Table 8 SURFACE TENSION OF SEVERAL BASE OILS Surface tension Liquid dyn/cm (N/m) Water 72 (× 10 –3 ) Mineral oils 30—35 (× 10 –3 ) Esters 30—35 (× 10 –3 ) Methylsilicone 20—22 (× 10 –3 ) Fluorochloro compounds 15—18 (× 10 –3 ) spinning drop apparatus. 34 Relationships in this unit are expressed in the following equation. σ = K( ρ 1 – ρ 2 ) (d) 3 / θ 2 (18) where ρ 1 = density of the heavy phase, ρ 2 = density of the light phase, d = drop width, θ = rotational speed, and K = constant characteristic of the test unit. With this unit interfacial tensions down to 10 –4 to 10 –5 dyn/cm (10 –7 to 10 –8 N/m) can be measured. The detergents and dispersants in many automotive lubricants are so effective at reducing interfacial tension that used crankcase oils contaminanted with 10 to 15% water form a stable emulsion which defies separation by techniques which do not involve distil- lation. These general techniques can be used to suspend graphite in motor oil, disperse calcium carbonate in over-based diesel lubes, or prepare a 95% water invert emulsion. Both surface tension and interfacial tension are altered by additives and by lubricant degradation. THERMAL STABILITY Thermal stability is the resistance of the lubricant to either molecular breakdown or rearrangement at elevated temperatures in the absence of oxygen. Stability in an ordinary air environment (oxidation stability) is covered in the next section. One method of measuring thermal stability involves the isoteniscope, a closed vessel with a manometer for measuring the rate of pressure increase at a specified heating rate. Thermal gravimetric and differential thermal analyses can also be used to evaluate thermal stability. Several thermal stability tests are described in Federal Specifications. 35-37 The test should allow for decomposition of a significant portion of the test sample and provide an analysis of the liquid and solid decomposition products as well as the gases formed. 37 Fluids such as mineral oils with a substantial percentage of C–C single bonds as the most vulnerable point for breakdown exhibit a thermal stability of about 650 to 700°F (343 to 371°C). Synthetic hydrocarbons prepared by a polymerization or aligomerization process and then hydrogenated involve the same basic structures as mineral oils, but exhibit a thermal stability of 50°F(28°C) or more below that of a mineral oil. In thermal breakdown, a mineral oil produces more moles of methane than of ethane and ethylene. That is, the molar quantities of the thermal decomposition product tend to decrease continuously with increasing molecular weight. A synthetic hydrocarbon made by polymerization will produce a significant quantity of the monomer from which it was made as a telltale fingerprint. Molecules containing only aromatic linkages or aromatic linkages with methyl groups as side chains show a thermal threshold of the order of 850 to 900°F (454 to 482°C). Polyphenyl ethers, chlorinated biphenyls, and condensed ring aromatic hydrocarbons fall in this category. With organic acid esters the functional group is the weak link in the molecule, and thermal stabilities range from 500 to 600°F (260 to 316°F). The presence of metals such as iron in 246 CRC Handbook of Lubrication 227-254 4/10/06 2:07 PM Page 246 Copyright © 1983 CRC Press LLC Table 9 SPECTRAL DATA OBTAINED FOR VARIOUS LUBRICANTS Extinction Extinction Wavelength coefficient Wavelength coefficient Lubricant λ (nm) ( ᐉ/g-cm) λ (nm) (ᐉ/g-cm) DEHS (orig.) ~280 NA ~220 NA DEHS (HMW) 277 7.19 219 13.69 MLO 7558 (HMW) 278 11.65 225 17.62 MLO 7219 (HMW) 275 48.47 223 73.18 MLO 7828 (HMW) 277 14.00 226 17.14 TMPTH (HMW) ~280 A ~220 A TDP (HMW) ~280 A ~220 A a nitrogen atmosphere tend to push the thermal stability limit of the common dibasic acid esters and polyol esters toward the low end of this range. An all-glass system 35 produces a thermal stability advantage for the polyol esters that is probably not reflected in use in a lubrication system. Methyl esters have thermal stability levels about the same as those of mineral oil. Polymers used as VI improvers tend to have thermal stability thresholds that are lower than smaller molecules of the same general structure. Polymethacrylates show thermal break- down at 450°F (232°C) and polybutenes at 550°F (288°C). In both cases, thermal breakdown is distinctly different from mechanical degradation. 