Cepstrum Analysis: An Advanced Technique in Vibration Analysis of Defects in Rotating Machinery

Conventional frequency analysis in machinery vibration is not adequate to find out accurately defects in gears, bearings, and blades where sidebands and harmonics are present. Also such an approach is dependent on the transmission path. On the other hand, cepstrum analysis accurately identifies harmonics and sideband families and is a better technique available for fault diagnosis in gears, bearings, and turbine blades of ships and submarines. Cepstrum represents the global power content of a whole family of harmonics and sidebands when more than one family of sidebands are presents at the same time. Also it is insensitive to the transmission path effects since source and transmission path effects are additive and can be separated in cepstrum. The concept, underlying theory and the measurement and analysis involved for using the technique are briefly outlined. Two cases were taken to demonstrate advantage of cepstrum technique over the spectrum analysis. An LP compressor was chosen to study the transmission path effects and a marine gearbox having two sets of sideband families was studied to diagnose the problematic sideband and its severity