Gear fault detection in rocket engine turbomachinery

A method for detecting anomalous gear behavior in rocket engine turbomachinery during acceptance hot firing is presented. The diagnostic procedure is based on a cepstrum analysis of steady-state gearbox accelerations. In this application, the cepstrum is defined as the inverse discrete Fourier transform of the log of the two-sided autospectral density, The vibration measurements used in the analysis are acquired during static hot fire tests from accelerometers mounted on the external surface of the turbopump gearbox. Following the ground tests, the cepstrum technique is used to provide insight into the differences between turbopumps that have functioned nominally and those with out-of-family signatures. The effectiveness of the method is demonstrated by comparing analysis results fmol an engine in good condition with a similar engine that suffered complete gear failure during its development test. This example is used to suggest that the cepstrum method cannot only help detect out-of-family vibration characteristics, but ran also pmr ide insight into the nature of a defect. A source mechanism is proposed to explain the unique spectral characteristics that appear due to the presence of the gear fault.