Radial internal clearance C4 is the bearing internal clearance group larger than C3, occupying the very wide clearance segment of the ISO 5753-1 classification scale, used for applications with large temperature differentials, extremely tight interference fits, or spherical roller bearings mounted via tapered adapter sleeves with high preload.
The ISO clearance scale from smallest to largest: C2 → CN → C3 → C4 → C5. C4 exceeds C3 by approximately 10–20 µm depending on size. See the full clearance group comparison at bearing internal clearance and shop deep groove ball bearings in C4 clearance for high-temperature motor applications. For ball bearings d = 60–80 mm: CN is 15–36 µm, C3 is 28–48 µm, C4 is 42–65 µm. This clearance is not intended for loose operation — it is engineered so that after all contracting factors (interference fit, thermal differential, centrifugal load) have acted, the actual operating clearance still remains slightly positive.
A common misconception: many engineers believe C4 is only for extremely high-power equipment. In practice, C4 also appears in medium-sized electric motors where an aluminum shaft seats in a cast iron housing — two materials with nearly 2× difference in thermal expansion coefficient (α aluminum ≈ 23 × 10⁻⁶/°C, α cast iron ≈ 12 × 10⁻⁶/°C). After 4–6 hours of continuous operation, thermal mismatch alone can account for 15–25 µm of clearance loss.
Typical Applications for C4
C4 is specified in four primary cases. First, cement rotary kilns and metallurgical equipment — outer housing temperature exceeds shaft temperature by 80–120°C. Second, large electric motors (above 200 kW) generating substantial self-heat under load. Third, large hollow shafts with m6/n6 interference fits — 40–60 µm interference nearly exhausts C3 clearance. Fourth, spherical roller bearings 22xxx series mounted via adapter sleeves on shafts with h9 tolerance.
Practical example: bearing 22220 EK/C4 (d = 100, D = 180, B = 46 mm, C = 365 kN) mounted via adapter sleeve H320 on an h9 shaft. After tightening the locknut to the correct position — shaft advances 1.5 mm on the 1:12 taper — initial C4 clearance of 48–70 µm reduces to approximately 20–30 µm, sufficient to absorb an additional 15–20 µm from 90°C operating temperature.
| Parameter | C3 | C4 |
|---|---|---|
| Pre-installation clearance — d = 60–80 mm | 28–48 µm | 42–65 µm |
| Difference vs. CN | +13 → +23 µm | +27 → +40 µm |
| Suitable temperature range | 60–100°C | 80–150°C |
| Suitable interference fit | k5, m5 (15–35 µm) | m6, n6, p6 (35–65 µm) |
| Typical applications | Pumps, fans, conveyors | Rotary kilns, crushers, large motors |