Chapter 18 Lubricating Grease

521

Chapter 18 LUBRICATING GREASE

In its general sense grease refers to a fatty solid, firm enough to hold its shape under its own weight but soft enough to flow under a low stress. These two characteristics determine what is known as the consistency of the grease. The consistency of a grease is the basis of its special utility as a lubricant and enables it to be applied to locations which cannot be supplied by a flow of liquid. In earlier times lubricating greases were actually solid fats, but modern greases are basically made up of a liquid oil and a solid thickener or gellant. The systematic study of the structure and the rheology of greases The falls mostly in the domain of colloid and physical chemistry. manufacture of greases is adequately treated in specialized books. A thorough treatment of greases in lubrication technology would require a None of these objectives can be specialized monograph of its own. achieved in the space of the single chapter t o follow, but the fundamental principles governing grease structure and the relations between structure and the utilization of greases are known well enough to be dealt with informatively. 18.1.

BASIC ASPECTS OF LUBRICATING GREASE STRUCTURE

The consistency of a grease is a complex of related properties, easily demonstrable empirically but difficult to define precisely. We can single out yield stress as a truly definable, pertinent property and then have a quantitative parameter in terms of which we can treat consistency. Criddle and Dreher [ l ] observed typical solid-body stress-strain behavior in greases, with an elastic region, a region of plastic deformation and an ultimate yield or rupture point. At rest grease behaves like a solid body; provided the specimen is not too big, i t will not flow under the force of gravity.* As stated above, a grease contains two basic components: an oil, which has the flow properties of a liquid; and a thickener o r gellant, which is a solid insoluble in the oil. I t will be shown later that the

*The behavior described above is idealized and simplified. In fact the yield point is affected by the rate of strain and there is slow creep at pressures within the nominal elastic range.

oil behaves as the continuous phase in this system. We must therefore explain the structure and the behavior of the system in which the continuous phase is a liquid and which responds to initial stress as an elastic solid. The structure and behavior is that of a gel. Following the reasoning of Hotten [ 2 ] , we shall call the solid component the g e e Lanf rather than the thickeneh. In this way we emphasize the fundamental structure of the grease as a gel and call attention to the difference between the action of a gellant and that of a polymeric viscosity improver, which functions by dissolving in the oil. Many different solids can be used to gel oils into greases. However, the gelling agents around which the standard technology of grease-making developed historically are the metallic soaps of fatty acids, and the bulk of the present-day output of greases consists of soap-gelled products. Therefore, when the nature of the grease is not explicitly specified otherwise, it will be understood that the gelling agent is fatty soap. The usual technique in making a soap-gelled grease is to form and disperse the soap in the oil at an appropriately elevated temperature and then to cool the mixture. The desired consistency in the finished product is achieved by a vigorous stirring and shearing process, known as milling. One of the forms in which soaps crystallize is a long-fiber phase. The structure of the soap phase in a grease is affected by the intrinsic nature of the fibers p e h b e , by the network which they form as they are generated and dispersed in the oil, and by the rearrangement of the fiber structure when the grease is milled. With the aid of electron microscopy, pictures showing soap fibers in great detail have become commonplace. Figure 18-1 shows some views of soap fibers separated from greases and converted to replicas by vacuum-shadowing with metals. The technique for making these replicas involves leaching the oil phase away from the soap with a volatile hydrocarbon solvent. While this treatment preserves the structure of the individual soap fibers adequately, it destroys the three-dimensional structure of the soap phase as a whole, upon which the characteristics of the grease depend. To preserve the structure of the soap network as it exists in the grease, the o i l phase must be displaced by a liquid that has the correct combination of critical temperature and critical pressure, so that i t can be evaporated from the oil-free soap without dissolving any of it and without forming a liquid film on the surface of the fibers which would disrupt their structure by surface tension forces. The soap skeleton resulting from this treatment is often termed an aerogel. Peterson and Bondi [31 described the sodium soap aerogel obtained from a grease by displacement of the oil with butylene as a chalky, pithy, rather brittle, opaque, white, porous

523

Figure 1 8 - 1 . Soap fibers from greases, photographs of replicas taken by electron microscopy. (a), ( b ) Lithium 12-hydroxystearate. ( c ) Lithium 1 nm oleate. ( d ) Calcium-sodium soap. Fiduciary marks:

524

solid which had maintained the bulk volume of the original grease to within 1%. Determination of the surface area of the aerogel by nitrogen adsorption and comparison with the area calculated from fiber diameter indicated the absence of pores in the fibers petr b e . The grease which was reconstituted by allowing the oil to permeate the aerogel in vacua experienced some shrinkage in gel volume. Key properties such as cone penetration value and durability of consistency in the roll test differed little for the original and the reconstituted grease. McClelland and Cortes [ 4 ] prepared aerogel specimens which could be examined directly in the electron microscope. These preparations were compared with soap specimens which had been prepared by the conventional solvent-washing techniques. All specimens prepared by the aerogel technique showed the typical closely interlocked network structure, whereas the soap structures resulting from solvent-washing were broken down, with dispersed fibers. Aerogels were also studied for non-soap gellants such as treated bentonite, purified attapulgite clay, esterified silicic acid and carbon black. I f structure is imparted by the gellant, what part does the oil play in the behavior of a grease? Let us think of an ideal grease as one with the fibers of the gellant interlocked to form a uniformly arranged system of passages filled with liquid oil as a continuous phase. Then let us consider a cubical element of this grease, restrained from gross motion as a whole but with the liquid phase free to move through the cube under the influence of the hydrostatic pressure p in the direction from face ABCD to face A'B'C'D', as illustrated in Fig. 18-2. There will be an ef-

D

A,

61

Figure 18-2. Gel structure of a grease and the flow of the oil phase. The oil will nove through the pores of the gel structure under the pressure p.

525

flux of oil through the pores of the gellant structure and out of the cubical element at the face A ' B ' C ' D ' . As a simplified analogue of a passage in the gel structure of grease let us examine the behavior of a liquid in a straight capillary tube of circular cross-section. I f we start with the capillary filled with liquid and displace a volume V in the capillary to form a drop, as shown in Fig. 18-3, two general cases arise. I f the liquid wets the wall of the

Figure 18-3. a grease.

Capillary analogue of oil flow through the gel structure of

capillary but movement of the column a distance h leaves the exposed wall dry, then the work done by the moving liquid must be equal to the work of wetting plus the work of creating new surface in the exuded drop. The energy thus required is given by the general relation n t2p h = Z n t h E , + A A E ~ (18-1) where p is the hydrostatic pressure, t the radius of the capillary, A A the increase in the surface area due to the exuded drop, E l the energy of wetting per unit area and E2 the surface energy of the liquid per unit area. I f the movement of the liquid in the capillary does not de-wet the wall, then ntI2ph

=

(2nt'h

+

aA')E2

(18-2)

Zqn 1 8 - 2 probably represents the behavior actually encountered, as complete de-wetting by retraction of the liquid is unlikely.

