C5 is the loosest group in ISO 5753-1. It is specified when thermal expansion is so large that even C4 would close — large kiln car bearings, furnace rollers, calender stacks and very large shafts running hot. Beyond C5 there is no standard group; the clearance is then agreed with the manufacturer as a special.
The classic case is a kiln car or a furnace roller: a large bearing on a shaft at several hundred degrees, with the housing insulated or cooled. The inner ring grows by a few tenths of a millimetre while the outer stays put, and only a C5 group leaves positive clearance at temperature. In these positions the bearing is also running slowly, which is what makes the large clearance tolerable.
At normal temperature a C5 bearing is visibly loose. Only two or three elements carry load, the shaft moves radially under load, and any attempt to run it fast ends in skidding and cage failure. If a C5 is being considered for a standard motor or gearbox, the real problem is usually misalignment or a wrong fit, not clearance.
| Meaning | Radial internal clearance greater than C4 |
|---|---|
| Position | Loosest standard group |
| Assumed temperature difference | Above 120 °C |
| Typical size range | Large bore, 100 mm and above |
| Typical use | Kiln cars, furnace rollers, calender stacks |
| Speed | Low |
| Availability | Limited — often special order |
| Beyond C5 | Special clearance agreed with the maker |
| Criterion | C4 | C5 | Special |
|---|---|---|---|
| Relative clearance | Looser | Loosest standard | Agreed value |
| Temperature difference | 80–120 °C | Above 120 °C | Calculated |
| Size range | Medium to large | Large | Any |
| Availability | Catalogue | Limited | Factory order |
| Lead time | Stock | Days to weeks | Weeks |
| Use | Dryer rollers | Kiln cars, furnace rollers | Extreme or unusual duty |
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Read →Technically yes, practically rarely. There is almost no application for a C5 clearance under 100 mm bore, so it is not stocked.
A special clearance, agreed with the manufacturer and marked on the drawing. There is no C6 in ISO 5753-1.
By calculation, not by feel: compute the expansion of the inner ring at operating temperature, add the fit interference, and compare with the clearance group. If the residual is still positive at temperature, the group is right.
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