The absence of significant peaks at the fundamental rotational frequencies, indicative of a particular defect such as an outer race fault, results in a failure of the power spectrum technique to detect and diagnose mechanical defects characterized by sounds of short duration. Based on the results of an incipient defect diagnosis, explanations are offered as to the cause of the missing fundamental in the power spectrum in terms of two different effects which may operate simultaneously; they are 1 An average and shift effect which causes a slow migration of the fundamental impact frequency from its computed value; and 2 An inter-modulation effect which translates defect related information to frequency locations unrelated to the fundamental impact frequency. The paper goes on to show that fault detection and diagnosis are possible despite the absence of a significant peak at the fundamental rotational frequency.

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