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TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL EMGINEERS.]

MEASUREMENTS OF THE FRICTION OF LUBRICATING OILS.

BY C. J. H. WOODBURY, BOSTON, MASS.

Read at the Annual Meeting, 1880.

The resistance existing between bodies of fixed matter, moving with different velocities or directions, presents itself in the form of a passive force which results in the diminution or the destruction of apparent motion. Modern science has demonstrated that this destruction is only apparent, being merely the conversion of the force of the moving body into the oscillation of the resisting obstacle, or into that molecular vibration which is recognized as heat. Direct friction refers to the case where the two bodies are in actual contact, and mediate friction where a film of lubricant is interposed between the surfaces; and it is this which applies to nearly every motion in mechanics where bodies slide upon each other. The coefficient of friction is the relation which the pressure upon moving surfaces bears to resistance. I have devoted some time to the examination of this subject, in the interests of the Mill Mutual Insurance Companies of New England. In this report of my work upon the measurement of friction of lubricating oils, I shall restrict myself to a description of the apparatus designed especially for the purpose, the method of its use, and the results obtained with a number of oils in our market which are used for lubricating spindles. Previous trials of nine different oil-testing machines in use showed that none of them could yield consistent duplicate results in furnishing the coefficient of friction. The operation of these machines, by their failure to obtain correct data, adduced certain negative evidence which established positive conditions as indispensable in the construction of a machine capable of measuring the friction of oils. The following circumstances must be known or preserved constant, -temperature, velocity, pressure, area of the frictional surfaces, thickness of the film of oil between the surfaces, and the mechanical

effect of the friction. In addition to the foregoing conditions, the radiation of the heat generated by friction must be reduced to a minimum, and the arrangement of the frictional surfaces must be of such nature that no oil can escape until subjected to attrition. To measure the frictional resistance at the instant of a given temperature, and at a time when both temperature and friction are varying, requires a dynamometer which is instantaneous and automatic in its action.

The apparatus consists of an iron frame supporting an upright shaft surmounted by an annular disc made of hardened tool steel. Upon the steel disc rests one of hard bronze (composed of the fol lowing alloy,-copper thirty-two parts, lead two parts, tin two parts, zinc one part) in the form of a cylindrical box. Water is fed in at one side and a diaphragm extending nearly across the interior produces a uniform circulation before discharge. Although this use of water is original with the writer in the method of its application, its first employment to control the temperature of the bearing surfaces of oil-testing machines is due to Monsieur G. Adolphus Hirn, and described by him in a paper on the subject of friction, read before the Société Industrielle de Mulhouse, June 28, 1854. M. Hirn, however, confined his attention chiefly to the determination of the mechanical equivalent of heat, as measured by the amount of heat imparted to the circulating water, expressed in the work of friction. His investigations of lubrication with this apparatus were confined to the friction of lard and olive oils, at the light pressure of about one and four-tenths pounds to the square inch. Mr. Chas. N. Waite, of Manchester, N. H., has independently, and I believe originally, made use of water in a friction machine, and has performed good work in the limit of his experiments.

A protection of wool batting and flannel, to guard the discs against loss of heat by radiation, diminishes the escape of heat to about two degrees per hour, which loss is not appreciable when observations are taken within a few seconds' interval. A thin copper tube, closed at the lower end, reaching through the cover, extends to the bottom of the disc; the bulb of a thermometer is inserted in this tube and measures the temperature of the discs; an oil tube runs to the center of the disc, and a glass tube at the upper end indicates the supply and its rate of consumption, and also serves to maintain a uniform head of oil fed to the bearing surfaces. The rubbing surfaces of both discs were made to coincide with the standard surface plates in the physical laboratory of the Institute of Technology, and their contact with each other is considered perfect.

After this surface was finished, the bronze disc was treated with

bi-chloride of platinum, which deposited a thin film of platinum upon the surface. Upon the application of the discs to each other the steel disc rubbed off the platinum from all parts of the surface, showing the perfection of contact. This nicety of construction enables a film of oil of uniform thickness to exist between the surfaces, and the resistances are not vitiated by the collision of projecting portions of the disc with each other. The rounded end of the upper shaft fits into a corresponding depression in the top of the upper disc. This method of connection retains the disc over the proper center, yet it is allowed to sway enough to correct any irregularity of motion caused by imperfection of construction or wear of the lower disc. To obtain the desired condition of pressure, weights are placed directly upon the upper spindle. The axes of the upper and lower spindles do not lie in the same straight line, but are parallel, being about one-eighth of an inch out of line with each other; such construction, giving a discoid motion, prevents the disc from wearing in rings and assists in the uniform distribution of the oil.. An arm is keyed through the lower part of the upper spindle and engages with projections upon the upper disc. Upon this arm, which is turned to the arc of a circle, whose development is two and one-half feet, a thin brass wire is wrapped upon this arc and reaches to the dynamometer, so that the tension of the dynamometer is tangential and the leverage is constant for all positions of the upper disc within its range of motion. The dynamometer consists of a simple bar of spring steel fastened at one end, and bent by the pull applied at the other. Its deflection is indicated by a pointer upon a circular dial, the motion of the spring being multiplied about eighty times by a segment and pinion. The whole is enclosed in a steamguage case.

When completed, the machine was subjected to a long series of tests with the same oil, to determine the accuracy of the results, and the best method of procuring them. The operation of the machine under equal conditions with the same oil gives results which are as closely consistent with each other as could be expected from such physical measurements. As an example, four tests of the Downer Oil Co. Light Spindle (Sample No. 7) at 100° Fah. and on different days gave .1145, .1094, 1118, .1094: mean, .1113. Another example (Sample No. 14) Heavy Spindle Oil, made by the same firm, yielded for a coefficient of friction as the result of five different trials, .1246, .1195, .1297, .1201, .1221: mean, 1233. Much of the irregularity, slight as it is, is due to the variable speed of the engine. Concurrent results were obtained under equal circumstances, but the coefficient of friction varied, not merely with the lubricants used, but also with the temperature, pressure, and velocity. The results

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of my own experiments on mediate friction do not agree with the laws of friction as given in works on mechanics, but the coefficient of friction varies in an inverse ratio with the pressure, as shown graphically on diagram C.

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Coëfficient of Friction at Different Pressures.

C

The variation of friction with the temperature is shown, graphically, on all sheets reporting the friction of each sample of oil. These curves belong to the hyperbolic class of a high degree; but I have not been able to deduce an equation which will answer to the conditions of more than one, because the law of the curves is modified by a constant, dependent upon the individual sample of oil used. A little difference in the sample would cause a difference in the line of curve. Reference is made to diagram D, showing the coefficient of friction under equal ranges of temperature and velocity, but with a different series of pressures.

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Coefficient of friction at 100° and 500 revolutions per minute :

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The ratio of the changing coefficient varies with the temperature at which the range of results is taken.

Friction varies with the area, because the adhesiveness of the lubricant is proportional to the area, and the resistance due to this cause is a larger fraction of the total mechanical effect with light, than it is with heavy pressures.

The limit of pressure permitting free lubrication varies with the conditions; for constant pressures and slow motion it is believed to be about five hundred pounds per square inch, while for intermittent pressures, like the wrist pin of a locomotive, the pressure amounts to three thousand pounds per square inch. It has been stated that about four thousand foot pounds of frictional resistance per square

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