Transactions A.S.M.E.

CONNELLY—INFLUENCE OF CRYSTAL SIZE ON WEAR OF HIGH-LEAD BEARING METAL 317Tin, per cent.....................................................84 to 86Lead, per cent.............................................. ....0.035 maxAntimony, per cent..........................................9.5 to 10.5Copper, per cent.......................................... ....4.5 to 5.5Sections from both halves of the bearing were given metallographicexaminations. A representative structure is shown inFig. 19 of this discussion. It contained extremely large cubesof Sn-Sb, and rather short thick needles of Cu-Sn in the alphasolidsolution of Sn and Cu and pseudoeutectic. At the magnificationused, this latter constituent could not be resolved, butthere was a considerable portion present.The conditions attending the pouring and cooling of thisbearing were not obtained and, since no analysis was made, it wasassumed that the composition was correct.That the pouring temperature was correct is revealed by therelatively short thick needles, some of which are arranged in astarlike shape. Also, the pouring temperature, except in so faras it affects the rate of cooling, is insignificant in its action towardthe formation of such large cubes of Sn-Sb.The size of these cubes, the area of each of which, as shown inFig. 19, is about 10 times normal and their uneven distributionthroughout the matrix are the only conditions present which canconceivably be the cause of the failure. The cause of thesevery large cubes can be attributed to the bearing alloy beingtoo slowly cooled. The reason for the failure of the lower halfwas probably due to the much higher loads it had to carry; theabsence of failure in the upper half was because of the comparativelylight loads to which it was subjected.The unsatisfactory service secured from this bearing appearsto have been due to the presence of extremely large, poorlydistributed crystals of tin-antimony compound in the bearingmaterial, resulting from too slow a cooling rate after pouring.This is one of many similar bearings we have examined and ourfindings would seem to substantiate very definitely the author’sdata.F. C. L in n .6 The author has presented an enlightening paperupon the crystalline structure and its effect upon wear and loadcarryingcapacity of the bearing when the operation is withinthe thin-film region.Under “Conclusions” he states that “ (a) crystalline structureis a factor and crystal size an important variable in thin-filmlubrication.” In reviewing Figs. 15 to 18, inclusive, it wouldappear that the linear dimension of the crystal has a very criticalrange, at which the rate of wear is large and the equilibriumpressure low. Outside of this range it is noted that the rate ofwear is relatively low and the equilibrium pressure high. Thisindicates that the size of the crystal should be held within reasonablelimits during the casting cycle.It is not quite understood how the author arrived at the conclusion(c) in which he states that “pressures at beginning andend of the lubricated wear are related.” A further discussion ofthis point would be appreciated.The writer requests that information be supplied on the following:1 What effect has hardness upon wear?2 What effect has hardness on rate of wear and equilibriumpressure?3 Referring to Fig. 6, on what surfaces were the wear testsmade?R. G. S im a b d .6 The scope of this investigation on the relation6 Turbine Engineering Department, General Electric Company,West Lynn, Mass. Mem. A.S.M.E.6 Research and Development Department , The Atlantic RefiningCompany, Philadelphia, Pa.of bearing-metal wear to structural characteristics is especiallycommendable and, in this writer’s opinion, constitutes a fruitfulapproach to the problem of wear phenomena. The directness ofthe method employed appears to have definite worth in fundamentalinvestigations of this nature. Although there is no pretensethat the work is in the category of simulated testing, thetime is not considered inopportune again to remind ourselvesthat wear resistance is a consequence of service conditions ratherthan an inherent property of matter. Application to practice ofthe results of this or similar investigations should be attendedonly with extreme caution.An investigation of the wear quality of lubricating oils in relationto representative bearing metals along lines suggested bythe work of Karelitz and Kenyon at Columbia has been underway for the last 3 years in the laboratories of the research divisionof the Atlantic Refining Company. A battery of 6 identicalmachines, carefully constructed to eliminate running inaccuracies,was employed for the purpose. The standard metalspecimens were prepared under rigid specification of compositionand casting procedure. Results soon indicated, however, thatreliance could not be attached to micrometer indication of rateof wear, since the depth of the wear groove was of such magnitudethat its measurement at the end of a 5:1 lever was seriously disturbedby temperature fluctuations and slight inaccuracies of theshaft. Measurements have accordingly been confined to comparativetotal wear after an operating period sufficient to insureattainment of equilibrium.The following data have been singled from our files as illustrativeof that which predicated the change in procedure and thesecomments:M etal..................Oil........................Shaft...................Rpm .................... 1760Load................... 60 lb (nominal)S.A.E. No. 62 bronze (hard cast)S.A.E. No. 30 acetone-benzol dewaxed nitrobenzenerefined motor oil (without film-strengthadditive) at 130 ± 5 F‘/s-in. ground tap and reamer stockMeasured Indicated CalculatedTime groove width, groove depth, groove depth,Unit (days; in. in. in.1 5 0.0139 0.0003 3.75 X 10“*2 7 0.0173 0.0002 5.63 X 10-»3 9 0.0149 0.0005 4.37 X 10"*4 11 0.0166 0.0004 5.63 X 10~*5 13 0.0154 0.0005 4.37 X 10-*6 13 0.0143 0.0002 3.75 X 10"*A u t h o r ’s C l o s u r eIn discussing the photomicrograph of the failed bearing, Mr.Hunter has opened up a field about which comparatively little isknown. Since preparing the original paper, the author has investigatedseveral additional methods of etching and is at presentof the opinion that no one method gives a complete picture of themetallographic structure for the high-lead metal described in thepaper. The electrolytic etch described by Weaver7 has beenquite instructive. Whether the same applies to the high tinmetal cannot be stated, but it is entirely possible that somefurther critical conditions were not revealed by the etching agentemployed.Conclusion (c), “pressures at beginning and end of the lubricatedwear are related,” was made after study of the data shownin Fig. 14 of the paper, where the ordinate is the pressure at thebeginning of lubricated wear (see Fig. 8) and the abscissa is thepressure at the end of lubricated wear. Mr. Linn raises thequestion of the effect of hardness on wear. Referring to Fig. 6,it is seen that CS-0, the specimen having the lowest rate of wear7 “Type Metal Alloys,” by F. E. Weaver, Journal of Institute ofMetals, vol. 56, 1935, pp. 209-233.