C1 is the smallest clearance group in ISO 5753-1, tighter than C2. It exists for precision instrument bearings where the designer wants the bearing to run with a slight preload once mounted, using the interference fit itself to close the residual gap. It is not a general-purpose choice and is not stocked in most sizes.
In an instrument bearing the shaft is small, the load is light and the requirement is stiffness, not load capacity. A small amount of preload removes axial play, raises stiffness and stops the rolling elements from skidding at very low speed. C1 lets the designer reach that state through the fit rather than through a spring, which keeps the assembly compact.
The margin between light preload and destructive preload is a few micrometres. If the shaft tolerance runs to the top of its band, or the housing runs hot, C1 becomes a preloaded bearing: temperature climbs, clearance closes further, and the bearing seizes. For anything other than a controlled instrument assembly, CN or C2 is the safer specification.
| Position in ISO 5753-1 | Tightest of the six groups |
|---|---|
| Relative to CN | Smaller |
| Typical use | Precision instruments, gyro and encoder bearings |
| Effect of mounting | Fit closes most of the remaining gap |
| Risk | Preload and heat if the fit is heavier than assumed |
| Availability | Special order in most sizes |
| Common alternative | C2 |
| Criterion | C1 | C2 | CN |
|---|---|---|---|
| Relative clearance | Smallest | Small | Normal |
| Mounted state | May run preloaded | Near zero at operating temperature | Positive clearance |
| Speed | Low to moderate | Moderate to high | Any |
| Noise | Lowest if preload is correct | Low | Reference |
| Risk of seizure | Highest | Low | Very low |
| Order route | Special | Catalogue in small sizes | Stock |
Yes. The order from tightest to loosest is C1, C2, CN, C3, C4, C5.
Usually yes in an instrument, at the cost of a little stiffness. Going the other way — fitting C1 where CN was specified — risks preload and seizure.
Rarely. Most distributors hold CN and C3, sometimes C2 or C4. C1 is normally a factory order with a lead time.
Send the designation, the three dimensions and the quantity — we reply with price, brand options and lead time within 30 minutes on working days.