size and a filter having absolute rating will black 100% of the particles of specified size. Coarse Filters are constructed out of wire mesh and are called strainers. They are often used as inlet filters. They are usually specified by their mesh size.


Table 15.1 Mesh Size 325 550 750

mm 30 10 6

Micro filters are used to filter particles as small aas 0.01 um. Micro Filter / Coalescent Filter Coalescent type of filters is used when it necessary to filter submicron size contaminates like oil. Hence, this type of filter gives us oil and moisture free air. Fig 15.2 shows a schematic sketch of this type of filter element. The filter medium is made up metal-wool’s wool’s which are kept compressed inside a stainless steel shell and the outer shell is made up of some porous material like ceramic, porcelain, etc., which has the capacity to absorb fince oil molecules. cules. The filtration medium is housed on a seat made of stainless steel. In this type of filters, when air is ed from center to the outside, it removes 99.9% of the contaminants down to 0.01 mm. • PRESSURE REGULATOR Figure 15.3 .3 shows the internal construction of a pressure regulator in a pneumatic system. The main function of this valve is to regulate the incoming pressure to the system so that the desired air pressure is capable of flowing at a steady condition. The valve has a metallic body (2) with the two openings primary and secondary opening. The pressure regulation is achieved by opening the poppet valve (5) to a measured amount commensurate with the desired pressure level to be achieved. This is done by an adjustable screw. rew. (1) The adjusting screw will move the diaphragm (4) upward and thus will make the poppet to unseat, thereby creating an opening to allow air to flow from the pressure of air flowing through it, will be directly proportional to the compression, more wi will be


the amount of opening and hence greater the pressure and vice versa. So in actual practice, the pressure regulator is but a pressure reducing valve and has immense application in pneumatic circuits to ensure desired pressure level at various parts of the system. In many cases, the valve has two vent hole openings through which the compressed air it let out into the atmosphere in case the secondary pressure increases to a level not desirable to the system. In most cases, the pressure once set by the screw should not be tampered with and lock-nut is tightened to ensure uninterrupted flow of air at desired pressure within the safe limit. The spring (6) at the other side of the poppet helps to act as a dampening device needed to stabilise the pressure. inlet pressure rating and down stream controlled range, as well as flow capacity must be determined before selecting a regulator. Port size should match piping size. Required response time, relieving capability, and type of adjustment are other considerations. •

LUBRICATOR In most of the pneumatic circuits, the compressed air is first filtered and then regulated to the operating pressure. Then a carefully controlled amount of oil is often added to this filtered and regulated compressed air immediately prior to use. It helps in lubricating the moving parts of pneumatic devices such as valves, cylinders, etc., This oil is introduced as a fine mist. To from the mist a lubricator unit is used. •

Principle of Lubricator All lubricators follow the principle of petrol – air mixing in a motor car carburettor. When the compressed air is made to flow through a venture / throttle, its velocity increases causing a drop in its pressure. simultaneously the inlet compressed air is also made to flow into the oil reservoir. The pressure differential between the oil reservoir and throttle zone causes oil to be drawn into the throttle zone.

Page | 135


•

Lubricator Construction and Working The constrictions of a typical lubricator is as shown iinn fig. 15.5. The compressed air from the regulator is allowed to enter the lubricator unit. The air then es through a narrow constriction and enters the glass bowl containing oil and also inside a small siphon tube. Slowly a pressure differential cause causes the oil to be drawn up a riser tube into the upper chamber. A check valve is installed in this riser tube to prevent the back flow of oil into the oil reservoir in the bowl. The oil drops are made to fall at the main venturi where the air will have high velocity. This high velocity air breaks the oil drops into tiny particles to from a mist of air and oil.

This air – oil mixture then flows to the actuator inlet. •

FRL UNIT / SERVICE UINT In pneumatic systems, an air filter with moisture separator, a pressure regulator, a pressure indicator and a lubricator are all frequently required. This need is so common that a combined device, encoming all these units is readily available. It is called as ‘Service unit’ or ‘Filter – Regulator – Lubricator FRL Unit’. The three main components of an FRL unit are: 1. Air Filter and Separator 2. Pressure Regulator with indicator 3. Lubricator Individual components comprising a service unit are shown in fig 15.6 (a) and the composite symbol of an FRL unit is shown in fig 15.6 (b).


FRL / Services unit

Fig. 15.6(b) : •

AIR DRYERS The air which contains moisture in atmospheric condition contains moisture even after fter compression. When this humid air flows into the system, there will be gradual loss heat to the cooler environment. This gradual cooling of compressed air, results in condensation of water vapour at every point in varying amount depending upon the localised loca effects. This water can cause a lot of damage such as 1. This condensed moisture can lead to corrosion and rusting of various pneumatic components. 2. This condensed moisture may mix with oil to form a sticky white emulsion which blocks the pneumatic pipe-lines. lines. This chocking may lead to the breakdown of the pneumatic system. 3. This condensed moisture washes away the lubricating film, thereby resulting in the untimely breakdown of these pneumatic devices. Hence, before this humid compressed air in fed to any control system, it requires to be dried in order to bring humidity and few point to reasonable level. Drying process is carried out by the following methods


Air Drying

Mechnical Method

Chemical Method using

Refrigerated Dryer Absorption type Deliquescent dryer

Absorption type Regenerative

Fig. 15.7: Classification of Air drying methods •

Refrigerated Dryers This incorporates a refrigeration unit (like that of a domestic fridge), through which the air es as shown in the fig. 15.8. It also contains controls to ensure that water which has been drained out of the air does not freeze inside the unit. The cooled air, as can be see, e, is re re-heated by the incoming air, cooling the latter and increasing the volume of the former. Refrigerated dryers give air with a due point sufficiently low for most processes. Hence, it is ideally suited for average industrial facility.

