Handbook of Lubrication Episode 1 Part 11 pptx

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
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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.
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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.
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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).
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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.
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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.
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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
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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
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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
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Copyright © 1983 CRC Press LLC
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
through a funnel.
THICKENERS
Variations in grease characteristics are largely determined by the material used to thicken
it. If the thickener can withstand heat, the grease will be usable at high temperatures. If the
thickener is unaffected by water, the grease will be also. The many different kinds of
thickeners used in commercial greases can be divided into two primary classes: soap and
nonsoap. Table 1 describes properties of greases with various thickeners.
Asoap is a metallic element reacted with fat or fatty acid. Metallic elements include
lithium, calcium, sodium, aluminum, barium, and others. Fats and fatty oils may be animal
or vegetable in origin, ranging from cattle, hog, fish, castor bean, coconut, cottonseed, or
flaxseed. Choice of these and reaction conditions offer a wide variety of soaps, and control
the characteristics of grease.
Among soap type greases, lithium accounts for the majority used in the U.S., followed
by calcium, aluminum, sodium, and others (mainly barium). Soap-type grease production

in 1981 were approximately 80% of the total, nonsoap thickeners accounted for about 10%
(see Table 2).
Lithium Soap Greases
Development of lithium greases on a large scale got its start just before and during World
War II. They can be made by virtually any conventional compounding procedure with no
unusual problems. Current products may be divided into those using 12-hydroxystearate and
those using organic acid radicals.
Lithium 12-hydroxystearate soaps can generally be dispersed at temperatures around 200°F
(93°C), while most of the other lithium soaps require temperatures in the range of 400°F
(204°C) or more. A large variation in fiber structure and grease properties results from using
soaps derived from organic acid compounds.
After soap dispersion, lithium greases are cooled in various ways. They can be transferred
to shallow pans, but more common methods include circulating through coolers, or slow
cooling and stirring in the kettle. Slow cooling, with or without stirring, will form long
fibers, increase mechanical stability and increase the bleeding tendency. Rapid, or quench
cooling yields fine fibers which result in poor mechanical stability but good oil retention.
Ideally, a grease should combine a variety of fibers to give a good compromise in stability
and bleeding. Milling (subjecting the product to very high shear) as a finishing touch is
desirable because it gives a more uniform smooth texture which is less likely to change in
consistency during service.
When properly formulated, lithium-base greases are very acceptable as multipurpose
products for automotive and industrial requirements. 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
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contact with food. To meet this need, calcium 12-hydroxystearate was developed. It does
not use water to stabilize the structure, so the maximum operating temperature ranges up
to 250°F (121°C). Some satisfactory services have been reported at temperatures near the

dropping point, which varies from 280 to 300°F (138 to 149°C), depending on formulation.
These products approach multipurpose greases in their temperature capability, but adhesion
and retention can be poor in some types of service because of the extremely smooth buttery
texture. Special formulations which provide unique characteristics and advantages have been
developed. Zinc oxide is frequently included when the grease is used in the food industry,
but the base oil must also be acceptable for incidental contact with food.
Calcium Complex Greases
Calcium soaps modified by a small quantity of calcium acetate provide characteristics
quite different from the normal water-stabilized product. Stability is usually improved, and
extreme pressure (EP) characteristics are enhanced without the use of additives. Other
complex calcium soaps have been developed, but the salts of acetic acid have been the
preferred complexing agent for performance improvement. Calcium complex greases fre-
quently perform under conditions that cannot be handled by any other soap-thickened product.
There have been, however, unsuccessful incidents. Bearing failures can result from excessive
thickening in use.
Sodium Soap Greases
These were originally developed to provide a higher service temperature limit than was
possible with conventional calcium soap. Even with the poor water resistance of most sodium
greases, they are still popular for some demanding applications in lubricating electric motor
bearings, particularly those with effective seals. Lighter grades are used in textile plants
since leaked product is easily removed from cloth in the normal washing process. Although
sodium base greases were the accepted “standard” in automotive wheel bearings for many
years, they are now being replaced by high-quality multipurpose greases with much better
water resistance.
Sodium greases are manufactured in the same type of equipment used for most types of
soap-thickened grease, but adequate kettle volume should be provided because of a tendency
to foam during early processing steps. The type of fatty acid or glyceride used is important
in determining characteristics of the finished product. Glycerine from the glyceride stabilizes
the grease, but reduces oxidation stability. Small amounts of water and/or glycerine are
common modifiers to control fiber structure and stability. The goal is to produce fibers as

