The lubrication of rollers II. Film thickness with relation to viscosity and speed

Abstract

Systematic measurements with a disk machine of the thickness of the hydrodynamic oil film between loaded rollers have been made with respect to load, rolling speed, sliding speed and oil viscosity. It has been found that the viscosity of greatest im portance with respect to film thickness is the viscosity of the oil at the surface temperature of the disks (tjs) ; the viscosity of the oil supplied by the lubricating jet and the viscosity attained by the oil on its passage through the pressure zone are unim portant. It has been shown that film thickness is independent of load at loads exceeding 7 x 10 ⁷ dyn cm ^-1 , but is dependent upon the rolling speed u (the mean peripheral speed of the disks) as well as upon rjs. To within +15 % all the results are expressed by h* = 0-8[(m^ ) /100] ^0.5 where h% is the film thickness in microns when utjs is expressed in dyncm ^-1 . Particularly is it noteworthy that this same expression remained true even when sliding was introduced. (The ranges of conditions covered by the experiments were loads from 7 x 10 ⁷ to 2 x 10 ⁸ dyncm ^-1 , rolling speeds from 32 to 1000 cm s ^-1 , sliding speeds up to 480 cm s ^-1 and values of rjs from 0.14 to 1.19P.) It is also shown that an implication of the insensitivity to sliding is that on the entry side there is little frictional heating of the oil due to the sliding up to a point where the pressure approaches 1 x 10 ⁹ dyncm ^-2 . The experimental results, have been compared with the theory of the elastohydrodynamic lubrication at the conjunction of the disks. The theory disregards frictional heating and predicts a film thickness proportional to (zb/s) ^0.7 in contrast with the exponent of 0.5 given by experiment. Evidence is cited that the difference is not due to frictional heat but it is suggested that the discrepancy is due to a specific effect of speed upon the increase in viscosity which oils exhibit under pressure.