Bearing clearance is the total distance the inner ring can move radially relative to the outer ring in a free bearing. It is classified into groups: C2 (tighter than normal), CN (normal — usually unmarked), C3 (larger than normal, the default for electric motors), C4 and C5 (high-temperature or heavy-interference applications). Ordering the wrong group causes overheating, noise and premature failure. This guide explains each group, the values behind them, and how to choose.
What bearing clearance actually is
Radial internal clearance is how much the inner ring shifts radially before the balls contact the raceway under a light measuring load. Every bearing leaves the factory with a defined clearance range, measured per ISO 5753 / GB/T 4604.
The critical point most buyers miss: factory clearance is not operating clearance. Two things eat into it during mounting and running:
- Interference fit — pressing the inner ring onto a shaft expands it, shrinking the clearance
- Thermal expansion differential — the inner ring runs hotter than the outer ring (heat path through the shaft), which also reduces clearance
By the time a motor reaches rated temperature, the operating clearance can be several micrometres less than what was measured on the bench. That is why clearance selection is done for the operating condition, not the free bearing.
The clearance groups — C2 to C5
| Group | Meaning | Typical use |
|---|---|---|
| C2 | Smaller than normal | Precision instruments, low-noise builds; risky with interference fits — verify before specifying |
| CN (normal) | Standard group; usually unmarked | General machinery, hand tools, low-temperature applications |
| C3 | Larger than normal | Electric motors, fans, pumps — the most widely specified group |
| C4 | Larger than C3 | High-temperature service, heavy interference fits (oven fans, hot-air equipment) |
| C5 | Larger than C4 | Special high-temperature / large temperature differential; rare, consult the factory |
For reference, the 6204 deep groove ball bearing (20×47×14 mm) has these indicative ranges per GB/T 4604:
| Model 6204 | Clearance range (µm) |
|---|---|
| CN | 4 – 11 |
| C3 | 11 – 21 |
| C4 | 21 – 31 |
Values above are indicative references from GB/T 4604 / ISO 5753. Different bore sizes have different ranges — larger bearings allow more clearance. Always verify against the current standard edition for your specific bore.
Why electric motors default to C3
A motor bearing carries two clearance-reducing effects at once:
- The interference fit. Motor shafts are typically machined to k5 or tighter. Pressing the inner ring on expands it — typically taking several micrometres out of the clearance (indicative range 4–6 µm for a 20 mm bore, depending on fit class).
- The temperature gradient. In operation, heat flows from the rotor through the shaft into the inner ring. The inner ring runs hotter than the outer ring and expands more (indicative 3–5 µm at motor temperature rise).
Start with CN and the operating clearance can approach zero: the bearing runs hot, cage stress rises, lubricant degrades early, and the failure looks like a "bad bearing" when it was actually a clearance specification problem. Starting at C3 leaves room for both effects — that is why it is the default for motors, fans and most rotating equipment.
C3 vs C4 — when to step up
C4 is not "a better C3". It exists for specific conditions:
- Sustained high ambient or raceway temperature — oven circulation fans, kiln cars, hot-air blowers
- Heavy interference fits — where mounting alone removes a large share of the clearance
- Large inner/outer ring temperature differentials — heat conducted in through the shaft while the housing stays cool
In a normal-temperature application, excessive clearance causes its own problems: the ball load zone shrinks, balls skid instead of roll, vibration and noise rise. More clearance is not more quality — it is a matched trade-off.
Clearance and precision are different things
Buyers frequently mix up the C groups and the P classes. They control completely different properties:
- C (clearance) — internal play: how much the rings can shift relative to each other
- P (precision) — manufacturing accuracy: dimensional tolerances and runout
The most common motor combination is P0 + C3: standard precision with enlarged clearance, because the motor's thermal behaviour demands it. See our bearing precision grades guide for the P-scale.
How clearance is measured and verified
Clearance is measured on a dedicated radial clearance instrument per ISO 5753 — the inner ring is fixed, a defined load shifts the outer ring, and the total displacement is the radial clearance.
At NLHB we measure clearance at two points:
- Before mounting — to confirm the shipped group matches the order
- After mounting (on request, for production batches) — because interference fit loss is real; the number that matters is what is left in the machine
If you are comparing suppliers, ask two questions: do you measure clearance before delivery, and can you provide the data with the shipment? Most trading companies cannot answer either.
Common clearance mistakes buyers make
- The drawing says C3, the supplier ships CN. Default-group bearings slip into orders easily — always confirm the group in writing.
- High-temperature application specified with C3. Everything looks fine on the bench; the machine eats bearings in three months.
- Assuming C4 is "better". Oversized clearance creates noise and skidding in normal-temperature applications.
- Mixing up clearance with precision. A P5 bearing with the wrong clearance still fails.
- Ignoring the shaft fit. The same C3 bearing behaves differently on a g5 shaft versus an m5 shaft. Fit and clearance must be chosen together.
Quick selection table
| Your application | Recommended group |
|---|---|
| Electric motor, fan, pump (normal temperature) | C3 |
| General machinery, hand tools | CN |
| Precision instrument, low-noise build | C2 — verify fit and temperature first |
| Oven fan, kiln, hot-air equipment | C4 |
| Extreme temperature differential | C5 — rare, consult factory |
If your application sits between categories, send us the speed, temperature range and shaft fit — the answer falls out of those three inputs.
FAQ
What is bearing clearance C3?
C3 means radial internal clearance larger than the normal (CN) group. It is the default for electric motors because interference fits and operating temperature reduce clearance — starting at C3 leaves room for both effects.
Can I use C3 instead of CN?
For motors, fans, pumps and most rotating equipment, yes — C3 is the safer default. For precision instruments with tight fits and low temperature rise, CN or C2 may be required.
Is C4 better than C3?
No. C4 only suits high-temperature service or heavy interference fits. In normal-temperature applications, excess clearance causes ball skidding, vibration and noise.
How is bearing clearance measured?
Per ISO 5753 on a radial clearance instrument: the inner ring is fixed and a defined measuring load displaces the outer ring. NLHB measures before delivery, and after mounting on request for production batches.
What clearance should I order for electric motors?
C3, unless the drawing says otherwise. It compensates for the interference fit and the inner-outer ring temperature difference at operating temperature.
Does clearance affect bearing noise?
Yes. Too little clearance causes heat and squealing; too much causes rattle and vibration. Clearance group and noise grade (Z1–Z4) are related but separate specifications.
Related resources
- 6204 Bearing Dimensions, Load Ratings and Sourcing Guide — includes the 6204 clearance table
- Bearing Precision Grades P0 to P5 — precision vs clearance explained
- Bearing Suffix Guide: ZZ, 2RS, C3 and More
- Bearing Finder — search by bore / OD / width