Alexander Wohlers

Contact mechanics and rubber friction for randomly rough surfaces with anisotropic statistical properties

In this paper we extend the theory of contact mechanics and rubber friction developed by one of us (B.N.J. Persson, J. Chem. Phys. 115, 3840 (2001)) to the case of surfaces with anisotropic surface roughness. As an application we calculate the viscoelastic contribution to the rubber friction. We show that the friction coefficient may depend significantly on the sliding direction, while the area of contact depends weakly on the sliding direction. We have carried out experiments for rubber blocks sliding on unidirectionally polished steel surfaces. The experimental data are in a good qualitative agreement with the theory.

Numerical and Experimental Investigation on O-Ring-Seals in Dynamic Applications

Abstract This paper presents a physically-based simulation approach to predict the friction force at oil lubricated contacts for rubber o-ring seals in dynamic applications. In the boundary lubrication regime the friction coefficient is calculated using a recently developed contact mechanics theory. The stress and strain fields in the rubber are calculated using the finite element analysis (FEA). In the FEA the temperature-dependent nonlinear rubber behaviour is considered. Loads due to the assembly process, thermal expansion, system pressure and tangential friction forces are included in the analysis. In the mixed and hydrodynamic lubrication regimes, the asperity-asperity and fluid-asperity interactions are determined from the Perssons dry-contact mechanics theory, the Reynolds-equation (gap flow) and the deformation model of the seal. To test the theory a test rig has been developed. Simulation results, carried out for an unpressurized o-ring seal system, are compared to the experimental data and, especially for small velocities where mixed lubrication prevails, the results are in good agreement.