Abstract: A cushion ring is provided between a transmission shaft and a transmission wheel's sleeve. The cushion ring has a number of centripetal teeth, thereby forming a number of chambers around the cushion ring. Inside each chamber, there is a slope section slanting towards an adjacent first tooth, and there is a spring element pressing a ball (or cylinder) against the slope section and the first tooth. The diameter of the ball or cylinder is such that it stands on the slope section to be slightly beyond the teeth and contacts an outside surface of the transmission shaft. As such, when the engine is ignited and a jitter causes the transmission shaft to turn in a reversed direction, the balls and cylinders are rolled towards the spring elements and the rotational force is thereby absorbed by the spring elements.