thin as possible for good shear stability.
Base oil selected depends on the end use. As usual, viscosity is the important consideration.
With a high-viscosity index, medium to light viscosity base oil, sodium base greases can
equal or surpass low-temperature performance of calcium or lithium greases. However, high-
viscosity oils give rise to variations in other grease characteristics.
Sodium soap greases for ball and roller bearings usually contain oxidation inhibitors. EP
additives such as sulfurized fatty oil, organic sulfur, and/or chlorine compounds will improve
load-carrying ability. Tackiness additives are seldom needed, and rust inhibition is not
normally required.
When sodium greases are mixed with other type greases, particularly with calcium type,
the combination can become soupy. Despite this apparent incompatibility, there are some
anomalies. Sodium-calcium soap mixed base greases are manufactured by special reactions
and used in wheel bearings and in ball and roller bearing service. The calcium is claimed
to shorten the fiber. Sodium-aluminum greases have been used in many industrial appli-
cations. The aluminum modifies the structure to give a smooth texture and a dropping point
of 375 to 425°F (191 to 218°C). At lower temperatures this grease has reasonably good
water resistance.
258 CRC Handbook of Lubrication
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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(116 to 177°C). Cooling is followed
by mild working prior to packaging. Care must be taken to avoid air bubbles which would
spoil the clear, translucent appearance of the grease. Various modifiers may be added during
the manufacture to change the structure, increase plasticity, or increase stringiness.
Aluminum base greases have a smooth texture and are water resistant. Their shear stability
is generally poor, however, and when exposed to temperatures above about 170°F (77°C)
the normal smooth structure becomes rubbery. In the rubber-like state, the grease pulls away
from metal and ceases to lubricate. They also tend to aerate badly when severely agitated

or churned.
Barium Soap Greases
These are among the least understood soap-oil systems, but they have unique and valuable
characteristics. Even impurities can affect the structure and performance. Typically, soap
content to achieve a given consistency is very high.
Barium soap greases were among the first multipurpose products with both high temper-
ature capability and good water resistance. They are frequently used to lubricate electrical
cables for power transmission lines. Wind, temperature, and current surges cause flexing
and stretching between the conductors and the sheath.
Nonsoap Thickeners
These fall into separate classifications: inorganic, organic, and “synthetic” materials.
Inorganic thickeners are very fine powders which have enough surface area and porosity to
thicken by absorbing oil. Silica and modified bentonite clay have been the most
successful commercially. Both types are very sensitive to water unless the thickener particles
are protected by a coating, which may break down at 300°F (149°C).
While clay-thickened greases usually are manufactured without the need for the hot cooking
or reaction cycle required for soaps, high-shear mechanical mixing or milling is needed.
Frequently dispersing aids are required during formulation; these aids are volatile and evap-
orate during processing.
Since these greases have no melting point, maximum continuous service temperatures
depend on the oxidation stability of the base oil and its inhibitor treatment. Rapid deterioration
is sometimes encountered with continued exposure of mineral oil-based grease to temper-
atures of 250°F (121°C). When properly inhibited against oxidation, however, inorganic-
thickened greases are successfully used in many high-temperature applications.
Greases of this type are considered multipurpose lubricants. Industrially, they have been
widely used in rolling contact bearings operating at moderate speeds and temperatures. In
automotive service they have been used as general purpose greases, but performance in
wheel bearings has varied widely.
Organic thickeners such as amides, anilides, arylureas, dyes and synthetic products used
in commercial products are also superior to soap based greases in high temperature appli-

