Radial internal clearance (RIC) is the total distance the inner ring of a bearing can be displaced in a direction perpendicular to the shaft axis, when one ring is held fixed and the other moves freely under no load.
This measurement is taken in the unmounted state at room temperature. After installation in a housing and onto a shaft (where interference fits reduce clearance) and during operation (where temperature rise reduces clearance further), the actual operating clearance is 10–40 µm smaller than the as-delivered clearance, depending on the application. Selecting the wrong clearance group is one of the top five causes of premature bearing failure according to SKF — see the full clearance selection guide at bearing internal clearance and shop deep groove ball bearings by clearance group.
Radial Clearance Groups
Standard ISO 5753 defines five clearance groups for deep groove ball bearings:
- C2: smaller than CN — used when the fit is loose or precise shaft positioning is required
- CN (standard): normal group — appropriate for most applications under standard conditions
- C3: larger than CN — mandatory for interference fits, operating temperatures above 70°C, or when electrical currents pass through the bearing (shaft grounding)
- C4: larger than C3 — very high temperatures or heavier interference fits
- C5: larger than C4 — special applications such as steam boilers and metallurgical equipment
The practical rule: operating clearance must be positive (clearance remaining) but not excessively large. Negative clearance (excessive contact pressure) increases friction and heat, causing early failure. Excessive clearance concentrates load on fewer rolling elements, raising local stress.
Practical Example: 6308 CN vs 6308 C3
The bearings 6308 CN (d = 40, D = 90, B = 23 mm) and 6308 C3 share identical dimensions but differ in clearance. A real case illustrates the difference:
At a yarn mill in Thai Binh, a 22 kW motor running at 2,900 rpm initially used 6308 CN bearings. After 6 months, bearings failed because the k6 shaft fit reduced clearance from 18 µm (CN) to −5 µm — negative clearance, bearing running above 100°C. Switching to 6308 C3 (initial clearance 28 µm), after installation with k6 fit approximately 10 µm positive remained — operating temperature stabilized at 65°C, and the bearing ran three years without replacement.
| Parameter | 6308 CN | 6308 C3 | Unit |
|---|---|---|---|
| Radial clearance (before fitting) | 11–25 | 18–36 | µm |
| Clearance reduction from k6 fit | ~18 | ~18 | µm |
| Estimated operating clearance | −7 to +7 | 0 to +18 | µm |
| Typical operating temperature | 75–85 | 60–70 | °C |
Selecting the Clearance Group for Your Application
Clearance selection is straightforward once you understand three key variables. Shaft fit: loose fit (g6, h6) → use CN; interference fit (k6, m6) → use C3 or higher. Operating temperature: below 50°C → CN; 50–80°C → C3; above 80°C → C4. Special conditions: electrical current through the bearing (shaft grounding applications) → always use C3 minimum to reduce contact stress.
When mounting two bearings on the same shaft, the fixed-end bearing uses CN or C3 depending on the fit; the free-end bearing uses C3 or C4 to accommodate thermal shaft expansion.