某汽车发动机在行驶过程中出现助力转向系统失效的现象，拆解后发现发动机的惰轮发生了开裂失效。通过断口分析、化学成分分析、硬度测试、金相检验、受力分析等方法，分析了该惰轮失效的原因。结果表明：惰轮铁轮本身实际承受的力大于惰轮的设计最大径向力，铁轮的设计强度不足，从而导致铁轮在使用过程中沿圆弧倒角发生低周疲劳开裂失效。最后通过改进惰轮的设计工艺，有效地解决了该问题。

The power-assisted steering system of an automobile engine failed in running process. After dismantling and disposal, it was found that the idler gear of the engine cracked. In order to find the failure causes of the idler gear, the fracture analysis, chemical composition analysis, hardness testing, metallographic inspection and the force analysis were adopted. The results indicate that:the real force burdened by the iron wheel of the idler gear was higher than the designed max radical force, and the designed strength of the iron wheel was insufficient, which led to the failure and low-cycle fatigue cracking along the circular arc chamfering surface during use. Finally, the problem was solved effectively by the improvement of technological design.