The basic model of a grease, then, is that of a self-supporting structure of interlocked fibers or aggregates of gellant, within which liquid is held by capillary forces. This model is consistent with the two characteristics most typical of grease: resistance to change of

526

shape below the yield stress and relatively easy flow at higher stresses. However we shall find that the actual behavior of real greases has a number of nuances and complexities which require modification and sophistication of this basic model. 18.2.

THE MANUFACTURE OF LUBRICATING GREASE

In principle the manufacture of a grease comprises two simple operations: dispersion of the gelling agent in the oil, and mechanical working o r stirring of the gel to give the grease the desired consistency. In practice there are many technological complexities, as the reader can discover by consulting a specialized text on the subject [ 5 1 . Our interest in the influence of technological manipulations in the manufacturing process on the properties of the grease is motivated by our concern with the relations between the properties of greases and their function as lubricants. The most important greases are those gelled by metal soaps of fatty acids. Table 1 8 - 1 gives a schematic outline of the typical processes by which soap-gelled greases are manufactured. These processes can have TABLE 1 8 - 1 .

SCHEMATIC OUTLINE OF MANUFACTURE OF SOAP-GELLED GREASE

I. Soap formed in the oil A. Saponification of glyceride

fats or other fatty acid esters

B. Saponification of fatty acids

1 . Dehydration 2. Dissolution or tempering of soap in the o i l 3 . Cooling and crystallization of the soap 4. Milling for consistency 11. Preformed soap added to the oil

1. Dissolution o r tempering Cooling and crystallization 3. Milling 2.

direct influences on the properties of the greases which they produce. For example, the direct saponification of a glyceride fat in the oil which is to be gelled into a grease means that the water which participates in the saponification and the glycerol released from the fat must be removed from the mixture. How well this is done is a function of the time allotted and of the configuration of the equipment. Thus, a grease gelled by preformed dry soap could have properties different from one made with soap saponified in b i t U and not fully purged of small amounts of retained water o r glycerol. After the soap has been made and dehydrated, the reaction mixture

is

527

tempered thermally to obtain the desired gel structure. To economize on heat in the commercial manufacturing process, the dehydrated soap is not dissolved. in the oil and then crystallized o n cooling. Instead, a thermal soak which gives the desired crystal structure to the soap in the solid phase is employed. The phase transitions involved in the production of lithium soap greases were investigated by Suggit 161, who showed that soaps prepared below a critical temperature characteristic for each fatty acid anion species crystallize as platelets which are mediocre gellants for grease. I f held above this critical temperature, the platelets are transformed to the type of fiber structure seen in Fig. 18-1. Suggit found the critical temperature for lithium stearate to be 468 K (195 C). The phase diagram for lithium stearate and white oil (390-440 molecular weight) as determined by Cox 171 and shown in Fig. 18-4 has 458 K (185 C) as the temperature of transition from the field representing a mixture of lithium stearate crystal I and isotropic solution to the field representing a waxy phase. I

250

I

I

E

I

I

I

I

I

I

~

/

00 200-/cLD

1 \

0

v

-II \

150-

B+E

3

Eg 100E

I-"

50, I

A+ E I I I

I

'

I

I

I

-

Mole-% Lithium Stearate

Figure 18-4. Phase diagram for the system lithium stearate-white oil. Crystalline lithium stearate I. B: Crystalline lithium stearate 11. C: Waxy phase. D: Liquid crystal phase. E: Isotropic solution. Data by D. B. Cox [71. A:

Both Suggit [6] and Cox [7] discussed those structural aspects of the waxy phase which would favor the formation of fibrous soap crystals. It is obvious that the fibrous soap structure desired in a grease at room temperature must be in a metastable, supercooled state. Cox commented'on the strong supercooling of the waxy phase 171. Vold, 3 z u and Bils 181 were able to demonstrate supercooling directly by comparison of heating and cooling curves in differential scanning calorimetry of the system lithium stearate in white oil. In modern greasemaking oil mixture is subjected to break up clumps of soap the grease. This stiffens

technology the to milling: i . and distribute the grease, a s

gel formed by cooling the soape . , passage through narrow gaps

the fibers uniformly throughout i s evidenced by decrease in the

cone penetration values; but even more important, it reduces gross leakage of oil out of the gel structure [91. Evidently reduction of the proportion of large passages in the free space of the soap structure is an important aspect of improving the utility of a grease. At least 90% of the grease produced in this country is gelled with soaps, and in these soaps the most prevelant cationic constituents are calcium, lithium and sodium. At the most, only 10% of the soaps have other cations, principally aluminum, barium, strontium, and perhaps lead. Glyceride fats are most likely to be tallow, animal grease,* hydrogenated lard oil, hydrogenated vegetable oils, liquid vegetable oils, or fish oils (liquid and hydrogenated). Mixed acids from fats and*oils are available commercially, as well as individual constituent acids such as stearic acid, palmitic acid, oleic acid, myristic acid, lauric acid, etc. fr fatty acid of particular importance is 12-hydroxystearic acid. Commercial grade fatty acids are usually contaminated by their nearest homologues. Commercial 12-hydroxystearic acid is likely to be the mixed hydrogenated acids from castor oil. It is thus apparent that one of the chief technological difficulties of making grease reproducibly is control of the quality of the starting materials. Non-soap gellants for grease add u p to a n impressive list, examples of which include the following: treated bentonite clays, treated attapulgite clays, terephthalamate salts, treated silica, aryl ureas, indanthrene, phthalocyanines, carbon blacks. The same general principles which govern soap-gelled grease structure hold for these non-soap gellants, but individual techniques for making the grease differ from one type of gellant to another. Another variation in grease formulation is the substitution of a synthetic organic liquid for the petroleum oil phase, particularly for use in severe environments. Silicones of various types-dimethyl, methyl phenyl, trifluoropropyl-have been incorporated into greases for such service. 18.3.

FURTHER CONSIDERATION OF GREASE STRUCTURE

Now that we have some idea about the composition of greases and how they are made, let us inquire into their structure a little further. In particular we wish to establish a basic connection between the structure of a grease and the reason it is used for lubrication service. 18.3.1.

Bleeding and Permeability

Booser and Wilcock 1101 viewed the function of grease in ball bearing lubrication as a controlled release of oil to the zone of contact of the balls with the races and the cage. Following this line of thought, A. E. *A mixture of solid stearine and liquid olein; not the lubricating grease made by gelling liquid lubricating oil.

