Measurement of Camshaft Wear—Wear and Kinematics of Overhead Camshafts 850442
Prevention of camshaft wear is a critical performance requirement for crankcase oils. As regards cam tip wear, an API specification requires that after an ASTM Sequence V-D test this wear should be below 25 μm for a crankcase oil to qualify. Wear at the cam tip, however, is only to some extent representative of the wear around the whole cam lobe, especially at low wear levels.
To gain a better insight into the Sequence V-D camshaft/cam follower wear phenomena a measuring technique has been developed which enables calculation of the wear of the relevant parts of the cam lobe. This technique involves application of two electronic transducers, one measuring cam lift, the other camshaft angle. Using a desk top computer, cam lobe profile data are stored in numerical form before and after an engine test. Subsequently, the data of the unworn and worn cam are processed, resulting in a wear graph and the calculated wear at the cam tip.
The results correlate well with conventional Sequence V-D cam tip wear measurements and the graphs give a complete picture of the wear occurring at both cam flanks and the tip. For the cam/follower combination investigated, maximum wear occurred 10 degrees past the tip on the valve-closing flank. Results show that, for cases with tip wear up to 50 μm, wear at the valve closing flank is always higher (up to five times) than wear at the tip. The location of maximum wear was predicted using kinematic analysis and taking elastohydrodynamic lubrication theory into account. The wear pictures confirm the anticipated location of maximum wear.