Lubricant And Lubrication Fundamentals 5aj38
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Mustafa Ali
LUBRICANTS AND LUBRICATION FUNDAMENTALS
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TABLE OF CONTENTS Types of Lubricants -----------------------------------------------------------------------------3 Solid Lubricants----------------------------------------------------------------------------------3 Liquid Lubricants--------------------------------------------------------------------------------4 Base Stock-------------------------------------------------------------------------------4 Additives---------------------------------------------------------------------------------6 Functions of Lubricant-----------------------------------------------------------------15 Fluid Film Lubrication-----------------------------------------------------------------16 Oil Characteristics----------------------------------------------------------------------17 Grease---------------------------------------------------------------------------------------------23 Filtration------------------------------------------------------------------------------------------26 Methods of lube Application ------------------------------------------------------------------27 Oil Testing----------------------------------------------------------------------------------------32 Useful Tables-------------------------------------------------------------------------------------35 Bibliography--------------------------------------------------------------------------------------51
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1. TYPES
Solid
Liquid
Gas
2. SOLID LUBRICANTS
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3. Liquid Lubricants 3.1 Composition of liquid Lubricants Base stock + Additives = Lubricant
3.1.1 Base Stock Crude oil is distilled to yield different fractions one of which is the base stock for finished lubricants. The base stock is manufactured by two processes namely separation and conversion.
Separation involves division of the crude oil into its different fractions while conversion takes into the different processes required to refine the base stock. Usually separation involves Pre-eating, Fractional distillation, Vacuum distillation, Propane de-asphalting, Furfural extraction, solvent de-waxing (through MEK) and clay/hydro-finishing. Conversion involves Vacuum distillation, Hydro-cracking (Group II and III)/ Hydrogen De-waxing (Group I and II) and Hydro-finishing.
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3.1.1.1 Classification of Base Stocks A Group I Solvent Refined II Hydro-finished III Hydro-cracked IV Poly Alpha Olephin V All Others
Saturates (%) <90 >90 >90
Sulfur (%) >0.03 <0.03 <0.03
VI 80-120 80-120 >120
B. According to Viscosity Index Type LVI MVI HVI VHVI UHVI
VI <35 35-80 80-110 110-130 130-150
(G-III) (G-III)
C. According to Composition Paraffinic Higher VI Higher FP Better Oxidation stability Better thermal stability
Naphthenic Better solvent Lower pour point No wax
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3.1.1.2 Mineral vs. Synthetic Base Stock Mineral BS contains numerous hydrocarbons and it works to the average of these hydrocarbons’ working ability. Synthetic BS refined to the extent that it contains one or two hydrocarbons. Examples include
Organic Esters Phosphate Esters Poly-glycols Synthesized Hydrocarbon Fluids (SHFs) Other synthetic Lubricating fluids
3.1.2 Additives
Surface Protecting
Performance Enhancing
Lubricant Protective
Surface Protecting
Performance Enhancing
Lubricant Protecting
Anti Wear (adsorb)
VI Improver
Oxidation Inhibitor
Anti Rust/ Corrosion (adsorb)
Pour point Depressant
Foam Inhibitor
Extreme Pressure (adsorb)
Demulsifier/ Emulsifier
Detergent/ Dispersant Tackiness Agent Lubricity Additive (adsorb)
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A. Pour Point Depressants Certain high molecular weight polymers function by inhibiting the formation of a wax crystal structure that would prevent oil flow at low temperatures. Two general types of pour point depressant are used: 1. Alkylaromatic polymers adsorb on the wax crystals as they form, preventing them from growing and adhering to each other. 2. Polymethacrylates co-crystallize with wax to prevent crystal growth. The additives do not entirely prevent wax crystal growth, but rather lower the temperature at which a rigid structure is formed. Depending on the type of oil, pour point depression of up to 50_F (28_C) can be achieved by these additives, although a lowering of the pour point by about 20–30 F_ (11–17 C_) is more common.
B. Viscosity Index Improvers VI improvers are long chain, high molecular weight polymers that function by causing the relative viscosity of an oil to increase more at high temperatures than at low temperatures. Generally this result is due to a change in the polymer’s physical configuration with increasing temperature of the mixture. It is postulated that in cold oil the molecules of the polymer adopt a coiled form so that their effect on viscosity is minimized. In hot oil, the molecules tend to straighten out, and the interaction between these long molecules and the oil produces a proportionally greater thickening effect. Note: Although the oil– polymer mixture still decreases in viscosity as the temperature increases, the decrease is not as great as it would have been in the oil alone. Under shear conditions the long molecules of the VI improver align themselves in the direction of the stress, thus reducing resistance to flow. When the stress is removed, the molecules return to their usual random arrangement and the temporary viscosity loss is recovered. This effect can be beneficial in that it can temporarily reduce oil friction to permit easier starting, as in the cranking of a cold engine. Permanent shear breakdown occurs when the shear stresses actually rupture the long molecules, converting them into lower molecular weight materials, which are less effective VI improvers. This results in a permanent viscosity loss, which can be significant. It is generally the limiting factor controlling the maximum amount of VI improver that can be used in a particular oil blend. VI improvers are used in engine oils, automatic transmission fluids, multipurpose tractor fluids, and hydraulic fluids. They are also used in automotive gear lubricants. Their use permits the formulation of products that provide satisfactory lubrication over a much wider temperature range than is possible with straight mineral oils alone.
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C. Defoamants The ability of oils to resist foaming varies considerably depending on type of crude oil, type and degree of refining, and viscosity. In many applications, there may be considerable tendency to agitate the oil and cause foaming, while in other cases even small amounts of foam can be extremely troublesome. In these cases, a de-foamant may be added to the oil. It is thought that the de-foamant droplets attach themselves to the air bubbles and can either spread or form unstable bridges between bubbles, which then coalesce into larger bubbles, which in turn rise more readily to the surface of the foam layer where they collapse, thus releasing the air. Silicone polymers used at a few parts per million are the most widely used defoamants. These materials are essentially insoluble in oil, and the correct choice of polymer size and blending procedures is critical if settling during long-term storage is to be avoided. Also, these additives may increase air entrainment in the oil. Organic polymers are sometimes used to overcome these difficulties with the silicones, although much higher concentrations are generally required.
D. Oxidation Inhibitors When oil is heated in the presence of air, oxidation occurs. As a result of this oxidation, both the oil viscosity and the concentration of organic acids in the oil increase, and varnish and lacquer deposits may form on hot metal surfaces exposed to the oil. In extreme cases, these deposits may be further oxidized to form hard, carbonaceous materials. The rate at which oxidation proceeds is affected by several factors. As the temperature increases, the rate of oxidation increases exponentially. A rule of thumb is that for each 10_C (18_F) rise in temperature, the oxidation rate of mineral oil will double. Greater exposure to air (and the oxygen it contains), or more intimate mixing with it, will also increase the rate of oxidation. Many materials, such as metals, particularly copper and iron and organic and mineral acids, may act as catalysts or oxidation promoters. Although the complete mechanism of oil oxidation is not too well defined, it is generally recognized as proceeding by free radical chain reaction. Reaction chain initiators are formed first from unstable oil molecules, and these react with oxygen to form peroxy radicals, which in turn attack the unoxidized oil to form new initiators and hydroperoxides. The hydroperoxides are unstable and divide, forming new 8
Mustafa Ali initiators to expand the reaction. Any materials that will interrupt this chain reaction will inhibit oxidation. Two general types of oxidation inhibitor are used: those that react with the initiators, peroxy radicals, and hydroperoxides to form inactive compounds, and those that decompose these materials to form less reactive compounds. At temperatures below 200_F (93_C), oxidation proceeds slowly and inhibitors of the first type are effective. Examples of this type are hindered (alkylated) phenols such as 2,6-ditertiary-butyl-4methylphenol (also called 2,6 ditertiary-butylparacresol, DBPC), and aromatic amines such as N-phenyl_-naphthylamine. These are used in products such as turbine, circulation, and hydraulic oils, which are intended for extended service at moderate temperatures. When the operating temperature exceeds about 200_F (93_C), the catalytic effects of metals become important factors in promoting oil oxidation. Under these conditions, inhibitors that reduce the catalytic effect of the metals must be used. These materials usually react with the surfaces of the metals to form protective coatings and for that reason are sometimes called metal deactivators. Typical of additives of this type are the dithiophosphates, primarily zinc dithiophosphate. Since the dithiophosphates also act to decompose hydroperoxides at temperatures above 200_F (93_C), they inhibit oxidation by this mechanism as well. Oxidation inhibitors may not entirely prevent oil oxidation when conditions of exposure are severe, and some types of oil are inhibited to a much greater degree than others. Oxidation inhibitors are not, therefore, cure-alls, and the formulation of a satisfactorily stable oil requires proper refining of a suitable base stock combined with careful selection of the type and concentration of oxidation inhibitor. It should also be pointed out that other additives can reduce oxidation stability in performing their design functions. Proper formulation requires the balancing of all the additive reactions to achieve the desired total performance characteristics.
