C4 is two groups looser than normal. It exists for the cases where the inner ring grows enough — through heat or through a heavy interference fit — that a C3 bearing would still end up preloaded. Typical owners are dryer rollers, kilns, large fans, calender rolls and any machine where the shaft is significantly hotter than the housing.
A dryer roller or a kiln car runs its shaft at 150–250 °C while the housing sits near 60 °C. The inner ring grows by roughly 11 µm per 100 mm of diameter per 100 °C, the outer ring by far less, and the difference has to come out of the internal clearance. A 120 mm bore bearing can lose 60–80 µm of clearance this way — more than a C3 group provides, which is why C4 exists.
With excessive clearance only a few rolling elements carry load at any moment, the load zone narrows, vibration rises and the cage is worked harder. At high speed the elements can skid rather than roll, which destroys the lubricant film. C4 is a correction for a specific thermal condition — not a general upgrade.
| Meaning | Radial internal clearance greater than C3 |
|---|---|
| Position | Two groups looser than CN |
| Assumed temperature difference | 80–120 °C |
| Assumed fit | Heavy interference, or interference on both rings |
| Typical use | Dryer rollers, kilns, large fans, calender rolls |
| Speed | Moderate — high clearance is not for high speed |
| Noise | Higher at light load |
| Availability | Catalogue item in medium and large sizes |
| Criterion | C3 | C4 | C5 |
|---|---|---|---|
| Relative clearance | Loose | Looser | Loosest |
| Temperature difference | 40–80 °C | 80–120 °C | Above 120 °C |
| Fit | Medium | Heavy | Very heavy or both rings |
| Speed | Any | Moderate | Low to moderate |
| Vibration at light load | Low | Noticeable | High |
| Typical use | Motors, pumps | Dryer rollers, kilns | Large kiln cars, extreme heat |
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Read →When the shaft runs more than about 80 °C hotter than the housing, or when the fit is heavier than k6, or when both rings are an interference fit on a large bearing.
The rating does not change, but the effective load zone narrows, so under light load the bearing behaves as if it were less stiff. Under the heavy loads these bearings normally carry, that is not the limiting factor.
Not usually. High clearance plus high speed causes element skidding and cage stress. If a position needs both heat tolerance and speed, the answer is usually a different bearing type, not a looser clearance.
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