Bearing condition parameters: peak, carpet level and normalisation
Early bearing damage shows up in a high-frequency impact signal long before the low-frequency vibration level rises. Peak value and base noise answer different questions.
Why ordinary vibration is late
Low frequency parameters rise only when the damage is quite severe. A rolling element passing a flaw makes a shock pulse that first shows in the high-frequency range, so special bearing parameters were developed. There is no international standard, so several different characteristic values are in use.
| Parameter | Answers | Rises with |
|---|---|---|
| Maximum shock value (dBm) | How severe are the individual shocks | Initial race damage |
| Carpet level (dBc) | How high is the base noise of the bearing | Lubrication problems, general wear, too little clearance, installation stress |
Compare like with like
Any bearing reading depends on rolling speed (size and rpm), signal damping, load and lubrication. That is why a reference reading in good condition, or a normalisation against bearing size and speed, is needed. Normalised readings let you use the same threshold scale for different bearings instead of setting a limit for each one.
- Initial value: what was the reading when the bearing was healthy?
- Tolerance: what rise is allowed before action?
- Rate: how fast is it rising?
Envelope step
To diagnose which part is damaged, filter the high-frequency band that contains the bearing signal and demodulate it to an envelope. If the spacing of repeating peaks in the envelope matches a bearing damage frequency (inner race, outer race, rolling element or cage), that part is damaged. The method needs the bearing geometry: diameters, number of rolling elements and contact angle.