hlay. 1926
INDUSTRIAL A V D ENGINEERIXG CHE‘MISTRY
463
The Role of Oiliness in Industrial Lubrication By W. C. Wilharm WESTINGHOUSE ELI:CTRIC & MANUFACTURINO Co.,
The importance of efficient lubrication is outlined and the mechanism and possibilities for improving this vital factor as it especially applies t o electrical machinery and such other types as are not subjected to high temperatures are discussed. The property of oiliness is considered as a means of improvement and after a brief review of the literature a modified inclined plane is described as a method of measuring this property. A number of determinations were made with a brass plate and steel slider lubricated with several typical lubricants. An analysis of the results shows a difference in the lubricating value of various lubricants that is not shown in the tests usually made and throws more light on the mechanism of the property of oiliness.
NASMUCH as the service requirements of bearings are becoming more and more exacting with the inevitable result that bearing troubles are on the increase, it seemed desirable to undertake a fundamental study of lubrication from the standpoint of the lubricants themselves. I n the electrical industry especially, where machines are being built larger and heavier to meet the demand of super-power, lubrication is a real problem. The conditions are often such that the bearings cannot be increased in size in the same ratio that the machines are made larger. As a result the bearing pressures are sometimes more than twice what was considered a safe load several years ago. With such conditions existing much more is expected from the lubricant than was formerly the case. The selection of lubricants is one of the most important and least understood factors in reducing friction. The usual tests are inadequate for judging intelligently the value of an oil as a lubricant. The general tests include such physical determinations as flash, fire, cold test, specific gravity, and viscosity. Some special tests, as emulsion tests, carbon residue, etc., are made to fit certain types of lubrication. Chemical tests are made to determine whether the lubricant is neutral and to detect admixtures such as fatty oils, soaps, and fillers. Of all the properties generally measured, viscosity is the only one that is actually a measure of lubrication value. The remaining physical tests are made to insure the proper viscosity under all conditions or to iiieasure some safety factor. The chemical tests a t present do not usually apply to the lubricating value, although fatty oils and soaps are admittedly better lubricants in some respects than straight mineral oils. Fluids of the same viscosity are known to vary greatly in their value as lubricbants, so it is obvious that a test other than viscosity is required. I n view of these facts, a satisfactory program of investigation should follow about the following lines:
I
(!) Find what is meant by an ideal lubricant and a means of identifying it without the necessity for service tests. (2) Search the present supply for the ideal. (3) In case the ideal lubricant is not available, find out if it can be produced. Mechanism of Lubrication
A bearing well designed, well machined, and running a t a fair speed runs the major portion of its life on a fluid film
of lubricant of appreciable thickness.‘
This is known as the
Kingsbury, Trans. A m Soc. Mech. Eng., 24, 143 (1903); Reynolds, Proc. Roy. Soc. ( L o n d o n ) , 40, 191 (1886); Goodman, Proc. I n s f . C. Eng.. 85, Pt. 111, 376 (1885-86). 1
EASTPITTSBURGH, PA.
fluid film regime of the bearing. The friction would be entirely in the fluid film, owing to the friction of the molecules themselves, called by the Germans “Innere Reibung (internal rubbing)” or in other words, viscosity. It therefore follows that with a lubricant of lower viscosity a more economical use of power will be effected, However, the bearing does not run at all times on a fluid film. I n certain periods of its operation the film ruptures and metal-to-metal contact occurs. I n this ruptured film regime the viscosity of the lubricant is no longer the important factor. It is in this regime that the property generally known as oiliness functions. Wilson and Barnard2 define it as that property of lubricants by virtue of which one fluid gives lower coefficients of friction than another fluid of the same viscosity, generally a t low speeds or high loads. Importance of Oiliness