E. Rust and Corrosion Inhibitors A number of kinds of corrosion can occur in systems served by lubricating oils. Probably the two most important types are corrosion by organic acids that develop in the oil itself and corrosion by contaminants that are picked up and carried by the oil. Corrosion by organic acids can occur, for example, in the bearing inserts used in internal combustion engines. Some of the metals used in these inserts, such as the lead in copper-lead or lead-bronze, are readily attacked by organic acids in oil. The corrosion inhibitors form a protective film on the bearing surfaces that prevents the corrosive materials from reaching or attacking the metal. The film may be either adsorbed on the metal or chemically bonded to it. During combustion in gasoline or diesel engines, certain materials in the fuel, such as sulfur and antiknock scavengers, can burn to form strong acids. These acids can then condense on the cylinder walls and be carried to other parts of the engine by the lubricant. Corrosive wear of rings and cylinder walls, and corrosion of crankshafts, rocker arms, and other engine components can then occur. It has been found that the inclusion of highly alkaline materials in the oil will help to neutralize these strong acids as they are formed, greatly reducing this corrosion and corrosive wear. These alkaline materials are also used to provide detergency. 9
Mustafa Ali Rust inhibitors are usually compounds having a high polar attraction toward metal surfaces. By physical or chemical interaction at the metal surface, they form a tenacious, continuous film that prevents water from reaching the metal surface. Typical materials used for this purpose are amine succinates and alkaline earth sulfonates. The effectiveness of a properly selected rust inhibitor is illustrated in Figure 3.3, where specimen 9 is rust free and the other specimens display varying degree of corrosion. Rust inhibitors can be used in most types of lubricating oil, but the selection must be made carefully to avoid problems such as corrosion of nonferrous metals or the formation of troublesome emulsions with water. Because rust inhibitors are adsorbed on metal surfaces, an oil can be depleted of rust inhibitor in time.
F. Detergents and Dispersants In internal combustion engine service, a variety of effects tends to cause oil deterioration and the formation of harmful deposits. These deposits can interfere with oil circulation, build up behind piston rings to cause ring sticking and rapid ring wear, and affect clearances and proper functioning of critical components, such as hydraulic valve lifters. Once formed, such deposits are generally hard to remove except by mechanical cleaning. Detergents and dispersants in the oil can delay the formation of deposits and reduce the rate at which they accumulate on metal surfaces. An essential factor with this approach is regular draining and replacement of the oil so that the contaminants in it are removed from the engine before the oil’s capacity to hold them is exceeded. Detergents are generally considered to be chemical compounds that chemically neutralize deposit precursors that form under high temperature conditions or as the result of burning fuels with high sulfur content or other materials that form acidic combustion byproducts. Dispersants, on the other hand, are chemical compounds that disperse or suspend in the oil potential sludge- or varnish-forming materials, particularly those formed during low temperature operation when condensation and partially burned fuel find their way into the oil. These contaminants are removed from the system when the oil is drained. There is no sharp line of demarcation between detergents and dispersants. Detergents have some ability to disperse and suspend contaminants, while dispersants have some ability to prevent the formation of high temperature deposits. The principal detergents used today are organic soaps and salts of alkaline earth metals such as barium, calcium, and magnesium. These materials are often referred to as metallo-organic compounds. Calcium and magnesium sulfonates, and calcium phenates (or phenol sulfides) are widely used. The sulfonates and phenates may be neutral or overbased; that is, they may contain more of the alkaline metal than is required to neutralize the acidic components used in diesel engine oils to neutralize the strong acids formed from combustion of the sulfur in the fuel. This neutralization reduces corrosion and corrosive wear and minimizes the tendency of these acids to cause oil degradation. Overbased materials are generally used at lower concentration in gasoline engine oils, where the fuel sulfur is much lower. The overbased materials are included in the formulation to help reduce corrosion in low temperature operation. Both neutral and overbased materials also act to disperse and suspend potential varnish forming materials resulting from oil oxidation, preventing these materials from depositing on engine surfaces. Metallo-organic detergents, on combustion, leave an ashy residue (see Section II.I, Sulfated Ash). In some cases, this may be detrimental in that the ash can contribute to combustion chamber deposits. In 10
Mustafa Ali other cases, it may be beneficial in that the ash provides wear-resistant coatings for surfaces such as valve faces and seats. Typical dispersants (also called polymeric dispersants and ashless dispersants) in use today are described as polymeric succinimides, olefin/P2S5 reaction products, polyesters, and benzylamides. These are based on long chain hydrocarbons that are acidified and then neutralized with a compound containing basic nitrogen. The hydrocarbon portion provides oil solubility, while the nitrogen portion provides an active site that attracts and holds potential deposit-forming materials to keep them suspended in the oil. While the primary use of detergents and dispersants is in engine oils, they are also being used in products such as automatic transmission fluids, hydraulic oils, and circulation oils for high temperature service. In these applications, the detergents and dispersants help to prevent the deposition of lacquer and varnish resulting from oil oxidation, thus supplementing the effects of the oxidation inhibitors.
G. Antiwear Additives Antiwear additives are used in many lubricating oils to reduce friction, wear, and scuffing and scoring under boundary lubrication conditions, that is, when full lubricating films cannot be maintained. As the oil film becomes progressively thinner as a result of increasing loads or temperatures, through the oil film is first made by minute surface irregularities or asperities. As these opposing asperities make , friction increases and welding can occur. As sliding continues, the welds break immediately, but the process can form new roughness through metal transfer, as well as wear particles, which can cause scuffing and scoring. Two general classes of materials are used to prevent metallic , depending on the severity of the requirements. Mild antiwear and friction-reducing additives, sometimes called boundary lubrication additives, are polar materials such as fatty oils, acids, and esters. They are long chain materials that form an adsorbed film on the metal surfaces with the polar ends of the molecules attached to the metal and the molecules projecting more or less normal to the surface. is then between the projecting ends of the layers of molecules on the opposing surfaces. Friction is reduced, and the surfaces move more freely relative to each other. Wear is reduced under mild sliding conditions, but under severe sliding conditions the layers of molecules can be rubbed off, with the result that their wearreducing effect is lost.
H. Extreme Pressure Additives At high temperatures or under heavy loads where more severe sliding conditions exist, compounds called extreme pressure (EP) additives are required to reduce friction, control wear, and prevent severe surface damage. These materials function by chemically reacting with the sliding metal surfaces to form relatively oil insoluble surface films. The kinetics of the reaction are a function of the surface temperatures generated by the localized high temperatures that result from rubbing between opposing surface asperities, and breaking of junctions between these asperities. Even with extreme pressure additives in the lubricant, wear of new surfaces may be high initially. In addition to the normal break-in wear, nascent metal (freshly formed, chemically reactive surfaces), time, and temperature are required to form the protective surface films. After the films have formed, relative motion is between the layers of surface films rather than the metals. The sliding process can lead to some film removal, but since replacement by further chemical reaction is rapid, the loss of metal is extremely low. This process gradually depletes the amount of EP additive available in the oil, although the rate of depletion is usually very slow. Thus, there will be sufficient additive left to provide adequate protection for the metal surfaces except possibly under severe operating conditions, where makeup rates are low and normal drain intervals are exceeded. The severity of the sliding conditions dictates the reactivity of the EP additives required for maximum effectiveness. The optimum reactivity occurs when the additives minimize the adhesive or 11
Mustafa Ali metallic wear without leading to appreciable corrosive or chemical wear. Additives that are too reactive lead to the formation of excessively thick surface films, which have less resistance to attrition, so some metal is lost by the sliding action. Since a particular EP additive may have different reactivity with different metals, it is important to match additive metal reactivity to the additives not only with the severity of the sliding system but also with the specific metals involved. For example, some additives that are excellent for steel-on-steel systems may not be satisfactory for bronze-on-steel systems operating at similar sliding severity because they are too reactive with the bronze. Another important function of EP additives is that because the chemical reaction is greatest on the asperities where is made and localized temperatures are highest, they lead to polishing of the surfaces. The load is then distributed more uniformly over a greater area, which allows for a reduction in sliding severity, more effective lubrication, and a reduction in wear. Extreme pressure agents are usually compounds containing sulfur, chlorine, or phosphorus, either alone or in combination. The compounds used depend on the end use of the lubricant and the chemical activity required in it. Sulfur compounds, sometimes with chlorine or phosphorus compounds, are used in many metal-cutting fluids. Sulfur–phosphorus combinations are used in most industrial and automotive gear lubricants. These materials provide excellent protection against gear tooth scuffing and have the advantages of better oxidation stability, lower corrosivity, and often lower friction than other combinations that have been used in the past.
