Mixed Friction: Critical Assessment of Engineering Load Sharing Equations Based on the Lambda-Parameter

The paper addresses the inconsistencies in the use of the Lambda-parameter for describing mixed friction in engineering applications. Commonly, a single roughness metric is used to define this parameter, especially root-mean-square (RMS) roughness, or average roughness. In this work, we used artificially generated surface roughness to highlight that the Lambda-parameter as generally used is not sufficient to describe the occurring friction coefficient in mixed lubrication. The generated surfaces have the same RMS and average roughness, but varying surface height distributions and roughness orientations. With numeric calculations for thermal elastohydrodynamic lubrication of a point contact we determine the traction. Additional roughness metrics in the definition of the Lambda-parameter are proposed and result in the collapse of the obtained traction curves.