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 following 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 steam guage 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 the crank. The precaution of turning off the bands successively as described, was rendered necessary to insure the safety of the bands from bursting by the shrinking strain, either in the weld or elsewhere in taking off the final cut, or had any of the inside bands yielded to the strain, it is clear, that the bands following would have to be removed to put on a new band, and if the first band had broken the whole operation of banding would have to be repeated, a loss of time we could not afford to risk. I was fortunate in not having any of the bands break. The pin was now ready for inserting in the crank. The after crank was then heated in the usual way to a red heat in the crown, and without the least difficulty the pin was inserted and adjusted to bring the key ways fair, the whole operation being performed without excitement and only in moderate haste. The crank was then gradually cooled with the hose, and when cooled and the debris removed, the divisions of key ways made by the bands (shown in the drawing) were cut away by sharp side chisels, the sides of key ways dressed, the new keys fitted and driven at this end of the pin, the chocks fitted and driven at the other end, the safety plate fastened over the ends of chocks, and the job was completed. This same pin is still in the cranks of the vessel, and apparently as sound and serviceable as the day it was put in. The actual time consumed in performing the job was something less than five days, working continuously day and night. It is proper here to remark that I was indebted for valuable support and assistance to Mr. Geo. H. Clarke, the chief engineer of the steamship Knickerbocker, and his able assistants. |