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NORTON, KNOTT, MUENGER—FLOW PROPERTIES OF LUBRICANTS UNDER HIGH PRESSURE 637Company, and the National Bureau of Standards donated materialor loaned equipment.BIBLIOGRAPHY1 “ Viscosity of Lubricants Under Pressure,” by M. D. Herseyand H enry Shore, Mechanical Engineering, vol. 50, 1928, pp. 221-232.2 “ Collected Results on Viscosity of Lubricants Under Pressure,I—F a tty Oils,” by M. D. Hersey and R. F. Hopkins, Journalof Applied Physics, vol. 8, 1937, pp. 560-566.3 “Experim ents by R obert Kleinschmidt on the Viscosity ofLubricating Oils Under High Hydrostatic Pressure,” Trans. A.S.M.E.,vol. 50, paper APM-50-4, 1928, pp. 2-5; Mechanical Engineering, vol.50, 1928, pp. 682-683.4 “Viscosity of Oil Under Pressure,” by Yoshio Suge (in Japanese),Bulletin of the Institute for Physical and Chemical Research,Tokyo, vol. 11, 1932, pp. 877-894; vol. 12, 1933, pp. 643-662.5 Discussion on “ Viscosity of Lubricating Oils,” by HenryShore, Mechankal Engineering, vol. 50, 1928, pp. 682-683.6 “ Changes in the Viscosity of Liquids W ith Tem perature,Pressure, and Composition,” by C. S. Cragoe, Proceedings, WorldPetroleum Congress, London, vol. II, 1934, pp. 529-541.7 Discussion on “ Teaching Lubrication,” by A. E. Norton,Journal of Engineering Education, vol. 30 (N.S.), June, 1940, pp. 880-882.8 “A New Consistometer and Its Application to Greases and toOils at Low Tem peratures,” by R. Bulkley and F. G. Bitner, Journalof Rheology, vol. 1, no. 3, 1930, pp. 269-282.9 “High-Pressure Capillary Flow,” by M. D. Hersey and G. H.S. Snyder, Journal of Rheology, vol. 3, no. 3, 1932, pp. 298-317.10 “ On Plastic Flow Through Capillary Tubes," by E. Buckingham,Proceedings American Society for Testing Materials, vol. 21,1921, part 1, pp. 1154-1156.11 “ Future Problems of Theoretical Rheology,” by M. D. Hersey,Journal of Rheology, vol. 3, no. 2, 1932, pp. 196-204.12 “tjber die Viskositat und Elastizitat von Solen,” by B.Rabinowitsch, Zeitschrift filr physikalische Chemie, Bd. 145, Abt.A-l, 1929, pp. 1-26.13 “ Explicit Formulas for Slip and Fluidity,” by M. Mooney,Journal of Rheology, vol. 2, no. 2, 1931, pp. 210-222.14 “ Flow Characteristics of Lime-Base Greases,” by J. F. T.Blott and D. L. Samuel, Industrial and Engineering Chemistry, vol.32, no. 1, January, 1940, pp. 68-72.15 “ Thixotropy,” by H. Freundlich, Herm ann, Paris, 1935.16 “ Thixotropy and Plasticity,” by E. L. McMillen, Journal ofRheology, vol. 3, nos. 1 and 2, 1932, pp. 75-94, 163-195.DiscussionL. J. Bradford.6 Advances in science are made in threestages: (o) New phenomena are observed, (b) these phenomenaare studied to discover and interpret their meaning, and (c) thephenomena are usefully applied. The late Prof. Norton and hisassociates have, in the work described in this paper, accomplishedthe first of these. The interpretation and application ofthese data will follow. In the development of these phases, allthose interested in the work should participate.Examination of Figs. 16 to 19, inclusive, indicates that in allcases the curve of rate of shear plotted against shearing stress issubstantially a straight line passing through the origin for apressure of 10,000 psi. It may be concluded that this is also truefor all lower pressures. At 14,000 psi this condition ceases. Therate of shear rises more rapidly than does the shearing stress, andthe curve is concave. Extrapolation of the curves for this andgreater pressures yields an intercept on the shear-stress axis.They are clearly the curves of plastic substances.Curves for 18,000 psi in Fig. 16, and for 18,000, 23,000, and27,000 psi in Fig. 17, show another peculiarity. It will be seenthat each is composed of two substantially straight lines joinedby a curve. It is quite possible that the other curves would showthe same characteristic had they covered wider ranges of shearstress. This suggests that the oils investigated pass from New­6 Professor of Machine Design and Research Assistant, The PennsylvaniaState College, State College, Pa.tonian liquids to plastic solids at some pressure between 10,000and 14,000 psi.These plastics are of the Bingham type and have a dual consistency,depending upon the rate of shear to which they aresubjected, the two types being connected by a transition region,lying roughly between rates of shear of 10,000 to 15,000 reciprocalsec.Another fact of considerable interest and importance which hasbeen noted is the relationship of time to the transformation of theoils from Newtonian liquids to Bingham solids. This is of importancebecause any attem pt to make use in bearing design of theelevation of viscosity caused by pressure must be limited to thechange possible while the oil is in the load-carrying region. Thisis usually only a fraction of a second. Quite possibly the pressureeffects will not appear at all.The work described by the authors is obviously incompleteand should certainly be continued. The range of the investigationinto the rate of shear versus shear stress should be considerablyextended. The effect of time and work should be thoroughlyinvestigated, and it might be found worth while to look into theeffect caused by repeated and rapid application of pressure.The Special Research Committee can perform a valuableservice to the science of lubrication by using its influence tofurther the investigation of the phenomena described by theauthors.R. B. Dow.7 The authors are to be congratulated as the first tooffer quantitative data on flow properties under high-pressuredifferences in the congealed state. It has been recognized for sometime in lubrication practice that “pumpability” at low temperatureis a property not described adequately in terms of viscosityof the lubricant alone. This paper indicates a start in the rightdirection and it is to be hoped that further work will eliminatesome of the errors and difficulties which were experienced by theauthors.It is to be pointed out, however, that these experiments give noinformation about solidification in the thermodynamic sense,and the nature of freezing as understood in the sense of Clapeyron’sequation must still remain an open question. It would be desirableto determine freezing of a lubricant by compression by thefree-piston method, a method which enables the volume changesto be followed. The writer has plans projected for an experimentof this kind. It is hoped that the sharpness and extent of freezingof a variety of lubricating oils can be studied and the results correlatedwith their various chemical and physical characteristics.Regarding the data of the present paper, it would appear thatfew generalizations can be made since the results show clearlythat the history of the pressure treatment is a vital factor, which,from the nature of the conditions, is to be expected, for example,Figs. 7 and 10. The data of Fig. 5 show that a 2-hr applicationgives uniform results and reproducibility; evidently equilibriumconditions are being approached in this case. However, it is tobe noted that the procedure followed does not distinguish betweenthe effect of magnitude of pressure and the effect of time ofapplication of pressure. A pressure of 500 psi, let us say, appliedfor 10 min, on a sample initially at atmospheric pressure at 0 Cwould produce quite a different effect from that produced by thesame pressure added to an already existing pressure which mayhave produced partial solidification. If successive increments ofpressure are increased according to the methods of the authors,for the same time intervals, it is clear that the state of solidificationwill be more complete at the higher pressures and this inturn will affect the flow characteristics. It is suggested that suddenpressure relaxations (10 min) during a test be avoided, and7 D epartm ent of Physics, The Pennsylvania State College, StateCollege, Pa.