Changes in surface topography during running-in of bearing steel contacts under mixed lubrication

Under mixed lubrication conditions, running-in is typically associated with a change in surface topography as surfaces conform at the very beginning of bearing operation. During this period, exposed roughness peaks of both bodies initially come into contact and can undergo plastic deformation or mild abrasive wear. This study focuses on running-in of rolling bearings, which typically have low initial roughness. Tests are performed on a twin roller machine at varying loads, entrainment velocities and slip ratios. To preclude the effect of additives, a synthetic base oil was used (PAO4). Due to the shape and low roughness of the samples a contacting profilometer was employed to measure the roughness. The variation in roughness between samples was much more than any difference measured before and after testing, indicating that low initial roughness limits the degree of running-in. The parametric analysis indicated reductions relating to entrainment velocity and contact pressure due their effect on film thickness and intensity of asperity interactions. The effect of slip can be attributed to increased shear cycles between the roughness peaks on the one hand but also appeared to be more complex, as the friction levels increase with slip and this in turn influenced the temperature and thus operating viscosity in the contact producing thinner films. Further, it could be demonstrated that the highest degree of roughness reduction occurs at small values of the relative lubricant film height. Consequently, the relative lubricant film height for all tests was adjusted to a similar level after completion, indicating that beyond a certain threshold of relative film height, the wear of surface roughness peaks ceases. As expected due to the low initial roughness of rolling bearings, the changes in roughness under mixed friction conditions found in the study are rather small. However, dependencies of the changes on the load parameters could be determined, which can form an important basis for future modeling.