Sidebands
If the defect is on the inner race of the bearing, the turning speed will amplitude modulate the bearing tones, and this will cause sidebands around the bearing tones, spaced apart at 1X, to appear. The amplitude modulation comes from the fact that the defect on the inner race moves in and out of the bearing load zone once per revolution. While in the load zone, the defect produces vibration at the ball pass frequency, but when it is out of the load zone, very little vibration is produced at this frequency. This accounts for the amplitude modulation of the bearing tone and the consequent sidebands. Sidebands spaced at 1X around bearing tones are a sure sign of advanced bearing wear. Sometimes, if a rotor is strongly out of balance, an inner-race bearing defect will not produce amplitude modulation or sidebands. This is because the centrifugal force due to imbalance keeps the inner race loaded at the same location on its periphery all the time.
Another example of sidebands in bearing spectra involves the Fundamental Train Frequency (FTF). This is the rate at which the cage holding the rollers rotates in the bearing. If one roller is spalled, cracked, or worse yet, in several pieces, it will make a lot of noise when it is in the load zone of the bearing, but will be quiet when not in the load zone. It will move in and out of the load zone at the FTF rate because it migrates around the bearing with the cage. This causes amplitude modulation of the bearing tones at the FTF rate, and the result is sidebands around the bearing tones spaced apart by the FTF.
The final stage of bearing wear is sometimes called the "thermal" stage, where the bearing becomes hot, breaking down the lubricant, leading to catastrophic failure which can include melting of the rolling elements and/or the races.
The key to effective predictive maintenance of bearings is the trending of bearing tone levels over time from their onset. Sometimes a bearing condition will progress from a very small defect to complete failure in a relatively short time, so early detection requires sensitivity to very small vibration signature components. The analyst should be aware that some types of machines will show bearing tones in the average spectra. Diagnosis is made on the basis of significant increases from these average values. Any significant bearing tone should be carefully watched for signs of worsening.
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