The determination of the pressure viscosity coefficient of a lubricant through an accurate film thickness formula and accurate film thickness measurements

The pressure viscosity coefficient is an indispensable property in the EHD lubrication of hard contacts, but often not known. A guess will easily lead to enormous errors in the film thickness. This paper describes a method to deduct this coefficient by adapting the value of the pressure viscosity coefficient until the differences between accurate film thickness approximation values and accurate film thickness measurements over a wide range of values, is at a minimum. Ten film thickness approximation formulas are compared in describing the film thickness of a test fluid with known value of the pressure viscosity coefficient. The measurement method is based on spacer layer interferometry. It is concluded that for circular contacts the newer more versatile expressions are not better than some older approximations limited to a smaller application field, and that the older fits are as least as appropriate to find the pressure viscosity coefficient of fluids, in spite of the limited data where they have been based on