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Sideband activity in spectrum

From: Jim P
Remote Name: 149.37.200.150

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I've come across a somewhat confusing (at least to me) FFT spectrum that I cant seem to find an explanation for. On several machines (one a ventilation fan and the other a motor generating set), I've noticed a series of sidebands extending out from each of the running speed harmonics (usually the first 7 or 8 harmonics) in the low frequency range (0-10 orders)spectrum. For one machine, the sidebands are spaced ~0.12R (R=running speed). For the other machine, the sideband activity is spaced ~0.09R. The sidebands decrease in amplitude on each side of the harmonic. In both cases, the harmonic content is relatively high compared to similar machines, suggesting looseness. In both cases, the high frequency energy is fairly normal- no significant indication of bearing damage. Question: Can someone explain why these sidebands appear? Does it have to do with the FFT process of a non-sinusoidal signal? Result of Modulation? Is it an indicator of severity? For each of the machines I've described above, this character has existed for years with no indication of increasing trend. Thanks in advance

Last changed: October 16, 2007