There are four main causes of bearing failure that can be traced back to improper lubrication:
- Lubricant contamination – this occurs when debris enters the bearing raceways and mixes with the lubricant. This gives the lubricant a grainy texture causing vibration and wear of the raceways and eventual failure.
- Unsuitable lubrication – if the grease used within the bearing is of the wrong viscosity or has incorrect additives for the operating conditions then failure is likely. A grease that is too thick gets trapped between the rolling elements and causes the raceways to run dry. This overheats the bearing causing failure.
- Aged lubricant– if the lubricant in the bearing is not replenished and the grease reaches the end of its useful life failure may occur. The lubricant is not able to distribute properly throughout the bearing. The raceways and rolling elements do not get an even film of lubricant and begin to wear quickly, generate heat, seize and fail catastrophically.
- Insufficient lubrication quantity – this is often because engineers fear blowing bearing seals by over-greasing and over pressurising the bearing. This over-caution results in the bearings running without sufficient lubrication and causes failure.
- Increased noise when running
- High levels of vibration
- High bearing/motor drive-end temperatures
- Lubricant seep from the drive-end shaft of the motor
The above signs are not all easy to spot through a visual inspection and do require measurement equipment to detect them accurately. Thermal imaging and vibration analysis probes will quickly pin-point any trouble.
High temperature at the motor drive-end and a high level of vibration indicates a bearing running without lubricant with contaminants in the raceways. This is common when bearings are over pressurised during re-greasing and seals have been blown. The grease then seeps out and the broken seals and allows dirt and dust to enter the bearing. Re-lubrication won’t resolve this in the long-term so it is a sensible precaution to decommission the motor and fully inspect the bearings and seals. They should be replaced so that temperature and vibration levels fall back to within normal tolerances for the bearing during operation.
Any lubricant you are using should be of the correct thickness and type. Improper viscosity can cause the bearing to skid if too high and dissipate too quickly if too weak.
For industrial applications, greases between NGLI Class 1 and 3 are typically used for re-lubrication of bearings. The bearing manufacturer can tell you the correct grease type to use and can match any wear additives and oils to specific application requirements.
Another good habit when carrying out maintenance is to ensure a tube of grease matched to that bearing set is provided to the workshop. This ensures it is used when re-assembling equipment and can be a big help in avoiding the use of incompatible grease.
For even more best-practice advice on lubrication of electric motor bearings, download ABB’s motor tips e-book