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Crest Factor

 

The Crest factor, also sometimes called the "peak-to-RMS-ratio", is defined as the ratio of the peak value of a waveform to its RMS value. It is a pure number, without units. The crest factor of a sine wave is Ö2, or1.414; i.e. the peak value is 1.414 times the RMS value. A typical vibration signal from a machine with a large imbalance and no other problems will have a crest factor of about 1.5, but as the bearings begin to wear, and impacting begins to happen, the crest factor will become much greater than this. The reason that the crest factor is so sensitive to the existence of sharp peaks in the waveform is that the peaks do not last very long in time, and therefore do not contain very much energy. The RMS value is proportional to the amount of energy in the vibration signal.

Examples of actual crest factors measured on a cam rider on a large low-speed machine are shown below. The cam rider contains a roller bearing. The vibration signature, which is scaled in velocity units (ips), shows a little irregularity that is no doubt due to some roughness in the bearing. There is essentially no low-frequency motion here because the accelerometer is oriented in the radial direction of the bearing shaft, which is at 90 degrees from the motion of the cam rider.

Note the RMS value is 0.017 ips, and the crest factor is 8.9. In other words, the peak value is 8.9 times the RMS value.

Radial Velocity Waveform Without Fault

The next figure, below, shows the same measurement point, at a later time. Here, the waveform shows that the bearing has an obvious fault in one of the races. The RMS level remains low at 0.086 ips, but the crest factor has risen to 19. This shows that a simple vibration meter that is only sensitive to RMS level is not able to detect a defective bearing, at least in this case.

 

 

The next figure, below, was measured at the same point on the machine as the previous waveforms, but in the direction of motion of the cam rider rather than perpendicular to it. The measurement was made before the bearing developed the fault. Here, we see the low-frequency content due to the movement of the cam. The small noise bursts are caused by the minor bearing damage as was shown in the first figure, above.

Note the RMS value is 0.45 ips and the crest factor is 1.7.

Tangential Waveform Without Fault

The figure below is from the same measurement point and direction as the one above, except it was recorded after the bearing fault developed, as in the second figure, above.

Tangential Velocity Waveform With Fault

Note that the bearing fault is clearly visible in the sharp spikes similar to the ones in the Radial with fault figure above. Note also that the RMS value is 0.45 ips, the same as in the previous figure, indicating that the bearing fault did not add significantly more energy to the vibration signature. The interesting fact here is that the crest factor of 1.8 is only slightly higher than before, even though the sharp vibration spikes are present. In this case, the large low-frequency signal masks the spikes and they do not show up as an elevated crest factor. This is a good illustration that the crest factor alone can sometimes be misinterpreted unless the vibration waveform is actually observed. This condition does no occur very often in practice, however.





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