1The grease breaks down
Heat and time age the grease: its oil separates and the thickener hardens, so the oil film between balls and races thins out. Metal starts touching metal, friction rises and the bearing runs hotter. Contamination, the wrong amount of grease, misalignment or stray currents from a variable-speed drive cause the same thing.
2Spalling on the outer race
Repeated contact stress fatigues the steel just under the surface of the race, and small flakes break out (spalling). Every ball clunks over the pits as it passes, at a set frequency of about 3 times shaft speed for this bearing, roughly 75 Hz. Vibration climbs, and the loose flakes get rolled into the other surfaces.
3Clearance grows and the rotor drops
The worn bearing gets loose inside, so the rotor sags and whirls off centre. The 0.5 mm air gap closes at the bottom. Magnetic pull is stronger where the gap is narrower, so it drags the rotor down even further.
4The rotor rubs the stator
The rotor touches the stator bore. Friction throws sparks and heats both cores, and smears the laminations together so eddy currents add more heat. The drag pulls more current, and the heat starts cooking the slot insulation, the first step to a burnt-out winding.
5How it is caught and prevented
- Here the overload relay finally trips on the extra current, too late to save the bearing or the stator bore.
- Vibration monitoring would have caught it at stage 2: an accelerometer on the bearing housing, readings in mm/s judged against ISO 10816 zones, and a spectrum that shows the bearing’s defect frequencies.
- Temperature sensors on the bearing housings (common on larger motors) alarm at around 90 °C.
- Regrease at the maker’s interval with the right amount and type, and keep belts and couplings aligned.
- On drive-fed motors, an insulated bearing or a shaft grounding ring stops stray currents from pitting the races.
- PTC thermistors in the windings protect the insulation if the rub heats the stator.