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Anti Ox
Anti
Anti
Anti
Rust
Corr
Wear
EP
VI Imp
Deter/
Anti
Friction
Disp
Foam
Mod
Petrol
Diesel
Steam Turbine
Compressor
Gears
PP Dep
Demul / Emul
(Spiral Bevel, Hypoid) Gears (Spur Bevel)
Gears
Hydraulic
ATF
(Worm) Machine Tool
Slideway
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4. Functions of Lubricant
Reduce friction and wear Dissipate heat Prevent rust and/or corrosion Act as a seal to outside contaminants Flush contaminants away from bearing surfaces
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5. Fluid Film Lubrication
Hydro-static Boundary (Internal Pressure) )
Hydro-dynamic
(External Pressure)
Elasto-hydrodynamic
(Elastic deformation)
(Metal
Fluid film thickness is directly proportional to viscosity and velocity and inversely proportional to load.
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6. Oil Characteristics
6.1 Carbon Residue It is the amount of deposit (wt %) left after evaporation and pyrolysis of the oil under prescribed conditions. Oils of naphthenic type usually show lower residues than those of similar viscosity made from paraffinic crudes. It decreases with the severity of the refining treatment. It is determined only for base oils used for engine oils, aircraft engines and some products of cylinder type such as reciprocating air compressors.
6.2 Color Visual comparison of light transmitted through a specified depth of oil with the amount of light transmitted through one of a series of colored glasses. Color differences in lubricating oils result from differences in crude oils, viscosity, and method and degree of treatment during refining and in the amount and nature of the additives used. It is a useful guide to the refiner to indicate whether processes are operating properly. In finished lubricants color has little significance except in the case of medicinal and industrial white oils which are often compounded into products where staining and discoloration would be undesirable.
6.3 Density and Gravity
The density of a substance is the mass of a unit volume of it at a standard temperature. The specific gravity (relative density) is the ratio of the mass of a given volume of a material at a standard temperature to the mass of an equal volume of water at the same temperature. API gravity is a special function of specific gravity (sp gr), which is related to it by the following equation:
API gravity = (141.5)/ (sp gr 60/60 F) - 131.5 . Density and gravity can be determined by means of hydrometers (Figure 3.4). The hydrometer can be calibrated to read any of the three properties: density, specific gravity, or API gravity. This property is widely used for control in refinery operations. It is also useful for identifying oils, provided the distillation range or viscosity of the oils is known. The primary use of API gravity, however, is to convert weighed quantities to volume and measured volumes to weight. In testing used oils, particularly used engine oils, a decrease in specific gravity (increase in API gravity) may indicate fuel dilution, whereas an increase in specific gravity might indicate the presence of contaminants such as fuel soot or oxidized materials.
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6.4 Viscosity It is the time required for a fixed volume of lubricant to flow through a capillary tube at a given test temperature usually 40 C or 100 C under the influence of gravity. It is the measure of a fluid’s internal friction or resistance to flow. The correct viscosity for a particular application would be thick enough to the load but not so thick so as to cause excessive fluid friction and a corresponding increase in temperature.
6.5 Viscosity Index Describes change in viscosity w.r.t temperature. Consider a high VI oil and a low VI oil having the same viscosity at, say, room temperature: as the temperature increased, the high VI oil would thin out less and, therefore, would have a higher viscosity than the low VI oil at higher temperatures. The viscosity index is an entirely empirical parameter which compares the kinematic viscosity of the oil of interest to the viscosities of two reference oils which have a considerable difference in sensitivity of viscosity to temperature. The reference oils have been selected in such a way that one of them has the viscosity index equal to zero (VI=O) and the other has the viscosity index equal to one hundred (VI=lOO) at lOOOF (37.8OC) but they both have the same viscosity as the oil of interest at 210°F (98.89OC), as illustrated in Figure 2.3. Since Pennsylvania and Gulf Coast oils have the same viscosity at 210°F (98.9OC) they were initially selected as reference oils. Oils made from Pennsylvania crude were assigned the viscosity index of 100 whereas oils made from the Gulf Coast crude the viscosity index of 0. The viscosity index can be calculated from the following formula: VI = (L - U) / (L - H) x 100 Firstly the kinematic viscosity of the oil of interest is measured at 4OoC ('U') and at 100°C . Then from Table (ASTM D2270), looking at the viscosity at 100°C of the oil of interest, the corresponding values of the reference oils, 'L' and 'H'
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Note that the viscosity index is an inverse measure of the decline in oil viscosity with temperature. High values indicate that the oil shows less relative decline in viscosity with temperature. The viscosity index of most of the refined mineral oils available on the market is about 100, whereas multi-grade and synthetic oils have higher viscosity indices of about 150 Alternatively
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6.5.1 Mono vs. Multi Grade Oils The oils without a 'W' suffix are called 'monograde oils' since they meet only one SAE grade. The oils with a 'W' suffix, which stands for 'winter', have good cold starting capabilities. For climates where the temperature regularly drops below zero Celsius, engine and transmission oils are formulated in such a manner that they give low resistance at start, i.e. their viscosity is low at the starting temperature. Such oils have a higher viscosity index, achieved by adding viscosity improvers (polymeric additives) to the oil and are called 'multigrade oils'. For example, SAE 20W/50 has a viscosity of SAE 20 at -18°C and viscosity of SAE 50 at 100°C as is demonstrated in Figure 2.18. The problem associated with the use of these oils is that they usually shear thin, i.e. their viscosity drops significantly with increased shear rates. This has to be taken into when deg machine components lubricated by these oils. The loss in viscosity can be quite pronounced, and with some viscosity improvers even a permanent viscosity loss at high shear rates can occur due to the breaking up of molecules into smaller units. The viscosity loss affects the thickness of the lubricating film separating the two surfaces and hence affects the performance of the machine. SAE classification of transmission oils is very similar to that of engine oils. The only difference is that the winter grade is defined by the temperature at which the oil reaches the viscosity of 150,000 [l. This is the maximum oil viscosity which can be used without causing damage to gears. The classification also permits multigrading.
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6.6 Pour Point It is the lowest temperature at which the lubricant will flow. In paraffinic oils, it is the result of crystallization of waxy particles. In naphthenic oils it is the result of the decrease in viscosity caused by a decrease in temperature.
6.7 Flash Point It is the lowest temperature at which enough vapors are given off to cause ignition. It is an indication of the fire and explosion hazard of particular oil. Also an indication of the evaporation loses that can be expected under high temperature applications.
6.8 Fire Point It is the lowest temperature at which vapors are given off in sufficient quantity to sustain combustion.
6.9 Total Base Number The total base number of an oil is the quantity of acid, expressed in of the equivalent number of milligrams of potassium hydroxide, that is required to neutralize all basic constituents present in one gram of oil. It is worth mentioning that new and used oil can exhibit both TAN and TBN (total base number) values.
6.10 Total Acid Number/ Neutralization Number It is a measure of the acidity of the oil and is the amount in milligrams of KOH required to neutralize 1 gram of oil. The total acid number (TAN) of an oil is synonymous with neutralization number. The amount of acid in the oil was expressed in of the amount of a standard base required to neutralize a specified volume of oil. This quantity of base came to be called the neutralization number (NN) of the oil. A relative increase in the NN indicates oxidation of the oil.
6.11 Sulfated Ash The sulfated ash of a lubricating oil is the residue, in percent by weight, remaining after three processes: burning the oil, treating the initial residue with sulfuric acid, and burning the treated residue. It is a measure of the noncombustible constituents (usually metallic materials) contained in the oil. New, straight mineral lubricating oils contain essentially no ash-forming materials. Many of the additives used in lubricating oils, such as detergents, contain metallo-organic components, which will form a residue in the sulfated ash test, and so the concentration of such materials in an oil is roughly indicated by the test. Thus, during manufacture, the test gives a simple method of checking to ensure that the additives have been incorporated in approximately the correct amounts. However, since the test combines all metallic elements into a single residue, additional testing may be necessary to determine if the various metallic elements are in the oil in the correct proportions.
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Metal - %age to Sulphated Ash
Zn
1.25
Na
3.1
Mg
4.5
Ca
3.4
Ba
1.7
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7. Grease Greases are most often used instead of fluids where a lubricant is required to maintain its original position in a mechanism, especially where opportunities for frequent relubrication may be limited or economically unjustifiable. This requirement may be due to the physical configuration of the mechanism, the type of motion, the type of sealing, or to the need for the lubricant to perform all or part of any sealing function in the prevention of lubricant loss or the entrance of contaminants. Because of their essentially solid nature, greases do not perform the cooling and cleaning functions associated with the use of a fluid lubricant. With these exceptions, greases are expected to accomplish all other functions of fluid lubricants.
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7.1 Consistency/ Hardness It is the measure of a grease’s resistance to deformation by an applied force and is the most important characteristic of a grease.. It should be soft enough to allow easy application but not so soft that it leaks out of the area being lubricated. NLGI values vary from 000 (soft) to 6 (hard). Penetration Number As above
7.2 Working Temperature Range 7.3 Dropping Point It is the temperature at which the grease becomes soft enough for a drop of fluid to fall from the grease. At or above the dropping point, the grease will act as a fluid. The grease may actually break down far below the dropping point. Most manufacturers list a usable temperature range along with the dropping point.
