Cyclic spectral analysis of rolling-element bearing signals: Facts and fictions

Rolling-element bearing vibrations are random cyclostationary. This property is so symptomatic when an incipient fault develops that it can be exploited for diagnostics. This paper discusses which cyclic spectral tools should be considered for that purpose. Specifically, it demonstrates the optimality of the cyclic coherence, which can not only evidence the presence of a fault in high levels of background noise, but can also return a relative measure of its severity. The estimation issue of the cyclic coherence is addressed in detail, as well as its use in a statistical test, and sub-optimal simplifications. Eventually, it is shown that the familiar squared-envelope spectrum happens to be a special case of the cyclic coherence with very similar diagnostic capabilities.