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Detecting bearing defects using vibration analysis – video

Vibration Experts

Detecting bearing defects using vibration analysis

Alan Friedman describes the detection of bearing defects using vibration analysis from the time waveform, the vibration spectrum and amplitude demodulation.  This video is appropriate for anyone interested in machinery vibration analysis who wants to know about detecting bearing defects using vibration analysis.

Bearing defects generate pulsations each time a ball or roller contacts or hits a defect on the race. These defects can be seen directly in the time waveform.  The period or the time between impacts can be used to calculate the frequency of impacting and relate that to the bearing defect frequency if desired.

The vibration spectrum or FFT converts the repetitive impacting in time, into a peak in the spectrum.  The spectrum is displayed in a graph of amplitude vs frequency, the peak related to the impacting will appear at the frequency or rate of impacting.

Because the impacting is repetitive but not sinusoidal, we will also see harmonics or multiples of its frequency in the spectrum. As the fault progresses, these harmonics may increase in both number and amplitude. As the bearing defect gets much worse the spectrum’s noise floor will rise.

Demodulation provides another way of detecting bearing defects using vibration analysis. Each impact the ball makes with the defect causes the bearing housing to ring like a bell. This ringing is a higher frequency than the frequency at which the defect is getting struck by the balls. It is also a higher frequency than most of the other vibration in the machine.

What we can do is filter out all of the other low frequency vibration from the machine using a high pass filter – now the only thing that remains is the sound of the balls hitting the defect. With all the other vibration gone, we can more clearly hear this ringing, allowing us to detect the bearing defect at a much earlier stage.

Demodulation then essentially asks “what is the rate at which this ringing is happening?” The answer to which is – “it is ringing at the rate the bearing defect is getting hit by the balls.” In other words, we just came up with the same bearing defect frequency but we did it in a different way that will allow us to detect the bearing defect at a much earlier stage.

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