Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/49

Rh LUBRICANTS 35 large proportion of the many lubricating compositions that are in use. For machinery where considerable pressure is exerted between the bearing surfaces the mineral oils are too thin, or, as it is termed, are too wanting in &quot; body &quot; to be quite suitable without being mixed with an animal or vegetable oil. Unless a lubricant has considerable &quot; body &quot; it is quickly pressed out of the bearing, and an unnecessarily rapid supply has to be provided. The same oil may be used several times over, and several ingenious designs of bearings for rotating shafts, such as Player Brothers or Taylor & Challen s, whereby the shaft itself as it runs round continually pumps up again to the top of the bearing the oil that has once been used, have been very successful in practice. If an automatic arrangement of this sort is not employed, the oil dripping from the bearing should be collected in a pan and used again to fill the oil-cups. The oil gets gradually worn out as a lubricant by becoming filled with dirt, partly the dust of the atmosphere and partly the minute iron and brass dust that is continually rubbed off the bearing surfaces. Oils that have been used two or three times can be to a certain extent repurified by washing in a solution of carbonate of soda or potash and chloride of calcium in boiling water. But it must not be supposed that with repurification an oil may be used an indefinite time as a lubricant. A large portion of it is actually evaporated by the heat caused by the friction at the journal, and the unevaporated portion seems to undergo some chemical change injurious to its lubricating properties. Vegetable oils are peculiarly rich in volatile constituents, and it is this fact probably, even more than the greater cheapness of mineral oils, that has Jed to the largely in creased use of the latter in the lubrication of machinery. The quality of an oil may be tested by chemical analysis ; by measurement of density and viscosity ; by observation of the temperature necessary for ignition in the atmo sphere, or, as it is called, the &quot; flashing &quot; temperature ; by observation of the succession of figured patterns produced when a single drop of the oil is let fall upon the surface of pure water in a clean dish ; by the measurement of the temperatures to which a journal rises when running at different speeds and under different pressures, and when supplied with a given amount of the lubricant per minute ; and by the measurement of the coefficient of friction at the same journal with varying speeds of rotation and pressures. The last two methods of test are the most interesting and directly useful from a mechanical point of view, i.e., considering the oil as a lubricant simply. The machine designed and used by Professor Thurston of the Stevens Institute of technology is the best that has yet been con structed to carry out these tests. In it a spindle is revolved in horizontal bearings by a belt from the main shaft of the workshop of the Institute. On the overhanging end of this spindle is formed a journal from which is hung a heavily-weighted rod. The bear ings in this rod by which it hangs on the journal are of brass, and the two halves are pressed down upon the journal with any desired pressure by me;ms of a spiral spring placed in the centre of the rod. The weight of this pendulum prevents it revolving along with the spindle, but the friction at the journal deflects the pendulum from the vertical through an angle whose sine is a measure of the fric- tional effort. There is also inserted in the bearings a thermometer by which the effect of the friction in increasing the temperature is observed. With this machine Professor Thurston has obtained extremely interesting results regarding the variation of the coeffi cient of friction with temperature, pressure, and velocity of rubbing. These are summar.zed as follows. With great intensity of pressure and low velocity, the friction increases as the temperature is raised; but for each low velocity the rate of increase of friction with tem perature becomes slower as the pressure diminishes, and becomes zero at a certain limit of pressure which is higher the higher the velocity is. With high velocities the variation of friction with temperature is in the opposite direction within the limits of pressure commonly used. Again at a given temperature and a given pres sure the friction first decreases very rapidly with increase of velocity, and then above a certain limit of velocity increases again slowly with further increase of velocity. The limit of velocity at which the direction of variation changes from -negative to positive does not appear to depend on the intensity of pressure, but the change occurs at much lower