Influence of surface topography on the tribo-characteristics of an eccentric-tappet pair

The cam-tappet pair is one of the most important components in an engine and determines the overall performance of the motor. Compared with modern car engines using rolling-sliding tappets for the transmission of the rapid up-down lifting motion of the cam to open and close the engine's inlet and exhaust valves, the flat-tappets are often subject to fatigue failure. Upon stripping down a damaged motor, the tappets are invariably found to be heavily pitted. This study performs a theoretical simulation of the surface topography influence on the tribo-characteristics of an eccentric-tappet pair. The asperities on the flat-tappet surface generate fluctuations of the pressure, film thickness and temperature distributions. The pressure fluctuations result in alternative stresses in the metal. The periodical variations of pressure, minimum film thickness and thermal rise are investigated by changing the eccentricity and roughness wavelength. The results suggest an explanation for the rapid occurrence of cracks or even complete fracture in the cam and flat-tappet pair.