Fault diagnosis on vibration oscillation of auxiliary power unit

In the vibration test and flight of an aircraft's auxiliary power unit (APU), it was found that there was a vibration oscillation phenomenon according to the vibration data. After analyzing the acceleration, strain and noise data, it was discovered that the vibration oscillation phenomenon was caused by APU itself. By performing fast Fourier transform on the test data with vibration oscillation, a spectrum diagram was obtained and the two similar frequencies causing the beat vibration were extracted. Vibration tests were carried out on the APU separately and the acceleration data were analyzed. It was found that the acceleration was significantly large at twice the rotor frequency and the acceleration trajectory showed a banana shape indicating typical rotor misalignment. Upon disassembling the APU gearbox and brushless starter generator (BSG), wear of the quill shaft was identified. Misalignment of the quill shaft resulted in wear, causing beating and leading to the vibration oscillation.