Fatigue failure analysis of the central-driven bevel gear in a turboshaft engine arising from multi-source excitation

The gas generator rotor of a small turboshaft aircraft engine, which operates at a relatively high speed, may have support stiffness that is weakened by the deliberate reduction of its structural weight, potentially causing a coupling vibration between the gas generator rotor and the engine's central-driven bevel gear (CDBG). The vibrating CDBG can be exposed to a complex pattern of excitation caused by lateral vibration of the rotor and meshing between gear teeth, resulting in possible fatigue damage of the CDBG. In experiments with a typical turboshaft engine, several fatigue fracture failures occurred, and the fatigue cracks were found to propagate radially toward the center of the disk, which is noticeably different from common fatigue failures of gear systems. In this paper, both the gas generator rotor of a turboshaft engine and the CDBG are investigated, and a dynamic model of the rotor-gear system that takes account of multi-source excitation is established. The vibration response of the CDBG, as well as the mechanism by which the fatigue crack is initiated and propagated, were studied. The research shows that in addition to high-frequency excitation arising from gear-teeth meshing, the lateral vibration of the gas generator rotor will also generate a 2× excitation on the CDBG. Thus, under multi-source excitation, the CDBG disc undergoes both a high-frequency 5th nodal diameter mode and a low-frequency swing-shaped vibration mode. The high-frequency excitation causes an initial crack at the root of the gear tooth, and the crack propagates under low-frequency vibration stress, eventually causing fatigue fracture of the CDBG structure.