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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 14, Peaks at Higher Frequencies that are not Harmonics

Rolling element bearing defects cause one or more peaks at higher frequencies that look like they may be harmonics but actually are not. Depending on the type of bearing defect and its location, frequencies are generated that cause one or more peaks.

However, the frequencies generated are non-synchronous and, therefore, not true harmonics. At the earlier stages of deterioration, the peak may be generated by the excitation of the resonant frequency of the bearing's inner or outer race. As the bearing deteriorates further, its frequencies often follow mathematical predictability, but the calculated results are still non-synchronous.

Cavitation. Sometimes a spectrum will show high peak amplitudes at several frequencies (relatively low frequencies to high frequencies). However, the peaks that result from cavitation are very poorly defined, are spread out over a much wider frequency range, and their amplitudes do not decrease to almost zero between major peaks. Instead, throughout the whole frequency spectrum, there is an appreciable vibration (except at very low frequencies where velocity amplitudes are small due to the very low frequencies or due to the low frequency limitations of the pickup). As cavitation produces non-synchronous frequencies, using synchronous time averaging is an excellent way to separate cavitation symptoms from other sources.

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