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When choosing bearings for small electric motors, internal clearance is one of the most misunderstood yet important factors. It may seem like a small detail, but the wrong clearance can cause overheating, noise, or premature failure. Let’s break it down in a practical, easy-to-understand way.
What Is Bearing Clearance?
Internal clearance (radial internal clearance) is the total amount one ring can move relative to the other before installation. In simple terms: It’s the “looseness” or play inside the bearing when it’s still in your hand.
Why Clearance Matters So Much in Small Motors?
After installation, the actual running clearance changes due to:
• Interference fits on the shaft and housing
• Thermal expansion during operation
• Actual load and speed conditions
If you start with the wrong clearance, the bearing may end up either too tight or too loose once running.
Common Clearance Types (Radial Internal Clearance)
C0 (Normal clearance / CN) – The standard choice for many general applications
C3 clearance – Larger than normal clearance
CM clearance – Motor-specific clearance (also known as Electric Motor clearance)
What makes CM special?
CM is specially optimized for electric motors and electric tools (often marked as EMQ – Electric Motor Quality).
• Its minimum value is slightly larger than CN’s maximum.
• Its maximum value is slightly smaller than C3’s minimum.
This ensures a narrower, more controlled range than CN or C3, leading to more consistent performance in mass-produced motors.
Clearance Grade Comparison (from smallest to largest – radial clearance)
C2 < CN (C0) < CM < C3 < C4 < C5
| Clearance Code | Meaning | Typical Applications |
| CN (C0) | Normal / Standard | General purpose applications |
| CM | Motor-specific | Electric motors, power tools (EMQ) |
| C3 | Greater than normal | High temperature, high speed, heavy load |
Why Do Electric Motors Need CM Clearance?
① Balance between temperature rise and noise:
Too little clearance can cause binding or overheating; too much clearance increases vibration and noise. CM strikes the ideal balance.
② Tighter tolerance range:
The min–max range of CM is narrower than CN or C3, ensuring more consistent performance in mass-produced motors.
When to Choose Each Clearance?
C0 (CN): Moderate loads, lower speeds, loose fits, low temperature rise. Typical: household appliances, low-speed motors.
C3: Higher speeds, tight shaft fits, elevated operating temperatures. Typical: pumps, fans, industrial equipment.
CM: Preferred for electric motors and tools where low noise, stable operation, and balanced thermal performance are required.
Commonly used in motors up to 3600 rpm (e.g., 60 Hz, 2-pole) or lower. For higher speeds, C3 may be required depending on fit and temperature rise.
Real-World Insight
In many small motor designs, engineers choose C3 or CM as default.
• C3 provides a larger safety margin for heat expansion.
• CM offers tighter control for quieter and more consistent running.
Both help prevent the bearing from becoming preloaded due to thermal expansion.
Common Mistakes to Avoid
✖ Using C0 in high-speed motors.
✖ Ignoring the combined effects of fit tolerances and operating temperature
✖ Selecting clearance based only on catalog values without considering real running conditions
✖ Assuming clearance grade alone guarantees performance, ignoring shaft and housing fit tolerances.
There is no one-size-fits-all solution.
The correct clearance depends on how the bearing performs after installation and under actual working conditions.
Bearing Model Examples
| Model Example | Meaning |
| 6007DDUCM | Deep groove ball bearing, double contact seals, CM motor clearance |
| 6204ZZCM | Deep groove ball bearing, double metal shields, CM clearance |
If you’re unsure, it’s always wise to consult a supplier who understands real motor applications.
About Welink Bearing
Welink Bearing offers a full range of miniature and deep groove ball bearings with various clearance options, including C0, C3, and CM (motor-specific). We help customers match the right clearance to their motor design, improving performance, reducing noise, and lowering failure rates.
Contact us for technical advice or a quick quotation.
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