Traction in Elastohydrodynamic Contacts

The magnitude of the traction in elastohydrodynamic lubrication is influenced by at least two non-Newtonian effects. At very high pressures, the lubricant becomes noticeably viscoelastic and behaves as an elastic solid when the shear stress is small. At high values of the shear stress, the relation between the stress and the shear rate becomes non-linear and, when thermal effects are insignificant, conforms to the Eyring expression for viscosity. In general, however, the elastic region, the non-linear isothermal region and the thermal region are not separate and distinct but merge gradually into each other. This makes it difficult, experimentally, to determine the magnitude of the parameters controlling the behaviour of the fluid and, conversely, to predict from them the shape of the traction curve. The present paper examines these problems and shows that the major uncertainty is caused by the dearth of knowledge of the thermal properties of fluids at very high pressures. [These have been measured in an associated investigation by Richmond et al. (1).] It is shown that, when the correct values of the thermal properties are used and allowance is also made for the merging of the vari