Acceptance Screening of Turbopump Gears Using the Cepstrum Method

Abstract

Vibration measurements are used to assess the health of components in rotating equipment across a wide range of industries. The typical practice in most machinery analysis is to establish a baseline for a specific machineand subsequently implement a regular monitoring schedule. In a process known as trend analysis, changes in relevant parameters are then tracked over the life of the machine. In some cases the comparison of measurements between different machines is not recommended because of variability in transmission path effects caused by either manufacturing tolerances or differences in the instrumentation setup. Although these concerns are valid, the constraints imposed by the particular application to acceptance screening of rocket engine turbomachinery make a comparison between engines useful. It is suggested that engine-to-engine comparisons of vibration-related parameters can be used to provide information on abnormal gear behavior. Support for this proposal is presented in the context of parameters developed from a cepstrum analysis of gearbox vibration data acquired during actual ground-test acceptance firings of a rocket engine. The results obtained are also used to verify the ability of the cepstrum method to diagnose gear tooth damage in turbopumps. In this application the cepstrum is defined as the inverse discrete Fourier transform of the log of the two-sided autospectral density.