529

Baker [ 1 1 1 regarded test data on the bleeding of oil out of a grease a s a significant evaluation of its lubricating quality. The bleeding test employed to arrive at this conclusion is a simple one [121. Ten grams of grease are placed in a wire mesh cone hung in a closed vessel which catches the oil that bleeds from the grease. Temperature and time can be fixed to suit the experimenter's requirements. Obviously the test is grossly empirical, and moreover its significance is obscured by complications which will be discussed later in this section. As pointed out in Secrion 18.1, grease is composed of a network of solid gellant fibers that hold the continuous liquid phase by interfacial and surface forces. Spontaneous exudation of liquid from this structure requires the unbalancing of these forces. The method of Sisko and Brunstrum 1 1 3 1 for measuring the permeability of grease to the movement of oil depends on driving the exudation artificially; the apparatus is illustrated in Fig. 18-5. The sample of grease is filled into the volume bounded by the retaining ring and the screen-supported Millipore filter;

Burette-

n

a

Gaskets Retainingring

Screens and millipore filters

Grease'Sample Figure 18-5. Apparatus for the grease by oil

determination

of

the

permeability

of

the thickness and the cross-sectional area of the sample are then known. clamp fastens the sample to the adapter that carries the burette. The adapter and the burette are filled with oil, preferably the same oil from which the grease was made. Under these conditions, the only liquid surfaces and interfaces of any consequence are those at the bottom surface of the sample tisk, and even those can be reduced to negligible proportions by having the bottom of the sample holder in contact with a pool of oil in a receiver. Movement of the oil will be governed by the viscous resistance of the passages in the gel structure and Darcy's law for the flow of liquids through porous beds is taken to be applicable: A

530

where u is the velocity of oil flow through the sample, p is the pressure is the viscosity of the oil. The perdrop across the thickness a, and meability B has the dimensions of area, but it can also be interpreted as the volumetric flow rate of the liquid through the unit cube of porous material under unit pressure drop. The experimental determination of permeability by the method of Sisko and Brunstrum [13] is obtained from the rate of fall of the level of the oil in the burette. Then from Eqn 18-3 we get h2

dh

- - _ = -

K2 d t

Bpgh

(18-4)

qa

where n is the radius of the burette, R the radius of the grease sample, h the height of the oil column above the sam le, p the density of the oil and cj the acceleration of gravity. Equation 18-4 can be rearranged to

g = -

2.303 vanL d(Racj h )

5R2

dt

where v is the kinematic viscosity. B = - k

(18-5) Collect ng the constants gives us

d(toy h)

dt

(18-6)

The validity of E q n 18-5 was tested experimentally by Ewbank and his co-workers 1141. They found that plots of h against t on semilogarithmic coordinates were consistently linear. The viscosity of the permeating 2 oil was varied over the range 0.00003-0.001 m / s (30-1000 cs), but fluctuations in the value of 8 stayed within the range 5-10% i f the oil was of the same general chemical type. Change in the type of oil sometimes gave deviations of up to 40% in the permeability. In some cases sensitivity to the thickness of the sample bed caused as much as 30% deviation, especially when the permeability of the qrease was intrinsically low * I t is a mistake, however, to ascribe deviations from the results expected by Darcy's law to weakness of the experimental technique. A more productive point of view is to assume that Darcy's law holds for the ideal gelled grease structure and to interpret deviations from predicted behavior as informative of the actual structure. For instance, Brown and Ewbank [15] proposed that permeation takes place predominantly through the larger pores of the grease structure and cited evidence that only about 25-50% of the oil held by a grease can be made to flow out of the structure. Persistent retention of the oil can be ascribed to physicochemical interaction with soap fibers, the effect of which is

531

relatively more pronounced in the narrower passages. Thus the strong effects of oil viscosity and oil type reported by Zakin and T u [ 1 6 1 can be given a rational explanation. Greases are often used to lubricate bearings at elevated temperatures, sometimes 4 7 8 K ( 2 0 5 C ) or higher. Therefore oil bleeding tests are frequently carried out at the anticipated working temperature of the grease. Two factors affect the influence of temperature on oil separation. One is the decrease in the viscosity of the oil as the temperature rises, thereby facilitating filtration of the separated oil away from the grease. The other factor is the phase state of the oil-soap system at from data by Hotten and Birdsall the test temperature. Figure 1 8 - 6 , [ 1 7 ] , shows the combined effect of these two influences on the bleeding of greases made with various lithium soaps in naphthenic oil. A l l the

I , , , , , , , ,

0 25 5 0 75 100 125 150 175 200

~

!5

Ternperature,deg . C

Figure 18-6. Eff ect of temperature on oil separation from greases made with lithium soaps: 1 2 % of the lithium soap in naphthenic oil: 3 hours vacuum filtration at the indicated temperature. Data by biotten and Birdsall [ ! 7 1 . greases exhibit an increase in the amount of oil separable by 3 hours vacuum filtration as the temperature rises to a critical value which depends on the soap. The maximum in the bleeding curve is followed by a decrease which is quite sharp for the greases made with oleate, stearate The grease or myristate soap but less so for the laurate soap grease. gelled with lithium 12-hydroxystearate shows the least sensitivity of bleeding to temperature and also has a high critical temperature. The sharp decrease in bleeding is probably associated with a transition to the waxy phase. I n Fig. 1 8 - 6 the dip in the oil separation curve is at

532

453 K ( 1 8 0 C); in Fig. 18-4 the transition for the system lithium stearate-white oil is at 463 K ( 1 9 0 C). The sharp rise in the bleeding of oil from the lithium 12-hydroxystearate grease is due to transformations to the liquid crystal and the isotropic liquid states. If controlled bleeding of oil is desired for the lubrication of bearings, then Fig. 1 8 - 6 illustrates the importance of the phase behavior of the soap component of grease with change of temperature. The reason for the superior utility of lithium hydroxystearate greases is apparent from the bleeding behavior. 18.3.2.

Consistency and Penetration

It has already been pointed out that the consistency of a grease is the characteristic most overtly connected with its functionality. In qualitative terms consistency can be thought of a s resistance to unwanted flow; in terms of performance it is manifest as antislumping behavior, the correct degree of channeling, resistance to ejection from a bearing by centrifugal force, etc. The physical test by which consistency is evaluated quantitatively is the ASTM cone penetration determination [ l a ] , which is illustrated schematically by Fig. 18-7. The cone and its attachments weigh 0.1500 kg. At the start of the test the cone and dial

I !

Figure 18-7.

initial position of the cone

Cone penetration of greases

assembly is adjusted so that the tip of the cone just touches the level, undisturbed surface of the grease sampie (AA') when the dial reads zero. The cone is then released and sinks into the grease. The distance A , read on the dial in tenths of millimeters, is the penetration value of the grease. The test is usually carried out at 298 K (25 C). The softer the grease the greater the penetration value. Table 18-2 gives the relation between the National Lubricating Grease Institute classification system and penetration values.

533

TABLE 18-2.

