MIDLIFE SCUFFING FAILURE IN AUTOMOTIVE CAM-FOLLOWER CONTACTS

There are several practically important engineering systems that fail by scuffing long after the initial running-in has been completed. A co mmon feature of these systems is repeated contact between the same poi nts on the mating surfaces. The processes leading up to such mid-life scuffing failures have been examined by monitoring the development of cam follower surface roughness and wear at regular intervals during a series of valve train wear tests in a fired engine. By the application of both statistical and numerical models for the elastic contact of r ough surfaces, the following stages in the scuffing process have been identified: (a) surface roughening in the mild-wear regime, which prog ressively increased maximum asperity contact pressures until (b) the e lastic shakedown limit was exceeded, causing plastic deformation wear, accelerated roughening and enlargement of valleys, leading eventually to (c) a transition to high rates of wear, probably resulting from hy drodynamic pressure loss and oil-film collapse.