ABSTRACT
We consider the problem of stick-slip and noise of lubricated rubber seals that sometimes occurs in master cylinders and clutches of car braking and transmission systems. The problem is investigated for different brake fluids with a special tribological device consisting of a steel ball moving oscillatory on the lubricated EPDM samples of different roughness. The stick-slip-noise appears reproducibly for selected brake fluids exclusively for sufficient roughness of the rubber sample. Chemical analysis on the influence of brake fluids on the EPDM-material resulted in neglible effects. Based on stationary friction measurements in a broad velocity range at different temperatures and loads it is demonstrated that the stick-slip-noise problem is present for selected brake fluids delivering a pronounced negative velocity gradient of the friction force in the mixed lubrication regime, which is known to produce instable sliding. We consider a scaling approach to interrelate the frequency of stick-slipsliding with the characteristic velocity range of mixed lubrication and the maximum roughness of the rubber sample. Obviously, stick-slip-noise can be avoided for all brake fluids if the predicted frequency lies out of the sensitivity range of the human ear. From the scaling relation we conclude that this is the case if the rubber surface is sufficiently smooth
