Experimental and numerical analysis of thermal elastohydrodynamic contacts

Abstract

The temperature behaviour of elastohydrodynamically lubricated contacts, such as those found in gears, cams and rolling bearings, is a key parameter relating to machine efficiency. This thesis presents an infrared microscopy technique capable of measuring the temperature of both the bounding surfaces and the oil film in an elastohydrodynamic contact between different surface materials and different lubricants. Based on this, the oil film temperature can, for the first time, be spatially resolved in three dimensions providing through-thickness oil temperature profiles. The contact was produced experimentally by loading a ball specimen against a transparent sapphire disc and viewing the film with an infrared microscope focussing through the sapphire disc. Two optical band pass filters were used to isolate the thermal radiation from the oil film and Planck’s Law was applied to analyse the radiation received. Experimental data analysis required the emissivity of the oil film to be determined during the initial calibration tests; this was measured in situ and shown to vary strongly as a function of film thickness and temperature. The measurement technique was adapted to overcome focussing issues due to the partially infrared transparent characteristics ofthe contact materials such as zirconia. It was validated under pure rolling conditions, when the temperature of the oil film was equal to that of the controlled lubricant reservoir, and compared to an equation commonly used to predict average oil film temperatures, confirming the value of the disputed constant. Results were used to infer the in-contact rheological behaviour of lubricants and the thermal effects of contact materials. These findings show how the thermal properties of the contact surfaces can be used to control the rheology and friction of Elastohydrodynamic lubrication. This is important since the temperature of elastohydrodynamic contact is critical in determining friction and hence the efficiency of machine components. This technique also provides much needed experimental validation and input data for numerical simulations. In addition, a numerical model of a circular contact under thermal elastohydrodynamic lubrication (TEHL) was built and solved by means of a finite difference method using Gauss-Seidel and line Jacobi approaches in the fluid domain to deliver pressure and temperature distributions. The model was run for the same range of contact materials, lubricants and sliding conditions as was measured experimentally. The resulting film thickness, friction and detailed temperature results were compared with experimental data. The thermal properties of the contact materials and lubricants used for the simulation process were obtained through thermal reflectance measurements. Very good agreements were obtained between numerical and experimental temperature and friction results over a wide range of contact conditions and for materials with different thermal properties. The results indicate that this numerical scheme provides an accurate and stable TEHL solution incorporating effective thermal relaxation schemes. The results also showed that the in-contact temperature rise is significant and cannot be neglected. The validation against experimental results has important implications with respect to building a 3D thermal elastohydrodynamic model based on the energy equation and thermal properties of test samples. Specifically, it suggests that a TEHL model can be applied to predict the temperature and film thickness profiles of EHL contacts for a wide range of materials with different thermal properties.