The fluid film regime is the ideal lubrication, but no bearing can run entirely on this film. It is when the fluid film ruptures that all the damage is done to rubbing surfaces. Most bearings go through a r6gime of ruptured film lubrication when starting and stopping, since the speed is not high enough a t these times to build and maintain the fluid film. T o insure the fluid film regime oils of higher viscosity are used, as the film-forming tendencies are mainly a function of the speed, viscosity, and load. This in turn materially increases the friction and consequently the power loss of the machine. If when the fluid film ruptures there is a property of the lubricant that materially reduces the friction and prevents abrasion of the surface, it is not so important to maintain the fluid film a t the expense of higher friction throughout the major portion of the bearing service. I n some electrical machinery the bearing pressures have become so high that it is doubtful if a fluid film forms a t all, and in these cases the oiliness property is all important. Gears,.pistons, cross heads, etc., fall in the same class, as there is no fluid film between rubbing surfaces of these types. The property would also be a good insurance against damage in the case of discontinued oil supply or some other mechanical neglect or mistake. Nature of Oiliness
It is well known that various lubricants differ in the property of oiliness, although the viscosities are about the same. This has been shown by field experience and by quantitative laboratory measurements. Fatty oils and acids invariably give lower values of static friction than mineral oils when two surfaces are rubbed together under conditions of ruptured film lubrication. It is also shown in machine work such as lathe turning and pipe threading. The mechanism of the property and the property itself are still the cause of much discussion. Wilson and Barnard2 describe the property as due to a constituent of the lubricant that forms on the surface of the metals as a tenaciously adsorbed film of colloidal dimensions preventing metal-tometal contact and thus lowering the friction. Hardy and collaborators3 in their classical researches on boundary lubrication, which is the ruptured film regime, Soc. Automofive E n & , 11, 143 (1922). Hardy, Phit. Mag., 38, 32 (1919); 40, 201 (1920); Hardy-Doubleday. Proc. Roy. Soc. ( L o n d o n ) , lOOA, 550 (1921-22); lOlA, 487 (1922); 1048, 25 (1923); Hardy-Bircumshaw, Ibid., 108A, 1 (1925). 8
May, 1926
I S D CSTRl A L A-Y D ESGINEERISG C H E X I S T R Y
mina grindings are made under a liquid of exceptionally poor lubricating properties on a plate glass covered with broadcloth. The spherical surfaces are polished by hand x i t h the same materials. The broadcloth is saturated with the grinding materials and wiped over the surfaces in much the same manner as in polishing shoes. To remove one lubricant and prepare the surfaces for another material necessitates sereral of these grinding operations with lubricant-free materials. The lubricants under examination are spread on the plate and slider alike. T'arious methods of obtaining a uniform film may be used.
Experimental
1 number of static friction measurements have been iiiacle with a steel slider on a brass plate. The surfaces were polished as described above and lubricated with sereral niaterials varying in physical properties and chemival constitution. When the results are tabulated they show fair agreement with each other. They also support some theories promoted by other investigators and suggest iiow points of J-iem of the mechariicm. The first series of tests, tabulated in Table I, was made with the brass plate and a slider with three steel pegs 0.10 inch in diameter ground and polished as flat as possible. Very small scratches running in the direction of the polishing motion were visible. The determinations were made so that these scratches in some cases crossed and in others ran parallel to the direction of motion. The lubricant was a straight steam-distilled paraffin-baie oil made from a Franklin, Pa., crude. It was not partially cracked to aid in den-axing. but was dewaxed by refrigeration and centrifuging. Its specific gravity a t 15' C. was 0.579, and it had a Yaybolt viscosity at 100" F. of 693 second.. and at 130" F. of 354 seconds. T a b l e I--.Pennsylvania L u b r i c a t i n g Oil Maximum Mean variation angle from Mean Loada Latent of slip mean No. of coeff of Grams period b Degrees Per cent detns. friction Oil spread on borh surfaces in an appreciably thick p o d 8.4 1 t o 20 minutes 9.5 8 0.141) 8.4 0 7 0.149 8.3 8.7 0.152 0 4 5.7 11.7 ... 16 hours 1 0.2Oi 0 8.8 9.1 6 0.160 0.0 3 0 10.0 0.176 n 11.5 9 10.4 0.18% 9.9 7. 1 0 12 0.173 0 10.0 7.0 6 0.176 0 10.3 6.8 6 0.182 8.9 10.1 0 0.1T9 6 0 to 1 hour 3.1 9.8 8 0.173 Oil wiped o f u,ilh clean clolh (cotton); no visible (lnt 3800 0 9.9 3.1 4 0.1T5 Washed suvfaces u'ith soap and wafer: dried with ethyl alcohol a n d surgical cotton 200 0 6.2 4.8 4 0.10s 3800 0 5.7 5.3 3 0,099 a The actual weight of the slider is ziven.. a s the exact area of contact is unknown. b The latent period marks the time the slider sets a t rest, after being placed in position, until the plane is started t o incline.