velocity -limits with low than with high tem peratures. Thirdly, with a given temperature and a given velocity the coefficient of friction, i.e., the ratio of friction to normal pres sure, at first decreases rapidly with increase of pressure at low pressures, and then at higher pressures increases again with the pressure. This law seems to hold for all temperatures and all velocities ; but how the limit of pressure at which the variation changes in direction is altered by alteration of temperature and velocity is not as yet certainly determined. It is thus seen that the variation of friction at lubricated journals is extremely complicated, and has no resemblance to the simple law of constant proportionality between the friction and the normal pressure which until lately was commonly believed to hold good for unlubricated fiat surfaces. This simple law is really true for many unlubricated surfaces and through a tolerably wide range of con ditions, but is not true for all such surfaces, or under all, and especially extreme, conditions of pressure, velocity, and temperature. Professor Thurston has endeavoured to represent these results in algebraic formulas, but the number of his experiments seems hardly sufficient to establish any general mathematical law which will be true under all circumstances, and the particular formulae which he has adopted give values differing very considerably from those given by some of his experiments. The friction at a lubricated journal depends really much more on the viscosity of the lubricant than on the frictional properties of either of the solids, which never come in contact when the lubri cation is carefully attended to. The layer of oil immediately in contact with either solid probably does not move at all relatively to the solid. The rubbing, therefore, in all probability takes place between two surfaces, or rather between an indefinitely large num ber of pairs of surfaces, pf oil. The viscosity of the oil, which hinders this relative motion, is, however, very likely affected by the adhesive force between the solid and liquid surfaces, because, especially if the intensity of bearing pressure be great and the film of lubricant consequently very thin, some at least of the liquid motion will take place within the sphere of action of the cohesive forces. It is of the greatest importance in order to secure economy in the use of lubricants to maintain the supply to each journal at a con stant uniform rate. To effect this, numberless &quot;automatic lubri cators&quot; have been invented. The common syphon oil-cup is very efficient so long as the rate of working is steady ; but the supply does not automatically vary with the requirements. The &quot; needle &quot; lubricator allows the oil to How down to the journal through a small straight tube in which is placed a wire which nearly blocks up the tube. When the wire is motionless the dimensions of the space between the wire and the tube are capillary, and no oil flows. When the shaft runs, however, its surface scraping on the end of the wire throws it into continual vibration, and this allows a slow stream of oil to pass downwards which is automatically regulated in accordance with the speed of revolution. The necessity of very perfect lubrica tion of the cylinders of gas engines, which run at a high speed and at a high temperature, has led to the adoption by Messrs Crossley Brothers of an extremely neat and perfect arrangement. A small crank on the end of a spindle, driven at a rate proportional to that of the engine, has suspended from the crank pin a short wire pendulum. At the lower part of the revolution this pendulum dips into a basin of oil and lifts a drop from it. In the upper half of the circular motion, the wire is dragged over a little scraper extending over the open mouth of a pipe. This scrapes the drop off the pendulum, and the drof&amp;gt; falls from the scraper into the tube, along which it flows to the surface to be lubricated. The number of drops is thus accu rately proportioned to the speed of the engine, and the size of drop can be varied by using smaller or larger wire for the pendulum. Table of Coefficients of Friction on Oast-Iron Journals at Tempera ture, 70 F, and, Velocity 750 Feet per Minute {from Thurston}. Name of Oil. Pressure in Ib per sq. in. 8. 16. 32. 48. Natural Summer Sperm 17 25 19 20 24 22 18 17 21 18 25 23 26 16 14 16 14 16 16 15 16 14 16 12 17 18 10 09 12 11 14 12 09 17 12 12 10 13 13 12 08 10 09 10 11 12 09 11 11 12 17 22 Winter Bleached ,, Winter Laid Extra Neatsfoot Tnllow Olive Refined Cotton Seed Rape Seed Menhaden KeroMnc Paraffin Other mineral oils than the kerosine and paraffin give a smaller coefficient. For mineral oils generally the lowest coefficient appears to occur at from 30 to 40 Ib pressure per square inch. The supply