Elastohydrodynamic Lubrication Traction Coefficient is Not a Property of the Liquid Alone

The slow progress in the prediction of elastohydrodynamic film thickness and friction may be attributed to the mistaken notion that a traction curve is equivalent to a rheological flow curve. A similar concept is being standardized by the Society of Automotive Engineers (SAE) for the traction coefficient of aerospace oils. Four standards teach that traction coefficient is an inherent physical property of oil and that full-film traction does not depend upon the roller material and that there can be traction measurements, which do not depend on the test rig. These traction coefficients are to be employed in models for thermal management analyses without the need for measuring primary properties. Through example calculations, using primary properties of turbine oil, it is shown here that elastohydrodynamic lubrication (EHL) friction is not a property of the liquid alone, and that it depends on at least two contact characteristics as well, namely, the aspect ratio and scale. Within the tribology literature, there is much evidence of the dependence of EHL friction on other contact parameters as well that are not characteristic of the fluid.