38 Additives used for lubrication improvement tend to have thermal stability limits below those of base oils. Zinc dialkyldithiophosphates used to improve boundary lubrication prop- erties show thermal degradation at 400 to 500°F (204 to 260°C). Generally, the more active the EP additive, the lower the thermal stability threshold. OXIDATION STABILITY Stability of a lubricant in the presence of air or oxygen is commonly its most important chemical property. Unlike thermal stability, oxidation stability can be altered significantly. Additives control oxidation by attacking the hydroperoxides formed in the initial oxidation step or by breaking the chain reaction mechanism. Aromatic amines, hindered phenols, and alkyl sulfides are compounds that provide oxidation protection by one of these mechanisms. A third type of oxidation control involves metal deactivators that can keep metal surfaces and soluble metal salts from catalyzing the condensation polymerization reactions of oxidized products to produce sludge and varnish. A number of bulk oxidation tests are described in the ASTM (D2272, D1313) and Federal Test Method Standards No. 791, Method No. 5308. These tests are good for measuring stable life or the effectiveness of oxidation inhibitors. Oxygen diffusion limits the value of these tests in correlations with many actual lubrication systems. The first step in oxidation of hydrocarbons is formation of a peroxide at the most vulnerable carbon-hydrogen bonds. This initiates a free radical chain mechanism which propagates formation of hydroperoxides. Further oxidation leads to other oxygen-containing molecules such as aldehydes, ketones, alcohols, acids, and esters. A similar peroxide path of oxidation has been shown for dibasic acid esters and polyol esters. Volume II 247 Note: DEHS — di-2-ethylhexyl sebacate, HMW — high molecular weight oxidation product, NA — no absorption at this wavelength, A — absorbs at this wave- length, but extinction coefficient not reported, MLO 7558 — paraffinic white oil, MLO 7828 — naphthenic white oil, and MLO 7219 — partially hydro- genated aromatic stock. 227-254 4/10/06 2:07 PM Page 247 Copyright © 1983 CRC Press LLC To monitor the oxidation process, a microoxidation test has been developed along with analytical procedures based on gel permeation chromatography (GPC) and atomic absorption spectroscopy (AAS). 39 In these tests, oxidations were carried out until 50% or more of the 248 CRC Handbook of Lubrication FIGURE 8. Oxidation of trimethylolpropane triheptanoate at 498 K. FIGURE 9. Oxidation stability as a function of temperature. 227-254 4/10/06 2:07 PM Page 248 Copyright © 1983 CRC Press LLC Table 10 INTERNATIONAL ORGANIZATION FOR STANDARDIZATION (ISO) VISCOSITY CLASSIFICATION SYSTEM FOR INDUSTRIALFLUID LUBRICANTS Viscosity grade ranges (cSt at 40°C) ISO viscosity grade numbersMinMax 21.982.42 32.883.52 54.145.06 76.127.48 109.0011.0 1513.516.5 2219.824.2 3228.835.2 4641.450.6 6861.274.8 10090.0110 150135165 220198242 320288252 460414506 680612748 1,0009001,100 1,5001,3501,650 original base oil was oxidized. The large molecules separated by GPC are found to be rich in metal corrosion products. These large molecular size products appear to be condensation polymers with a characteristic beta keto conjugated unsaturation (–C=C–C–) which can be found in oxidation products from dibasic acid esters, polyol esters, monoesters, and mineral oils. These fluids all show oxidation products with the same general UVabsorption patterns as shown in Table 9. In Figure 8 the rates of oxidation for the same polyol ester show that a copper catalyst has an inhibiting effect, while lead and iron accelerate the primary oxidation rate. The effect of temperatures on stable life of lubricants is illustrated in Figure 9. This extrapolation system relating log of life to temperature provides a design guideline for the limiting bulk lubricant temperatures in a system. LUBRICATION SPECIFICATIONS Several widely used specifications include SAE engine oil grades, SAE gear lubrication grades, ASTM/International Organization for Standardization (ISO) grades for industrial Volume II 249 Note: The viscosity grade numbers for the ISO System are identical to those shown for the ANSI/ASTM system (ASTM D 2422, ISO 3448 — 1975). 227-254 4/10/06 2:07 PM Page 249 Copyright © 1983 CRC Press LLC 250 CRC Handbook of Lubrication Table 11 TYPICAL MILITARY SPECIFICATIONS FOR HYDRAULIC FLUIDS AND LUBRICANTS Specification designation Properties MIL-H-27601 MIL-H-83282 MIL-L-6387 MIL-L-7808 MIL-L-23699 cSt viscosit at 98.9°C (219°F) 3.2 (min) 3.5 (min) 4 5 (min) 3.0 (min) 5.0—5.5 (min-max) 54.4°C (130°F) — — 10.0 (min) — 37.8°C (100°F) — 16.5 (min) — 11.0 (min) 25.0 (min) –40°C (–40°F) 4.000 (max) 2.800 (max) 1,500 (max) — 13.000 (max) –54°C (–65°F) — — 7,500 (max) 13,000 (max) — Viscosity index 89 (min) — — 140 130 COC flash point (°C) 182 (min) 202 (min) 177 (min) 205 (min) 246 (min) Pour point (°C) –54 (max) –54 (max) –60 (max) −60 (max) –54 (max) Total acid no. 0.20 (max) 0.10 (max) 0.2 (max) — 0.05 (max) Note: MIL-H-27601 — Hydraulic fluid, petroleum base, high temperature, flight vehicle, MIL-H-83282 — Hydraulic fluid, fire resistant synthetic hydrocarbon base, aircraft, MIL-L-6387 — Lubricating oil, synthetic base, MIL-L-7808 — Lu- bricating oil, gas turbine, aircraft, and MIL-L-23699 — Lubricating oil, aircraft turbine engine, synthetic base. 227-254 4/10/06 2:07 PM Page 250 Copyright © 1983 CRC Press LLC Table 12 PHYSICALPROPERTIES OFSEVERALFLUIDS Table 13 PROPERTIES OFTYPICALSAE GRADE LUBRICANTS fluid lubricants, and military specifications. Examples of these standards and classifications are shown in Tables 10 and 11 and in pertinent chapters of Volume I. These specifications define the lubricants in terms of physical properties and in some cases, particularly the Volume II 251 Note: For automotive oil specifications, sec “Automobile Engines” and subsequent chapters in Volume I. 227-254 4/10/06 2:07 PM Page 251 Copyright © 1983 CRC Press LLC military specifications, with respect to oxidation stability, thermal behavior, and wear characteristics. General specifications for a fluid type do not imply that all fluids meeting the requirements are of equal quality. Relative quality must be determined by the ultimate user in his particular application. Asummary of some properties for several classes of fluids with potential use in the formulation of lubricants is shown in Table 12. Properties of some typical SAE grade lubricants are shown in Table 13. Characteristics of a variety of commercial lubricants are also provided in the chapter on “Lubricant Properties and Test Methods” in Volume I. NOMENCLATURE _ B = Isothermal secant bulk modulus B s = Adiabatic bulk modulus B r = Isothermal tangent bulk modulus ΔE = Energy of activation F = Force h = Planck’s constant L = Length ᐉ = Depth N = Rotational speed N _ = Avagadro’s No. n = Power law index n D 20 = Refractive index P = Pressure R = Gas constant r = Radius T = Temperature t = Time V = Volume V _ = Molecular volume VI = Viscosity index α = Viscosity-pressure coefficient γ = Shear rate η = Viscosity in centipoise θ = Angle v = Viscosity in centistokes ρ = Fluid density α = Surface tension; interfacial tension τ = Torque ω = Angular velocity 252 CRC Handbook of Lubrication 227-254 4/10/06 2:07 PM Page 252 Copyright © 1983 CRC Press LLC REFERENCES 1. Fredrickson, A. G., Principles and Applications of Rheolagy, Prentice-Hall, Englewood Cliffs, N.J., 1964, 118. 