January, 1932
ISDUSTRIAL AND ENGIKEERIKG CONCLUSION
A research laboratory is a n organization made up of human beings with all the advantages and limitations which that involves. I n my opinion, the more really human are the members of a research organization, the more successful it will be. Or, to put i t another way, the lese; it operates along the lines of discipline, such as we think of in connection
CHEMISTRY
71
with the army, the more successful it will be. The enthusiasm of a worker-must never be dampened, although properly it 111a) directed. H e should always be encouraged when he shows a thirst for knowledge. All that may Le necessary is tactfully to guide his thirst for knowledge to the particular problems on which he should be working. RECEIVED .4ugust 29, 1931.
Flow of Petroleum Lubricating Greases4 34. H. ARVESON, Standard Oil Company of Indiana, Whiting, Ind.
I
of rate of shear is involved in the T HAS long been recognized PETROLEU?II lubricating greases' are disuse of lubricants. I n dispensing that the actual lubrication pensed and used as lubricants under a variety and in feeding from cups and of rubbing surfaces must be of conditions in which the factors determining the boxes the rate of shear involved accomplished by a fluid film if flow characteristics are of primary importance. is usually in the comparatively satisfactory results are to be Based on a norel principle, a aiscometer which low range from 0-1000 seconds- ' obtained. Xot so generally recand in the lubricating film from ognized is the fact that, a-hen predetermines the rate of shear, and which is 1000 seconds-' to indefinitely greases are employed, the luespecially designed f o r the purpose of measuring large values. brication must be accomplished the $0:. characteristics of lubricants, is described. The foregoing shows the imby a fluid film, rather than by The data at 77" F. (25" C.) on several worked portance of providing an instruthe soap structure itself. As a cup greases and a p u l p oil are presented in ment capable of producing varirule, greases are used as a matter ous rates of shear over a very of convenience in handling and graphical form. The large range of rafe qf shear large range. The numerous conapplication, rather than for any reported (0.08 to 132,000 reciprocal seconds) couers trol instruments, though very peculiar lubricating properties. the complete practical range of use. Among useful in their fields, are not hlany m e c h a n i s m s are conothers, the following conclusion is drawn: The suitable for this research because structed so that oil lubrication apparent aiscosity of greases decreases uith inof their restriction t o a narrow would be either impractical or range of rate of shear, usually extravagant. For this reason creasing rates qf shear in a manner characteristic very low in magnitude. I n most the lubricant used must possess of the particular soap used, approaching in the cases it is impossible, in the abthose characteristics which perlimit a value higher than, but of the same order sence of other data, to translate mit it t o be held in containers of magnitude as, the oil in the grease. the data from such instruments which are not liquid-tight, and to absolute units because of the to feed to the surfaces only when actual motion occurs. TLe consistency requirernents of differ- complex function of rates of shear involved. Particularly illustrative are the Saybolt tube, with its variable head, and ent mechanisms are almost as varied as the mechanisms themselves. I n the past no basic data of sufficient completeness the A. S. T. M. cone penetrometer, with its double-taper q imahave been available which mould permit accurate e-t' cone. The hfachfichael and Stormer instruments come a little nearer to being applicable to the problem but are only tion of the needs of any particular case. suitable for use on thin greases. and cover only a comparaIMPORTANCE OF RATEOF SHEAR tively narrow range in rate of shear. The use of applied pressure on liquids and plastics in capilThe viscosity of a lubricating oil is its most important property, being fixed if the temperature and total pressure lary-tube viscometers to effect various rates of flow is well are fixed. (Variation with total pressure is ~ e r ysmall in known ( 2 , 6). Pressure-riscosity data have been obtained the normal range.) The analogous property of a grease, on a variety of compositions, including paints, lacqucrs, termed the "apparent viscosity," is not fiied under the above emulsions, and gels of many kinds ( I , 2, 6). I t is the comconditions but varies with the rate of shear. Table I shows mon experience of workers in this field that, when these the rate of shear in a simplified case of a film between a 2- methods are applied to lubricating greases, certain difficulinch (5-cm.) journal and concentric with its bearing when ties are frequently encountered. Grease lumps and air pockets alter the rate of flow, and blow holes occur in the the speed and clearances are as specified. charge. These occurrences are evident only from the fact TABLE I. RATEOF S H E 4 R IN CONCENTRIC I3EARING that the data do not check a previously determined value. (Speed, 1800 r. p m.) Bulkley and Bitner ( 5 ) managed to circumvent the difficulty CLE4RAXCE RATEOF SHEAR of irregular flow by attaching a perfectly horizontal caliInch (cm) Seconds-' 18,800 0 . 0 1 (0.025) brated capillary through a trap to the metering capillary 188,000 0.001 (0.0025) and noting the rate of movement of a mercury plug therein. 1,880,000 0.0001 (0.00025) Their instrument has the further advantage that the same I n the simple case of a material flowing through a tube of sample can be used repeatedly. l/a-inch (0.32-cm.) bore at a rate of 3 cc. per second (0.4 CONSTANT-SHEAR VISCOMETER pound per minute), the rate of shear is approximately 1000 seconds-'. It appears then that an extremely large range To avoid the above difficulties and to effect greater precision and ease of operation, a method, believed to be novel, 1 The term grease is used throughout in the restricted eense of a soaphas been developed, consisting simply in fixing the rate of thickened mineral lubricating oil.
