Paper 3: Film Thicknesses in Elastohydrodynamic Lubrication by Silicone Fluids

Abstract

In an earlier paper it was shown that the thicknesses of lubricant films in elastohydrodynamic contacts, as deduced from the measured electrical capacities between two heavily loaded rotating discs, agreed well with theoretical values. Exceptions to this result were observed for a polymer solution and a silicone fluid; the discrepancy between experimental and theoretical film thicknesses with these fluids was tentatively attributed to non-Newtonian behaviour. This paper describes further work carried out with the object of investigating this point more closely. Polydimethylsiloxanes (silicone fluids) covering a wide range of nominal viscosities were used, and the thicknesses of the films formed by these fluids in a disc machine were measured over a wide range of rolling and sliding speeds and of temperatures. Since continuous shear viscometry of these materials is impossible under the conditions encountered in the disc machine, use was made of the relation between the behaviour of visco-elastic substances in oscillatory and in continuous shear. There is a good experimental and theoretical basis for this relation. Published information on the properties of silicones in oscillatory shear was used to predict their behaviour in continuous shear, under the conditions of interest in elastohydrodynamic lubrication. This information has been used to predict the film thicknesses in elastohydrodynamic lubrication, and to compare these predictions with experimental results. Although there were differences in detail, experiment and theory were in approximate agreement over most of the range covered. The reasons for some of the discrepancies are discussed.