2. Fenske, M. R., Klaus, E. E., and Dannenbrink, R, W., The comparison of viscosity-shear data obtained with the Kingsbury tapered plug viscometer and the PRL high shear capillary viscometer. Special Tech. Publ. No. 111, Symposium on Methods of Measuring Viscosity at High Rates of Shear, Tech. Publ. 111, American Society for Testing and Materials, Philadelphia, Pa., 1950, 45. 3. Gerrard, J. E., Steidler, F. E., and Appeldoorn, J. K., Viscous healing in capillaries, Ind. Eng. Chem. Found., 4, 332, 1965; 5, 260, 1966. 4. Ewell, R. H. and Eyring, H. J., Chem. Phys., 5, 726, 1937. 5. Fresco, G. P., Klaus, E. E., and Tewksburg, E. J., Measurement and prediction of viscosity-pressure characteristics of liquids, J. Lubr. Tech., Trans. ASME, 91, 454, 1969. 6. Kuss, E., The Viscosities of 50 Lubricating Oils Under Pressures up to 2000 Atmospheres, Rep. No. 17 on Sponsored Res., (Germany), Department of Scientific and Industrial Research, London, 1951. 7. ASME, Pressure-Viscosity Report, American Society of Mechanical Engineers, New York, 1953. 8. Klaus, E. E., Johnson, R. H., and Fresco, G. P., Development of a precision capillary-type pressure viscometer, ASLE Trans., 9, 113, 1966. 9. Kim, H. W., Viscosity-Pressure Studies of Polymer Solutions, Ph.D. thesis, Pennslyvania State University, University Park, Pa., 1970. 10. So, B. Y. C. and Klaus, E. E., Viscosity-pressure correlation of liquids, ASLE Trans., 23, 409, 1980. 11. Jones, W. R., Johnson, R. L., Sanborn, D. M., and Winer, W. O., Viscosity-pressure measurements for several lubricants to 5.5 × 10 8 N/m 2 (8 × 10 4 psi), and 149°C (300°F). Trans. ASLE, 18, 249, 1975. 12. Novak, J. and Winer, W. O., Some measurements of high pressure lubricant rheology, J. Lubr. Technol. Trans. ASME, 90, 580, 1968. 13. Jakobsen, J., Sanborn, D. M., and Winer, W. O., Pressure-viscosity characteristics of a series of siloxanes, J. Lubr. Technol., Trans. ASME, 96, 410, 1974. 14. Appledoorn, J. K., Okrent, E. H., and Philippoff, W., Viscosity and elasticity at high pressures and high shear rates, Proc. Am. Pet. Inst., 42(3), 1962. 15. Foord, C. A., Wedeven, L. D., Westlake, F. J., and Cameron, A., Optical elastohydrodynamics, Proc. Inst. Mech. Eng., 184, 487, 1969/1970. 16. Nagaraj, H. S., Sanborn, D. M., and Winer, W. O., Surface temperature measurements in rolling and sliding EHD contacts, ASLE Trans., 22, 277, 1979. 17. Nagaraj, H. S., Sanborn, D. M., and Winer, W. O., Direct surface temperature measurements by infrared radiation in EHD, and the correlation of the Blok flash temperature theory, Wear, 49, 43, 1978. 18. API, Technical Data Book — Petroleum Refining, 3rd ed., American Petroleum Institute, Washington, D.C., 1977. 19. Johnston, W. G., A method to calculate the pressure-viscosity coefficient from bulk properties of lubricants, ASLE Trans., 24, 232, 1981. 20. Alsaad, M., Bair, S., Sanborn, D. M., and Winer, W. O., Glass transitions in lubricants: its relation to EHD lubrication, J. Lubr. Technol. Trans. ASME, 100, 404, 1978. 21. Bair, S. and Winer, W. O., Shear strength measurements of lubricants at high pressure, J. Lubr. Technol., Trans. ASME, 101, 251, 1979. 22. Dubois, G. B., Ocvirk, F. W., and Wehe, R. L., Natl. Advisory Committee for Aeronautics, Contract No. NAw6197, Prog. Rep. 9 (revised), August 1953. 23. Klaus, E. E. and Duda, J. L., Effect of Cavitation on Fluid Stability in Polymer-Thickened Fluids and Lubricants, Sp. Publ. 394, U.S. National Bureau of Standards. Washington, D.C., 1974, 88. 