Reason for bolt fracture of grinding roller shaft in medium speed coal mill

During the operation of the first grinding roller shaft of the B coal mill in Unit 1 of a certain coal-fired power plant, the bearing temperature increased. After shutdown and disassembly in spection, it was found that all three bolts connecting the grinding roller bracket and the grinding roller shaft were broken. The cause of bolt fracture was analyzed using macroscopic observation, metallographic examination, chemical composition analysis, scanning electron microscopy analysis, hardness testing. The results show that there was a significant stress concentration at the thread root and the transition between the bolt head and the screw along the variable cross-section of the bolt. The hardness at the thread root of the bolt was relatively high, and the core tissue was abnormal, resulted in a decrease in the fatigue resistance of the bolt. Under the action of cyclic vibration loaded, microcracks were generated at the stress concentration point, and bolt cracks continued to propagate, ultimately led to fatigue fracture of the bolt. Improper bolt pre tightening force and assembly process also promoted the initiation and propagation of cracks.