Arcording to Table I, the coefficient of friction appear< to be to a certain extent a function of the load and also of the latent period. The coefficient rises until the load beconies about 500 grams and then becomes fairly constant with a value of 0.176. The latent period also seems to have a similar effect, but owing to the small number of tests made with long latent periods, too much stress should not lie placed on them. The effect of the load is shown graphically in Figure 2. When the excess or thick film of oil was iemoved the coefficient mas the same as for the thick pool. When the film of oil was washed from the surfaces with soap and water, the coefficient dropped to a new low point. What no doubt happened was that the metal surfaces ad-
465
sorbed a film of the soap and the readings obtained were the coefficients for the soap. The surfaces were then reground under mater in the same manner as before and another series of tests was made with sweral lubricants (Table 11). T a b l e 11-Several
Lubricants Max. Mean variation angle from Load Latent of slip mean No. of Coeff. of Grams period Degrees Per cent detns. friction S o lubricant applied 200 0 37.8 12.7 8 0.776 Surfaces rubbed with surgical cotton satuvaled ?cith redistilled denatured alcohol 200 0 26.1 9.2 4 0.491 Crude oil spread o n both surfacesa 200 0 10.7 30.0 9 0.188 Oleic acid Yubbed into crude oil til?% 200 0 7.3 12.3 8 0.128 3000 0 6.8 10.3 3 0.119 Same crude from which the oil used in first series was refined. Specific gravity at 15' C., 0.868; viscosity a t 100' F., 108 seconds; a t 130' F., 71 seconds.
The close agreement of the coefficient for the crude oil (Table 11) with that for t.he refined oil (Table I) made from this crude indicates that the refining process has not been detriment'al to the lubricating property of the oil, and the great variation in the viscosities of the two emphasizes the belief that there are other properties than viscosity to consider when judging the value of an oil as a lubricant.
Lood- y o m s
of F r i c t i o n - F r a n k l i n L u b r i c a t i n g Oil 3-O.t Inch Flat S t e e l C o n t a c t Areas o n Brass S u r f a c e
F i g u r e 2-Coefficient
The brass plate mas again reground to a mirror finish and the slider with the flat surfaces was replaced by three 1-inch steel balls mounted rigidly on a plate in triangular form. A series of readings was taken using the refined Pennylvanin oil. P e n n s y l v a n i a Oil a n d S t e e l Balls Max Xean variation angle from Latent of slip mean S o . of Coeff. of period Degrees Per cent detns. friction N o lubricant applied 0 23.9 10.8 10 0.445 Pvartklin lirbricotrnp o?l spread on surface 0 to 27 minutes 10.T 16.0 10 0.188 Excess oil washed or with ethev and cotlan 0 10.6 10.6 10 0.187 0 10,s lZ.5 10 0,191 0 9.0 J . .I 10 0.158 0 to 1 hour 9.0 i.8 12 0.158 0 9.1 6.6 0.160 10 0 9.4 Y . .5 10 0.165 0 9 . i 6.2 11 0.170
T a b l e 111-Refined
Load Grams 309
369 369 369 369 369 369 369 369
The values obtained in thi, caqe check those obtained before for the same oil with flat suifaces and heavy loads. The decreased area of contact in the case of the spherical 4 d e r produces the same effect as the heavy load with the flat-surfaced slider. A large number of determinations were made without repolishing the steel contact areas and each.
INDUSTRIAL A N D ENGI MEERING CHEMISTRY
466
determination was made on a new, unused part of the brass surface. The coefficient drops after a number of these determinations have been made, as shown in Figure 3. The variations between the separate determinations also fall in magnitude simultaneously with the decreasing coefficient. The curve slowly rises again after dropping to the low coefficient, but no explanation of this can be offered a t present. The contact area of the steel showed as a brass spot when examined under a microscope and the brass showed scratches in the paths of motion. I
Figure 3-Coefficient
of Friction-Franklin Lubricating Oil Steel Balls on Brass Surface
After some changes in the apparatus as outlined previously -namely, insulation from vibrations and the contact method for detecting the initial motion-another series of determinations was made with lard oil as the lubricant. The oil was diluted with ethyl ether and applied to the surfaces with a cotton sponge. Table IV-Effect
Load Gram
369 369 369 369 369
Latent
Mean angle of slip Degrees
of Lard Oil Max. variation from mean Per cent
period 0 7.1 8.1 1 hour Cleaned surfaces with ether 6.6 0 One hour after washing 7.6 0 Again cleaned with ether 0 6.7
4.2
...