PENETRATION VALUES FOR NLGI GREASE GRADES

NLGI Grade NO. 000 00 0 1 2

3

4 5 6

Penetration at 2 9 8 K (a) 445-475 400-430 335-385 310-340 265-295 220-250 175-205 130- 1 6 0 85-115

(a) ASTM D 2 1 7 (worked 6 0 strokes). The ASTM penetration value is not a measure of yield stress. The theoretical difficulties in the determination of the true yield stress of a grease have been discussed by Evans, Xutton and Matthews [ 1 9 1 . Greases do not really have a sharp stress boundary between elastic and plastic deformation; there i s always some residual creep although the flow rate may be slow enough to be treated as zero for practical purposes. The apparent viscosity of grease is dependent on rate of shear, and slow lowering of.the penetrator at a uniform rate will give results different from those of the quick-release ASTM method. Also, the buoyancy due to the behavior of the grease a s a fluid must be taken into account. Evans e t al. developed a relation for a simple right circular 90' cone as an indenter: Pb =

w

- W'

-

where on the in the ing as

A

(18-7)

pb is the bearing pressure which supports the cone, W is the load cone, W ' is the buoyancy force and A is the cross-sectional area plane of AA'. I f T~ denotes the yield stress of the grease behava plastic solid, then

where c is a constant of proportionality which in theory is obtainable Therefore p b is from the solution of the plastic deformation problem. fundamentally proportional to the yield stress of the grease, whereas the empirical ASTM penetration value is not. Nevertheless, it is the ASTM penetration numbers that are the generally accepted measure of the consistency of greases, and numerous investigations are on record where the penetration values are used to assess the influence of the type of gellant, the concentration of the gellant, the method of manufacture, the working of the grease (milling, shearing, flow), etc. The empirical character of such information has

534

TABLE 18-3.

EFFECT OF FIBER LENGTH ON GREASE CONSISTENCY ~~~

Nylon fibers

Soda soap grease

Axial ratio

Micropenetration, fiber diameter of

L/D

300/1 100/1 70/1 50/1 6/ 1

95

Axial ratio

Micropenetration

170/1

120 200

L/D

118 125 -

165 208 208 Fluid

80/1

Data by Bondi e t a L . [ 2 0 ] . not seriously affected its utility for technological purposes. A direct demonstration that the geometry of the gellant fibers influences the penetration response of grease is found in the work of Bondi and his collaborators [20], who gelled a turbine oil into a grease by incorporating 10% of completely insoluble nylon fibers. The consistency of the resulting product depended on the ratio of length to diameter ( L / D ) of the suspended nylon particles, as shown by the data in Table 18-3. The same trend holds for a grease gelled by a soda soap. R. H. Leet [211 obtained the ratio of the average fiber length L to the average diameter V for nine greases of different composition, samples of which were worked i n various ways o r allcwed to harden by aging and then tested for penetration. The relations between ASTM penetration value and L / V are shown diagrammatically in Fig. 18-8. The influence of the individual nature of the grease is evident, but there is also a consistent linear

400

I

I

c

g e c

fv)a a"

C

& \ \ \ 0 0 3 g

200

A ,

535

relation between ASTM penetration and L/D for each grease. Other data showing the influence of L / D on penetration for a grease gelled by 7% lithium 12-hydroxystearate have been reported by Borg and Leet [22]. All greases suffer a degradation in consistency when subjected to prolonged shearing action. One of the devices commonly used to shear greases in stability testing is the ASTM grease worker, a full description of which is given in the ASTM penetration Method D 217 [l8]. In essence the grease worker is a circular plate 2-15/16 inches in diameter with 41 1/4-inch holes. The plate is worked up and down with a stroke length of 2-5/8 inches through the grease sample in a cup 3.000 inches in diameter. The instructions f o r standard working call for 60 double strokes [18]. Prolonged working may amount to 100,000 strokes or more. Table 18-4 gives data by Woods and Trowbridge [231 showing the effect of prolonged working on penetration value for six different types of soapgelled greases. Another method of working greases is with the roll tester [241, which consists of a cylinder 3-35/64 inches i n diameter and 7-3/32 inches long, i n which are placed a 50-gram sample of grease and a solid roller 2-3/8 inches in diameter, 6-15/16 inches long, weighing 5 kg. The cylinder is driven o n a set of idlers at 165 rpm. The grease is sheared by the motion of the roller on the inner surface of the cylinder. Loss of consistency is monitored by a quarter-scale penetration test. Figure 18-9 shows some characteristic behavior. Comparison of Fig. 18-9 with Table 18-4 shows that the results of the roller test and the grease worker test do not rank the greases in the same relative order. 18.4.

THE FLOW OF GREASES

I n lubrication with a liquid, the rheology of the lubricant at the functioning site is the dominant behavior; transportation of the lubricant to that site, e . 5 . a bearing o r a gear, by piping or spray nozzle is on the whole a routine matter and rarely affects the selection

TABLE 18-4.

EFFECT OF WORKING ON PENETRATION OF SOAP-GELLED GREASES

Type of soap

ASTM penetration 60 strokes

100,000 strokes

277 295 282 270 269 268

286 335 354 358 370 380

Li 12-hydroxystearate Ca tallow Na tallow/stearate A 1 stearate Li stearate Na tallow Data by Woods and Trowbridge [231.

536

240

200

s s

E

160

W C

a

2

P 120 80

I0

I 24

I

t

48 72 Rolling Time, hours

96

Figure 18-9. Roll testing of grease consistency. Gellants: A . Lithium stearate. B. Lithium 12-hydroxystearate. C. Calcium tallow soap. D. Sodium tallow-stearate. E. Aluminum stearate. From data by Woods and Trowbridge [23]. of the lubricant. But when the lubricant is a grease, rheological problems associated with getting i t to the functioning site are often a s important as those directly connected with its primary lubricating action. These two aspects of grease behavior are frequently at variance with each other. For instance, in a bearing it is preferred that the grease remains packed in place and not be bodily moved around by stress, whereas for transportation in a piping system, gross flow is desired. Let us look at the behavior wanted of grease in a rolling element bearing. Some lubricant must get into the conjunction zone between the ball or the roller and the race, small though the quantity required may be. There are a number of reasonable models f o r how the lubricant gets into the conjunction zone. If we postulate that it is the total grease which flows into the conjunction and functions there as the lubricant

0

2.5

a Q 2.0

(u

< 1.5

1 E 3i 8

F First cycle

300 seconds

E--

Sleady stale after 1000 sec preshearmg

1.0

8 0.5

10

0.0

I 8

I

1

I

I 1

I

160 8 160 8 Rote of Sheor,.i, lo3 sec-'

I 16

Figure 18-10. Flow behavior for lithium stearate grease. Bauer, Finkelstein and Wiberly [261.

