Lubrication of elastohydrodynamic contacts, mainly concerning low temperature

Abstract: The limiting shear stress coefficient has been studied in a impacting ball apparatus for four different lubricating oils at -30ºC and 20ºC. The results show that the most interesting effects are; oil type, maximum contact pressure, temperature, and the interaction between the above mentioned parameters. A theoretical study has been used to investigate the possibility of using optical interferometry for pressure and viscosity measurements in a fully flooded EHD point contact. From the theoretical study it can be concluded that a film thickness map obtained from an optical interferometry method can be used both for pressure and for viscosity measurements. However, to get reliable results from the viscosity calculations, the measurements must be very accurate. Using photographs from a high-speed video camera, it was confirmed that the main part of a lubricating film will appear at the end of the contact time for a contact simultaneously subjected to squeeze and sliding motion. This corresponds with earlier findings using totally different equipment for electric detection of the asperity contact. The present investigation used a ball and glass disc apparatus. A simple iterative method is used for the computation of the central film thickness in a fully flooded elastohydrodynamic point contact lubricated with a polymeric viscosity-index-improved base oil. The shear stress dependence of the VI-improved base oil is included in the iterative method. Experimental data are presented on temporary viscosity loss and central film thickness at different temperatures from -8ºC to 40ºC. A good approximation of the central film thickness is achieved by using an iterative method. A ball and disc apparatus was used to investigate the lubricant replenishment of a grease-lubricated elastohydrodynamic point contact. In starved conditions in the grease-lubricated contact, a primary reservoir is formed at the sides of the contact. This reservoir governs the replenishment of the contact. The slide to roll ratio and the friction torque was measured for a grease lubricated roller bearing at -30ºC and 20ºC. A numerical model for the roller motion was also built; only the rheology of the base oil to the grease is used in the model. It could be concluded that the degree of sliding, in the unloaded zone, is mainly dependent on the local distribution of the lubricant and the position of the roller. Some differences are also seen in slide to roll ratio between lubricants due to different roller diameters.