The development of cross-flow single overhead camshaft designs of engines led to the introduction of pivoted cam followers with pads that were subjected to uni-directional rolling/sliding under heavy contact loads. Such components were prone to wear failure by a mechanism involving severe surface roughening. The initiating wear mechanism was eventually shown to be a form of “mild” wear and the Archard wear equation was used successfully to model the pattern of wear seen on cams and followers.
The use of rigs to assess the wear performance of different lubricants has hitherto been a very poor predictor of engine performance, because of the complex interaction of materials, kinematics and forces in real engines. As a result, most automotive lubricant development relies on engine testing, which is expensive and time-consuming. Also, the complexities of the engine environment make it difficult to obtain much scientific insight into the tribological processes involved. The latter problem has been tackled by developing a rig test that simulates key features of a valve train system in a controllable manner.
The test is based on the use of a highly modified twin-disc Amsler test machine, with one disc replaced by a reciprocating block to simulate the kinematics of valve train systems. This test has been used to evaluate the wear performance of the industry standard reference oils for the Peugeot TU3 valve train test. The wear data obtained in the Reciprocating Amsler test correctly ranked the two oils over a range of loads. Furthermore, the test successfully reproduced the patterns of wear, surface topography and surface film formation seen on cam followers from engine tests.