From

data

by

537

without segregating into oil and soap, then the principles of elastohydrodynamic lubrication can be applied. A detailed analysis is given by Kauzlarich and Greenwood [ 2 5 ] ; what interests us particularly in this work is the viscosity function used for the grease. An informative study of the flow behavior of soap-gelled grease is that of Bauer, Finkelstein and Wiberly [ 2 6 ] . Figure 1 8 - 1 0 shows the relation between the rate of shear and shear stress for a grease gelled with 12% lithium stearate. The three diagrams show the behavior for the first cycle of shearing, the second cycle, and the ultimate steady State. The first cycle is characterized by pronounced hysteresis as the rate of shear is increased from zero to the maximum of the viscometer and then decreased back to zero. In the second cycle the hysteresis loop is much smaller. A shearing time of 100 seconds at a shear rate of approximately 975 5-l removed the hysteresis loop from the flow curve having a cycle period of 300 seconds. With the hysteresis eliminated, the flow diagram clearly shows the existence of a yield stress. Bauer and his co-workers [ 2 6 ] proposed the following equation for the flow relation of greases: T

-

To

= Cli

+

-n C2y

where c l and c2 are constants and T~ is the yield stress. Greenwood [ 2 5 1 generalized the flow function to T

-

To =

f(i)n

(18-9)

Kauzlarich and

(18-10)

in order to facilitate manipulation of the elastohydrodynamic computations. The evaluation of f(:)n required for a specific problem can be obtained from experimental data by graphic or numerical methods. Usually it is assumed that hysteresis has been eliminated. But obviously it is incorrect to use steady-state grease rheology when measuring starting torque.after a long rest or evaluating flow resistance in slow reciprocation. The thixotropic properties of grease and the relation of structure to thixotropy, as exemplified by the work of Eyring and his collaborators C27, 2 8 1 , can be highly pertinent aspects of the flow of grease, although the details of an adequate treatment are too involved for the space available here. Just a s complex is the viscoelasticity of grease. The work of Forster and Kolfenbach [291 indicates a relation between viscoelastic behavior and structure. The flow of grease in piping or tubing brings up two questions of technological importance: (a) How much pressure drop is there along the run of pipe? (b) Given the driving pressure at the pump, what is the flow rate at any selected distance along the piping? Both of these questions can be treated adequately in terms of the general principles of

538

viscometry discussed in Chapter 4 and the flow relations for grease given above. The following expression (see Chapter 4, Eqn 4 - 1 4 ) can be used to obtain 2, the volumetric rate of flow through a tube:

where A p is the driving pressure, R the radius of the tube, L its length is a functional relation between the shear stress and the rate and T"/+ By one means of shear such as shown in the flow diagram of Fig. 1 8 - 1 0 . or another the steady-state flow diagram can be made to yield a value for A capillary flow determination such as ASTM Method D the exponent n'. 1092 I 3 0 1 yields values of 2 for preselected values of Ap, R and L . Furthermore, Eqn 4 - 1 of Chapter 4 gives the expression below for the shear stress at the wall of the tube o r piping through which the grease is flowing: ApR

T = -

2L

Substitution into Eqn 1 8 - 1 0 gives T - T o = -

APR (18-12)

2L

The yield stress T~ can be obtained by the rotational viscometry technique as shown in Chapter 4. Thus from two sets of rheological determinations, one the capillary flow technique and the other rotational viscometry, enough information can be obtained to treat the problems associated with the flow of grease in pipes. A quantity cited fairly often in discussing the properties of greases is the apparent viscosity. This is the empirical ratio of measured shear stress to rate of shear: T

nappatlent =

Y

(18-13)

Apparent viscosity is useful as an evaluation of the difficulty of pumping a grease. Since greese moves by non-Newtonian flow, knowledge of the rate of shear and also of the yield stress required to initiate flow is necessary for apparent viscosity value to have any utility. 18.5.

GREASE AS A LUBRICANT IN SERVICE.

Greases are used as lubricants i n a wide range of service conditions, The lubricathe more important of which are summarized in Table 1 8 - 5 . tion of high-speed rolling-element bearings is probably the most familiar service use of grease, but other applications are the lubrication of

539

TABLE 18-5.

TYPES OF SERVICE FOR LUBRICATING GREASES ~

Motion and contact

Examples

Unidirectional Fast, continuous Slow, continuous Slow, intermittent

Rolling element bearings Bearings, cams Gears, cams

Oscillatory or reciprocating Slow to moderately fast

Suspensions, linkages, cams

slow-speed plain bearings a s well as of large, slow-acting gears which i t is impractical to enclose in a case that holds an oil lubricant. Automotive suspensions, which cannot be sealed adequately against oil leakage, are also advantageously lubricated with grease. Basically the lubrication of a high-speed rolling element bearing is no different with grease than with oil. Booser and Wilcock [ l o ] studied the running life of No. 306 ball bearings turning at 3600 rpm under a radial load of 7 1 1 . 7 N 1 1 6 0 lb) when lubricated with oil in submilligram quantities. With less than 0.35 mg of oil on the bearing, running life was gi’ven by the expression

t

=

0.66

w

2 being the life of the bearing in hours and LU the quantity of oil in milligrams. For quantities of oil greater than 0 . 3 5 mg, bearing life was given by

I t was calculated that 0 . 3 5 rng of oil would cover the balls, races and F o r bearings retainer pockets with a film 51 nm ( 2 rnicroinches) thick. of conventional precision (Grade ABEC-1) lubricated with one milligram of oil, a running time of 4 . 7 hours was observed, and with high-precision Grade ABEC-5 bearings the running time was 1 4 . 5 hours. With fully packed open bearings lubricated with grease the life times were 2000 and 4000 hours respectively. The grease is believed to function a s a source of oil by controlled bleedincj a s it is worked by the motion of the roIling elements in the bearing.

However, there are n o a phiahi grounds for excluding the possibility that the whole grease instead of just the exuded oil is the functioning lubricant. Dyson and Wilson I 3 1 1 and also S . Y. Poon [ 3 2 1 have published film thickness data for the elastohydrodynamic lubrication of rolling disks with grease. This may be regarded a s a close experimental approximation of a roller acting against a race in a bearing. Figure 18-11 is a diagram of typical results obtained. Curve A shows the ratio of

540

0

oc"=

1.0

ln ln 0,

c

1

0

._ E

LL

0.0 0 10

20

30 40

50 90

Time, minutes

Figure 18-11. Elastohydrodynamic film ments. 10% Lithium 12-hydroxystearate sure; 335.3 cm/s surface velocity at base oil film thickness 2.201 Um. A : before experimental r u n . Data by Poon

behavior of grease in disk experigrease; 965.3 MPa contact pres800 rpm; temperature 308 K (35 C ) ; Original grease. B: Presheared [321.

