Role of Inertia in Hydrodynamic Lubrication

Abstract

THE letter from Messrs. Shaw and Strang states that the behaviour of lubricated parallel surfaces, which I reported in a paper to the Institution of Mechanical Engineers in January 1945, is mainly due to the inertia and not, as I suggested, to thermal expansion of the lubricating fluid. While agreeing that the influence of thermal expansion must be included in the analysis of film lubricated bearings, they suggest that its effect is only of the order of 1/5 to 1/10 of a corresponding geometrical wedge. I should like to ask on what basis this comparison has been made. Has it, as seems probable, been made on the basis of a given mean film thickness with corresponding relative velocity and oil viscosity? If so, I suggest that this is not the correct basis for comparison, since the parallel surface bearing will run with a much lower film thickness before contact takes place than will a fixed taper or tilting pad bearing, for a given degree of surface finish. The fixed-taper surface will obviously fail by contact at the trailing edge when the mean film thickness is relatively large, while experiment shows that the tilting pad bearing, also, fails in the same way. It has, in fact, been shown experimentally that, if taper bearings are cautiously failed a number of times so that the bearing metal is wiped away from the trailing edge without deterioration of the surface condition and a parallel portion established along the surface, the load capacity is thereby increased.