Modeling and experimentation of the rheological behavior of liquid lubricants in elastohydrodynamic lubrication

The rheological properties of liquid lubricants in elastohydrodynamic lubrication (EHL) strongly affect the performance of machine elements such as rolling element bearings, gears and traction drives. Traction machines are frequently used in experimentation to cover a wide operating range and to realistically represent actual EHL conditions. Rheological parameters averaged over the contact area are commonly used to model lubricant behavior, however, the use of the average quantities may lead to inaccurate modeling due to the highly non-linear nature of the fluid rheology under high pressures and temperatures.; The objective of this thesis research is to develop theoretical and experimental rheological modeling approaches using traction machines and local, or non-averaged, rheological quantities. The thesis consists of three parts. The first part describes the development of a novel line-contact EHL traction test rig that is used to conduct the experiments for rheological modeling. The second part presents the development of a practical and accurate approach to modeling the viscous behavior of the lubricants. Unlike conventional traction-machine methods, this approach relates local shear stress to local shear strain rate, pressure and temperature, making use of first-principle theories and traction measurements. This method retains the advantages of traction-machine method while allowing lubricants with any ambient viscosity and with an uncertain pressure-temperature-viscosity relationship to be studied over a wide range of operating conditions. The third part describes the development of a non-averaging method to determine elastic and plastic lubricant properties as manifest in EHL contacts under heavy loading and small sliding velocity, conditions typical of traction drives. By choosing a sensible pressure-property expression, the parameters of the expression are determined making use of the initial slopes and peak values of measured traction curves at a number of contact loads. The elastic shear modulus and limiting shear stress of a lubricant corresponding to a single, local pressure, rather than to an averaged pressure, can be calculated over a range of pressures and temperatures.; The modeling approaches are evaluated by applying them to the study of a number of synthetic liquid lubricants. The evaluations suggest that the current modeling approaches, accounting for the variation of pressure and temperature within the contact, are reasonably valid and can be used to extract the lubricant rheological properties over a wide range of operating conditions of practical interest. The rheological parameters derived through the modeling approaches can be used to characterize the rheological behavior of the lubricants, and to predict the lubrication conditions in machine elements.