At Stage 1 of bearing failure, the bearings are still considered within normal operation. They are still functioning normally, but small pits begin to form in the bearing race and the rolling elements’ impact will be audible at ultrasonic frequencies. Vibrations at ultrasonic frequencies ranging between 20,000 and 60,000 Hertz (Hz) indicate that there is a bearing-to-raceway contact caused by a lack of lubrication.

This stage can be avoided beforehand by checking the lubrication levels and making any necessary amendments.

At Stage 2 of bearing failure, bearings exhibit advanced micro-pitting as a result of increased friction from the low bearing lubrication. In this condition, resonance detectable in the range of 2,000 to 8,000 Hertz (Hz) will be generated. At this stage, it is best to replace bearings in extremely critical machines.

At Stage 3 of bearing failure, bearings show obvious defects. Harmonic frequencies and sidebands will appear on the spectrum. When the bearings are taken off, the defects in the raceways or rolling components are easily visible. Therefore, it is advised to replace the bearings in both critical and non-critical machinery at this stage.

At Stage 4, a noticeable noise floor increases at all levels, which then creates a random, broad spectrum. This stage also indicates that the bearings are nearing the end of their lifespan. Bearing failure is almost inevitable, and no machine should ever be pushed to this limit.