Spectrum analysis provides a great deal of information about
the health of rotating machinery. But you should consider the spectrum as a summary
of the vibration within the machine.
The Fast Fourier Transform takes the time waveform and
computes how much of each frequency is present and displays that as a line in
the spectrum (grossly summarized, but that is basically the case).
Therefore, if the vibration from the machine is generated by smooth periodic
motion, then the spectrum provides a very good representation of what is
happening inside the machine. But as damaged gears mesh together, and rolling
elements pass over damaged areas on the raceway of the bearing, and as the pump
vanes push through the fluid causing turbulence or cavitation, the vibration
generated is not smooth and periodic. And there are a lot of other fault
conditions that likewise do not generate smooth and periodic vibration. Thus,
the only way to really understand what is happening inside the machine is to
study the time waveform.
The time waveform is a record of exactly what happened from
moment to moment as the shaft turns, the gears mesh, the vanes pass through
fluid, and the rolling elements roll around the bearing. Each minute change
that results from impacts, rubs, scrapes, rattles, surges, and so much more is
recorded in the time waveform and then summarized in the spectrum. Therefore,
it is critical to record the time waveform correctly and analyze it when you
have any suspicion that a fault condition exists.
This tip is provided by Jason Tranter, Managing Director of Mobius Institute.
For more condition monitoring tips, continue to visit the IMVAC – International Machine Vibration Analysis Conference – blog every week. To learn more about IMVAC and the event nearest to you, visit vibrationconference.com.
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