Rolling-element Bearings

Discrete faults in ball or roller bearings cause a series of impacts at a frequency determined by the location of the fault, e.g. outer race, inner race, rolling-element, etc. in the bearing.

At the initial stages when the fault is still microscopic the impulses are so short that the frequencies can extend up to 300 kHz. These impacts excite structural and other resonance, including the resonance of the accelerometer, and produce a series of bursts, with a frequency content dominated by these resonances. This bearing signal is masked by other background vibrations from the machine, and the basic problem is to find a frequency range where the bearing signal is dominant over background vibration.

The repetition rate is indicated better by analyzing the envelope of the bursts rather than the raw time-signature. It is possible to calculate the repetition frequency of the bursts knowing the bearing data and using simple classical mechanics, but this calculation assumes purely a rolling action, whereas there is really some sliding action as well. Therefore the equation should be regarded as only approximate. Amplitude modulations may also produce sidebands.

Modern predictive maintenance software programs have built-in bearing databases from various manufacturers to simplify these calculations, and the resultant bearing damage frequencies can be superimposed onto the spectrum or envelope curve as an additional bearing analysis aid.