Bearing clearance, also referred to as internal clearance, is the relative movement possible between the inner and outer rings of a bearing before it is mounted onto a shaft or into a housing. It is a vital parameter that significantly affects the operational performance and longevity of rolling bearings. Depending on the direction of movement, clearance is categorized as either radial clearance or axial clearance. When the bearing is in service, this clearance is referred to as operating clearance.
1. Importance of Proper Bearing Clearance
The operating clearance of a bearing directly influences several performance aspects, including:
Rotational precision
Rolling fatigue life
Heat generation
Noise and vibration levels
Improper clearance can lead to significant issues:
Excessive clearance results in fewer rolling elements sharing the load, which increases the load on individual elements, degrades rotational accuracy, raises vibration levels, and shortens the service life.
Insufficient clearance increases friction and operating temperature, accelerates wear, and in severe cases, may cause the bearing to seize during operation.
Therefore, precise adjustment and control of bearing clearance during assembly are essential.
Cylindrical Roller Bearing - used for Rolling Mills
Rod Mill Bearings
Rolling Mill Bearings
Rolling Mill Rollers
2. Radial Internal Clearance Classifications
Bearings are manufactured with various internal clearance classes, generally classified as follows (values in micrometers for cylindrical bore bearings):
| Clearance Class | Min (μm) | Max (μm) |
|---|---|---|
| C2 (less than normal) | 10 | 95 |
| C0 (normal) | 20 | 145 |
| C3 (greater than normal) | 35 | 390 |
| C4 | 45 | 470 |
| C5 | 60 | 570 |
Other notations:
C1 – Tighter than C2
CN – Standard (often omitted in naming)
MC3 – Standard radial clearance for miniature and small-size ball bearings
MC1 – MC6 – Radial clearance classes for miniature bearings, ranging from tighter (MC1) to looser (MC6)
For cylindrical roller bearings:
CC1 to CC5 – Similar progression as above, with CC3 typically considered standard.
Special clearances such as CM (used in electric motor applications) and CT also exist for specific bearing types.
3. Selecting the Right Bearing Clearance
Selecting the appropriate clearance depends on several factors:
Application type (e.g., electric motors, machine tool spindles)
Operating temperature and speed
Load type and direction
Lubrication conditions
Noise and vibration sensitivity
General guidelines:
High-speed, high-temperature, or low-friction applications → larger clearance (e.g., C3, C4)
Precision applications (e.g., spindles) → smaller clearance (e.g., C1, C2)
Bearings with expected tight shaft and housing fits → larger initial clearance to offset fit-induced reductions
Note: The operating clearance after mounting is usually smaller than the original unmounted clearance due to interference fits and thermal expansion.
4. Adjustment Techniques for Bearing Clearance
Adjusting bearing clearance is crucial during assembly to ensure optimal performance. There are two main approaches:
A. Mechanical Adjustment
Use of shims, adjusting nuts, or spacer rings
Precision fitting and alignment
Suitable for most standard and tapered roller bearing assemblies
B. Preload Adjustment
Applying axial force to eliminate clearance entirely
Used when high rigidity or positioning accuracy is required
Can be applied via springs or threaded preloading systems
5. Practical Examples of Adjustment
Cylindrical and Elliptical Journal Bearings:
Side clearance adjustment: Manually scrape the surface or add shims at split lines, then machine and scrape.
Top clearance adjustment: Same as above.
Fixed Multi-Wedge Bearings:
Not recommended for scraping or manual clearance adjustment.
Replace with new bearing if clearance is not within specification.
Tilting Pad Bearings:
Pads are not to be scraped.
Clearance is adjusted by adding stainless steel shims behind the pads or altering the thickness of the support blocks.
Thickness variation between pads in a set should be within 0.01 mm.
Tapered Roller Bearings:
Check alignment: shaft should be horizontal, and bearing rings perpendicular.
Rotate shaft to observe rolling element movement:
4–5 rollers sliding → excessive axial clearance
All rollers rolling → too tight
Ideal: 2–3 rollers sliding → optimal clearance
Tapered Roller Bearings
Double row Tapered Roller Bearings
Four-row Tapered Roller Bearings
Single-row Tapered Roller Bearing
6. Final Considerations
During adjustment, multiple methods should be used in conjunction for verification. It is essential to consider the effects of:
Operating temperature
Lubrication quality
Thermal expansion during operation
Post-adjustment, a no-load trial run followed by detailed inspection and final correction is recommended. Precision in bearing clearance adjustment not only extends service life but also ensures reliable and efficient machinery performance.
Conclusion
Bearing clearance is more than a technical specification-it is a dynamic factor that shapes the lifespan, performance, and safety of rotating machinery. Whether in high-speed spindles or heavy-load agricultural equipment, understanding and adjusting clearance properly is essential. Engineers and technicians must develop a deep familiarity with clearance classifications, measurement, and adjustment techniques to maintain equipment at peak performance.