Simulation Model and Ultrasound Study for Engineering Interfaces

Friction and wear are experienced both in terms of machine operation and finance. They impact, amongst other things, energy consumption and material loss, product manufacture and component maintenance. Liquid lubrication is widely used on machinery interfaces to separate the loaded surfaces, with the ultimate aim of reducing friction and wear from direct metallic contacts. However, in most engineering applications, the lubricant film thickness is not thick enough to fully separate matching surfaces because of too high loads or low speeds. Therefore, the interface contains isolated asperity contacts with the surrounding gaps filled by liquid, which is known as mixed lubrication. The aim of this project is to investigate engineering interfaces using both theoretical and experimental approaches.
A statistical mixed lubrication model has been built in this study. The friction force, load sharing proportions, film formation and normal contact stiffness can be simulated for engineering interfaces under varying operating conditions. The model was applied to a greased contact of a landing gear articulating pin joint and an oil lubricated contact of a ball-on-disc. Friction forces, load sharing ratios, contact stiffness, and mean film thickness from asperity contact and lubricant layer have been predicted and compared with experimental measurements.
The ultrasonic method was applied to the greased pin joint contact and the oil lubricated ball-on-disc contact. Reflection measurements have been linked to the interface characteristics, including contact pressure, lubricating film thickness and contact stiffness. A start-up process of a ball sliding on disc was studied to extract contributions from asperity interaction and trapped lubricant on the interface. Finally, a shear study on a dry Perspex contact was carried out using the ultrasonic technique. The shear stiffness in the stationary and stick-slip process was studied which presents the shear performance of contacting asperities under normal pressure.
Results of these investigations show that the mixed lubrication model and the ultrasonic technique could be used to accurately determine the interfacial characteristics for real engineering contacts. Agreement between analytical model and ultrasonic measurement is found to be good, indicating the applicability of the technique for analysis of contact size, contact pressure and interfacial stiffness.