7.4 Apparent Viscosity Grease is a non-Newtonian material that does not begin to flow until a shear stress exceeding a yield point is applied. If the shear stress is then increased further, the flow rate increases more proportionally, and the viscosity, as measured by the ratio of shear stress to shear rate, decreases. The observed viscosity of a non-Newtonian material such as grease is called its apparent viscosity. Apparent viscosity varies with both temperature and shear rate; thus, it must always be reported at a specific temperature and flow rate. Apparent viscosity is used to predict the handling and dispensing properties of a grease. In addition, it can be related to starting and running torque, in grease lubricated mechanisms, and is useful in predicting leakage tendencies.
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8. Oil Filtration
Gravity (Separation/ settling of contaminants that are heavier than oil) Centrifugal (rotation-viscosity and size determine degree of purification) Mechanical (filter-Beta Rating) Coalescence (cartridge combines small water particles into larger ones which are filtered to leave behind dry oil) Vacuum (Application of heat and vacuum 100-140F) Adsorption (Fullers Earth to remove oil oxidation products)
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9. Oil Application 9.1 Oil-Bath Lubrication The conventional oil-bath system for lubricating bearings is satisfactory for low to moderately high speed applications. Because this type of system is non-circulating, the static oil-level should never be higher than the center of the lowest positioned rolling element in the bearing being lubricated. A greater amount of oil can cause churning, increase the fluid friction within the bearing and result in excessive operating temperatures. Unless the running level of the oil is known, oil level should be checked only when equipment is shut down as the running level can drop considerably below the static level depending on the speed of the application. Because speed, sealing effectiveness, temperature and type of oil are factors that influence the refilling cycle, regular inspection is necessary to determine the frequency of refilling. Applications of this type generally employ sight gages to facilitate inspection.
9.2 Wick-Feed Lubrication Wick-feed oilers, one of the older methods of applying oil to bearings, still enjoy a certain popularity. Properly designed, applied and maintained, then are effective and inexpensive. Functioning as a filter and quantity regulator, the wick employs either capillary action, or gravity (see illustration) to transfer the oil from the reservoir to bearing. Paraffinic lubricating oils may also be used with this type oiler although they have a tendency to deposit wax crystals on the wick fibers, destroying the effectiveness of the wick. Because napthetic and synthetic oils do not exhibit this tendency, they are preferred for wick oilers.
9.3 Drip-Feed Lubrication Another one of the older methods of lubrication of oiling bearings is the drip-feed system. This system has been applied successfully to applications where moderate loads and speeds are encountered. The oil introduced through a filter-type, sight feed oiler, has a controllable flow rate which is determined by the operating temperature of the particular application.
9.4 Oil -Splash Lubrication This system of lubrication is used primarily in gear cases where the bearing and gear lubricant is common. The lubrication of bearings in a gearbox, other than one of slow speed, is usually not critical as the oil splash from gear teeth is sufficient to lubricate the bearings. Because of the constant problem of the oil carrying wear debris, the use of filters and magnetic drain plugs is helpful in reducing the possibility of wear debris contaminating the bearings. In applications where heavy oil flow or splash is encountered, bearings equipped with shields to reduce the quantity of oil reaching the bearings are sometimes necessary to prevent overheating caused by fluid friction where the bearing is flooded. In systems where normal splash or washdown is expected to be marginal, oil feeder trails should be designed into the case to direct case washdown into the bearings.
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9.5 Circulating-Oil Lubrication This type of system utilizes a circulating pump to assure a positive supply of lubricant to the bearing and can be used for low to moderately high speed and high temperature power transmission applications. The flow path of the oil in this system is important because bearing churning in a captive amount of oil can generate temperatures capable of causing lubricant breakdown and bearing damage. Due to the inherent possibility of contamination from wear debris in heavy duty applications, suitable oil filters and magnetic drain plugs are necessary to prevent damage to the bearings.
9.6 Oil-Jet Lubrication In applications where a bearing is heavily loaded and operating at high speed and temperatures, a sophisticated variation of circulating oil lubrication, called oil-jet lubrication, may be required. In such cases, it is necessary to lubricate each bearing location individually, under pressure, and to provide adequately large scavenging drains to prevent the accumulation of oil after age through the bearing. In certain high speed applications where the bearing itself creates a pumping action, the flow of oil must be adjusted to assure age through the bearing. This is extremely important where the flow of oil from the jet opposes the pumping action within the bearing.
9.7 Oil-Mist Lubrication Oil-Mist Lubrication systems are used in high-speed, continuous operation applications. This system permits close control of the amount of lubricant reaching the bearing. The oil may be metered, atomized by compressed air and mixed with air, or it may be picked up from a reservoir using a venturi effect. In either case, the air is filtered and supplied under sufficient pressure to assure adequate lubrication of the bearings. Control of this type of lubricating system is accomplished by monitoring the operating temperatures of the bearings being lubricated. The continuous age of the pressurized air and oil through the labyrinth seals used in the system prevents the entrance of contaminants from the atmosphere into the system. To insure “wetting” of the bearings and to prevent possible damage to the rolling elements and races, it is imperative that the oil-mist system be turned on for several minutes before the equipment is started. The importance of the “wetting” the bearings before starting cannot be overstressed and has particular significance for equipment that has been idle for extended periods of time. The successful operation of this type of system is based upon the following factors: Proper location of the lubricant entry ports in relation to the bearings being lubricated Avoidance of excessive pressure drops across void systems within the system The proper air pressure and oil quantity ratios to suit the particular application The adequate exhaust of the air-oil mist after lubrication has been accomplished
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10. Oil Testing Problems
Tests
Over heating =>Oxidation
Carbonyl group -------Spectroscopy Pentane Insolubles
Unusual -------------------------------------------------------
Some part is too hot
Depletion of Additives
Calorimetric, Gravimetric, Polarimetric, Potentiometric spectroscopy for wear metals
Contamination with Fuel
Fractional distillation
(Leakage, condensation, over-rich mixture)
Gas Chromatography
=>reduction in viscosity
Flash point test
Contamination with solvents
If low then flash point test
Contamination with solids
Pentane insolubles, Benzene Insolubles
Water contamination
Crackle Test >110 C
10.1 Wear Analysis A. Spectroscopy 1. Emission Oil + Electric arc from C electrode (frequency and intensity of light)
Rotode
Inductively coupled plasma (I)-----(nebula + hot argon plasma)
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B. Magnetic Plug Techniques Equipment Magnetic plug with electrodes Chip detector to indicate metal parts Electric heater to burn small fuzz If warning
Not cancelled => large chip flakes =>danger
Occurring rapidly => fine and continuous debris
Cancelled => fine but broken debris
C. Ferrography
Wear particles are deposited on the slide. The magnet below the chip retains them even when the substrate has been washed away from the chip. Stronger particles are deposited first and along the axis.
D. Rotary Particle Depositor
The oil is dropped on the centre of the magnet from where it moves radially outwards. 33
Mustafa Ali The largest and most highly ferromagnetic particles gather in the inner most circle. Particles of decreasing susceptibility deposit in the outer circles.
10.2 Contaminant Warning Levels
Test
Warning Limit
Viscosity -cSt @ 40C
25% change vs. the new oil viscosity
-cSt @ 100 C
15% change vs. the new oil viscosity
Coolant
Any positive identification
Water
Greater than 0.1 %
Dilution
Greater than 5%
Insolubles
0.5% or more
Total Acid Number
More than 5 units (motor oil) or 1 unit (industrial oil)
Total Base Number
No lower than 3 to 4 units
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11. USEFUL TABLES
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12. BIBLIOGRAPHY 1. 2. 3. 4. 5.