24. Bhatia, R., Mechanical Shear Stability and Blending Efficiency of Polymers in Lubricant Formulations, M.S. thesis, Pennsylvania State University, University Park, Pa., 1978. 25. Myers, H. S., Jr., Volatility Characteristics of High-Boiling Hydrocarbons, Ph.D. thesis, Pennsylvania State University, University Park, Pa., 1952. 26. Beerbower, A. and Zudkevitch, D., Predicting the evaporation behavior of lubricants in the space en- vironment, ACS Meet. 8, C-99, Div. Pet. Chem., American Chemical Society, Los Angeles, April 1963, preprint. 27. Klaus, E. E. and Bieber, H. E., Effects of some physical and chemical properties of lubricants on boundary lubrication, ASLE Trans., 7, 1, 1964. 28. Fein, R. S., Chemistry in concentrated-conjunction lubrication, in An Interdisciplinary Approach to the Lubrication of Concentrated Contacts, National Aeronautics and Space Administration, Washington, D.C., 1970, chap. 12. 29. Maxwell, J. B., Data Book on Hydrocarbons, D Van Nostrand, New York, 1950. Volume II 253 227-254 4/10/06 2:07 PM Page 253 Copyright © 1983 CRC Press LLC 30. Klaus, E. E. and O’Brien, J. A., Precision measurement and prediction of bulk-modulus values for fluids and lubricants, J. Basic Eng., ASME Trans., 86 (D-3), 469, 1964. 31. Wright, W. A., Prediction of bulk moduli and pressure-volume-temperature data for petroleum oils, ASLE Trans., 10, 349, 1967. 32. Wilkinson, E. L., Jr., Measurement and Prediction of Gas Solubilities in Liquids. M.S. thesis, Pennslyvania State University, University Park, Pa., 1971. 33. Beerbower, A., Estimating the solubility of gases in petroleum and synthetic lubricants, ASLE Trans., 23, 335, 1980. 34. Cayias, J. L., Wade, W. H., and Schecter, R. S., The measurement of low interfacial tension via the spinning drop techniques, Adsorption at Interfaces, ACS Symp. Ser. No. 8, American Chemical Society, Washington, D.C., 1975. 35. Military Specification, MIL-L-23699B, Lubricating Oil, Aircraft Turbine Engine, Synthetic Base, U.S. Department of Defense, Washington, D.C., 1969. 36. Federal Test Method Standards No. 791, Lubricants, Liquid Fuel, and Related Products; Methods of Testing, U.S. Bureau of Standards, Washington, D.C., 1974. 37. Military Specification MIL-H-27601A (USAF), Hydraulic Fluid, Petroleum Base, High Temperature, Flight Vehicle, U.S. Department of Defense, Washington, D.C., 1966. 38. Klaus, E. E., Tweksbury, E. J., Jolie, R. M., Lloyd, W. A., and Manning, R. E., Effect of Some High Energy Sources on Polymer-Thickened Lubricants. Spec. Tech. Publ. No. 382, American Society for Testing and Materials, Philadelphia, Pa., 1965, 45. 39. Lockwood, F. E. and Klaus, E. E., Ester oxidation under simulated boundary lubrication conditions, ASLE Trans., 24, 276, 1981. 254 CRC Handbook of Lubrication 227-254 4/10/06 2:07 PM Page 254 Copyright © 1983 CRC Press LLC [...]... texture and a dropping point of 375 to 425°F (19 1 to 218 °C) At lower temperatures this grease has reasonably good water resistance Copyright © 19 83 CRC Press LLC 255-268 4 /11 /06 11 :58 AM Page 259 Volume II 259 Aluminum Soap Greases These are manufactured by mixing dry, powdered aluminum stearate in the base oil as the temperature is increased to the range of 240 to350°F (11 6 to 17 7°C) Cooling is followed... High-quality products have no serious deficiencies in any of these applications except for very severe extremes of temperature, speed, loads, and pressures They have given good service in journal and antifriction bearings Copyright © 19 83 CRC Press LLC 255-268 4 /11 /06 258 11 :58 AM Page 258 CRC Handbook of Lubrication contact with food To meet this need, calcium 12 -hydroxystearate was developed It does not use... thickeners Oxidation is commonly limited by the base fluid While rust protection is poor, they are water resistant and in manufacture