Bearing faults are categorized into four stages, based on the types of frequencies produced by rolling elements impacting defects in the inner or outer races.
At the first stage
Small pits appear in the bearing race and impacts of rolling elements appear at ultrasonic frequencies in a spectrum. Stage 1 is still considered normal operation. At this stage, your bearings should still be functioning normally. Defects appear around 20,000-60,000 Hz. Although not cause for replacing a bearing, Stage 1 may indicate a lack of lubrication between the races and rolling elements. According to the Technical Associates Vibration Diagnostic Handbook, “Bearing defect frequencies will appear in HFE spectra but will not in normal vibration spectra in Stage 1.”
In Stage 2
Defects start to ring at the bearing’s natural frequencies. This generally occurs between 500 and 2,000 Hz. These natural frequencies may be resonances of either bearing support structures or components of the bearings themselves such as races or rolling elements. Sideband frequencies also start to appear above and below these frequencies. Repair should be scheduled for critical machines at this stage.
At Stage 3
Levels increase at bearing defect frequencies and harmonics appear on the spectrum. As this stage progresses, defect frequency harmonics and sidebands increase. If the bearing is removed, defects can be clearly seen in the raceways. A deterioration rate should be determined. Bearings at this stage should be replaced both in critical and non-critical machinery at this stage.
Appears toward the end of a bearing’s life. In this stage, the noise floor is raised at all frequencies, creating a random, broadband spectrum. Amplitudes of both high frequency noise floor and HFE may actually decrease, but this is a sign that the bearing is about to fail. Hopefully, your bearings never reach this stage, but if they do, it’s very important that you replace them immediately.
Being able to identify what stage a bearing is in will hopefully help you implement a more accurate predictive maintenance plan. This, in turn, will help you to keep machines and their components running smoothly with the least amount of downtime possible.
If you are interested in learning more about the signs of bearing failure, watch the video below or check out some of our related blogs.
Vibration Analysis: Bearing Replacement within the 4 Stages of Bearing Failure
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