An Experimental Verification of the Significance of the Reciprocal Asymptotic Isoviscous Pressure for EHD Lubricants

The classical elastohydrodynamic film thickness solution which assumes a purely exponential variation of viscosity with pressure requires a pressure-viscosity coefficient. In general, liquid lubricant viscosity varies with pressure such that the local pressure-viscosity coefficient is a function of pressure, making the determination of the pertinent parameter for film thickness difficult. Blok proposed a reciprocal asymptotic isoviscous pressure α* as a replacement for the initial pressure viscosity coefficient α0 based on the Weibull pressure transformation. The pressure-viscosity characteristics of seven liquid lubricants were measured with a falling body viscometer to pressure of 0.76 GPa and α0 and α* were calculated. Viscosity and glass transition results were applied to a model in current usage. The EHD film thickness was measured using interferometry supported by the measurement of the refractive index at elevated pressure. The reciprocal asymptotic isoviscous pressure was found to be more correct as a film thickness parameter than was the initial pressure viscosity coefficient.