Low-speed Radar Gearbox
This example is the acceleration spectrum of a large low speed (4 RPM) gear driving a long-range radar antenna. The frequency range is from 0Hz to 6Hz. Note that the spectrum is noisy, and it is difficult to tell it contains anything else but noise.
The next graph shows the synchronous-averaged waveform from which this spectrum came :
Note that the time record length is 90 seconds.
There is a sharp peak every 15 seconds. Since the gear speed is 4 RPM, 15 seconds is the time it takes to make one revolution. The peaks are the result of one defective tooth in the gear. Each time the tooth contacts the pinion gear an impulse is generated. The information is immediately assimilated form the waveform, while the spectrum is nowhere near as informative.
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