measured film thicknesses for a grease gelled with 10% lithium 12hydroxystearate and for the base oil a s a function of the duration of running. At the beginning, the thickness of the grease film was 1.6 times that of the base oil, but after approximately 50 minutes running the ratio dropped to a steady 0.7. The part that degradation of gel structure by flow through the conjunction zone played was assessed by preshearing the grease in a gear mill before the disk experiment. The initial film thickness ratio for grease/oil decreased to 1.1 and a final ratio of 0.7 was attained in 30 minutes. The film thickness of the ungelled base oil was 2.201 urn ( e 7 microinches); hence a ratio of 0.7 c o r responds to 1.549 Urn ( 6 1 microinches) for the grease film. Even though the film of grease was thinner than that of the base oil, it was still thick enough to prevent contact of the bearing surfaces. Poon [321 explained the film behavior of grease in terms of viscoelastic behavior a s the lubricant passes through the conjunction zone. Kauzlarich and Greenwood [251 suggested that because of its gel structure grease heats u p by shear faster than oil and loses the heat by conduction more slowly. I n their estimation, a thermal rather than an isothermal treatment of the elastohydrodynamic problem is required. Elastohydrodynamic action of the total grease and selective bleeding of the oil are not mutually exclusive mechanisms in the lubrication of I n this respect grease i s a versatile rolling-element bearings. lubricant, for i f the flow of the grease a s a whole suffers a temporary interruption, then the residual oil coating the bearing can protect the system from damage for a while. Still another mechanism for lubrication by soap-gelled greases stems from the fact that soap is a "boundary" lubricating agent. Using a slow-

541

speed pin-on-disk type tribometer, Godfrey [33] showed that the friction with dry soaps such as calcium stearate or sodium stearate, as influenced by temperature in the range 373-473 K (100-200 C), resembled that of the greases made with these soaps and was quite distinct from that of the base oil. Simple dispersions of silica, bentonite o r calcium carbonate in the base oil showed maxima in the coefficient of friction at 473 K , whereas the soaps and the greases made with them gave minima at that temperature. Horth, Sproule and Pattenden [341 compared the frictional torque of a 120' journal bearing lubricated with greases against the torque obtained with the base oil; their results are summarized in Fig. 18-12. Speed-governed transitions to hydrodynamic action are apparent for the base oil and for the greases gelled with aluminum soap,

I

1

I

I

I

L

I

1

I

,

0.05 0.1 0.2 0.5 1.0 2.0 5.0 10 Journal Speed, cm/s Figure 18-12. Speed-dependent torque behavior of greases in a journal bearing, steel journal in bronze bearing: load 3718 N , temperature 311 K (37.8 C ) . A: Base oil 300 SUS at 37.8 C. B: Aluminum soap grease. C: Calcium soap grease. D: Sodium soap grease. E: Lithium soap grease. Data by Horth, Sproule and Pattenden [341.

calcium soap or sodium soap, but there seems to be no identifiable transition speed for the lithium soap grease. When the torque values are converted to coefficients of friction in the non-hydrodynamic region, there is a reliable difference between the friction with the base oil hnd the greases. Interpretation of these data would be even better i f flow curves of shear stress vs. rate of shear were available for the greases. Although elastohydrodynamic transport of grease through the conjunction between the rolling element and the race is the operative mode of flow for the basic lubrication process, the overall movement of the lubricant in a grease-packed rolling-element bearing is more complicated in than that. Consider the ball bearing shown diagrammatically Fig. 18-13a. The movement of the inner ring rotates the ball in the direction shown, but there is also partition of sliding so that the entire ball assembly held by the retainer will also rotate slowly. The

542

Grease within

Shield vent-

-&

Retainer Inner ring

Figure 18-13. Diagram of ball bearing. (a) View showing modes o f motion which interact with grease lubrication. (b) Cross-sectional view with shields mounted. grease packed into the bearing between the rings is subject to gross stirring by the following kinds of action: (a) There will be viscous shear, most probably turbulent, of the grease in the gap between the inner and the outer rings as the inner ring rotates. (b) There will be turbulent mixing of the grease as the ball and retainer assembly plows (c) Superimposed on the movement of the grease due to the through it. rotating of the bearing components around the axis of the shaft there will be movement due to the rotation of the balls around their individual axes. In addition to these large-scale movements of the grease there are two other modes of motion, which under favorable circumstances should proceed by streamline flow of grease through the conjunctions between the balls and the races, and flow of grease between the balls and the retainer. The gross flow of soap-gelled grease in rolling-element bearings was studied by O'Halloran, Kolfenbach and Leland [ 3 5 ] . They used a shielded ball bearing, a cross-sectional diagram of which is shown in Fig. 18-13b. A weighed sample of grease treated with two kinds of dye, one an'oilsoluble blue and the other a water-soluble sodium fluorescein which dissolved in the soap phase, was packed into the bearing proper, and a weighed portion of undyed grease was put on the shields. The mixing of the qrease was monitored by assaying the migration of the dye into the grease on the shields. The results showed that the grease moved as an entity; i.e. soap and oil moved at the same rate. Soft greases mixed readily, whereas with harder greases the moving components of the bearing tended to plow channels and leave portions of the grease unmixed.

543

0

10 20 30 40 50 Running Time, minutes

60

Figure 18-14. Grease consistency and the running temperature of a rolling element bearing. I: Grease with initial worked penetration 200. 11: Grease with initial worked penetration 260. Spindle speed 10,000 rpm. Data by Horth, Norton and Pattenden [361.

TABLE 18-6.

DISTRIBUTION OF GREASE IN SHIELDED BEARINGS Grease I (a)

Grease I 1 (a)

Total original packing

3.00 grams

3.00 grams

Total grease in bearing within cage outside cage

1.10 0.32 0.78

1.43 0.34 1.08

Total grease on shields on shield faces in shield cups

1.92 1.04 0.87

1.53 1.15 0.37

(a) One hour at 10,000 rpm. 1361.

Data by

Horth,

Norton

and

Pattenden

The extent cf mixing is reflected in the gross temperature behavior of the running bearing. Figure 18-14 shows the two extreme types of behavior as observed by Horth, Norton and Pattenden [361. In Curve i the temperature of the bearing quickly rises to a maximum and then gradually stabilizes at a level less than 2.9 K above ambient. The time interval t ' is often designated as the clearing time because i t is associated with the expulsion of the excess grease from locations where it can be churned by the moving components of the bearing and thus generate heat. In Curve I1 the clearing phenomenon is absent: the temperature of the bearing rises to a level 33 K above ambient. Horth e k a e . 1361 demonstrated that the distribution of grease in the disassembled bearings was consistent with this explanation of the temperature behavior (see Table 18-6). With Grease I , once the excess was displaced from the path of the moving ball/ retainer assembly, little back mixing took place, whereas Grease I 1 readily slumped back into the inter-ring annulus. This was demonstrated

544

by dye-partition experiments, which showed that there was only 33% back mixing of Grease I from the faces of the shields into the bearing in the course of an hour's running, but 100% back mixing of Grease 11. The worked penetration of Grease I when i t was packed into the bearing was 200, that of Grease I 1 260. However, Horth e t at. 1361 found that the penetration value was not sufficient by itself to predict the behavior of a grease in an operating bearing. Soap content is another important parameter which is related to the tendency of a grease to channel in the bearing and therefore to r u n cooler. Channeling greases are characterized by sharply defined yield stress, by shear hardening in the bearing and by short fibers. It is the entire complex of rheological properties which influences the functioning of a grease in service. SO far n o all-encompassing generalizations have been developed. In lieu of an overall rationale, there are numerous empirical tests for evaluating greases as lubricants for rolling-element bearings. A. Schilling [37] published descriptions of 14 different bearing tests with outlines of the failure criteria and diagrams of the operating principles of the testers. These tests are concerned with the ability of the grease to prevent bearing failure o r its ability to withstand prolonged shearing without intolerable loss of consistency o r both. Bearing failure may be signalized by increased power required to drive the test rig at the rated speed, o r a sharp increase in the temperature of the bearing after establishment of steady-state operation, o r persistent unusual noise. Other manifestations of failure are: scuffed locations on the races o r the rolling elements (balls or rollers), spalled races, spalled balls o r rollers, worn o r broken retainers, excessive wear of races o r rolling el emen t s

.

