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Internal Clearance and Operating Clearance
Internal clearance is the total distance one bearing ring can move relative to the other before mounting under defined measuring conditions. It is not the same as bearing precision: a tolerance class and a clearance class describe different properties.
The clearance required before mounting depends on the clearance lost through fits and differential thermal expansion, plus the operating clearance needed by the application.
Clearance Classes
| Designation | Relative range | Selection note |
|---|---|---|
| C2 | Less than CN | Use only after calculating the required operating clearance |
| CN | Manufacturer’s normal range | Check fit and thermal effects before treating it as the default |
| C3 | Greater than CN | May compensate for fit or thermal clearance reduction |
| C4 | Greater than C3 | Requires the same operating-clearance calculation; temperature alone is insufficient |
| C5 | Greater than C4 | Order only when the calculated requirement and exact model data support it |
The actual micrometre range varies with bore size and bearing type. Use the table for the selected model rather than comparing suffixes alone.
How Mounting Changes Clearance
Interference on the shaft or in the housing changes ring geometry and reduces internal clearance. Calculate the reduction using the bearing manufacturer’s method and the actual fit, shaft or housing geometry, material, and wall thickness.
For tapered-bore bearings, record the initial clearance and the measured drive-up or clearance reduction. A target taken from another bearing size should not be reused.
Thermal Effects
If the inner and outer rings operate at different temperatures, their thermal expansion changes the clearance. Use measured or calculated ring temperatures and the shaft and housing materials.
Process heat can enter through either the shaft or housing, so the direction of the temperature difference should not be assumed. Start-up and transient conditions may also differ from the steady-state measurement.
F80 and Screen-Duty References
Some bearing manufacturers use F80 or another screen-duty clearance reference. Confirm the actual range for the selected model and manufacturer; F80 should not be translated automatically into CN+, C3+, C3, or C4.
Screen selection should account for speed, acceleration, amplitude, fit, temperature, cage design, lubrication, and the equipment maker’s mounting instructions.
Data Needed for Selection
| Input | What to record | Why it matters |
|---|---|---|
| Exact model and suffix | Brand, complete marking, bore type, clearance code | Establishes the published initial range |
| Shaft and housing | Diameters, tolerances, material, wall thickness | Determines fit-related reduction |
| Temperature | Inner ring, outer ring, continuous and peak values | Determines thermal change |
| Duty | Load, speed, misalignment, vibration | Establishes required operating behavior |
| Lubrication | Grease or oil, viscosity, supply method | Affects temperature and film formation |
| Mounting | Drive-up or clearance-reduction record | Confirms the as-mounted condition |
Send these details, together with quantity and destination, when requesting a C3, C4, F80, or other clearance comparison.