Motor-driven Centrifugal pump
The first case history is interesting in that it shows that demodulation is very powerful in improving diagnostic accuracy compared to conventional vibration analysis. The machine is a direct-coupled motor pump combination. Conventional vibration spectral analysis over a frequency range of 20 orders of run speed detected a bad bearing, but the spectrum alone could lead one to the wrong bearing in the machine.
Here is the vibration spectrum measured at the coupled end of the motor. Note the non-synchronous bearing tone at just over 100 Hz and 92 VdB (0.022 ips) level. There is also a second harmonic of this component visible. This strongly indicates a defective rolling element bearing.
The next spectrum was measured at the free end of the pump, and is shown below: Note that the same bearing tone and second harmonic are present here, but the level of the tone is 12 dB (4 times) higher in level than it was on the motor coupled end. This is an indication that the bad bearing is in the pump and not the motor. The fact that the bearing tone appears in both locations is because this is a small machine, and the bearing vibration travels across the machine.
The next pair of spectra tells a different story. Following is an amplitude-demodulated spectrum taken at the pump free end. The frequency span of the
demodulated spectrum is 20 orders. Note that there are no bearing tones visible.
Next is the demodulated spectrum from the motor-coupled end, shown below: Here we have a series of bearing tone harmonics that rise above the noise floor more than 15 dB! This is a sure-fire indication that the problem bearing is in the motor and not the pump. This is a good indication of the power of demodulation to localize vibration sources, especially in the case of rolling element bearings.
the higher-level bearing tone in the conventional vibration spectrum of the pump free end? If we look at the noise floor near the bearing-tone frequency in the conventional spectrum from the pump free end, we see a hump, or “haystack” that is no doubt caused by a mechanical resonance. This resonance amplifies the noise floor by about 15 dB. The bearing tone is also amplified by the resonance, accounting for its elevated amplitude.
An important aspect of the demodulation process is that the signal actually being demodulated is very high in frequency since it has been passed through a high-pass filter, usually above 2.5 kHz or so. This high-frequency energy does not travel very well through mechanical structures, so the information in the demodulated spectrum comes from very close to the accelerometer. This is the reason the demodulated spectrum from the motor-coupled end is not affected by the mechanical resonance that amplifies the bearing tone in the base band spectrum.
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