Fundamentals of Fluid Film Lubrication- Marcel Dekker-2004. Lubrication Fundamentals-D.M.Pirro & A.A.Wessol-Exxon Mobil Corporation-2001. Mechanical Engineer’s Reference Book-E. H. Smith-Butterworth Heinemann-1994 Lubrication and Lubricant Selection-A.R.Lansdown-TIPS Series-2004 Lubrication and Maintenance of Industrial Machinery-Robert Gresham & G.E.TottenTaylor and Francis Group-2009 6. Lubricant Additives Chemistry and Application-Leslie R. Rudnick- Taylor and Francis Group-2009 7. U.S Army Lubricants-U.S. Army Corps of Engineers 8. Timken Lubrication Guide-Torrington & Fafnir 9. Petro Canada Lubricants Handbook-2005 10. Engineering Tribology-G.W.Stachowiak & A.W.Batchelor-Elsevier-1993 11. Tribology in Machine Design-T.A. Stolarski-Butterworth Heinemann-1990 12. Tribology: Friction, Wear and Lubrication-Bharat Bhushan-CRC Press-2000
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LUBRICANTS AND LUBRICATION FUNDAMENTALS
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TABLE OF CONTENTS Types of Lubricants -----------------------------------------------------------------------------3 Solid Lubricants----------------------------------------------------------------------------------3 Liquid Lubricants--------------------------------------------------------------------------------4 Base Stock-------------------------------------------------------------------------------4 Additives---------------------------------------------------------------------------------6 Functions of Lubricant-----------------------------------------------------------------15 Fluid Film Lubrication-----------------------------------------------------------------16 Oil Characteristics----------------------------------------------------------------------17 Grease---------------------------------------------------------------------------------------------23 Filtration------------------------------------------------------------------------------------------26 Methods of lube Application ------------------------------------------------------------------27 Oil Testing----------------------------------------------------------------------------------------32 Useful Tables-------------------------------------------------------------------------------------35 Bibliography--------------------------------------------------------------------------------------51
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1. TYPES
Solid
Liquid
Gas
2. SOLID LUBRICANTS
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3. Liquid Lubricants 3.1 Composition of liquid Lubricants Base stock + Additives = Lubricant
3.1.1 Base Stock Crude oil is distilled to yield different fractions one of which is the base stock for finished lubricants. The base stock is manufactured by two processes namely separation and conversion.
Separation involves division of the crude oil into its different fractions while conversion takes into the different processes required to refine the base stock. Usually separation involves Pre-eating, Fractional distillation, Vacuum distillation, Propane de-asphalting, Furfural extraction, solvent de-waxing (through MEK) and clay/hydro-finishing. Conversion involves Vacuum distillation, Hydro-cracking (Group II and III)/ Hydrogen De-waxing (Group I and II) and Hydro-finishing.
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3.1.1.1 Classification of Base Stocks A Group I Solvent Refined II Hydro-finished III Hydro-cracked IV Poly Alpha Olephin V All Others
Saturates (%) <90 >90 >90
Sulfur (%) >0.03 <0.03 <0.03
VI 80-120 80-120 >120
B. According to Viscosity Index Type LVI MVI HVI VHVI UHVI
VI <35 35-80 80-110 110-130 130-150
(G-III) (G-III)
C. According to Composition Paraffinic Higher VI Higher FP Better Oxidation stability Better thermal stability
Naphthenic Better solvent Lower pour point No wax
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3.1.1.2 Mineral vs. Synthetic Base Stock Mineral BS contains numerous hydrocarbons and it works to the average of these hydrocarbons’ working ability. Synthetic BS refined to the extent that it contains one or two hydrocarbons. Examples include
Organic Esters Phosphate Esters Poly-glycols Synthesized Hydrocarbon Fluids (SHFs) Other synthetic Lubricating fluids
3.1.2 Additives
Surface Protecting
Performance Enhancing
Lubricant Protective
Surface Protecting
Performance Enhancing
Lubricant Protecting
Anti Wear (adsorb)
VI Improver
Oxidation Inhibitor
Anti Rust/ Corrosion (adsorb)
Pour point Depressant
Foam Inhibitor
Extreme Pressure (adsorb)
Demulsifier/ Emulsifier
Detergent/ Dispersant Tackiness Agent Lubricity Additive (adsorb)
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A. Pour Point Depressants Certain high molecular weight polymers function by inhibiting the formation of a wax crystal structure that would prevent oil flow at low temperatures. Two general types of pour point depressant are used: 1. Alkylaromatic polymers adsorb on the wax crystals as they form, preventing them from growing and adhering to each other. 2. Polymethacrylates co-crystallize with wax to prevent crystal growth. The additives do not entirely prevent wax crystal growth, but rather lower the temperature at which a rigid structure is formed. Depending on the type of oil, pour point depression of up to 50_F (28_C) can be achieved by these additives, although a lowering of the pour point by about 20–30 F_ (11–17 C_) is more common.
B. Viscosity Index Improvers VI improvers are long chain, high molecular weight polymers that function by causing the relative viscosity of an oil to increase more at high temperatures than at low temperatures. Generally this result is due to a change in the polymer’s physical configuration with increasing temperature of the mixture. It is postulated that in cold oil the molecules of the polymer adopt a coiled form so that their effect on viscosity is minimized. In hot oil, the molecules tend to straighten out, and the interaction between these long molecules and the oil produces a proportionally greater thickening effect. Note: Although the oil– polymer mixture still decreases in viscosity as the temperature increases, the decrease is not as great as it would have been in the oil alone. Under shear conditions the long molecules of the VI improver align themselves in the direction of the stress, thus reducing resistance to flow. When the stress is removed, the molecules return to their usual random arrangement and the temporary viscosity loss is recovered. This effect can be beneficial in that it can temporarily reduce oil friction to permit easier starting, as in the cranking of a cold engine. Permanent shear breakdown occurs when the shear stresses actually rupture the long molecules, converting them into lower molecular weight materials, which are less effective VI improvers. This results in a permanent viscosity loss, which can be significant. It is generally the limiting factor controlling the maximum amount of VI improver that can be used in a particular oil blend. VI improvers are used in engine oils, automatic transmission fluids, multipurpose tractor fluids, and hydraulic fluids. They are also used in automotive gear lubricants. Their use permits the formulation of products that provide satisfactory lubrication over a much wider temperature range than is possible with straight mineral oils alone.
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C. Defoamants The ability of oils to resist foaming varies considerably depending on type of crude oil, type and degree of refining, and viscosity. In many applications, there may be considerable tendency to agitate the oil and cause foaming, while in other cases even small amounts of foam can be extremely troublesome. In these cases, a de-foamant may be added to the oil. It is thought that the de-foamant droplets attach themselves to the air bubbles and can either spread or form unstable bridges between bubbles, which then coalesce into larger bubbles, which in turn rise more readily to the surface of the foam layer where they collapse, thus releasing the air. Silicone polymers used at a few parts per million are the most widely used defoamants. These materials are essentially insoluble in oil, and the correct choice of polymer size and blending procedures is critical if settling during long-term storage is to be avoided. Also, these additives may increase air entrainment in the oil. Organic polymers are sometimes used to overcome these difficulties with the silicones, although much higher concentrations are generally required.
D. Oxidation Inhibitors When oil is heated in the presence of air, oxidation occurs. As a result of this oxidation, both the oil viscosity and the concentration of organic acids in the oil increase, and varnish and lacquer deposits may form on hot metal surfaces exposed to the oil. In extreme cases, these deposits may be further oxidized to form hard, carbonaceous materials. The rate at which oxidation proceeds is affected by several factors. As the temperature increases, the rate of oxidation increases exponentially. A rule of thumb is that for each 10_C (18_F) rise in temperature, the oxidation rate of mineral oil will double. Greater exposure to air (and the oxygen it contains), or more intimate mixing with it, will also increase the rate of oxidation. Many materials, such as metals, particularly copper and iron and organic and mineral acids, may act as catalysts or oxidation promoters. Although the complete mechanism of oil oxidation is not too well defined, it is generally recognized as proceeding by free radical chain reaction. Reaction chain initiators are formed first from unstable oil molecules, and these react with oxygen to form peroxy radicals, which in turn attack the unoxidized oil to form new initiators and hydroperoxides. The hydroperoxides are unstable and divide, forming new 8
Mustafa Ali initiators to expand the reaction. Any materials that will interrupt this chain reaction will inhibit oxidation. Two general types of oxidation inhibitor are used: those that react with the initiators, peroxy radicals, and hydroperoxides to form inactive compounds, and those that decompose these materials to form less reactive compounds. At temperatures below 200_F (93_C), oxidation proceeds slowly and inhibitors of the first type are effective. Examples of this type are hindered (alkylated) phenols such as 2,6-ditertiary-butyl-4methylphenol (also called 2,6 ditertiary-butylparacresol, DBPC), and aromatic amines such as N-phenyl_-naphthylamine. These are used in products such as turbine, circulation, and hydraulic oils, which are intended for extended service at moderate temperatures. When the operating temperature exceeds about 200_F (93_C), the catalytic effects of metals become important factors in promoting oil oxidation. Under these conditions, inhibitors that reduce the catalytic effect of the metals must be used. These materials usually react with the surfaces of the metals to form protective coatings and for that reason are sometimes called metal deactivators. Typical of additives of this type are the dithiophosphates, primarily zinc dithiophosphate. Since the dithiophosphates also act to decompose hydroperoxides at temperatures above 200_F (93_C), they inhibit oxidation by this mechanism as well. Oxidation inhibitors may not entirely prevent oil oxidation when conditions of exposure are severe, and some types of oil are inhibited to a much greater degree than others. Oxidation inhibitors are not, therefore, cure-alls, and the formulation of a satisfactorily stable oil requires proper refining of a suitable base stock combined with careful selection of the type and concentration of oxidation inhibitor. It should also be pointed out that other additives can reduce oxidation stability in performing their design functions. Proper formulation requires the balancing of all the additive reactions to achieve the desired total performance characteristics.