KO.of
Coeff. of
detns.
friction
7 1
0.143
and cotton 4.5 5 with ether 0 2 and cotton 4.5 5
0.125
0.116 0.134 0.117
The changes in apparatus reduced the margin of error, making maximum deviations less than 4.5 per cent in the case of the lard oil. The close agreement of these results with those obtained with oleic acid (Table 11) is not surprising since the active principle in lard oil is oleic acid. The coefficient for the heavy load on oleic acid agrees with the readings on the ether-washed surfaces for lard oil, and also the lighter load oleic acid test agrees with the lard oil tests taken after the surfaces have been allowed to stand long enough to recover from the effects of the ether wash. Theory
On examining under a microscope the surfaces where motion has occurred, it has been noted that in every case the softer and weaker metal has been cut and the harder metal has built up a spot of the softer metal over the area of contact. I n the particular cases noted the brass plate has been cut in the path of the motion. The cut shows uniformity of width and depth as far as can be seen with the microscope. The steel balls built up a brass coat, fairly round, a t the area of contact. This same effect was noted by Marvin' in making oil tests with brass and steel surfaces. The question is raised as to why the surfaces should become altered in this manner if they are actually separated from metal-to-metal
'J.
SOC.Automotive Eng., 17, 287 (1925).
Vol. 18, No. 5
contact by an adsorbed film of lubricant, one or more molecules thick. It is generally agreed that oiliness is a function of some sort of adsorbed film, although there is a difference of opinion regarding the exact character and thickness of the film. This cutting of the softer metal seems to be in agreement with the theory of Bingham8 on the mechanism of cutting fluids. The theory is that the tool cutting a piece of metal is exerting no more than a wedging action of the tool between the parent metal and what becomes the chip. I n other words, the metal is pried loose and the action of the lubricant takes place a t the rubbing surfaces of the chip and the tool and the tool against the parent metal. The conditions encountered in the motion of surfaces over each other can be satisfied by applying this theory. The energy expenditure in this motion-i. e., the friction-may be due, in part a t least, to this cutting action. Even the polished surfaces used in the static friction tests are evidently not smooth. They must be a mass of projecting asperities that interlock and subsequently wedge off the weaker metal. The lubricant functions in these minute cutting-tool effects in the same manner as it does in actual lathe cutting. Coulomb, back in 1785, proposed the theory that friction was due to interlocking asperities, the actual surfaces being frictionless. This theory has met with much opposition from several investigators. Rayleigh9 disputed the theory that a polished surface was covered with such projections. HardylO agrees with Rayleigh in saying that the projections of a polished surface are insensible and attributes the friction to the attraction of one metal surface toward another. He goes further and applies Amonton's law-that the friction is independent of the area of contact and directly proportional to the load. The reduction of friction by various chemical substances is caused, according to him, by dissipating the attractive force of the two surfaces through the adsorbed film of these substances. Assuming there is a n attractive force, as there no doubt is, and that the surfaces are contaminated or covered with a film of lubricant one or more molecules thick, there should be no tearing of the surfaces. It is not conceivable that the attractive force of two surfaces acting through an interface of lubricant would be greater than the cohesive force of the internal molecules of the mass of metal. I n practical cases there must be interlocking asperities, even with the highly polished surfaces used in the foregoing static friction tests. If it were possible to make surfaces so that there would be no interlocking, Amonton's law would hold, but when speaking in terms of practical lubrication the friction would be also a function of the area of contact. Possibly this theory would explain the effect shown in Figure 2 and also the facts brought out in Table 111. According to the laws of fluid friction there would be some increase in friction due to the thinning of the film by increasing the load. It is also possible that this decrease in film thickness allows more of the projections to interlock, causing an additional rise in friction. The fact that the curve straightens to a fairly constant value would indicate that after a definite pressure the film has thinned to the limit and higher pressure, within the scope used in the tests, cannot cause further interlocking of asperities. When the spherical slider is used the values are equivalent to the heavy load values of the flat-surfaced slider. The spherical surface has a much smaller area of contact and, being heavier, no doubt exerts a pressure equivalent to or greater than t h e flat surfaces with heavy loads. The time factor or latent period may produce the same effect by gradually squeezing (1
8 '0
Bur. Standards, Tech. Paper 204, Proc. Roy. Inst. Gt. Britain, March, 1901. Proc. Roy. Soc. (London), lOOA, 565 (1922).
