Application of the Flash Temperature Concept to Cam and Tappet Wear Problems

Abstract

In the lubrication of a modern automotive engine the cams and tappets are among the most sensitive parts and are expected to become more difficult to lubricate as engine speeds increase. This paper deals with the two types of cam and tappet distress that occur mainly at high engine speeds; scuffing and polishing (or burnishing). The occurrence of these phenomena is believed to be controlled by the temperatures in the contact zone, and in order to treat these in a quantitative manner the flash temperature theory of Blok has been applied to cam and tappet systems. For this purpose, some preliminary measurements have been made of the coefficient of friction between cam and tappet. The contact temperature reaches a maximum at three points in the valve operating cycle; at the cam nose and at the two points of zero tappet acceleration. The relative values of these maxima depend on the cam contour, spring forces, materials of construction, etc. If the temperatures at these points are excessive, either scuffing or polishing is liable to occur. A case of polishing wear is examined, and it is shown that most of the observed details may be explained on the flash temperature theory. It seems necessary to postulate that a steel surface with a low velocity relative to a rubbing contact will scuff if the contact temperatures are high enough, but that the surface will become more liable to fail by polishing wear as this relative velocity increases. This is related to some phenomena observed with steel spur gears. The theory may also be used to evaluate the relative scuffing properties of different cam profiles, with certain limitations which are explained in the paper, the metallurgy being constant. Some indications as to the design of a cam profile to avoid scuffing are also given, but there is no experimental backing for these.