A Study of the Lubrication of EHL Point Contact in the Presence of Longitudinal Roughness

This work investigates the effect on elastohydrodynamic lubrication of roughness ridges oriented along the rolling-sliding direction, such as may be present on rolling bearing raceways. The roughness of the three specimens tested is characterised by the RMS of surface heights and a dominant wavelength. Optical interferometry and a ball-on-disc set-up were employed to map the oil film thickness. The paper first describes a novel procedure to carry out optical interferometry measurements on rough surfaces. Film thickness maps from the central part of the contact were obtained for a range of speeds in pure rolling and rolling-sliding conditions. The evolution of the film distribution with increasing speed along with the in-contact RMS and the real area of contact was calculated. The film maps show that the lift-off speed increases when roughness is introduced compared with smooth surfaces, while the average film thickness remains very close to the smooth case. The general horseshoe film shape that becomes visible at higher speeds is discussed. Using an inverse solution approach based on measured in-contact roughness, the pressure distribution is estimated in a rough, lubricated contact and its evolution with speed is explained. The findings provide important insights into the transition from boundary, through mixed, to full EHL lubrication for longitudinal roughness.