Breakdown of grease by prolonged shear to the point of excessive l o s s of the whole grease by leakage o r of the oil by bleeding is undesirable because of the danger of leaving the bearing inadequately lubricated and because of soiling or contamination of the surroundings by the leaking A standard test for this type of leakage is ASTM Method lubricant. D 1263 [38]. This test is a credible analogue of service in an automotive front wheel bearing assembly. Stokely and Calish [391 noted that shearing the grease in an ASTM worker o r the roll tester does not necessarily predict its behavior in an overpacked bearing. The ASTM leakage test is carried out with an overpacked assembly. In contradistinction to rolling-element bearings, the lubrication of automotive suspensions, which is the second major service area for greases, is characterized by lower contact pressures, lower rubbing speeds and intermittent o r oscillatory motion. In 1960 Brunstrum and Hayne [ 4 0 ] published the results of a road test with four different greases used on 15 different vehicles. Cumulative plots of the distribu-

545

tion of wear showed no consistent trend assignable to a particular grease or a particular vehicle. Wear was low, c a . 4 mg average l o s s per 1000 miles at the most sensitive locations in the suspension. Tests with these four greases in a laboratory oscillatory contact rig also showed no differentiation among the four greases, but in the same type of test a special lithium grease gave a cumulative distribution diagram with a significantly lower level of wear. Another point of view on the function of grease in automotive suspensions is found in the laboratory ball joint tests reported by Gilbert, Verdura and Rounds [ 4 1 1 . A convex surface is oscillated against a concave housing under load to simulate the ball joint action in an automotive front wheel suspension. The desired standard of performance is torque stability at a predetermined level: high, erratic torque is symptomatic of “ride harshness,” but torque at too low a level adversely affects the damping required in the suspension system. Results obtained with various commercial greases indicated significant differences in perf ormance. There are three ASTM extreme-pressure bench tests specifically applicable to greases [ 4 2 1 . Considerable empirical bench test data is to be found in the literature for greases compounded with extreme-pressure additives. A particularly interesting study along such lines was carried out by Silver and Stanley [ 4 3 1 . Greases were made u p from a base oil with various concentrations of lithium 12-hydroxystearate or a treated bentonite a s gellants and with the following substances as additives: dibenzyl disulfide, chlorinated wax, and tricresyl phosphate. A special set of greases was made up with 15% ground graphite a s the gellant. The behavior of these greases in the four-ball machine was tested by two procedures: a one-hour wear test under 15 kg load and the standard ASTM extreme-pressure test. Table 18-7 shows the results. The data in Table 1 8 - 7 do not permit a firm interpretation of additive action in a complex system such as grease but they do offer a number of suggestive hints. For the greases without additives there is no discernible relation between penetration or gellant concentration and the 15-kg wear scar. With the exception of graphite, which seems to have no influence whatsoever on the 15-kg wear, the type of gellant makes only minor differences in the wear results. But in the EP test graphite has the strongest influence, while the mean Hertz loads for the other two gellants group at substantially the same level. The basic behavior of additives in the ungelled oil is given by the following ranking of effecdibenzyl disulfide = tiveness in terms of the 15-kg wear test: chlorinated wax < tricresyl phosphate. Ranking in terms of the mean Hertz load is tricresyl phosphate < chlorinated wax < dibenzyl disulfide. Adsorption of the additive by the gellant was determined by filtering

546

TABLE 18-7.

FOUR BALL TESTING OF GREASES COKPOUNDED WITH E? ADDITIVES No additive

Gellant

Dibenzyl disulfide (a)

Chlorinated wax (b)

Tricresyl phosphate (c)

Wear scar, mm (d) None Bentonite (treated), 2.5% worked penetration >385 Bentonite (treated), 5.0% worked penetration 197 Bentonite (treated), 7.5% worked penetration 167 Li 12-hydroxystearate, 4% worked penetration >385 Li 12-hydroxystearate, 8% worked penetration 320 Li 12-hydroxystearate, 12% worked penetration 219 Graphite, 15% worked penetration >385

