Synchronizer Spring Failure Due to Gear Shift Loads - Investigation and Design Recommendations

Abstract

In manual transmission, the vital function of synchronizer pack is to synchronize the speed of the target gear for smooth gear shifting. The synchronizer pack consists of various elements and each of these elements has specific function. These elements are baulk rings, shifter sleeve, hub, synchro key, synchro springs etc. The function of synchronizer can be affected due to failure of any one of these elements. This work focuses on the failure of synchronizer pack due to synchro spring failure.

The function of synchronizer spring is to exert the required force, to index the synchronizer ring before the movement of shifter sleeve over synchronizer ring. During the shifting of shifter sleeve from one gear to another gear, the springs deflect in both shifting directions. This causes fatigue failure of synchronizer springs. The manufacturing variations, and part quality issues results in very early fatigue failure of synchronizer springs. The failure of synchronizer springs affects the smooth gear shifting or it does not allow the shifting at all. The vehicle becomes off road due to the spring failure. To evaluate the spring fatigue strength, a new accelerated test was developed to get the result in shorter time. Each failure mode is investigated and based on the findings design improvements were carried out. The improvements in the spring design and the corrective actions in interfacing components help to meet the fatigue life of the spring.