Running-in of rolling-slidiing contacts

Abstract

Running-in of two fresh and unworn surfaces in contact is a transient phase where friction and wear vary considerably in time. During running-in the surface properties of the components are adjusted. If the initial surface roughness of the rubbing surfaces is correctly chosen, the running-in changes into the steady-state phase. At this stage, the rubbing surfaces are in general smoother and their wear rate is low and constant. On the other hand, an inappropriate choice of roughness may lead to a rapid deterioration of the rubbing surfaces. The micro-geometry of the surface is an important factor in determining the life of mechanical components. During the running-in phase, the highest asperities are “flattened”, thereby increasing the number of asperities in contact and, as a result, increasing the load-carrying capacity of the surface. Fundamental studies that attempt to consider the details of running-in phenomena are relatively rare. This research is conducted with the aim of exploring the running-in phase for the rolling, sliding and rolling-sliding contact. Finite element simulations are conducted to calculate the stress distributions for the three types of contact motions during the running-in phase. The evolution of the contact pressure for a certain rolling or sliding distance is studied to unravel the running-in phase. During running-in of rolling contacts, the change in the surface topography results in the transformation from a rough surface to a smoother surface: the flattening of the high asperities induces a reduction in surface roughness. This flattening of asperities is due to plastic deformation and causes a higher equivalent residual stress at the surface. The transition of the running-in phase to the steady-state phase of a rolling contact is governed by the transition of plastic to elastic deformation on roughness level. In sliding contacts, the proposed finite element (FE) model combined with the Archard wear equation successfully predicts the contact pressure evolution and change in the topography on a macroscopic point of view. The change in the topography in a sliding contact is mainly caused by wear. A new FE model, with respect to the artificial and real surface roughness, is discussed. It is found that the proposed model is a useful tool to study the running-in of a surface on roughness level. The changes on macroscopic and on microscopic level of the surface are also discussed in the running-in of rolling-sliding contacts considering two aspects: wear and plastic deformation. The geometrical change of the contacting surface due to wear is predicted using the present FEM model, combined with the Archard wear equation, and has been compared with results from the literature. Calculations are performed to predict the wear of an artificial rough hemisphere in rolling-sliding contact with a smooth cylinder. The model also predicts the change of real rough surfaces which were in good agreement with the experimental results. The change of a rough surface, represented by an arithmetic average surface roughness, Ra, is predicted for lubricated rolling-sliding contacts using the load-sharing concept. The results obtained are in good agreement with experimental results. A FEM based model has been developed to study the running-in of rolling, sliding and rolling-sliding contacts on macroscopic level as well as on roughness level. However, the transition between the running-in phase and the steady-state phase for sliding and rolling-sliding contacts cannot be determined by considering only one single parameter; likewise for the rolling contact situation. Wear is an ongoing process.