72
INDUSTRIAL AND ENGINEERING CHEMISTRY
flow (with a given capillary, therefore the rate of shear) and measuring the resultant pressure at the entrance to the capillary. This principle is illustrated diagrammatically in Figure 1. When a piston, PI is driven mechanically a t a constant rate into a cylinder, C, the sample, S, is forced downward against the mercury bed, H g , and out through capillary 0 mounted in the removable support, D. Mercury is displaced through a duct to the gage until equilibrium is reached, a t which time all
FIGURE 1. PRINCIPLE OF CONSTANT-SHEAR VISCOMETER
the flow is through the capillary. The rate of efflux (or the rate of flow in centimeters per second) is thus predetermined by the speed of the piston and the bore of the cylinder, and the pressure may be read as frequently as desired. This gives a continuous direct means of observing the conditions existing in the capillary. The following definite advantages were. expected and all have been realized when pressures were determined on the mercury bed: 1. Passage of air bubbles-and grease lumps can be detected and the data interpreted accordingly. 2. A large number of observations a t equilibrium are possible and are the only readings required during the run. 3. One capillary can be conveniently operated orer a large range of efflux rites. 4. No blow holes occur. 5. A source of constant pressure over a large range is not required. 6. The capillaries are readily interchangeable. 7. The pressure differential across the capillary is measured directly.
Vol. 24, No. 1
cylinder I , forcing the oil out of cylinder through duct J , through pressure block K into viscometer L,forring disk M downward. The mercury from the bed about capillary N moves through duct 0, then through the pressure block, P , to the manometer at R until equilibrium is reached. The sample between disk M and capillary support N - 1 is forced out through the capillary, N - 3 (see small detail). The method of mounting the capillary N -3 is shown. The glass capillary, 3, is ground to a shoulder as shown a t 2, and cemented into place a t 4 in the support, 1. The bottom of the support is threaded so that the whole acts as a bolt to be fastened into position. The top opening in the viscometer is closed by a plug, X , consisting of two parts, 1 and 2, separated on the flat face by a rubber washer. The viscometer is in a thermostat, V , controlled by mercury regulator 5". The bath liquid was thoroughly agitatcd. Oil was first used as the liquid, but thi. was found to be unsatisfactory. A 15 per cent soluble oil emulsion was used with excellent results. The heat transfer was satisfactory, and no corrosion of the steel occurred. The pressure blocks, K and P , are constructed so that gages, $9,and manometers both funttion with the Bridgman plug in the back position, while only the gages function when the plug is in the foraard position. The plugs are operated by two valve wheels as shown &. A detail, TY, of the Bridgman tube connections is shown. The block has R duct, 3, a plain portion, 1, into ahich 4 is fitted, and a threaded section, 2, to hold plug 8. Between head 4 and plug 8, t w o copper \\ashers, 6, and a rubber washer, 7, are placed. Tension on the rubber seal is furnished by plug 8. When pressure is built up in the system, it applies t o the full end surface of 4, producing a force which is transmitted to the rubber washer, 7, mid causing pressures higher than the internal pressure, thus maintaining a seal. For further comments on Br idgman's developments in high-pressure construction, see his original discussion (S).
