The use of wavelets for analyzing transient machinery vibration

Transient events are sometimes buried in continuous machinery vibration data. Conditions causing these transient events include: a bearing ball rolling over a defect, a pit or chip on the face of a gear tooth, clearance in a bearing that allows a repetitive pounding, engine rod or main bearing knock and piston slap. Detection of the magnitude and timing of these events can be valuable for diagnostics. These can be identified with time-frequency methods that show frequency content and time of occurrence on a two-dimension contour plot. Wavelets can also be used to detect magnitude and timing. The orthogonal wavelet is inexpensive to compute and has the potential to display something new. The code decomposes the signal into wavelets, the location and magnitude of which identify the transient. Partial inverse transformation also shows regions of the signal where different wavelet levels build up to recreate the transient in the signal. The recent availability of orthogonal wavelets and MATLAB&reg wavelet code has made such analyses convenient. This article explains these methods and the signal processing calculations involved.