Experimental investigation into the influence of roughness on friction and film thickness in EHD contacts

Abstract

The roughness of machined surfaces such as those in bearings and gears is characterized by asperities which cover a wide spectrum of wavelengths and heights. When the height of the asperities becomes comparable to the lubricant film thickness, the roughness is known to influence friction and wear behaviour. This thesis reports an experimental investigation into the effect of roughness on the film thickness and friction in EHD contact. This work focuses on the particular roughness case of ridges oriented parallel to the rolling-sliding direction, such as the ones produced on raceways during the manufacturing process of rolling element bearings. To carry-out this research a ball-on disk test rig was used to model the contact between the ball and the ring of a ball bearing. The disks were made of glass to enable the lubricant film thickness to be measured through an optical technique based on optical interferometry. The ball specimens were made of AISI 52100 steel and they were roughened with a cutting tool, resulting in longitudinally oriented roughness ridges showing a dominant wavelength and amplitude. The friction was measured through a torque meter attached to the ball shaft. A duo-chromatic system using two LEDs was developed in order to be able to measure the film thickness over a wider range of film thickness. A novel procedure was also introduced to enable the film thickness in rough EHD contacts to be measured accurately. The film thickness and the friction produced by specimens with various amplitude and wavelength were measured under pure rolling and rolling-sliding conditions. The roughness was found to have a big influence on both film build-up and friction. In particular, under the same operating condition, a rough specimen will generate a thinner minimum film and a higher friction compared to a smooth specimen. At the top of the asperities, a micro-EHD film was found to form. As suggested by friction and pressure measurements, the conditions in this micro film are severe enough to reach the limiting shear stress of the lubricant.