E. Rust and Corrosion Inhibitors A number of kinds of corrosion can occur in systems served by lubricating oils. Probably the two most important types are corrosion by organic acids that develop in the oil itself and corrosion by contaminants that are picked up and carried by the oil. Corrosion by organic acids can occur, for example, in the bearing inserts used in internal combustion engines. Some of the metals used in these inserts, such as the lead in copper-lead or lead-bronze, are readily attacked by organic acids in oil. The corrosion inhibitors form a protective film on the bearing surfaces that prevents the corrosive materials from reaching or attacking the metal. The film may be either adsorbed on the metal or chemically bonded to it. During combustion in gasoline or diesel engines, certain materials in the fuel, such as sulfur and antiknock scavengers, can burn to form strong acids. These acids can then condense on the cylinder walls and be carried to other parts of the engine by the lubricant. Corrosive wear of rings and cylinder walls, and corrosion of crankshafts, rocker arms, and other engine components can then occur. It has been found that the inclusion of highly alkaline materials in the oil will help to neutralize these strong acids as they are formed, greatly reducing this corrosion and corrosive wear. These alkaline materials are also used to provide detergency. 9
Mustafa Ali Rust inhibitors are usually compounds having a high polar attraction toward metal surfaces. By physical or chemical interaction at the metal surface, they form a tenacious, continuous film that prevents water from reaching the metal surface. Typical materials used for this purpose are amine succinates and alkaline earth sulfonates. The effectiveness of a properly selected rust inhibitor is illustrated in Figure 3.3, where specimen 9 is rust free and the other specimens display varying degree of corrosion. Rust inhibitors can be used in most types of lubricating oil, but the selection must be made carefully to avoid problems such as corrosion of nonferrous metals or the formation of troublesome emulsions with water. Because rust inhibitors are adsorbed on metal surfaces, an oil can be depleted of rust inhibitor in time.
F. Detergents and Dispersants In internal combustion engine service, a variety of effects tends to cause oil deterioration and the formation of harmful deposits. These deposits can interfere with oil circulation, build up behind piston rings to cause ring sticking and rapid ring wear, and affect clearances and proper functioning of critical components, such as hydraulic valve lifters. Once formed, such deposits are generally hard to remove except by mechanical cleaning. Detergents and dispersants in the oil can delay the formation of deposits and reduce the rate at which they accumulate on metal surfaces. An essential factor with this approach is regular draining and replacement of the oil so that the contaminants in it are removed from the engine before the oil’s capacity to hold them is exceeded. Detergents are generally considered to be chemical compounds that chemically neutralize deposit precursors that form under high temperature conditions or as the result of burning fuels with high sulfur content or other materials that form acidic combustion byproducts. Dispersants, on the other hand, are chemical compounds that disperse or suspend in the oil potential sludge- or varnish-forming materials, particularly those formed during low temperature operation when condensation and partially burned fuel find their way into the oil. These contaminants are removed from the system when the oil is drained. There is no sharp line of demarcation between detergents and dispersants. Detergents have some ability to disperse and suspend contaminants, while dispersants have some ability to prevent the formation of high temperature deposits. The principal detergents used today are organic soaps and salts of alkaline earth metals such as barium, calcium, and magnesium. These materials are often referred to as metallo-organic compounds. Calcium and magnesium sulfonates, and calcium phenates (or phenol sulfides) are widely used. The sulfonates and phenates may be neutral or overbased; that is, they may contain more of the alkaline metal than is required to neutralize the acidic components used in diesel engine oils to neutralize the strong acids formed from combustion of the sulfur in the fuel. This neutralization reduces corrosion and corrosive wear and minimizes the tendency of these acids to cause oil degradation. Overbased materials are generally used at lower concentration in gasoline engine oils, where the fuel sulfur is much lower. The overbased materials are included in the formulation to help reduce corrosion in low temperature operation. Both neutral and overbased materials also act to disperse and suspend potential varnish forming materials resulting from oil oxidation, preventing these materials from depositing on engine surfaces. Metallo-organic detergents, on combustion, leave an ashy residue (see Section II.I, Sulfated Ash). In some cases, this may be detrimental in that the ash can contribute to combustion chamber deposits. In 10
Mustafa Ali other cases, it may be beneficial in that the ash provides wear-resistant coatings for surfaces such as valve faces and seats. Typical dispersants (also called polymeric dispersants and ashless dispersants) in use today are described as polymeric succinimides, olefin/P2S5 reaction products, polyesters, and benzylamides. These are based on long chain hydrocarbons that are acidified and then neutralized with a compound containing basic nitrogen. The hydrocarbon portion provides oil solubility, while the nitrogen portion provides an active site that attracts and holds potential deposit-forming materials to keep them suspended in the oil. While the primary use of detergents and dispersants is in engine oils, they are also being used in products such as automatic transmission fluids, hydraulic oils, and circulation oils for high temperature service. In these applications, the detergents and dispersants help to prevent the deposition of lacquer and varnish resulting from oil oxidation, thus supplementing the effects of the oxidation inhibitors.
G. Antiwear Additives Antiwear additives are used in many lubricating oils to reduce friction, wear, and scuffing and scoring under boundary lubrication conditions, that is, when full lubricating films cannot be maintained. As the oil film becomes progressively thinner as a result of increasing loads or temperatures, through the oil film is first made by minute surface irregularities or asperities. As these opposing asperities make , friction increases and welding can occur. As sliding continues, the welds break immediately, but the process can form new roughness through metal transfer, as well as wear particles, which can cause scuffing and scoring. Two general classes of materials are used to prevent metallic , depending on the severity of the requirements. Mild antiwear and friction-reducing additives, sometimes called boundary lubrication additives, are polar materials such as fatty oils, acids, and esters. They are long chain materials that form an adsorbed film on the metal surfaces with the polar ends of the molecules attached to the metal and the molecules projecting more or less normal to the surface. is then between the projecting ends of the layers of molecules on the opposing surfaces. Friction is reduced, and the surfaces move more freely relative to each other. Wear is reduced under mild sliding conditions, but under severe sliding conditions the layers of molecules can be rubbed off, with the result that their wearreducing effect is lost.
H. Extreme Pressure Additives At high temperatures or under heavy loads where more severe sliding conditions exist, compounds called extreme pressure (EP) additives are required to reduce friction, control wear, and prevent severe surface damage. These materials function by chemically reacting with the sliding metal surfaces to form relatively oil insoluble surface films. The kinetics of the reaction are a function of the surface temperatures generated by the localized high temperatures that result from rubbing between opposing surface asperities, and breaking of junctions between these asperities. Even with extreme pressure additives in the lubricant, wear of new surfaces may be high initially. In addition to the normal break-in wear, nascent metal (freshly formed, chemically reactive surfaces), time, and temperature are required to form the protective surface films. After the films have formed, relative motion is between the layers of surface films rather than the metals. The sliding process can lead to some film removal, but since replacement by further chemical reaction is rapid, the loss of metal is extremely low. This process gradually depletes the amount of EP additive available in the oil, although the rate of depletion is usually very slow. Thus, there will be sufficient additive left to provide adequate protection for the metal surfaces except possibly under severe operating conditions, where makeup rates are low and normal drain intervals are exceeded. The severity of the sliding conditions dictates the reactivity of the EP additives required for maximum effectiveness. The optimum reactivity occurs when the additives minimize the adhesive or 11
Mustafa Ali metallic wear without leading to appreciable corrosive or chemical wear. Additives that are too reactive lead to the formation of excessively thick surface films, which have less resistance to attrition, so some metal is lost by the sliding action. Since a particular EP additive may have different reactivity with different metals, it is important to match additive metal reactivity to the additives not only with the severity of the sliding system but also with the specific metals involved. For example, some additives that are excellent for steel-on-steel systems may not be satisfactory for bronze-on-steel systems operating at similar sliding severity because they are too reactive with the bronze. Another important function of EP additives is that because the chemical reaction is greatest on the asperities where is made and localized temperatures are highest, they lead to polishing of the surfaces. The load is then distributed more uniformly over a greater area, which allows for a reduction in sliding severity, more effective lubrication, and a reduction in wear. Extreme pressure agents are usually compounds containing sulfur, chlorine, or phosphorus, either alone or in combination. The compounds used depend on the end use of the lubricant and the chemical activity required in it. Sulfur compounds, sometimes with chlorine or phosphorus compounds, are used in many metal-cutting fluids. Sulfur–phosphorus combinations are used in most industrial and automotive gear lubricants. These materials provide excellent protection against gear tooth scuffing and have the advantages of better oxidation stability, lower corrosivity, and often lower friction than other combinations that have been used in the past.
