Bench Test of Cam and Tappet Wear in Additive Oils

Abstract

Differences in the wear of automotive cams and tappets were determined in a bench-type tester with a group of oils used earlier in an extensive field car study. The rate of wear in all oils decreased with running time, providing scuff failure had not occurred. The incidence of scuff failure during the bench tests was similar to that in field cars. The wear of hardenable cast iron cams and tappets increased with the concentration of zinc dialkyl dithiophosphate. Cam wear was increased when barium phenate was added to an oil and decreased when a sulfurized olefin was present. Neither of these additives affected tappet wear. These effects, with the exception of the increased wear in high concentration zinc dithiophosphate oils, were too small to be significant in dynamometer and field car tests. However, the direction and magnitude of the zinc dithiophosphate effect was similar in the bench and field tests. Metallurgy and design factors were shown to have a major effect on valve train wear and must be controlled when the additive effects are being determined.