Internal Clearance Review Before RFQ
Internal clearance is the total distance one bearing ring can move relative to the other in the radial direction. It is not the same as bearing precision (ABEC/ISO tolerance class) — a P4 precision bearing and a P0 general-purpose bearing can both have C3 clearance.
This article is a sourcing preparation guide. Table-led clearance selection and final C3/C4 recommendations should be handled on the dedicated technical clearance page when it is produced. For RFQs, TFL Bearing reviews the current model, fit, temperature, speed, load, and application before recommending a clearance path.
Internal clearance is critical because it determines:
- How the bearing handles thermal expansion
- The actual operating clearance after mounting
- Load distribution among the rollers
- Noise, vibration, and operating temperature
The Clearance Classes
| Designation | Clearance Range | When to Use |
|---|---|---|
| C2 | Less than normal | High-precision applications where minimum play is required; very low speed; controlled temperature environment |
| CN (Normal) | Standard range | General industrial applications with normal fits and moderate temperatures (< 80°C) |
| C3 | Greater than normal | Standard for most industrial applications; interference fits on shaft; normal temperature differential between inner/outer rings |
| C4 | Greater than C3 | High-temperature applications; heavy interference fits; large temperature differentials; vibrating screen applications |
| C5 | Greater than C4 | Extreme temperature applications; very heavy interference fits; special applications |
How Mounting Changes Clearance
The clearance printed on the bearing box is the unmounted clearance. When the bearing is pressed onto a shaft and into a housing, the clearance reduces:
- Interference fit on shaft: Reduces internal clearance by approximately 80% of the interference amount
- Interference fit in housing: Reduces internal clearance by approximately 70% of the interference amount
A bearing with CN clearance and a tight shaft fit may have zero operating clearance — leading to overheating and seizure. This is why C3 has become the de facto standard for most industrial spherical roller bearing applications.
Thermal Effects on Clearance
When a bearing operates, the inner ring typically runs hotter than the outer ring because:
- The inner ring has a smaller surface area for heat dissipation
- The shaft conducts heat from the process into the inner ring
- The housing dissipates heat from the outer ring to the environment
This temperature differential causes the inner ring to expand more than the outer ring, further reducing clearance. For a 100mm bore bearing with a 10°C differential, the clearance reduction is approximately 0.012mm — significant for precision applications.
The F80 Clearance for Vibrating Screens
Vibrating screen bearings use a special clearance designation: F80 (also written as CN+ or C3+). This is larger than standard C3 and is essential because:
- The high-frequency vibration generates heat unevenly through the bearing
- The rapid acceleration requires additional clearance for oil film formation
- Standard C3 clearance bearings will seize prematurely in screen applications
Do not treat a standard clearance bearing as a direct substitute in a vibrating screen application without review. F80 or application-specific clearance requirements should be checked against the screen model, speed, acceleration, shaft fit, and operating temperature before quotation.
Practical Selection Guide
| Application | Shaft Fit | Housing Fit | Recommended Clearance |
|---|---|---|---|
| Electric motor, pump, fan | j6, k6 | H7, J7 | C3 |
| Conveyor pulley | k6, m6 | H7, J7 | C3 |
| Steel mill roll | m6, n6 | J7, K7 | C4 |
| Paper machine dryer | m6, n6 | G7, H7 | C4 |
| Vibrating screen | m6, n6 | J7 | F80 |
| High-temperature fan | k6, m6 | G7, H7 | C4 or C5 |
When in doubt, prepare the current bearing model, fit details, operating temperature, equipment type, and quantity for RFQ review. TFL Bearing can compare C3, C4, F80, and other clearance paths against the application instead of quoting from the suffix alone.