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Anti Ox
Anti
Anti
Anti
Rust
Corr
Wear
EP
VI Imp
Deter/
Anti
Friction
Disp
Foam
Mod
Petrol
Diesel
Steam Turbine
Compressor
Gears
PP Dep
Demul / Emul
(Spiral Bevel, Hypoid) Gears (Spur Bevel)
Gears
Hydraulic
ATF
(Worm) Machine Tool
Slideway
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4. Functions of Lubricant
Reduce friction and wear Dissipate heat Prevent rust and/or corrosion Act as a seal to outside contaminants Flush contaminants away from bearing surfaces
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5. Fluid Film Lubrication
Hydro-static Boundary (Internal Pressure) )
Hydro-dynamic
(External Pressure)
Elasto-hydrodynamic
(Elastic deformation)
(Metal
Fluid film thickness is directly proportional to viscosity and velocity and inversely proportional to load.
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6. Oil Characteristics
6.1 Carbon Residue It is the amount of deposit (wt %) left after evaporation and pyrolysis of the oil under prescribed conditions. Oils of naphthenic type usually show lower residues than those of similar viscosity made from paraffinic crudes. It decreases with the severity of the refining treatment. It is determined only for base oils used for engine oils, aircraft engines and some products of cylinder type such as reciprocating air compressors.
6.2 Color Visual comparison of light transmitted through a specified depth of oil with the amount of light transmitted through one of a series of colored glasses. Color differences in lubricating oils result from differences in crude oils, viscosity, and method and degree of treatment during refining and in the amount and nature of the additives used. It is a useful guide to the refiner to indicate whether processes are operating properly. In finished lubricants color has little significance except in the case of medicinal and industrial white oils which are often compounded into products where staining and discoloration would be undesirable.
6.3 Density and Gravity
The density of a substance is the mass of a unit volume of it at a standard temperature. The specific gravity (relative density) is the ratio of the mass of a given volume of a material at a standard temperature to the mass of an equal volume of water at the same temperature. API gravity is a special function of specific gravity (sp gr), which is related to it by the following equation:
API gravity = (141.5)/ (sp gr 60/60 F) - 131.5 . Density and gravity can be determined by means of hydrometers (Figure 3.4). The hydrometer can be calibrated to read any of the three properties: density, specific gravity, or API gravity. This property is widely used for control in refinery operations. It is also useful for identifying oils, provided the distillation range or viscosity of the oils is known. The primary use of API gravity, however, is to convert weighed quantities to volume and measured volumes to weight. In testing used oils, particularly used engine oils, a decrease in specific gravity (increase in API gravity) may indicate fuel dilution, whereas an increase in specific gravity might indicate the presence of contaminants such as fuel soot or oxidized materials.
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6.4 Viscosity It is the time required for a fixed volume of lubricant to flow through a capillary tube at a given test temperature usually 40 C or 100 C under the influence of gravity. It is the measure of a fluid’s internal friction or resistance to flow. The correct viscosity for a particular application would be thick enough to the load but not so thick so as to cause excessive fluid friction and a corresponding increase in temperature.
6.5 Viscosity Index Describes change in viscosity w.r.t temperature. Consider a high VI oil and a low VI oil having the same viscosity at, say, room temperature: as the temperature increased, the high VI oil would thin out less and, therefore, would have a higher viscosity than the low VI oil at higher temperatures. The viscosity index is an entirely empirical parameter which compares the kinematic viscosity of the oil of interest to the viscosities of two reference oils which have a considerable difference in sensitivity of viscosity to temperature. The reference oils have been selected in such a way that one of them has the viscosity index equal to zero (VI=O) and the other has the viscosity index equal to one hundred (VI=lOO) at lOOOF (37.8OC) but they both have the same viscosity as the oil of interest at 210°F (98.89OC), as illustrated in Figure 2.3. Since Pennsylvania and Gulf Coast oils have the same viscosity at 210°F (98.9OC) they were initially selected as reference oils. Oils made from Pennsylvania crude were assigned the viscosity index of 100 whereas oils made from the Gulf Coast crude the viscosity index of 0. The viscosity index can be calculated from the following formula: VI = (L - U) / (L - H) x 100 Firstly the kinematic viscosity of the oil of interest is measured at 4OoC ('U') and at 100°C . Then from Table (ASTM D2270), looking at the viscosity at 100°C of the oil of interest, the corresponding values of the reference oils, 'L' and 'H'
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Note that the viscosity index is an inverse measure of the decline in oil viscosity with temperature. High values indicate that the oil shows less relative decline in viscosity with temperature. The viscosity index of most of the refined mineral oils available on the market is about 100, whereas multi-grade and synthetic oils have higher viscosity indices of about 150 Alternatively
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6.5.1 Mono vs. Multi Grade Oils The oils without a 'W' suffix are called 'monograde oils' since they meet only one SAE grade. The oils with a 'W' suffix, which stands for 'winter', have good cold starting capabilities. For climates where the temperature regularly drops below zero Celsius, engine and transmission oils are formulated in such a manner that they give low resistance at start, i.e. their viscosity is low at the starting temperature. Such oils have a higher viscosity index, achieved by adding viscosity improvers (polymeric additives) to the oil and are called 'multigrade oils'. For example, SAE 20W/50 has a viscosity of SAE 20 at -18°C and viscosity of SAE 50 at 100°C as is demonstrated in Figure 2.18. The problem associated with the use of these oils is that they usually shear thin, i.e. their viscosity drops significantly with increased shear rates. This has to be taken into when deg machine components lubricated by these oils. The loss in viscosity can be quite pronounced, and with some viscosity improvers even a permanent viscosity loss at high shear rates can occur due to the breaking up of molecules into smaller units. The viscosity loss affects the thickness of the lubricating film separating the two surfaces and hence affects the performance of the machine. SAE classification of transmission oils is very similar to that of engine oils. The only difference is that the winter grade is defined by the temperature at which the oil reaches the viscosity of 150,000 [l. This is the maximum oil viscosity which can be used without causing damage to gears. The classification also permits multigrading.
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6.6 Pour Point It is the lowest temperature at which the lubricant will flow. In paraffinic oils, it is the result of crystallization of waxy particles. In naphthenic oils it is the result of the decrease in viscosity caused by a decrease in temperature.
6.7 Flash Point It is the lowest temperature at which enough vapors are given off to cause ignition. It is an indication of the fire and explosion hazard of particular oil. Also an indication of the evaporation loses that can be expected under high temperature applications.
6.8 Fire Point It is the lowest temperature at which vapors are given off in sufficient quantity to sustain combustion.
6.9 Total Base Number The total base number of an oil is the quantity of acid, expressed in of the equivalent number of milligrams of potassium hydroxide, that is required to neutralize all basic constituents present in one gram of oil. It is worth mentioning that new and used oil can exhibit both TAN and TBN (total base number) values.
6.10 Total Acid Number/ Neutralization Number It is a measure of the acidity of the oil and is the amount in milligrams of KOH required to neutralize 1 gram of oil. The total acid number (TAN) of an oil is synonymous with neutralization number. The amount of acid in the oil was expressed in of the amount of a standard base required to neutralize a specified volume of oil. This quantity of base came to be called the neutralization number (NN) of the oil. A relative increase in the NN indicates oxidation of the oil.
6.11 Sulfated Ash The sulfated ash of a lubricating oil is the residue, in percent by weight, remaining after three processes: burning the oil, treating the initial residue with sulfuric acid, and burning the treated residue. It is a measure of the noncombustible constituents (usually metallic materials) contained in the oil. New, straight mineral lubricating oils contain essentially no ash-forming materials. Many of the additives used in lubricating oils, such as detergents, contain metallo-organic components, which will form a residue in the sulfated ash test, and so the concentration of such materials in an oil is roughly indicated by the test. Thus, during manufacture, the test gives a simple method of checking to ensure that the additives have been incorporated in approximately the correct amounts. However, since the test combines all metallic elements into a single residue, additional testing may be necessary to determine if the various metallic elements are in the oil in the correct proportions.
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Metal - %age to Sulphated Ash
Zn
1.25
Na
3.1
Mg
4.5
Ca
3.4
Ba
1.7
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7. Grease Greases are most often used instead of fluids where a lubricant is required to maintain its original position in a mechanism, especially where opportunities for frequent relubrication may be limited or economically unjustifiable. This requirement may be due to the physical configuration of the mechanism, the type of motion, the type of sealing, or to the need for the lubricant to perform all or part of any sealing function in the prevention of lubricant loss or the entrance of contaminants. Because of their essentially solid nature, greases do not perform the cooling and cleaning functions associated with the use of a fluid lubricant. With these exceptions, greases are expected to accomplish all other functions of fluid lubricants.
