The temperature, pressure and time dependence of lubricant viscosity

The general form of the pressure (Proc. Am. Acad. Arts Sci. 77 (1949) 117) and temperature (Physical properties of molecular crystals, liquids and glasses (1968) 350) dependence of viscosity has been known for at least 50 years. Viscosity varies with temperature in a greater than exponential manner and temperature–viscosity equations generally allow for an unbounded viscosity at some characteristic temperature. At high-pressures the pressure–viscosity response is likewise greater than exponential, often following a less than exponential response at low-pressures. In spite of this known behavior, tribologists working in EHL have generally assumed less than exponential pressure response as a means of applying the Eyring stress aided thermal activation theory to the viscous regime of EHL traction. As justification, time dependence of the lubricant properties in the response to a pressure transient has been advanced. We present acoustic, capillary and impact measurements for timescales less than EHL. While time dependence of properties may be important in the viscoelastic regime of traction, this paper will show that for the timescale of viscous response, a significant time dependence of viscosity is unlikely.