No trouble of any kind has been experienced with the seals. The system is absolutely tight. The eight speeds of the piston differ by approximately equal logarithmic steps, making i t possible to cover an extremely large range in rate of shear with a given capillary on one sample. The speed of the piston covers the entire practical experimental range, since the time required for a full stroke for the highest speed is 4 minutes, and for the lowest, 6 days. Each one of the equilibrium pressures was determined in ascending order, so that the manometer readings might be obtained without
When the rate of shear is to be increased by reducing the radius of the capillary, the pressures required to produce the specified rate of flow increase very rapidly. This, coupled with the fact that plugging of the capillary causes the rapid development of high pressures, necessitated rugged construction. The design was for 20,000 pounds per square inch (1400 kg. per sq. em.) on the piston, and pressures of that magnitude have been developed in use. As shown in Figure 2, a synchronous motor, A , drives an 8-speed transmission, B , which in turn drives a pair of worm
gears, C, functioning as nuts drawing forward the screws, P , attached to the head, G, riding on the rails supported hy plates D and E. The piston, H , constructed on the Bridgman principle (3) is attached to head G , and in operation is forced into
L FIGURE
2.
SCHEMATIC
DRAWING OF CONSTANT-SHEAR
VISCOMETER
(I+'igurf:6) M U Im.p:m:d whidi sliuws liiies of constant shear for tlie relation hetween the npparcnt viscosity and the percentage of soap. Figure 7 is an apparent 'I rate of shear diagram of tlie data on and a pulp oil (aluminum oleate soap era1 oil), all containing a parafin oil of lower viscosity than tlint in the greases shown in Figure 5. I t must lie einpliasized that the charts are Tile ertrenie variations in apisity should be noted. For inapparent viscosity of grease VI (Figure 5 ) was 100,000 lxri shear of 0.1 set:ond-', and only 8.4 poises at 100,000 seconds-', as compared wit11 the oil hasing a constant viscosity of I .e poises, a t the fixed tcniperatine 7 i " F. ('25' Tile agrecmeiit between cajii1larir:s (indica& ing tire absence of slilipage a t the junctions) may In! noted from a study of the points for the several capillaries, althnugli ilicrc seeins to lit: some evidence that slippage riecurred :it tlie loner rates oil capillary 3. With the liiiriier I'rcases diliicultv \vas esimienced. rriving to failure of the nicrcury to deform the grease, whicli allo~veilthe latter to enter the gage connection iind intorfere nit,h the tiecessnry rapid transfer of pressiires. The miicdient adopted wts to read tlie pressures aliove tlic di,& ill, wit,li and without tlie capillary in position. The iiso of t,lie first pressure value gave the upper, and the dill'eerencc garc the lower limit of pressure rctpired tluuugh the capillary. I n later work on
e.).
I, = 1711 = P = S =
length of e&!!:n?; em. rate of eEltius, e m 3 per secund stress, lJy.i>csper cm.2 rate of sheer, secinids-'
The syrobols are those convcntional in the scic:t!ct! of rhedogy. T h e di~nensiimrcquircd were detcrniiiird in the
,.
conventional manncr. I he mnasurciiietits were entirely in th Furtlieriiiore, the kinetic-eirrgy rorrwtioi and ni, eorrrctiiin was ixtntie for end cff have l m n irtade for tlic c n p i h r y head ~ v l i e n r q i i i r i d . It was found that, as the prcxssores increasctl, it dcereasc ill the visaisity of oils iicciirrd, cnx-inx to tlic d of Iicat i n t,lia eapikiry. Suflicictit dirt:% weue oiitaincd to permit errrectirrns for this e t h t wer the liirver ranges. The percentage of decrease was propi~rtioiiaionly to the pressure ihserred Ijiid amounted to I0 per cent at olxs!rvcd prcssurr of 3iK) pounds pcr squ:irc inch ('21 kg. per srl. r n . ) The data are given graphically and m r e iilitairied oil ~ o r k e dgreases (60 stnkcs i n tho A. 9. T. M. greasc churn) at one ternperatore, 77" F. (23" C.). APSAItENT ~'ISCOSIl'Y, A 14'USCNOS OB'
Ih'XN
OF SHEAR
Figure 5 is an apparent viscosity-rate of drear dingr;lm of a series of cup greases (caleium s o a p of mixed fatty acids in mineral oil) containing the same oil, but varying amounts of soap. Interpolating from this fipurr, R plot
F~curm4.
