Failure analysis of fractured motor bolts in high-speed train due to cardan shaft misalignment

摘要

Motor of high-speed train is normally connected with suspending devices through bolts. This paper presents an investigation of six motor connecting bolts, based on GB/T 3077 standard and strength grade of 8.8, which fractured at the thread close to the contact surface of motor and suspending device before the designed service life. The investigation includes macro examination, chemical compositions analysis, tensile tests, hardness tests, metallographic examinations were used to analyze the cause of fracture. The results revealed that the strength and material of bolts met the standard, no metallurgical defect was found in examination, and the bolts fractured were due to alternating load. According to the spectrum of acceleration signal measured by the sensor mounted on the motor housing and subsequent inspection of the cardan shaft and rotor of motor, revealed the alternating load is caused by misaligned cardan shaft. Further dynamic analysis shows that the rotation of misaligned cardan shaft resulted in huge unbalance force. To confirm the findings, finite element analysis on the bolts subjected to pre-tensile load reveals that the materials at the bottom surface of the first thread suffered high alternating stress and reaches the fatigue limit. From the above findings, this paper proposes an innovative strategy to avoid similar accidents of high-speed train suspending devices.