respond well to inhibition Copyright © 19 83 CRC Press LLC 255-268 4 /11 /06 260 11 :58 AM Page 260 CRC Handbook of Lubrication OIL PHASE In grease, the lubricating petroleum oil or synthetic fluid is the main component and makes a most important contribution in structure, performance,... blends Dyes Dyes are nonperformance additives that may be used to identify grease, improve color, * Trademark for high molecular weight iso-butylene polymer Copyright © 19 83 CRC Press LLC 255-268 4 /11 /06 264 11 :58 AM Page 264 CRC Handbook of Lubrication and mask slight variations between batches They may camouflage changes in color that do not detract from performance SELECTION FACTORS Performance characteristics... REFERENCES 1 Boner, C J., Additives used in lubricating greases, in Standard Handbook of Lubricating Engineering, O’Connor, J J and Boyd, J., McGraw-Hill, New York, l968, 11 2 Rebuck, N D., Naval Air Development Center, private communication, March, 19 81 ADDITIONAL REFERENCES 3 Boner, C J., Manufacture and Application of Lubricating Greases, Hafner Publ Co., New York 4 Bailey, C A and Aarons, J S., The Lubrication. .. separation of a limited amount of oil is not harmful If none separated, lubricant starvation could result In some cases separation permits lubricant to creep into narrow clearances by capillary action where soap won’t go Pressure and vibration promotes separation; this may cause trouble with soap plugging in central lube systems and pressure grease cups Copyright © 19 83 CRC Press LLC 255-268 4 /11 /06 266 11 :58... lubricants, indicates areas of application in Table 1, discusses their particular advantages and limitations, and shows how they can be used in the four main ways which are outlined in Table 2 Reviews of solid lubricants are also given in References 1 to 5 Discussions of the nature and influence of surface films, boundary lubrication, and wear mechanisms are covered in earlier handbook chapters Many factors...255-268 4 /11 /06 11 :58 AM Page 255 Volume II 255 LUBRICATING GREASES — CHARACTERISTICS AND SELECTION I W Ruge GREASES Grease is a semisolid lubricant consisting essentially of a liquid mixed with a thickener; the liquid does the lubricating, the thickener primarily holds the oil in place and provides varying resistance to flow It may be hard enough to cut into blocks, or soft enough to pour... Press LLC 255-268 4 /11 /06 266 11 :58 AM Page 266 CRC Handbook of Lubrication viscosity of the grease can be very high Once motion begins and the rate of shear is increased, the apparent viscosity of grease approaches, but never reaches, the viscosity of the fluid component In industrial applications, the apparent viscosity is useful in predicting: 1 2 3 4 How grease might actually perform in a bearing... This is known as the weld-point Copyright © 19 83 CRC Press LLC 255-268 4 /11 /06 11 :58 AM Page 267 Volume II 267 A variation in these methods utilizing similar specimens (one ball rotating against a nest of three), but not under the high load conditions, is the Four-Ball Wear Tester (ASTM D2266) It measures relative wear performance of grease on the steel-on-steel balls by measuring scar diameters rather . numbersMinMax 21. 982.42 32.883.52 54 .14 5.06 76 .12 7.48 10 9.0 011 .0 15 13. 516 .5 2 219 .824.2 3228.835.2 46 41. 450.6 68 61. 274.8 10 090. 011 0 15 013 516 5 22 019 8242 320288252 460 414 506 680 612 748 1, 00090 01, 100 1, 50 01, 35 01, 650 original. point of 375 to 425°F (19 1 to 218 °C). At lower temperatures this grease has reasonably good water resistance. 258 CRC Handbook of Lubrication 255-268 4 /11 /06 11 :58 AM Page 258 Copyright © 19 83. Special Tech. Publ. No. 11 1, Symposium on Methods of Measuring Viscosity at High Rates of Shear, Tech. Publ. 11 1, American Society for Testing and Materials, Philadelphia, Pa., 19 50, 45. 3. Gerrard,