Investigation of manual transmission synchronizer failure mechanism induced by interface material/lubricant combinations

To better understand the effects of interface material/lubricant combinations on the failure mechanism of a certain type of manual transmission synchronizer, a gear shift experiment was performed on the custom-built apparatus. After the experiment which ran for thousands of simulated shifts, the surfaces of friction material were analyzed by a non-contact surface profiler, and the Fe content contained in gear oil was measured by atomic emission spectrometry. The results show that with an increase of the number of shifts the surfaces became smoother, which in turn caused a decrease in the contact temperature and local asperity pressure. The increment of Fe content in the gear oil decreased gradually, which signified that a thick oil film had formed between the contact surfaces. Therefore, the oil film became very much stable. The contribution of solid/solid interactions on the friction coefficient continued to reduce as the liquid gradually dominated performance. Due to the low friction coefficient, quick synchronization became unavailable and when any gear shocks occurred, it meant that the synchronizer was not performing as intended. •A custom wear tester was built to simulate transmission shifting conditions.•The running surface became smooth and made oil film rupture more difficult.•The solid contribution to the friction coefficient rose with increasing number of shifts.•The continuing gear shocks observed were due to a low friction coefficient.