Improved Miniature Penetrometer Cones fok Determination of lubricating Grease Consistency B. W . HOTTEN A \ D C , . I T . KIBLER California Research Corporation, Richmond, Calif.
THE accepted niethod for determining the consistencies of greases cannot. be applied to very small quantities of greaye, as has been pointed out (3,4). Two methods devised to circunivent the objectioiis to t,he A.S.T.M. method have not proved to be entirely satisfactory-for example, although the micropenetration procedure of Kaufman et al. (1, 5) requires only a very small (4-gram) sample, its reproducibility is somewhat less than is frequently desired, and its correlation with standard 150-gram cone penetratioiis varies somewhat with grease type. McFarlane’s microcone method (4)also shows some variation in correlation with the A.S.T.M. cone as grease texture is varied, and the sample size required is nearly one fourth as great as the quantity needed for the accepted -4.S.T.M. method. The purpose of the present nork was to devise penetrometer cones requiring oidy small samples of grease but giving penetmtion measuremerits re nearly related to the A.S.T.31. penetration over t h e entire range of grease texture tyFes and consistency grades. It was t,houyht that previously proposed small-size penetrometer ( m n c ~did not completely achieve this result because they did not have the proper shape and weight relationships to give the s m e shear rates during the penetration test as were obtained mith the .\.S.T.lI. cone. The cones described below were carefully designed to maintain A.S.T.31. cone shear rates under all perwtrntion conditions. I)ESCRI€’TION OF APPARATUS
In Figure 1 :ire shoi1.n details of construction of the new cones.
(.i,uciblcshas other advantages-they are uniform as to top dimensions and capacity, readily available, and easily filled with grease without occlusion of air. The quarter-scale cone is used with a magnesium penetrometer shaft, giving a total falling Teight of 9.38 grams, and a Coors Kc. 0000 crucible as sample container. Various relationships between each reduced-scale cone and the A.S.T.M. cone are showii in Table I. EXPERIMENTAL RESULTS
The relationship betveen miniature cone penetrations and A.S.T.JI. cone penetrations was invcstigated by obtaining coniparative data on the most common types of greases over the entire Iiational Lubricating Grease Institute consistency range. The standard method of test ( Z ) was used in measuring both the A.S.T.M. cone penetration and the miniature cone penetrations, except that in the case of worked penetrations with the miniature cones it was necessary to transfer grease from the worker to t h e crucible grease cup prior to measurement. Working was done in the A.S.T.M. worker for both miniature and A.S.T.M. penetrations, u-hirh were made on separate portions of the same worked sample. To obtain a truly comparative set of measurements, penetrations Kith the standard cone and the miniature cones were made ~ i t the h same penetrometer a t nearly the same time. By using the light-weight plungers and interchanging cones and supplemental weights, it was possible to make the two measurements within 30 seconds of each other in each case. Worked penetrations were obtained 15 minutes after completion of the working process to avoid making the measurements during the period of most rapid consistency change. Most greases stiffen very rapidly immediately after being agitated and more slowly thereafter. I n Table TI are listed the various types of greases upon which consistency measurements were made, and their corresponding, consistency ranges. The greases studied include both fibrous and unctuous types and were made from oils of both low and liirh viscosity.
Each linear dimension of the “half-scale” cone, except the shaft diameter, is exactly one half the corresponding dimension of the 150-gram A.S.T.11. cone. Similarly, the “quarter-scale” cone has dimensions one quarter those of the A.S.T.M. cone, except for the shaft. Thr shaft diameters of the tn-o cones are equal to that of the A.S.T.31. cone, so that they will fit the standard penetrometer. Th(, sizes of thv I I P W cones relative to the A.S.T.M. cone are shown in Figure 2. The shaft and point’ of the half-scale cone are of steel; the skirt is machined from Duralumin. The quarter-scale cone is of niagriesium except for the steel point,. The half-scale cone fits the . aluminum shaft used in the Kaufman niicropenetration 0.640’-4 procedure. A supplemental weight is used to bring the total moving weight to 37.5 grams. (A second supplemental weight. was selected which with the Icaufman aluminum shaft and A.S.T.M.cone totals 150grams, 0.025thus permitting an A.S.T.>LI. penetration and a half-scale cone penetration to I F measured a t nearly the s:me time.) lo“ 1.190” The sample cup with which the half-scale cone is used is a Coors KO,0 crucible. The choice of ,555 crucibles for grease cups was influenced bv the fact that the diameter of the rim of this size of crucible is very close to one half the diameter of the cup used with the A.S.T.M. cone. It was believed that this diIO’ mension was the critical on(’ and that neither the depth nor the sloping sides of the crucible mattered, provided that the bottom and sides did not themQUARTER -SCALE selves interfere mith the movement of the cone. These asHALF- SCALE CONE CONE sumptions wer? confirmed by Figure 1. Miniature Penetrometer Cones experiment. The usc of thesix ~~