0.77 0.55

0.61 0.63

0.63 0.47

0.26 0.33

0.67

0.63

0.43

0.47

0.56

0.54

0.48

0.50

0.56

0.60

0.52

0.29

0.61

0.65

0.46

0.49

0.53

0.59

0.37

0.48

0.72

0.61

0.53

0.43

% Left ~

~~~

None Bentonite (treated), 2.5% 5.0% 7.5% Li 12-hydroxystearate, 4% 8% 12% Graphite, 15%

__ __ -_ -__ __ ---

in oil (e)

__

__

__

95 68 69 98 89 96 86

86 81 65 99 92 95 96

70 44 20 87 83 87 67

~~

Mean Hertz load, kg None Bentonite (treated). 2.5% 5.0% 7.5% Li 12-hydroxystearate, 4% t 8% I 12% Graphite, 15% 11

,I

16.6 22.3 22.8 23.7 20.3 21.4 21.0 27.4

41.4 30.7 34.0 41.1 28.9 36.4 33.3 55.1

37.7 34.0 27.0 31 .O 28.3 30.8 32.7 48.0

27.6 22.4 19.6 23.3 20.8 24.5 24.5 34.1

(a) 1 .7% in grease. ( b ) 51% C1, 1.7% in grease. (c) 1.7% in grease. (d) 1 hour, 15 kg load, 1500 rpm, 50 C. (e) At end of wear test, after' separation of oil from gellant by 0.45 pm Millipore filter. Data by Silver and Stanley [431.

off a known proportion of the oil and assaying it for additive content. The treated bentonite was the strongest adsorbent for all the additives. But close scrutiny of Table 18-7 reveals enough irregularity and anomaly to make the simple concept of reduction of the effective concentration of the EP additive by competitive adsorption on the gellant untenable. Each system seems to have its individual characteristics. For instance, the effect of interaction between gellant and dibenzyl disulfide seems to be

541

minor with respect to the 15-kg wear test and irregular with respect to the mean Hertz load. The antiwear action of tricresyl phosphate in the 15-kg test is noticeably inhibited when i t is compounded in a grease: the same holds true for the mean Hertz load except for the grease thickened with graphite. The reader will have noted that the emphasis in this chapter has been on basic modes of behavior and that specific discussion of grease of particular compositions and types has been limited. This is not for lack of available information. However, a great deal of such information is empirical and non-systematic. The fundamental aspects of a complex colloidal system such as grease are difficult to investigate; therefore it is not surprising that the major attention has been directed to studies with direct technological applications. The intent of the presentation in this chapter is to equip the reader with enough basic background so that he can examine the results published in the technical literature of grease composition, manufacture and use with the insight to appreciate a l l the implications he finds there. REFERENCES 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

D. W. Criddle and J. L. Dreher, NLGI Spokesman, 2 3 ( 1 9 5 9 ) 9 7 - 1 0 1 . B. W. Hotten, in Advances in Petroleum Chemistry and Refining, J. J. McKetta, Editor, Interscience, New York, 1 9 6 4 , Volume 9, Chapter 3. W. H. Peterson and A. Bondi, J. Phys. Chem., 57 ( 1 9 5 3 ) 3 0 - 3 5 . A . L. McClelland and 3. Cortes, jr., NLGI Spokesman, 2 0 ( 1 9 5 6 ) NO. 6 , 1 2 - 1 6 . C. J. Boner, Manufacture and Application of Lubricating Greases, Reinhold Publishing Co., New York, 1 9 5 4 . R. M. Suggit, NLGI Spokesman, 24 ( 1 9 6 0 ) 3 6 7 - 3 7 5 . D. B. Cox, J. Phys. Chem., 6 2 ( 1 9 5 8 ) 1 2 5 4 - 1 2 5 6 . M. J. Vold, Y. Uzu and R. F. Bils, NLGI Spokesman, 3 2 ( 1 9 6 9 ) 3 6 2 367. S. F.

E.

Calhoun, NLGI Spokesman, 2 9 ( 1 9 6 6 ) 3 2 8 - 3 3 2 . R. Booser and D. F. Wilcock, Lubrication Eng., 9 ( 1 9 5 3 ) 140-143,

156- 1 5 8 . A . E. Baker, NLGI Spokesman, 2 2 ( 1 9 5 8 ) 2 7 1 - 2 7 7 . A . E. Baker, E. G. Jackson and E. R. Booser, (1953) 249-253.

Lubrication

Eng.,

9

kT. Sisko and L. C. Brunstrum, NLGI Spokesman, 2 5 ( 1 9 6 1 ) 7 2 - 7 6 . W. J. Ewbank, J. Dye, J. Gargaro, K. Doke and J. Beattie, NLGI Spokesman, 2 7 ( 1 9 6 3 ) 7 5 - 8 2 . W. L. Brown and W. J. Ewbank, NLGI Spokesman, 2 9 ( 1 9 6 5 ) 7 7 - 8 3 . J. L. Zakin and E. H. Tu, NLGI Spokesman, 2 9 ( 1 9 6 6 ) 3 3 3 - 3 3 7 . B. W. Hotten and D. H. Birdsall, Ind. Eng. Chem., 4 7 ( 1 9 5 5 ) 4 4 7 - 4 5 1 . ASTM Method D 2 1 7 - 6 7 , Cone Penetration of Lubricating Grease, ASTM Standards Book, Part 17-Petroleum Products, American Society for Testing and Materials, Philadelphia. D. Evans, J. F. Hutton and J. B. Matthews, Lubrication Eng., 13 A.

( 1 9 5 7 ) 341-346.

A. Bondi, A. M. Cravath, R. J. Moore and W. H. Peterson, NLGI Spokesman, 13 ( 1 9 5 0 ) No. 12, 1 2 - 1 8 . R. H. Leet, NLGI Spokesman, 19 ( 1 9 5 5 ) No. 1, 2 0 - 2 3 . A . C. Borg and R. H. Leet, Lubrication Eng., 15 ( 1 9 5 9 ) 4 5 0 - 4 5 4 . H. A. Woods and H. M. Trowbridge, NLGI Spokesman, 19 ( 1 9 5 5 ) No. 5, 2 6 - 3 1.

548

24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

43.

ASTM Method D 1831-64, Roll Stability of Lubricating Grease, ASTM Standards Book, Part 17-Petroleum Products, American Society for Testing and Materials, Philadelphia. J. J. Kauzlarich and J. A. Greenwood, ASLE Trans., 15 (1972) 269277. W. H. Bauer, A. P. Finkelstein and S. E. Wiberly, ASLE Trans., 3 (1960) 215-224. S. J. Hahn. T. Ree and H. Evrins, NLGI SDokesman 23 (1956) 129-136. H. Utsugi; K. Kim, T. Ree-and-H. Eyring, NLGI Spokesman, 25 (1961) 125- 13 1. E. 0. Forster and J. J. Kolfenbach, ASLE Trans., 2 11959) 13-24. ASTM Method D 1092-62, Apparent Viscosity of Lubricating Greases, ASTM Standards Book, Part 17-Petroleum Products, American Society for Testing and Materials, Philadelphia. A. Dyson and A. R. Wilson, Proc. Inst. Mech. Eng., 184 (1969/1970), Part 3F, 1 - 1 1 . S. Y. Poon, J. Lubrication Tech. (Trans. ASME), 94F (1972) 27-34. D. Godfrey, ASLE Trans., 7 (1964) 24-31. A. C. Horth, L. W. Sproule and W. C. Pattenden, NLGI Spokesman, 32 (1968) 155-161. R. O'Halloran, J. J. Kolfenbach and H. L. Leland, Lubrication Eng., 14 (1958) 104-107, 117. A. C. Horth, J. H; Norton and W. C. Pattenden, Lubrication Eng., 27 (1971) 380-385. A. Schilling, NLGI Spokesman, 30 (1967) 388-400, 420-432. ASTM Method D 1263-61, Leakage Tendencies of Automotive Wheel Bearing Greases, ASTM Standards Book, Part 17-Petroleum Products, American Society for Testing and Materials, Philadelphia. J. M. Stokely and S. R. Calish, NLGI Spokesman, 19 (1955) No. 9, 1215. L. C. Brunstrum and W. L. Hayne, jr,, NLGI Spokesman, 23 (1960) 394400. A. W. Gilbert, T. M. Verdura and F. G. Rounds, NLGI Spokesman, 29 (1966) 356-365. (a) ASTM Method D 2266-67, Wear Preventive Characteristics of Lubricating Grease (Four Ball Method). (b) ASTM Method D 2509-66, Measurement of Extreme-Pressure Properties of Lubricating Grease (Timken Method). (c) ASTM Method D 2596-67, Measurement of Extreme-Pressure Properties of Lubricating Grease (Four Ball Method). ASTM Standards Book, Part 17-Petroleum Products, American Society for Testing and Materials, Philadelphia. H. B. Silver and I. R. Stanley, Tribology Int., 7 (1974) 113-118.