Transactions

Flow P roperties of L ubricantsU nder H igh P ressureBy A. E. NORTON,1 M. J. KNOTT,2 a n d J. R. MUENGER3In th is paper, results are given o f a prelim inary stu d y o fthe rate-of-shear versus shear-stress relationsh ip for severaloils know n to undergo apparent solidification w hensubjected to h igh pressure. Lard, rapeseed, sperm , andone m ineral oil were tested under a tem perature range o f —5C to 20 C w hile subjected to pressures up to 50,000 psi.Experim ental curves o f flow versus pressure difference wereobtained for capillary flow, and th ese curves were tra n s­form ed m athem atically to th e desired curves o f rate o fshear versus shear stress. A b rief d iscu ssion o f som e o fthe problem s inheren t in capillary testin g o f p lastic m a ­terials is included in th is report.FOREWORD BY M. D. HERSEY4T HE following contribution is one of a series of investigationson the properties of lubricants under high pressure,conducted by the Special Research Committee on Lubricationof the Society. These studies were begun at Harvard Universityin 1915, and reported in various publications dating from1916. They were more completely outlined in a paper by HenryShore and the writer at the Annual Meeting of the Society in1927 (l),5 and in a joint paper with R. F. Hopkins (2).A phenomenon cautiously termed “apparent solidification,”produced by increasing the pressure on a lubricating oil at constanttemperature, was briefly described in the first of these twopapers. Future experiments were recommended in order to determinethe flow or shear characteristics of the lubricant—in aword, its consistency—while in that condition. Is it a hard solidlike that formed by the freezing of water into ice, or a soft jellymore like an oil at its pour point? And how does its consistencyvary when the pressure is increased well beyond the critical valuefor solidification?This phenomenon was confirmed by Robert Kleinschmidt atHarvard (3) and by Yoshio Suge in Tokyo (4). Shore found anempirical relation connecting solidifying pressures with temperatures(5), while Cragoe discussed the question theoretically inthe light of Clapeyron’s equation (6). I t remained for ProfessorNorton, assisted at the start by Knott and later by Muenger, tocarry through the first quantitative measurements.It appears that castor oil and naphthene-base mineral oilshave not been solidified by pressure, and that the only lubricatingoils for which pressure solidification has been reported are lard,horse, rapeseed, whale (including sperm), mineral oils containingsufficient paraffin wax, and compounded oils. We may add tothis list crude petroleum, oleic acid, and any pure substance whosefreezing points have been determined under pressure.The experiments to be described constitute a preliminary phaseof the project. They are reported at this time to provide a recordof the work accomplished under Professor Norton’s supervision.The report was compiled by Mr. Muenger in consultation withMr. Knott and others concerned. Through the courtesy of DeanWestergaard and Professor Den Hartog of the Graduate Schoolof Engineering, Harvard University, arrangements have beenmade for the continuation of the research for a limited period.The principal difficulties outstanding are due to the relativelylarge pressure differences thus far employed in the observationsof capillary flow, and to the further fact that the lubricant undertest is neither unworked nor completely worked but is in someintermediate, undefined, partially worked condition. In spite ofthese uncertainties, the progress report at hand reveals the orderof magnitude of the effects in question, thus providing a firstapproximation to the data required. These results are given inabsolute units, with rate of shear plotted against shearing stress,in the last four diagrams of this paper.Professor Norton believed that such investigations are of educationalas well as scientific and industrial value. This will beevident from the closing paragraph in his discussion of a recentpaper on “Teaching Lubrication” (7):“Any graduate course should aim not only to prepare engineersfor advancing the science and art of lubrication but also togive these men a unified knowledge of materials. If properlytaught, the subject of lubrication can be allied with the studyof elasticity and plasticity, especially with the latter, sincethe rate of shear is an important feature of both liquids andplastic materials.”INTRODUCTIONWhat is perhaps the most important characteristic of a lubricatingmaterial can be defined by the relationship between theshear stress S, applied to the material and the resulting rateof shear R. For example, a Newtonian liquid is a material whoseshear behavior can be represented by a straight line passingthrough the origin. Its viscosity S/R and fluidity R/S are constantat all values of shear stress for any given pressure and1Late Gordon McKay Professor of Applied Mechanics, Harvard temperature. Most lubricating oils at ordinary pressures andUniversity, Cambridge, Mass. Mem. A.S.M.E. Deceased, February temperatures have this type of graph.24, 1940.Other materials which may be represented by a nonlinear2 Brown & Sharpe Manufacturing Company, Providence, R. I.curve, passing through the origin, are known as non-NewtonianJun. A.S.M.E.a Assistant in Mechanical Engineering, Harvard University, Cambridge,Mass.terials whose graphs have intercepts on the axis of shear stressliquids. Rubber suspensions fall into this class. Yet other ma­4 Research Director, Morgan Construction Company, Worcester, are known as plastic solids. They require an initial value of shearMass. Fellow A.S.M.E.stress known as the “yield” shear stress So to start the flow. If1 Numbers in parentheses refer to the Bibliography at the end o fthe paper.the plastic solid has a straight-line relationship, it is known as aContributed by the Special Research Committee on Lubrication Bingham solid and can be represented by two parameters, itsand presented at the Annual Meeting, New York, N. Y., December yield shear stress So and “mobility” R/(S — So), which is analogousto the fluidity of liquids.2 -6 , 1940, of T h e A m e r ic a n S o c i e t y o r M e c h a n i c a l E n g i n e e r s .N o t e : Statements and opinions advanced in papers are to beunderstood as individual expressions of their authors and not those of Typical curves for these various types of materials are indicatedin Fig. 1. There are other definitions of ideal materials and,the Society.631