What Is Internal Clearance
Internal radial clearance (IRC) is the total radial movement possible when the outer ring is fixed and the inner ring is moved from one side to the other, with no load applied. It is measured at manufacture and is reduced in service by:
- Interference fit on the shaft: Pressing the bearing inner ring onto the shaft expands the inner ring and squeezes the rolling elements against the outer ring — reducing clearance by a predictable amount (typically 80% of the interference value)
- Thermal expansion: If the inner ring runs hotter than the outer ring (typical in electric motors and gearboxes), the inner ring expands more, reducing clearance. A 10°C temperature difference reduces clearance by approximately 1 µm per 10 mm of bore diameter
- Elastic deformation under load: Load compresses the contact, reducing effective clearance slightly
The critical rule: operating clearance must never fall below zero (preload). Preloaded bearings generate excessive heat and dramatically reduced life — unless intentionally preloaded for precision purposes (machine tool spindles) with the correct bearing type and cooling.
Clearance Groups
| Group | Radial clearance vs Normal | When to specify | Typical applications |
|---|---|---|---|
| C2 | Below normal (tighter) | Preloaded arrangements, high precision, very light interference fit, cooler inner ring operation | Machine tool spindles (combined with angular contact), precision instruments |
| CN (Normal) | Normal (baseline) | Light interference fits (k5/m5 shaft tolerances), moderate operating temperatures, general industrial bearings at <80°C housing temp | General industrial use, fans, light-duty motors |
| C3 | Greater than normal | Standard for electric motors; heavy interference fits; inner ring significantly hotter than outer ring; high ambient temperatures; large shaft diameters | Electric motors (almost universal), heavy fans, gearboxes, elevated temperature applications |
| C4 | Even greater than normal | Very heavy interference fits, very high temperature differentials (>50°C between inner and outer ring), very large bearings | Dryer roll bearings, kiln support bearings, hot strip mill applications |
| C5 | Greatest standard | Extreme temperature differentials | Specialised high-temperature industrial processes |
Interference Fit Clearance Reduction
The reduction in internal clearance due to shaft interference fit can be estimated as:
ΔCr = reduction in radial clearance (µm)
δ = diametral interference of inner ring on shaft (µm)
(0.8 factor accounts for elastic deformation of the ring)
Example: 6310 bearing, bore = 50 mm, k5 shaft tolerance = +0.018 mm interference. ΔCr = 0.8 × 18 µm = 14.4 µm clearance reduction. CN clearance for 6310 (50 mm bore): 11–25 µm radial clearance. After fit: 11−14 to 25−14 = range drops to potentially zero or negative. This is why C3 (20–36 µm CN+) is required for press-fit mounting.
Temperature Effect on Clearance
For electric motors, the inner ring typically runs 10–25°C hotter than the outer ring due to heat conducted from the shaft. Additional clearance reduction:
d = bearing bore diameter (mm)
ΔT = temperature difference between inner and outer ring (°C)
Example: d = 50 mm, ΔT = 20°C: ΔCr = 0.0115 × 50 × 20 = 11.5 µm additional reduction. Combined with interference fit reduction of 14.4 µm: total reduction = ~26 µm. A CN bearing with minimum clearance of 11 µm would be preloaded by 15 µm — guaranteed premature failure. C3 (minimum 20 µm) after 26 µm reduction still provides a small positive clearance margin — correct choice.
Clearance Selection Guide
| Application | Recommended clearance |
|---|---|
| Electric motors — standard (k5 shaft fit, typical ambient) | C3 |
| Electric motors — large frame (>200 frame) or high ambient | C3 or C4 |
| Fans — belt-driven, normal temperature | CN or C3 |
| Pumps — direct-coupled, normal temperature | CN |
| Gearbox — input and output shaft bearings | CN or C3 depending on fit |
| Machine tool spindles — preloaded angular contact pairs | C2 (standard for angular contact preload) |
| High temperature (>120°C housing) | C4 |