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p2
1574
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1575
V O L U M E 2 2 , NO. 12, D E C E M B E R 1 9 5 0
where P i s the A.S.T.M. penetration and p is the miniature cone penetration. Because the percentage error of a reading inA.S.T.M. Half-Sde _ Qnuartsr-Soale _Cone ~ cuo creases somewhat with decreasing cone size, the cone cuo C"" cons half-scale cone may be preferable to the qusrtervohlme ml. .. 200 .. 19 .. 4.8 scale cone when sufficient sample is available. Approa.' weight of a ~ m .. 260 17 4.3 Pie, g. If some intermediate size cone should he desired, 85 76.5 32:s 30 1613 20 Diameter mm. Total ialijng weight. 6. 150 37.5 9.3s m e could also provide it hy adhering to the prinSurface &re&,89. mm. 7070 4600 1768 1190' 442 283 c.iple of maintaining the s a e o shear rlttus as occur Ratio, t o t d falling weight to are& 0.021 0.033 0.021 0.032 0.021 0.03a with the cones described abovo. The good corRelative diameter 1 .. 0.5 .. 0.25 .. neibtive falling weight 1 .. 0 25 .. 0.0626 .. relation obtained over the wide siac range of the Ratio, cone diameter to cones described indicates t h t this niet,l>odof scal0.86 cup diameter 0.85 ing down cone sizes is generally useful. Ratio cone surface area 1.56 1 .54 to ;"D suriaee area There is also a considerable need for a small worker havine eood correlation with the A.S.T.M. grease worker. However, difficulty may he enrount,evod in reproducing oxactly the shear rates of the larger worker.
Table I.
Comparison of Reduoed-Scale Cones and Sample Cups with A.S.T.M. Cone and Cup
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SUMMARY
Two modifications of an improved mcthod of mensuring the consistency of small samples of lubricating greme are described. The penetrometer cones used have the same shape as the A.S.T.M. cone, but their diameters and moving weights and the diameters of the sample cups have heen redured in a proportionat,e manner, so that the shear rates of the cones falling through the grease ~
Figure 2.
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A.S.T.M. (left), Half-Scale (center), and Quarter-Scale (right)Cones
The relationship hetwecn A.S.T.M. penetrations and the new miniature cone penetrations is shown in Figure 3, which summarizes the data of this investigation. No distinction is made in this figure between points for worked and unmorked penetrations; because comparative values only were desired. Kor does the figure indicate differences a8 to the oil type or texture of the grease, inaamuoh as the distinction betwoen a high and low viscosity oil and between a fihrous and an unctuous grease is hard to draw in many eases. The 6gure does indicate that linear relationships exist between the standard and miniature cone measurements over the entire range of grease consistencies and that the conversion from miniature cone penetrations to A.S.T.M. values can he made rapidly. and accurately. The deviations from the best straight lines through the points are small and of the same order of magnitude as the allowable error in an A.S.T.M. penetration measurement. The figure shows also that the N.L.G.I. consistency grade of a grease can be readily determined hy the rninin.t,ure -..nenet.romet,er ~ ~methods - ~ discussed - ~ here. The relationship beta;em the miniature penetrations and the A.S.T.M. cone penetrst ions may also be expressed mathematically by the equations ((ietermined from Figure 3): Half-scale eone Quarter-scale! cone
Figure 3.
Correlation of Miniature Cone Penetration with A.S.T.M. Penetration
samples are the same as those occurring when the full-sized A.S.T.M. oone is used. As a result, rectilinear relationships between reduced-scale cone and A.S.T.M. cone penetrations are obtained with smdl fractions (one 6fteenth for the half-scale cone and one sixtiefh for the quarter-4cale cone) of the sample required by the latter method.
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I 7 = 1.ssp 22 I ' =. 3.851, 4-22
ACKNOWLEDGMENT
The authors wish t o thank B. R. Farringtoil and R. T. Maedonald ior their interest and ztdviw regarding this investigation. LITERATURE CITED
Table 11. Types of Greases Studied Type of Soap Aluminum Caloium
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Caioium h t t ) Lithium Lithium Sodium Sodium and aluminum Sodium
Texture
A.S.T.M. Penetration Range
Am. Soo. Testing Materials. Committee D-2on Petroleum Produots and Lubneants. A.S.T.M. Bull.. No. 147,81-5 (1947). (2) Am. Soo. Testing Materials. "Standards on Petroleum Producta and Lubricants," Desisnation 217-48,p. 143 (1948). (3) Ihufman, G., Finn. W. J.. and Harrinaton. R. F.. IND.ERG. C ~ E MANAL. .. Eo.. 2, 108-10 (1939). 141 MeFarlsne, R. P..Inst. Spokesman (National Lubricatine Grease Inst.), 6,No. 12.1. P5 (1943) (1)
REOEIVED June 28, 1050.