cations. These thickeners are chemically and structurally stable over a wide temperature
range and do not act as catalysts for the oxidation of the base oil. Current use is mainly for
high temperature ball bearing greases and special synthetic greases for military and aerospace
use.
Generally, they have dropping points above 500°F (260°C) and are serviceable down to
very low temperatures. The oxidation rate at all temperatures is typically lower than for
greases prepared with other 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.
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OILPHASE
In grease, the lubricating petroleum oil or synthetic fluid is the main component and
makes a most important contribution in structure, performance, and stability.
Petroleum Oils
Petroleum oils used as lubricants in grease vary widely in type. Various crude sources
and refining methods result in differing oil characteristics relating to hydrocarbon types.
Naphthenic oils are most common despite their relatively low viscosity index. Their low-
temperature fluidity and ability to combine readily with soap contributes to their wide use.
Abase oil viscosity range of 65 to 175 cSt at 40°C (approximately 300 to 800 SUS at 100°F)
is the most widely used. Greases for low-temperature or high-speed use may have lower
viscosity base oils while greases used for particularly low speed, high loads, and shock
loading will be higher in viscosity.
Viscosity, viscosity index, and chemical characteristics are each important. Low-tem-
perature pumpability and handling ease are mostly a result of properly selected base oil
viscosity. Viscosity is usually more significant than pour point. Viscosity-temperature re-
lationship of the base oil is important where service conditions involve a wide temperature
range. Load-carrying ability at moderate to high shear rates is mainly due to base oil viscosity,
particularly in the absence of EPadditives.

The two main classes of petroleum base oils, paraffinic and naphthenic, have different
effects on thickeners. With some soaps, the gel structure is weaker with paraffinic oils. Due
to the relative stability of paraffinic oils, they are less likely to react chemically during
grease formation. Paraffinic oils are poorer solvents for many additives used in greases.
Naphthenic oils, particularly when some unsaturates are present, can function chemically
during manufacture. When controlled, this can be advantageous and explains in part why
naphthenic base oils are popular.
Synthetic Fluids
These have proven to be particularly well suited for extreme conditions. Among soap-
type greases, probably more synthetic fluids are thickened with lithium soap than any other.
Tailoring the desired characteristics of the grease can only be achieved by careful selection
of both thickener and fluid. Synthetic fluid greases are normally designed for improved
performance in some extreme temperature range, either high or low. By suitable compromise,
reasonable service performance can be achieved over wide ranges of temperature. Products
of this type find their greatest application in high-performance aircraft, missiles, and space
vehicles. When thickeners and fluids are both synthetic, use is almost exclusively in such
high-performance equipment. For some missile applications, a service life of minutes or
less might be adequate. Avariety of synthetic military specification greases and their tem-
perature ranges are indicated in Table 3.
ADDITIVES AND FILLERS
Additives of the types indicated in Table 4 are often needed to augment or improve
performance, or meet special needs. Some modify soaps; others enhance natural character-
istics of the oil, give it longer life, or improve its ability to protect equipment.
Antioxidants
Antioxidants (oxidation inhibitors) are the most common additives and must be selected
to match the individual grease. The original objective of inhibitors was to protect the grease
during storage prior to use. Most multipurpose greases and greases designed for high tem-
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Tackiness Additives
Products such as Paratac* are sometimes added to increase adhesion and cohesion, impart
stringy texture, and give better bearing retention. The latter characteristic is also occasionally
improved by polyethylene. When used with lower viscosity base oils (which may be desired
for low temperature performance), tackiness agents give the appearance and structure of
higher viscosity base oils.
Extreme Pressure (EP) Additives
EP additives provide improved load-carrying ability and give added protection under shock
loads. In most rolling contact bearings other than tapered rollers, EP characteristics have
little beneficial effect. In tapered roller bearings where thrust loads are high, EP additives
are often needed to prevent scoring and wear of roller ends. In other extreme pressure
conditions, the EP agents react with steel surfaces to form a surface or interface which will
act like a solid lubricant and prevent metal-to-metal contact or welding.
EP additives for greases generally contain various combinations of sulfur and phosphorus.
They are similar, if not identical, to those used in industrial oils and gear lubes.
Fillers
Fillers are usually, but not always, fine micron-size solids employed for special functions.
As a rule of thumb, particles less than one half the size of the bearing clearance give little
trouble with wear, but rolling element bearing clearance may be as low as 0.0001 in.
Nevertheless, a proper filler can improve grease performance under adverse conditions. In
food processing and food canning, an alkaline filler helps protect bearings against the
corrosive action of food acids. In some applications, the cosmetic effect of the color or
pleasing appearance may be more important than any change in performance.
There are decreasing numbers of fillers in common use. Ranging from popular to almost
extinct are graphite, molybdenum disulfide, metal oxides, metal flakes, carbon black, talc,
and mica. Another material that might be classed as a filler is yarn.
If enough is present, graphite can minimize metal-to-metal contact and wear of sliding
surface bearings. Artificial graphite is most often used because it is free of abrasive impurities.
But even pure graphite deposits tend to promote wear in rolling element bearings, so graphite
grease should usually be avoided in them. Molybdenum disulfide (Moly) is popular as a

