Calculation of mixed friction conditions in large-Scale rolling-Sliding contacts for different surface structures

The lubricant significantly influences the performance of rolling-sliding contacts. On the one hand, the normal force is transmitted by the lubricant in consequence of hydrodynamic effects and on the other hand, the solid friction between the contacting asperities can be reduced. In this report, a novel calculation approach for mixed lubrication conditions is presented. Besides the consideration of measured surface structures, the calculation approach can be applied to application-oriented rolling-sliding contacts with large contact areas. Based on the elastic half-space theory according to boussinesq/love/hartnett, an adapted solving algorithm for the micro contact calculation is presented. Instead of the established adhesion boundary condition, which leads to a pressure of zero in the area of the roughness valleys, a lubricant boundary condition is introduced. Based on this lubricant boundary condition, the influence of the fluid pressure on the elastic deformation of the rough surface can be directly regarded. The resulting cross influences of deformation affect the solid contact pressure as well as the size of the lubrication pockets. The proposed solving algorithm is extended to a method for calculating the pressure distribution under mixed lubrication. The calculation method is validated by friction force measurements on a disc-on-disc test rig, whereby the normal force as well as different surface structures are specifically varied. The method confirms the changes of the friction coefficient for different normal forces as well as for different surface structures. Furthermore, the chart of solid-contact-load-ratio can be derived, which is mostly a missing link in tribological calculation models for different surface topographies.