Yr~w OP 1 3 1 . m ~ Shown viscnmt:lcr. conirols. pofenw ~ ~ ~ ~ ~ r n Expansion elcrs. v a h e from ammonia coo~pressorat upper right. lioincI,cr,
~ n t l~
other hard greases, the simple expedient w:is used of irrcreasing thc depth of the bed aiid mcasuring tlie ]iressures directly on the lied. The upper- and lomr-limit valiirs are sliown by srilit points in t,he diagrams, the curve being drawn intermediate in a manner indicated by thorough study of the conditions involved. The upper-limit value would be the one obtained in a pressurr-viscometer of the fixed-pressure type. The spread
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 24, No. 1
between the split points shows the inaccuracies that may be thus encountered and definitely stresses the importance of obtaining the pressure at the entrance to the capillary whenever possible when plastics are concerned. The mercury bed described makes this possible.
in the same region. I n order to demonstrate that these diagrams represent, as the variation of one property, several characteristics described by the rather vague terms of body, consistency, toughness, tenderness, false body, etc., commonly used in the trade, the following exDeriments are described: The pulp oil, when ilaced in a beaker and tilted, will flow very easily, while the No. 0 cup grease will not. This is represented by that portion of the curve a t very low rates of shear; e. g., S = 0.1. The cup grease is said to have more body. If the two samples are stirred very slowly with a rod, the same will be said to be true to a lesser extent. The rate of shear is somewhat higher; e. g., S = 1 t o 4. If now the two samples are stirred vigorously, the pulp oil will offer as much, if not more, resistance than the cup grease. The cup grease is said to be tender while the pulp oil is relatively tough. This involves a still higher rate of shear; e. g., S = 300 to 1000. Occasionally the term “false body” is used as synonymous with “tenderness,” though it is usually used in a similar connection when 0.0 IO 100 I000 l0,OW 100,000 I,OW,WO worked and unworked consistencies are inR A T E OF SHEAR -SECONDS-’ volved. VISCOSITY-RATE OF SHEAR DIAGRAM OF SERIES OF FIGURE 5. APPARENT With the lower and upper ranges of rate of CUP GREASES shear fixed by the reauirements of use. that LIMITINQVALUE OF APPARENTVISCOSITY grease having the lowest apparent viscosity a t intermediate ranges of rate of shear a t the temperature of operation will A survey of Figures 5 and 7 serves to demonstrate beyond be the most economical of pqwer in bath-type lubrication. any doubt that, as the rate of shear increases, the apparent viscosity of the grease approaches some limiting value, higher Is THEREOKE GENERALRELATIONFOR EACH KIND OF than, but of the same order of, magnitude as that of the oil SOAP? in the grease. This may seem obvious, even in the absence A close examination of the curves in Figure 5 suggests of the data, but does not seem to be universally accepted. To illustrate, certain investigators were surprised t o find that they may all be part of one general relation, which is that, while the initial torque of ball bearings freshly filled characteristic of each class, and involves, besides the rate with the proper grease was dependent on the consistencyin other words, on apparent viscosity a t low rates of shearthe torque and rise in temperature a t equilibrium conditions were primarily dependent upon the viscosity of the oil in the grease. These findings are obvious in light of the diagrams given. I n general, the apparent viscosity a t low rates of shear governs the dispensing and feeding characteristics of the lubricant, and diagrams such as Figures 5 and 6 may be used to predict what changes would be effected by certain changes in the grease. For example, if a certain piece of dispensing equipment used on grease I11 (Figure 5) required pressure of 200 pounds per square inch (14 kg. per sq. cm.) a t the drum, and it was desired to change to grease IV, and the rate of shear was known to be approximately 100 seconds-’, the pressure required for grease IV could he calculated as follows: 52
X 200 lbs. per sq. in. = 560 lbs. per sq. in.
(39.4kg. per sq. cm.)
The values 145 and 52 are the apparent viscosities of greases IV and 111, respectively. The similarity in shape of the curves fortunately makes the accurate estimation of the rate of shear unnecessary in this type of problem where a change between greases of one class is involved. I n the above example if the rate of shear should have been either 40 or 400 seconds-1, the pressures required for grease IV would be 630 and 475 pounds per square inch (44.3 and 33.4 kg. per sq. cm.), respectively. The pulp-oil curve in Figure 7 was included to illustrate that different kinds of soap may produce different trends
FIGURE6. APPARENT VISCOSITY-PER CENT SOAPDIAGRAM. Rate of shear as parameter. of shear, factors characteristic of the oil and soap, It is impossible to set up this relation a t the present time, but, as additional data are accumulated, it is expected that such general relations may be evolved which will be characteristic of greases of a given type.
