## Abstract

An experimental study has been made of the effect of temperature on traction in elastohydrodynamic conditions with a range of nine fluids. The pressure coefficient of viscosity falls with increase of temperature and this, combined with the fall in the viscosity at atmospheric pressure, leads to very great changes in the effective viscosity within the elastohydrodynamic contact. Thus whereas at low rates of shear and low temperature the fluid in the high pressure zone may behave as an elastic solid, when the temperature is raised it reverts to its liquid form with a viscosity of the order of 10$^{2}$ Pa s or less. At high rates of shear, the fluid behaves as a non-Newtonian liquid of the Eyring type at all temperatures within the range of the experiments, i.e. from 30 to 110 degrees C. The viscosity, $\eta $, of a fluid of the Eyring type can be expressed as $\eta =\frac{\eta _{0}\exp (\alpha p)}{\sinh (\tau /\tau _{0})}\frac{\tau}{\tau _{0}}$, where $\eta _{0}$ is the viscosity at atmospheric pressure, $\alpha $ is the pressure coefficient, p is the pressure, $\tau $ is the shear stress and $\tau _{0}$ is a characteristic shear stress determining the non-Newtonian behaviour. At high rates of shear, and high pressures, sinh ($\tau /\tau _{0}$) approximates to the exponential form exp ($\tau /\tau _{0}$) and the value of the traction coefficient (T/W) is then given by the expression $\frac{T}{W}=\overline{\alpha}\tau _{0}-\frac{\tau _{0}}{p}\ln \frac{\tau _{0}}{2\eta _{0}\dot{s}}$, where $\dot{s}$ is the rate of shear and $\overline{\alpha}$ is the mean value of $\alpha $ over the range of pressure. It has been found that $\overline{\alpha}\tau _{0}$ is sensibly independent of temperature which suggests that the limiting coefficient of traction at sufficiently high pressure should also be independent of temperature. Since $\overline{\alpha}$ falls and $\tau _{0}$ rises with temperature, the effect of an increase in temperature is to raise the pressure at which a given value of the traction coefficient is achieved. In some fluids the molecules appear to associate into clusters. The clusters dissociate when the temperature is raised and for these fluids $\tau _{0}$ rises nearly exponentially with temperature. For two of the fluids the unit of flow is the individual molecule. $\tau _{0}$ is then directly proportional to the absolute temperature in agreement with the Eyring theory. At low pressures and high temperatures, the approximate relation of equation (II) ceases to be applicable. The full expression of equation (I) has then to be used and the variation of pressure over the region of contact has to be taken into account.