Bearings for Electric Motors & Drives
Motor bearings are chosen for low noise, long grease life and the ability to survive shaft voltages from variable-frequency drives. The front position usually carries the belt or coupling load and needs the heavier duty; the rear position mostly carries radial load from the rotor and is where electrical erosion shows up first.
Typical duty
- Speed from 1,500 to 6,000 rpm on mains motors, up to 20,000 rpm on servo and spindle drives.
- Light to moderate radial load, with an axial component when a helical gear or worm is fitted.
- Continuous running with grease life as the limiting factor - 20,000 to 40,000 hours is typical.
- Shaft voltages from PWM drives, which discharge through the bearing and erode the raceway.
- Low noise and vibration targets, especially on fan-cooled and pump-mounted motors.
Recommended types & suffixes
Type, reason and the suffix that follows from it
| Type | Why it suits the application | Suffix guidance |
|---|
| Deep groove ball bearings | Standard choice for both positions: low friction, quiet, cheap to replace. | C3 clearance for the fit and running temperature, 2Z or 2RS sealing, and an insulated variant on the drive end. |
| Cylindrical roller bearings | For large motors above roughly 100 kW where radial load and stiffness dominate. | NU/NJ types with a brass or polyamide cage; NJ adds axial location in one direction. |
| Insulated or hybrid bearings | Break the circulating current path that destroys grease and raceways. | Ceramic-coated outer ring, or hybrid with ceramic balls and steel rings. |
| Ball bearing units | Mounted plummer and flange versions for belt-driven auxiliary equipment. | Cast housing with a self-aligning insert; choose the seal for dust or washdown. |
Models for this application
From the live catalogue, 10–120 mm bore
Models are listed for reference. For a specific duty we confirm the clearance, seal and lubricant before quoting -
send the application.
Installation & maintenance notes
- Set the clearance for running temperature: a C3 bearing can sit at effectively normal clearance once the inner ring is hot.
- Grease life halves roughly for every 15 degrees C above 70 degrees C, so motor temperature drives the relubrication interval.
- For inverter-fed motors, fit an insulated bearing at the non-drive end and a shaft grounding ring at the drive end.
- Respect the recommended shaft and housing fits: too loose a housing fit lets the outer ring creep and wear the seat.
- Balance the rotor and align the coupling - unbalance is often misdiagnosed as a bearing defect.
Common mistakes in this industry
- Fitting a standard bearing on the drive end of an inverter-fed motor and losing it to fluting within a year.
- Replacing a C3 bearing with a normal-clearance one in a hot application, then finding the bearing preloaded on assembly.
- Mixing grease types during relubrication, which can thicken or separate and starve the raceway.
Further reading
Frequently asked questions
Which clearance should a motor bearing have?
C3 radial internal clearance is the usual choice for motors because the inner ring expands with rotor heat and the interference fit closes the clearance. Small, cool-running motors are often supplied with normal clearance, so match what the motor maker fitted.
Why do motor bearings fail on variable-frequency drives?
Fast switching in the drive creates a common-mode voltage on the shaft. Current discharges through the bearing, melting grease and pitting the raceway - the washboard pattern called fluting. Insulated or hybrid bearings plus a shaft grounding ring are the standard fix.
What does the Z or 2Z suffix mean on a motor bearing?
Z is a single metal shield on one side, 2Z shields on both sides: non-contact protection that keeps grease in and coarse dirt out with almost no extra friction. 2RS is a contact rubber seal with better protection but a lower speed limit.
Sourcing for Motors?
Send the model number, quantity and destination - we reply within 30 minutes on working days.