Fracture fault analysis of tail rotor drive shaft on a helicopter

To identify the cause of the high-frequency abnormal vibrations and tail rotor control failure in a certain type of helicopter, fault localization and analysis were conducted. Through analysis of the flight parameters and vibration data, the fault was located as the fracture of the horizontal short shaft in the tail drive shaft assembly of the transmission system. A fault tree was constructed with the fracture of the horizontal short shaft of the tail drive as the top event, and fault tree analysis and bottom event verification were carried out to determine that the fault attributed to the divergence of self-excited vibration in the horizontal short shaft of the tail drive. An analysis of the self-excited vibration mechanism of the tail horizontal short shaft was performed. Contact models for mating internal and external spline teeth pairs, as well as a lumped parameter model for the shafting-spline system, were established. A stability analysis of the horizontal short shaft of the tail drive under the radial friction force of the spline was conducted, clarifying the conditions for system instability and the measures against instability. The dynamic characteristics test of the tail horizontal short shaft was carried out, confirming that self-excited vibration occurred under conditions of inadequate lubrication at the spline pair position, wear on the spline tooth surfaces and locating section of the flange, etc. Analysis of the failure mechanism and test results indicated that the friction in the spline locating section was the fundamental cause of the self-excited oscillation in the horizontal short shaft of the tail drive. The divergence of this self-excited vibration ultimately led to the fracture of the shaft. This finding could provide theoretical guidance for optimizing drive shaft design, maintenance, and failure prevention.