January, 1932
IXDUSTRIAL AND ENGINEERING CHEMISTRY
A study of the application of equations for plastic flow published by Bingham (d), Buckingham (4), de Waele ( 8 ) , Williamson (9), Reiner ( 7 ) , and others, and the equations presented by Peak and MacLean before the Society of Rheology in December, 1930, but as yet unpublished, has been made, but is too involved for complete discussion at this time. It was not found possible to fit any of these equations to the data. The Williamson equation gave qualitative agreement but could not be fitted accuratelv. Empirical equations have been found to fit some of the‘ data and will be published later.
CONCLUSIONS 1. A viscometer has been developed which has proved very useful in obtaining data on the flow of greases in capillaries over a very large range of rate of shear. The range reported is from 0.08 to 132,000 seconds-’. 2. The apparent viscosity of petroleum lubricating greases a t any given rate of shear increases with increasing concentrations of soap in a manner shown by the figures.
CONCEPTION OF GREASESTRUCTURE I n the course of this work it was noticed that lime-soap greases, which can be worked to substantially constant consistency in the A. S. T. 14. grease churn, could be forced through the capillary of the viscometer without any noticeable additional loss in consistency. This mas true even when very high rates of shear were employed. Further, the curves expressing the relation between the apparent viscosity and the rate of shear (see Figures 5 and 7 ) are obriously members of a definite system which could not well exist if actual severing of elements of the structure occurred. This behavior suggests that the structure of 0’ I I 1 4M OJ 10 IW iOW 10,000 100.000 ~,000.000 a lime-soap grease may consist of the interlacing R A T E O F SHEAR, SECONDS-’ of flexible solid members, which may be in FIGURE 7. APPARENT VISCOSITY-RATE OF S H E 4 R DIAGRAM OF SEVERAL tlitniselves deformed or merely separated by GRE.~SES.They contain an oil common to all. the shearing action. but which are not actuallv 3. The apparent viscosity of greases decreases with inruptured. I n other words, any shearing acticln merely distorts the individual members and reduces the average amount creasing rates of shear in a manner characteristic of the of interlacing. which accounts for the tendency toward work- particular soap used, approaching in the limit a value higher ing to a constant consistency representing the condition of than, but of the same order of magnitude as, the oil in the maximum possible deformation of the members under that grease. particular shearing condition. If, furthermore the members 4. The diagrams of the type illustrated may be used to be elastic, they will tend, after severe deformation, to assume predict the behavior of greases in use. their previous forms. This, in general, explains the fact 5 . The nature of the apparent viscosity-rate oi shear that a worked grease approaches a limiting value a t very diagrams suggests that one general relation for each type high rates of shear-the lowest conceivable limit being at of grease may exist. Additional data are required to subleast slightly above the viscosity of the oil--but resumes stantiate this. its worked consistency immediately upon issuing from the 6. Sone of the equations for plastic flow to be found in capillary. Elasticity of the members surrounded by a vis- the literature appear to be applicable to the data obtained. cous medium would also explain the tendency of worked lime-soap greases to increase in consistency on aging. LITERAT~R CITE:D E The common soda-soap greases have much longer and coarser struclures than do the lime-soap greases, as is evi- (1) Barr, ”Monograph on Viscosimetry,” Oxford University Press London, 1931. denced by their fibrous nature, opacity, and tendency to (2) Bingham, ”Fluidity and Plasticity,” McGraw-Hill, 1922. leak oil in storage. On working, these greases do not give (3) Bridgman, Proc. Am. Acad. A r t s Scz., 49, 6 2 7 4 3 (1913-14). constant penetrations in 60 strokes in the A. S T. 11. grease (4) Buckingham, P r o c . 4 m . Soc. Testing 3faferzaZs, 21, 1154 (1921). churn, but work down to very thin, less fibrous, products. (5) Bulkley and Bitner, Bur. Standards J . Research, 5 , 83 (1930). It appears then that the individual structural members of (6) Hatschek, “Viscosity of Liquids,” Van Nostrand, 1928 (7) Reiner, J . Rheology, 1, 11 (1929). these greases differ in size, shape, and structural strength (8) Waele, de, J . 011Colour Chenb. .4ssocn , 6 , 33 (1923). from those of the lime-soap greases and are apparently (9) Williamson, ISD.Exo. CHEM.,21, 1108 (1929) actually ruptured on working. Though quite distinct, this conception doea have points RECEIVEDAugust 24, 1931. Presented before the Division of Petroleum of similarity with the conception of pseudoplastic flcm pro- Chemistry a t the 82nd Meetlng of the American Chemioal Society, Buffalo, N. Y., August 31 t o September 4, 1931. posed by Williamson (,9).
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