•

Deliquescent dryer It is an absorption type of chemical dryer. The chemical descend used for drying is of deliquescent type. It gets its meaning from. De-away, away, liquescere –melt: melt: So ‘deliquesce’ means to become liquid by absorption of moisture from the air. The chemicall is placed in a bed in a pressure vessel. The bed is made of non-corrosive corrosive material, perforated to allow the free flow of air. The compressed air enters below the . The inlet is baffled to mechanically liquid water and gross soild. The air then flows vertically up through the dessicant bed. After some time, the Dowest payer of dessicant bed begins to dissolve as it absorbs water vapour. The mist of liquid desiccant that is fared removes small particles of soild, oil and other contami contaminants. nants. The remainder of the desiccant bed also to cleaning and drying the air but its effect is relatively miner. This liquid dessicant at the bottom of the unit from where it can be drained.


The dessicant material is used up during this process needs to bbee replaced at regular intervals. Also the maximum temperature of the system should not exceed 38-40 38 oC as a temperature above this results in excessive dessicant consumption. •

Absorption dryer / Regenerative dryer It is an adsorption type of chemical dryer. The chemical desiccant used does not physically change on absorbing the water vapour like deliquescents do. The dessicant use are silicon dioxide, copper sulphate, activated alumina or silica gel, etc., The moisture in the adsorption material can be released released by heating. Thus the desiccant can be regenerated and can be used again and again. It consists of two columns. At any time, one column is drying air while the other is being regenerated by heating and the age of a low pure air stream. •

ILENCER MUFFLER / SILENCER When the used compressed air in the pneumatic system is exhausted / vented to the atmosphere, it generates high intensity sound having the same frequency as normal conversation. The increased use of compressed air in industry has created this noise problem. 1. Excessive exposure to these noises can cause loss of hearing without noticeable pain or discomfort. 2. Noise exposure also causes fatigue and lowers production. 3. It also blocks out warning signals, thus causing accidents. Hence, it is necessary to muffle uffle out [deader] these noise. The pneumatic device used to control the noise caused by a rapidly exhausting airstream into the atmosphere is called as muffler or silencer. Mufflers are attached to the exhaust ports of air valves, pneumatic cylinders and air motors. Mufflers are designed to reduce noise levels without creating back pressure sufficient to reduce the operating efficiency of the system. The air path through a typical muffler is as shown in Fig. 15.9 and indicates the cancelling effects of noi noise se causing vibration. Appropriate sizing for each application is necessary to assure maximum performance. •

AFTERCOOLER The compressed air delivered by the compressor is at a very high temperature and contains a fair bulk of moisture. 1. This high temperature of the compressed air is required to be reduced as high temperature reduces can damage the metal components as well as increase input power requirements. 2. On cooling, the moisture will condense and is eventually carried along to the various pneumatic devices. It chockes the pneumatic pipe lines, washes away the lubrication causing excessive wear and decreased efficiency. To tackle these problems an after cooler is placed in the air line between the compressor and the air-receiver. receiver. It reduces the air temperature temperature to about 100o F and acts as the first stage in the removal of moisture (about 80%) from the compressed air.


An after cooler basically consists of a beat exchanger with water as the cooling medium. Water flow is opposi opposite te to compressed air flow. The internal baffles provide proper water velocity and turbulence there by resulting in high heat transfer rates. After ing through the tubes, the cooled compressed air enters the moisture separating chamber which effectively ly traps out the condensed moisture. •

COMPONENTS OF A TYPICAL COMPRESSED AIR SYSTEM Fig 15.11 shows a compressed air system with the commonly used components. These components are: 1. Air-Inlet Filter: Free air from the atmosphere enters the compressor through an airinlet filter which will essentially keep out the dust and air from entering the system. 2. Prime mover: It can be either an electric motor or an internal combustion engine which is used to provide the drive to the compressor. 3. Compressor: It is used to compress the air from atmospheric pressure to the desired higher pressure level. It can be single stage or multi multi-stage in operation. 4. Intercooler: If the compressor is multistage in operation, then an intercooler is used to decrease the heat of compression between stages. 5. After cooler: The high pressure air discharged from the second stage is then made to through an after cooler, where both the temperature and moisture content in the compressed air are drastically reduces.


6. Air-Drier: It completely dries the compressed air before directing it into the air airreceiver. 7. Air Receiver: It is an air reservoir and its function is similar to that of a flywheel. 8. FRL unit: The compressed air before being used by the various pneumatic devices is filtered, lubricated and regulated by the FRL unit. 9. Pneumatic Valves: Pneumatic valves are required to control air direction, pressure and flow rate. They are responsible for the smooth and precise control of the ppneumatic actuator, and also for the safe operation of the system. 10. Pneumatic Actuator: Pneumatic actuators will convert the high pressure energy of the compressed air into mechanical force or torque to do useful work. Actuators can either be pneumatic cylinders ders to provide linear motion or pneumatic motors to provide rotary motion. 11. Pipes and hoses: They carry the compressed air from one location to another.

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