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7.1 Consistency/ Hardness It is the measure of a grease’s resistance to deformation by an applied force and is the most important characteristic of a grease.. It should be soft enough to allow easy application but not so soft that it leaks out of the area being lubricated. NLGI values vary from 000 (soft) to 6 (hard). Penetration Number As above
7.2 Working Temperature Range 7.3 Dropping Point It is the temperature at which the grease becomes soft enough for a drop of fluid to fall from the grease. At or above the dropping point, the grease will act as a fluid. The grease may actually break down far below the dropping point. Most manufacturers list a usable temperature range along with the dropping point.
7.4 Apparent Viscosity Grease is a non-Newtonian material that does not begin to flow until a shear stress exceeding a yield point is applied. If the shear stress is then increased further, the flow rate increases more proportionally, and the viscosity, as measured by the ratio of shear stress to shear rate, decreases. The observed viscosity of a non-Newtonian material such as grease is called its apparent viscosity. Apparent viscosity varies with both temperature and shear rate; thus, it must always be reported at a specific temperature and flow rate. Apparent viscosity is used to predict the handling and dispensing properties of a grease. In addition, it can be related to starting and running torque, in grease lubricated mechanisms, and is useful in predicting leakage tendencies.
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8. Oil Filtration
Gravity (Separation/ settling of contaminants that are heavier than oil) Centrifugal (rotation-viscosity and size determine degree of purification) Mechanical (filter-Beta Rating) Coalescence (cartridge combines small water particles into larger ones which are filtered to leave behind dry oil) Vacuum (Application of heat and vacuum 100-140F) Adsorption (Fullers Earth to remove oil oxidation products)
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9. Oil Application 9.1 Oil-Bath Lubrication The conventional oil-bath system for lubricating bearings is satisfactory for low to moderately high speed applications. Because this type of system is non-circulating, the static oil-level should never be higher than the center of the lowest positioned rolling element in the bearing being lubricated. A greater amount of oil can cause churning, increase the fluid friction within the bearing and result in excessive operating temperatures. Unless the running level of the oil is known, oil level should be checked only when equipment is shut down as the running level can drop considerably below the static level depending on the speed of the application. Because speed, sealing effectiveness, temperature and type of oil are factors that influence the refilling cycle, regular inspection is necessary to determine the frequency of refilling. Applications of this type generally employ sight gages to facilitate inspection.
9.2 Wick-Feed Lubrication Wick-feed oilers, one of the older methods of applying oil to bearings, still enjoy a certain popularity. Properly designed, applied and maintained, then are effective and inexpensive. Functioning as a filter and quantity regulator, the wick employs either capillary action, or gravity (see illustration) to transfer the oil from the reservoir to bearing. Paraffinic lubricating oils may also be used with this type oiler although they have a tendency to deposit wax crystals on the wick fibers, destroying the effectiveness of the wick. Because napthetic and synthetic oils do not exhibit this tendency, they are preferred for wick oilers.
9.3 Drip-Feed Lubrication Another one of the older methods of lubrication of oiling bearings is the drip-feed system. This system has been applied successfully to applications where moderate loads and speeds are encountered. The oil introduced through a filter-type, sight feed oiler, has a controllable flow rate which is determined by the operating temperature of the particular application.
9.4 Oil -Splash Lubrication This system of lubrication is used primarily in gear cases where the bearing and gear lubricant is common. The lubrication of bearings in a gearbox, other than one of slow speed, is usually not critical as the oil splash from gear teeth is sufficient to lubricate the bearings. Because of the constant problem of the oil carrying wear debris, the use of filters and magnetic drain plugs is helpful in reducing the possibility of wear debris contaminating the bearings. In applications where heavy oil flow or splash is encountered, bearings equipped with shields to reduce the quantity of oil reaching the bearings are sometimes necessary to prevent overheating caused by fluid friction where the bearing is flooded. In systems where normal splash or washdown is expected to be marginal, oil feeder trails should be designed into the case to direct case washdown into the bearings.
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9.5 Circulating-Oil Lubrication This type of system utilizes a circulating pump to assure a positive supply of lubricant to the bearing and can be used for low to moderately high speed and high temperature power transmission applications. The flow path of the oil in this system is important because bearing churning in a captive amount of oil can generate temperatures capable of causing lubricant breakdown and bearing damage. Due to the inherent possibility of contamination from wear debris in heavy duty applications, suitable oil filters and magnetic drain plugs are necessary to prevent damage to the bearings.
9.6 Oil-Jet Lubrication In applications where a bearing is heavily loaded and operating at high speed and temperatures, a sophisticated variation of circulating oil lubrication, called oil-jet lubrication, may be required. In such cases, it is necessary to lubricate each bearing location individually, under pressure, and to provide adequately large scavenging drains to prevent the accumulation of oil after age through the bearing. In certain high speed applications where the bearing itself creates a pumping action, the flow of oil must be adjusted to assure age through the bearing. This is extremely important where the flow of oil from the jet opposes the pumping action within the bearing.
9.7 Oil-Mist Lubrication Oil-Mist Lubrication systems are used in high-speed, continuous operation applications. This system permits close control of the amount of lubricant reaching the bearing. The oil may be metered, atomized by compressed air and mixed with air, or it may be picked up from a reservoir using a venturi effect. In either case, the air is filtered and supplied under sufficient pressure to assure adequate lubrication of the bearings. Control of this type of lubricating system is accomplished by monitoring the operating temperatures of the bearings being lubricated. The continuous age of the pressurized air and oil through the labyrinth seals used in the system prevents the entrance of contaminants from the atmosphere into the system. To insure “wetting” of the bearings and to prevent possible damage to the rolling elements and races, it is imperative that the oil-mist system be turned on for several minutes before the equipment is started. The importance of the “wetting” the bearings before starting cannot be overstressed and has particular significance for equipment that has been idle for extended periods of time. The successful operation of this type of system is based upon the following factors: Proper location of the lubricant entry ports in relation to the bearings being lubricated Avoidance of excessive pressure drops across void systems within the system The proper air pressure and oil quantity ratios to suit the particular application The adequate exhaust of the air-oil mist after lubrication has been accomplished
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10. Oil Testing Problems
Tests
Over heating =>Oxidation
Carbonyl group -------Spectroscopy Pentane Insolubles
Unusual -------------------------------------------------------
Some part is too hot
Depletion of Additives
Calorimetric, Gravimetric, Polarimetric, Potentiometric spectroscopy for wear metals
Contamination with Fuel
Fractional distillation
(Leakage, condensation, over-rich mixture)
Gas Chromatography
=>reduction in viscosity
Flash point test
Contamination with solvents
If low then flash point test
Contamination with solids
Pentane insolubles, Benzene Insolubles
Water contamination
Crackle Test >110 C
10.1 Wear Analysis A. Spectroscopy 1. Emission Oil + Electric arc from C electrode (frequency and intensity of light)
Rotode
Inductively coupled plasma (I)-----(nebula + hot argon plasma)
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B. Magnetic Plug Techniques Equipment Magnetic plug with electrodes Chip detector to indicate metal parts Electric heater to burn small fuzz If warning
Not cancelled => large chip flakes =>danger
Occurring rapidly => fine and continuous debris
Cancelled => fine but broken debris
C. Ferrography
Wear particles are deposited on the slide. The magnet below the chip retains them even when the substrate has been washed away from the chip. Stronger particles are deposited first and along the axis.
D. Rotary Particle Depositor
The oil is dropped on the centre of the magnet from where it moves radially outwards. 33
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10.2 Contaminant Warning Levels
Test
Warning Limit
Viscosity -cSt @ 40C
25% change vs. the new oil viscosity
-cSt @ 100 C
15% change vs. the new oil viscosity
Coolant
Any positive identification
Water
Greater than 0.1 %
Dilution
Greater than 5%
Insolubles
0.5% or more
Total Acid Number
More than 5 units (motor oil) or 1 unit (industrial oil)
Total Base Number
No lower than 3 to 4 units
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11. USEFUL TABLES
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12. BIBLIOGRAPHY 1. 2. 3. 4. 5.
Fundamentals of Fluid Film Lubrication- Marcel Dekker-2004. Lubrication Fundamentals-D.M.Pirro & A.A.Wessol-Exxon Mobil Corporation-2001. Mechanical Engineer’s Reference Book-E. H. Smith-Butterworth Heinemann-1994 Lubrication and Lubricant Selection-A.R.Lansdown-TIPS Series-2004 Lubrication and Maintenance of Industrial Machinery-Robert Gresham & G.E.TottenTaylor and Francis Group-2009 6. Lubricant Additives Chemistry and Application-Leslie R. Rudnick- Taylor and Francis Group-2009 7. U.S Army Lubricants-U.S. Army Corps of Engineers 8. Timken Lubrication Guide-Torrington & Fafnir 9. Petro Canada Lubricants Handbook-2005 10. Engineering Tribology-G.W.Stachowiak & A.W.Batchelor-Elsevier-1993 11. Tribology in Machine Design-T.A. Stolarski-Butterworth Heinemann-1990 12. Tribology: Friction, Wear and Lubrication-Bharat Bhushan-CRC Press-2000
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