filler in lubricating greases. While the advantages at low concentrations are open to question,
higher concentrations of 3% and over can be demonstrated to provide a protective film when
grease lubrication is difficult to maintain.
Zinc and magnesium oxides have been used in food processing industries. The light color
and ability to neutralize acid are the main advantages. For best performance, these oxides
must be dispersed in the grease by milling.
Metal flakes and powdered soft metals such as lead, tin, zinc, and aluminum are used in
pipe thread and antiseize compounds. These compounds typically are greases with a high-
filler content. At one time, talc was widely used as a die and drawing lubricant, and in roll
neck lubricants for older type mill stands. Carbon black, because of its fineness, also must
be vigorously milled to get complete dispersion. Although usually acting only as fillers,
some carbon blacks have thickening characteristics.
Wool yarn is still used, mostly in large open bearing boxes. It is exactly what its name
implies: wool yarn cut into short lengths to help give structural depth to grease. It is
occasionally blended with curled horsehair, but avoid synthetic fibers in blends.
Dyes
Dyes are nonperformance additives that may be used to identify grease, improve color,
* Trademark for high molecular weight iso-butylene polymer.
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and mask slight variations between batches. They may camouflage changes in color that do
not detract from performance.
SELECTION FACTORS
Performance characteristics used in selecting and specifying greases are best described
by commonly used test procedures. These tests use finished greases, but base oil viscosities
are often included in specifications because of their obvious importance. Abrief description
of the American Society for Testing and Materials (ASTM) test procedures for grease follows.
Penetration (ASTM D-217, ASTM D-1403)
This test is an arbitrary measure of hardness, consistency, or shear strength. It is defined

as the depth in tenths of a millimeter that a precisely standardized cone penetrates the grease
in a specified cup under prescribed conditions of time and temperature.
Greases are graded by the National Lubricating Grease Institute (NLGI) classifications
based on ASTM penetration, see Table 5. Note that there is only a 30-point spread for each
NLGI grade, with a 15-point gap between each grade. The 30-point spread in a given grade
may often make minor variations in penetration appear unduly important. In selecting a
suitable grease, penetration is but one of the many factors which must be considered. The
correlation between penetration and pumpability is so poor that penetration cannot be used
for predicting the behavior of a grease under various temperature conditions. As Table 5
indicates, the grading system separates the lubricants into two groups, then classifies them
into two semifluid grades and from soft to hard in six grades.
Grades 000 and 00 are most frequently used in relatively low-speed gear boxes, usually
larger ones with rudimentary or inefficient seals. These grades are not appropriate for grease-
lubricated antifriction bearings because they would leak. Antifriction bearings are most
frequently lubricated with #2 grease because of ideal oil release and good feeding ability.
Grades 0 and 1 are most suited for central lube systems with lengthy tubing runs. Number
3 grease may be better for large rolling element bearings which hold substantial quantities
of grease. Harder greases may be used on large open gears or large shaft bushings in which
a block of grease is resting on the rotating element. Greases harder than #3 constitute a
very minor proportion of lubricating greases.
Dropping Point (ASTM D-566)
This is the temperature at which a drop of fluid forms and falls from a grease under test
conditions established by ASTM. Although dropping points are poor predictors of service
performance of lubricating greases, they do represent a limiting temperature for most ap-
plications. The limiting temperature for prolonged exposure is well below the dropping point.
This is particularly true of products containing volatile components or additives. A common
example is straight calcium grease that is stabilized by a small amount of water. While the
dropping point is below 200°F (93°C), water loss can become a problem at about 125°F
(52°C) and a serious one above 150°F (66°C).
Bleeding (ASTM D-1742)

