Mechanical Analysis of Lubrication and Lubricants

Abstract: This thesis comprises three different topics within the field of tribology. First, design functions for analyses of slider bearings are developed. Second, a lubricant model applied to elastohydrodynamically lubricated (EHL) line contacts considering wall-slip is presented. Finally, an experimental apparatus for investigations of lubricants at high pressures is presented. The design of hydrodynamic bearings usually requires a total numerical analysis of the pressure distribution and the corresponding design quantities such as load capacity and power loss. The objective of this part of the thesis was to determine curve-fitted functions describing each design quantity. Three different kinds of geometry were analysed; rectangular tilting-pad thrust bearings, sector-shaped tilting-pad thrust bearings and journal bearings with two axial oil grooves. The approximate design functions obtained are shown to be in very good agreement with the numerically calculated results. The functions are intended to be implemented in short computer programs. A wall-slip model including limiting shear stress is presented. The lubricant model was applied to EHL line contacts using isothermal conditions. The main part of the model concerns the lubricant velocity at each surface that is decoupled from the corresponding surface velocity giving two new variables in the EHL equations. The lubricant velocity at the surface is related to the corresponding shear stress. As long as the value of the shear stress is below the limiting shear stress, the lubricant velocity is equal to the surface velocity. However, when the shear stress reaches the limiting shear stress, interfacial slip appears and the lubricant velocity differs from the surface velocity. Both smooth and wavy surfaces were used in the calculations and the influence of the wall-slip model on the results compared to a Newtonian model was investigated. Results from a high pressure chamber are presented. It is possible to use the apparatus for a number of different measurements. The compressibility variation with the pressure for five different lubricants was investigated for pressures up to 2.7 GPa. The density variation for each lubricant is presented as a curve-fit.