May, 1926
INDUSTRIAL AND ENGINEERING CHEMISTRY
out part of the film. In the case of the facts shown in Table I11 the reduction of the friction and the decrease in margin of error are possibly due to the building up of the brass spot on the steel contact area, smoothing over the irregularities. This same effect is manifested in bearings. It is well known that a bearing runs with less friction after it has l.)een worn in. If this assumption--namely, that there are interlocking projections-is correct, several conclusions that throw more light on the property of oiliness may be drawn: (1) The secret of good oiliness would be to have a tenaciously adsorbed film of such thickness t h a t the projecting asperities could not interlock. ( 2 ) The friction would be a function of the attractive forces of the metals, the tensile strength of the metals, and the internal friction of the lubricant. This may partly explain why soft bearing metals such as babbitts give lower resistance than the harder metals.
Conclusions
Accepting the property of oiliness as being important in the selection of lubricants for industrial lubrication, a method
467
of measuring this property has been developed and found t o give fairly reliable results. With brass and steel surfaces it is evident that there is an appreciable difference between various commonly used lubricants, although this difference is not shown in the tests usually made. The values obtained are in the same order of magnitude that experience in the field and other investigators, working with other materials for surfaces, have found them to be. A comparison of a Pennsylvania crude with a lubricating fraction of the same showed the process of refinement was not detrimental to the oiliness property of the oil. h study of the observations made on the static friction tests throws more light on the mechanism of the property of oiliness. Acknowledgment
The writer desires to express his appreciation to D. R. Kellogg and his associates for their constructive criticisms and helpful suggestions that aided in no small way in making this paper possible.
Some Little Understood Factors Affecting Lubrication By E. G. Gilson RESEARCH LABORATORY. GENERAL ELECTRIC Co., SCHENECTADY, N. T.
Changes in friction cannot always be satisfactorily explained by changes in viscosity of the oil due to a change of its temperature. Oil-film friction is shown to be influenced by change of one of the metals between which the film is working, and also by changing from a n oxidizing to a nonoxidizing atmosphere. It is demonstrated by means of a complete bearing within an enclosure how the friction is affected by the atmosphere surrounding the bearing. In conclusion, it is pointed out that the facts shown cannot be explained by the viscosity-temperature changes of the oil, and it is suggested that efficient lubrication may be dependent upon a reaction between the metals of the bearing and the oil, the nature of this reaction being influenced by the atmosphere in which the bearing is operating.
N ALL bearing problems the viscosity of the lubricating oil receives great consideration. There is no doubt about the importance of viscosity in this connection, but there is some question as to the extent of our knowledge of the influence of this factor. In an effort made several years ago to obtain some definite information about oils, and incidentally bearing materials, great difficulty was experienced in getting results to check. It was finally established that differences found in friction were coincident with differences in room temperature, as shown by Figure 1, which contains typical friction curves taken on a standard machine. The only variable is the room temperature. Through an operating range up to 35 kg. per sq. cm. (500 pounds per square inch) there is a change in friction amounting to as much as 20 per cent. It is important to note that the ultimate temperature is practically the same in each case. The natural assumption from these curves is that the higher friction a t the lower temperature is due to the greater viscosity of the oil a t that lower temperature. However, a consideration of the method of the test will tend to weaken this conclusion. The test journal was 9.7 cm. (3.8195 inches) in diameter by 10.2 cm. (4inches) long. The test block was a comparatively
large piece of bronze in which were placed strips of the bearing material 1.27 cm. (0.5 inch) wide by 9.5 cm. (3.75 inches) long, spaced about 2.5 cm. (1 inch) apart. The long dimension was parallel to the axis of rotation. The oil was s u p plied to the under side of the journal by means of a wick extending its whole length. This wick fed the oil from a reservoir in which a constant level was maintained from a large supply tank. The speed of the journal was 500 r. p. m. The oil had a viscosity of 270 Saybolt seconds at 38" C. (100' F.). Fresh oil was supplied all the time, no oil being used over except that which stuck to the journal. Under these conditions it is hard to conceive of this very thin film of oil, supplied by the wick, not coming to the temperature of the journal during the approximate one-half revolution i t made before passing under the bearing surface. T h e tem-
I
0 005
1 IO
20
30
Lood- K 9 . p e r . q cm. 40
50
60
1
1
1
I 1
70
perature curves show that the ultimate temperature was approximately the same in each case; in other words, the viscosity of the oil, when it was effective as a lubricant, must have been approximately the same. Another factor, however, not recognized at 'the time, may have exerted considerable influence, and will be discussed later.