Bleeding is a characteristic tested mainly for evaluation, not as a batch or shipment
acceptance. Practically all greases will separate oil under certain conditions. In most appli-
cations, the 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 sep-
aration; this may cause trouble with soap plugging in central lube systems and pressure
grease cups.
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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. How grease might actually perform in a bearing
2. Leakage tendency from a journal bearing
3. Performance at low temperature
4. Pumpability and flowability
Unfortunately, apparent viscosity data from a pressure viscosimeter are of little value in
predicting performance of the grease at high temperatures.
Mechanical Stability
These tests also find their way into specifications. The Grease Worker used in conjunction
with penetration testing (ASTM D-217) involves the penetration cup which contains about
a pound of grease. A sturdy disc with 51 holes is forced through the grease in the closed
cup for a specified number of strokes. Changes in penetration from unworked grease are
recorded. Some may soften, some harden.
Wheel Bearing Test (ASTM D-1263)
This test involves placing a weighed amount of grease, properly packed into an automotive
type front wheel hub and spindle. Instead of installing a seal, the large end of the hub is
encircled by a trough or collector ring. The assembly is placed in an enclosed box equipped

with heaters. It is belt driven by an electric motor outside the box. Temperature, time, and
speed are specified. The grease that leaves the hub is slung into the ring; its weight is
recorded for comparison with performance parameters.
Roll Stability Test (ASTM D-1831)
This test, sometimes referred to as the Shell roll test, is also used in conjunction with the
penetration test, which is run before and after a prescribed number of hours. A small sample
of grease is weighed into a cylinder. A weighted steel roller whose outside diameter is
about two thirds the inside diameter of the cylinder is inserted after the grease is distributed
in the cylinder. The cylinder is rolled on a horizontal axis for a timed interval (usually 2
hr), starting at essentially room temperature. The difference in penetration before and after
rolling is considered to be a measure of shear stability.
Extreme Pressure
These tests are frequently seen in specifications, although correlation with actual service
is sometimes lacking.
Timken Test (ASTM D-2509)
The Timken test involves rotating a bearing race against a fixed block at a prescribed
speed under increasing load while a constant flow of grease is maintained. Results, expressed
as pounds lever load, will differentiate between low, medium, or high levels of extreme
pressure performance.
Four-Ball Methods (ASTM D-2596, ASTM D-2783)
These tests, likewise, do not necessarily correlate with service results. In these procedures,
one steel ball is rotated at a controlled speed against a nest of three stationary steel balls of
identical size. Load is increased incrementally until seizure occurs. This is known as the
weld-point.
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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 D-
2266). It measures relative wear performance of grease on the steel-on-steel balls by meas-

