The Effect of Load (Pressure) for Quantitative EHL Film Thickness

New quantitative numerical simulations of the elastohydrodynamic lubrication (EHL) film thickness using realistic pressure and shear-dependent rheology and realistic compressibility have indicated that the dependence of central film thickness upon Hertz pressure (or load) for the classic Newtonian, slightly compressible solution is merely a lower limit with magnitudes three times as great being possible. Experimental measurements of central film thickness employing Hertz pressures from 1.0 to 2.6 GPa confirm that for a neat mineral oil, the classical pressure dependence is accurate, while for two gear oils the experimental pressure dependence is much larger. Shear-dependent viscosity is a major factor and compressibility plays a lesser role, while there is evidence that mechanical degradation is also important. New experimental evidence of the enhanced scale sensitivity resulting from shear-thinning has also been obtained. These results for the pressure and scale dependence have dire implications for the usual practice of extrapolation of film thickness from experimental measurements at large scale and low pressure using effective pressure–viscosity coefficients. For machines of small scale and high pressure, the extrapolation will sometimes result in substantially overestimated film thicknesses.