In the modelling of the leakage rate, friction force or contact pressure distribution of hydraulic seals is quite common to assume the mating surfaces to be characterized by a random isotropic roughness. However, due to different surface finishing methods, such as coating, grinding or polishing, roughness with anisotropic characteristics is often generated.
In this paper a first experimental investigation of the influence of such anisotropic surfaces on the sliding friction is provided. For this purpose, a test rig has been designed and set up to investigate a soft, lubricated line contact representative of a generic reciprocating hydraulic seal. In particular, an O-ring cord is squeezed into contact with a steady rotating rigid cylinder. In order to adopt a cylinder-on-flat configuration, the diameter of the rigid cylinder is chosen to be significantly larger than the O-ring (cross-section) diameter. Furthermore, three cylinders with different surfaces are used: One (sandblasted) isotropic surface and two anisotropic surfaces roughness, scratched perpendicularly or along the azimuthal direction. Therefore, under temperature control, Stribeck curves have been measured at different squeezing loads and surface roughness, showing a neat influence of the surface roughness characteristics on the friction force. Finally, the experimental results are compared with the predictions provided by a recent mean field theory of soft contact (e.g. rubber) lubrication.