uring scar diameters rather than measuring the load at seizure.
Falex Tests (ASTM D-3233)
These tests consist of determining load carrying limits as indicated by shear pin breakage.
A small journal is rotated between two vee blocks which are clamped against the journal
and immersed in the lubricant. Load is steadily increased by clamp levers. Performance is
measured in pounds force when the shear pin fails. A variation of the method consists of
increasing the load incrementally, maintaining a constant load for 1 min at each step. Load
at the time the shear pin breaks is also the criteria in this procedure.
Other Selection Factors
Numerous in-house and special tests are performed for qualification and product accept-
ance. Among better known performance tests is the Lincoln ventmeter which measures
pumpability and ventability of greases in a central system. It consists of a coil of
1
/
4
in.
tubing with a pressure gage at one end, a grease fitting at the other, and valves at both ends.
The tubing is filled with grease at a high pressure, and the entire system is allowed to
stabilize to a desired temperature. The venting valve is opened for 30 sec and the pressure
is read. This gives a clue about relative pumpability, and helps to determine either NLGI
grade or type of grease that will function, or to select pipe size for the central system.
COMMENTS
All the expertise that goes into the development, manufacture, selection, and application
of the grease does not necessarily assure performance. One other most important consid-
eration is cleanliness. Contaminated grease is faulty grease. Keep containers intact, closed,
stored in a clean, dry place. Be sure fittings, plugs, and nozzles are clean before application.
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, 1981.

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 Engineers Manual, 1st ed., United States Steel, Pitts-
burgh, Pa.,
5. Technical Bulletin, Lubrication, Texaco Inc., Houston.
6. Quigg, J. S., Grease survey, paper presented at 49th Ann. Meet. Natl. Lubr. Grease Inst., Hilton Head,
South Carolina, 1982.
7. Witco. Inc., Target Talk and Technical Letter, Southwest Petro-Chem., Inc., Wichita, Kan.
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SOLID LUBRICANTS
J. K. Lancaster
INTRODUCTION
Asolid lubricant is “any material used as a powder or a thin film on a surface to provide
protection from damage during relative movement and to reduce friction and wear.” This
chapter describes the various materials currently being used as solid 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 influence friction and wear of polymers and have
been reviewed elsewhere.
6,7
MATERIALS
Important features needed in a solid lubricant are tabulated in Table 3. The extent to
which all of these can be realized in various groups of materials is covered in the following
discussion.
Lamellar Solids
These materials crystallize with a layered structure in which interatomic bonding between

the layers is weaker than that within them. Graphite and MoS
2
are the best known examples.
Graphite and MoS
2
Bragg first suggested in 1923 that the low friction of graphite, typically 0.05 to 0.1, might
be a consequence of its lamellar structure with easy shear occurring between basal planes.
Subsequently, low friction was found only in environments containing water or other con-
densable vapors and interlamellar bonding in vacuum was greater than that attributable to
weak Van der Waals forces alone. These forces are now believed to be supplemented by
π-electron interactions and the intrinsic interlamellar shear strength of graphite is thus not
particularly low. Most recent ideas attribute the low friction of graphite to its basal planes
being low energy surfaces.
8
During sliding the basal planes orient themselves almost parallel
to the surface and adhesion between them is very low. However, once a basal plane becomes
damaged, high-energy edge sites are exposed and adhesion and friction increase appreciably
unless these edge sites can be neutralized by adsorbed vapors.
Thermal stability of graphite is extremely high (>2000°C), but its use in ordinary envi-
ronments is limited by onset of oxidation in the 500 to 600°C range; the greater the degree
of graphitic order in the crystallites, the higher the oxidation temperature. The necessity for
adsorbed vapors to maintain low friction can restrict the use of graphite to much lower
temperatures. In air, the amount of physically adsorbed water may decrease at around 100°C
to such an extent that low friction can no longer be maintained. Organic vapors are very
effective substitutes for water and may be available as contaminants in the surrounding
environment, or be derived from organic material introduced deliberately or accidentally
into the graphite itself. Some added inorganic compounds are also able to extend the tem-
perature range over which low friction occurs. PbO, CdO, Na
2
SO

4
, and CdSO
4
have all
been shown to be effective on nickel alloy substrates to around 550°C.
9
Although graphite continues to play a major role in metal working lubrication and in
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