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How to Select the Right Bearing for an Electric Motor

Almost every electric motor runs on rolling bearings, and choosing the right one comes down to a short list of decisions: bearing type, size and life, internal clearance, sealing, lubrication and speed — plus, on inverter-driven motors, protection against electrical damage. This guide walks through each in the order an engineer actually decides them.

1. Start with the Bearing Type

For the great majority of motors, a single-row deep-groove ball bearing is the right answer. It handles radial load and moderate axial load, runs at high speed with low friction, and is inexpensive and widely stocked. You only move away from it for specific reasons:

• Angular-contact ball bearings when a defined axial load must be carried in one direction.
• Cylindrical roller bearings on larger motors with heavy radial loads (for example belt-driven machines).

Most motors use a locating / non-locating arrangement: one bearing is fixed axially to position the rotor, and the other is allowed to slide so the shaft can expand with heat without preloading itself. Getting this right prevents one of the most common causes of motor bearings running hot.

2. Size the Bearing and Check its Life

The bore must match the shaft, and the bearing must be large enough that its rated life suits the duty. The standard measure is basic rating life (L10) — the life 90% of identical bearings will reach under a given load and speed. You can estimate it with the L10 life calculator using the dynamic load rating from the catalogue and your applied load. If the life comes out short, step up to the next series (for example 6300 instead of 6200) for more capacity.

3. Choose the Internal Clearance — and Why Motors Often Use C3

Internal clearance is the small gap inside the bearing before it is fitted. Two things in a motor reduce that gap in service: the interference fit on the shaft, and the inner ring running hotter than the outer ring. If too little clearance is left, the bearing preloads itself and overheats. That is why C3 (greater than normal) clearance is a common default for electric motors.

Rule of thumb: a warm-running or tightly-fitted motor bearing usually wants C3. A cool, lightly-fitted one is fine on normal (CN) clearance.

4. Match the Seal to the Environment

The suffix tells you the sealing:

• Open — relies on the motor end-shield and external seals; used where the motor has its own sealing and re-greasing.
• Shielded (ZZ / 2Z) — metal shields, non-contact, low friction; good for clean, higher-speed duty.
• Sealed (2RS) — contact rubber seals, better protection against dust and moisture, at the cost of a little more friction; common on smaller "sealed-for-life" motors.

Choose shields for clean and fast, seals for dirty or damp.

5. Check the Speed

Make sure the motor speed is sensible for the bearing size and lubricant. A quick screen is the speed factor n·dm — run it through the speed check calculator. Standard grease-packed bearings cover the vast majority of motor speeds; only high-speed or large bearings need oil or a special cage.

6. Get the Lubrication Right

Small motors are usually grease-for-life; larger ones are re-greasable on a schedule. The two mistakes to avoid are using the wrong grease for the temperature, and over-greasing — too much grease churns and drives the temperature up, especially at speed. If a motor bearing runs hot, over-lubrication is one of the first things to check.

7. Inverter-Driven Motors: Protect Against Bearing Currents

Motors run from a variable-frequency drive (VFD) can suffer electrical erosion — stray currents pass through the bearing and, over time, etch fine grey "fluting" marks into the raceway and darken the grease. If you are specifying bearings for a VFD motor, plan for this from the start:

• Fit an insulated or hybrid (ceramic-rolling-element) bearing on the affected end.
• Provide a shaft grounding ring and proper earthing.
• See electrical erosion in the Failure Library for how to recognise and prevent it.

Quick Selection Checklist

• Type — deep-groove ball unless there's a reason not to.
• Locating / non-locating arrangement set for thermal expansion.
• Size chosen so L10 life suits the duty.
• Clearance — C3 if warm-running or tightly fitted, else normal.
• Seal matched to the environment.
• Speed factor within the guideline.
• Correct grease, correct amount.
• VFD motor? Add insulation/grounding.

Frequently Asked Questions

What bearing clearance is best for an electric motor?

C3 (greater than normal) is a common default, because the interference fit and the hotter inner ring both reduce the running clearance. Cool, lightly-loaded motors can use normal (CN) clearance.

Should an electric motor use ball or roller bearings?

Deep-groove ball bearings suit most motors. Cylindrical roller bearings are used on larger motors with heavy radial loads, such as belt-driven applications.

What is the most common cause of motor bearing failure?

Lubrication problems and contamination lead the list, followed by mounting errors and — on inverter-driven motors — electrical erosion.