The first step to ultrasound bearing monitoring is establishing a baseline dB level for each machine. There isn’t a decibel domain a bearing should reside within which is why each machine needs its own baseline to be established. This can be done a few different ways. For example, analysis can be performed on a new, healthy bearing or it can be performed on other, similar bearings which operate under similar loads.
Establishing accurate baselines for healthy equipment is very important to your bearing condition monitoring program as it creates a reference point for multiple aspects of future bearing analysis.
Next, it’s time to take readings with your ultrasound device. Your sensor should be placed on top of the bearing housing for the most accurate measurement. Record your measurement and look for dB gains that could indicate a mechanical fault. As a rule of thumb anything over an 8 dB gain requires investigation. This gain could be a result of under or over lubrication or possibly the early stages of a bearing failure. A dB gain that eclipses 12-16 over the baseline is an indication of bearing degradation. When dB gains get anywhere above 30 over baseline it is a warning of a potential catastrophic failure. Thus, maintenance should be planned immediately to avoid the failure.
More advanced analysis can be done in the FFT, timewave form of the recording. This can help determine what the cause of the dB gain might be. Consistent, repetitive dB spikes could be the indication of imperfections on the bearing, which will worsen over time if left unchecked. Whereas relatively inconsistent dB spikes could be the result of under or over lubrication.