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

Under mixed lubricaiton conditions, running-in is typically associated with heighten material removal rates as surfaces conform and equalize in roughness at the very beginning of bearing operation. During this period, the roughness peaks of both bodies initially come into contact and undergo topographical changes due to plastic deformation and mild abrasive wear. This study focuses on running-in of rolling element bearings, which typically have low initial roughness, and the effect of running-in conditions on surface roughness. Tests are performed on a Phoenix Tribology TE74S twin roller machine varying load, entrainment velocity and slip ratio. To preclude the effect of additives the lubricant employed is a synthetic base oil (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. The parametric analysis indicated reductions relating to entrainment velocity and contact pressure, which were related to film thickness. 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 by a factor of around five for any level slip above pure rolling and this likely influenced the temperature and thus viscosity in the contact producing thinner films. Interestingly plotting the Lambda values indicates that before testing the wide spread (planned based on wide range of entrainment velocities) becomes ‘normalised’ to a narrow band (around 1.5) in the mixed lubrication regime after testing. In this study investigating the influences of running-in, the changes seen are very small and for the majority decreases in roughness, this is due to the low initial surface roughness, which is typical of bearing surfaces, where the ‘bigger’ (relatively) changes are observed are when initially the lubrication is in the boundary regime and this results in smoother surfaces ending the mixed regime.