Pressure-Viscosity Behavior of Lubricants to 1.4 GPa and Its Relation to EHD Traction

Abstract

A new falling body viscometer was developed to extend the measurement of the limiting low shear viscosity to 1.4 GPa in pressure and 105 Pas in viscosity for temperatures above ambient. Rather complete characterizations of the temperature-pressure-viscosity behavior of three synthetic base oils were performed. Free volume models can reproduce and provide a physical explanation for the inflection observed in pressure viscosity isotherms and provide a relationship between viscosity and state behavior. Examples of traction calculations indicate that used alone, traction curves cannot be used to deduce flow curves. The interpretation of a traction curve in terms of shear rheology is ambiguous without knowledge of at least the pressure-viscosity behavior up to the maximum Hertz pressure. A pressure-viscosity correlation capable of providing accurate values for α at these pressures may be very different from a correlation which has proven useful for film thickness analysis. A need exists to extend the flow curves obtained from rheometers to the highest EHD pressures in order to model friction losses in machines and the tractive performance of continuously variable transmissions. It should be possible to do this with viscosity data and correlations of the kind presented here, together with traction obtained from say a twin disc machine. A well-proven rheological model such as the Carreau Yasuda equation could then be used in a regression analysis to obtain the shear rheological response to very high pressure. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in Orlando, Florida, October 11–13, 1999