Motor Bearings Guide
Bearing failure is the most common cause of motor failure. Learn about bearing types, lubrication, failure modes, and how to extend bearing life.
Bearing Types in Motors
- Ball bearings: Most common for small to medium motors. Deep groove ball bearings handle both radial and axial loads. Used on both ends of the motor.
- Roller bearings: Used for high radial loads. Cylindrical roller bearings. Common on the drive end of larger motors.
- Angular contact ball bearings: Handles combined radial and axial loads. Used where significant thrust is present.
- Sleeve bearings: Used on very large motors and high-speed applications. Oil-lubricated. More complex but can handle higher speeds than rolling element bearings.
Lubrication
- Greased for life: Small motors often have sealed bearings with grease that lasts the bearing's lifetime. No relubrication needed or possible. Replace the bearing when it fails.
- Regreasable: Larger motors have bearings with grease fittings and drain plugs. Follow the manufacturer's relubrication schedule. Use the correct grease.
- Oil lubrication: Large motors may use oil-bath or circulating oil lubrication. Check oil level and condition regularly.
Over-greasing causes failure
Pumping grease until it comes out the seal overfills the bearing cavity. The bearing runs in a bath of grease, generating heat from churning. The grease can also get into the motor windings, causing insulation failure. Use the manufacturer's specified grease quantity. A typical motor bearing needs only 10 to 30 grams of grease per relubrication.
Failure Modes
- Lubrication failure: Insufficient grease, wrong grease, or over-greasing. The bearing overheats and the rolling elements and raceways are damaged.
- Contamination: Dirt, water, or abrasive particles entering the bearing. Causes abrasive wear. Ensure seals are intact. Clean grease fittings before use.
- Misalignment: Motor-to-load misalignment causes uneven loading on the bearing, accelerating wear on one side. Laser-align after installation and maintenance.
- Electrical damage (EDM): Stray currents from VFDs pass through the bearing, causing electrical discharge machining (EDM) pitting on the raceways. Install shaft grounding rings or use insulated bearings.
- Overloading: Excessive radial or axial load beyond the bearing's rating. Causes fatigue spalling. Check for belt over-tension, misalignment, or process overloading.
- Improper installation: Hammering the bearing on the shaft causes brinelling (dents in the raceway). Use proper bearing pullers and heaters. Never strike a bearing.
Extending Bearing Life
- Follow the relubrication schedule with the correct grease and quantity.
- Clean grease fittings before applying grease.
- Laser-align motor and driven equipment after any disturbance.
- Check belt tension (for belt-driven loads). Don't over-tension.
- Install shaft grounding if using a VFD, or use insulated bearings.
- Protect bearings from contamination with proper seals and motor enclosures.
- Monitor vibration and temperature. Trend the data. Rising trends indicate developing problems.
- Use proper installation tools (bearing heaters, pullers). Never hammer bearings.
Tip: For motors on VFDs, consider insulated bearings on the non-drive end and a shaft grounding ring on the drive end. This combination breaks the circulating current path and prevents EDM damage. The cost is small compared to the cost of bearing replacement and downtime.