某电梯曳引机用螺栓在使用过程中发生了断裂。采用化学成分分析、金相检验、硬度测试及断口分析等方法，对螺栓断裂的原因进行了分析。结果表明：由于原材料、设计、热处理等方面的原因使螺栓各项性能下降，齿根部位形成应力集中，在长期交变应力的作用下发生了疲劳断裂。最后提出了控制螺栓原材料的质量，严格进行调质热处理的改进措施。

The bolt of an elevator traction machine broke in the process of use. The causes of bolt fracture were analyzed by means of chemical composition analysis, metallographic examination, hardness test and fracture analysis. The results show that due to raw materials, design, heat treatment and other reasons, the properties of bolt were reduced, stress concentration occured at the tooth root, and fatigue fracture occurred under the action of long-term alternating stress. The improvement measures of controlling the quality of bolt raw materials and strictly carrying out quenching and tempering heat treatment were put forward.