Radial internal clearance is the amount of movement between the inner and outer rings of a bearing in the radial direction before mounting. It plays an important role in bearing performance because the clearance changes once the bearing is installed and reaches its operating temperature.
Clearance groups such as C2, CN, C3 and C4 indicate different ranges of initial radial clearance. Choosing the right group depends on several factors, including shaft and housing fits, operating temperature, bearing size and working conditions.
Too little operating clearance can lead to excessive heat, vibration and premature bearing failure. Too much clearance may affect load distribution and running stability. Understanding the differences between clearance groups helps engineers and maintenance teams select bearings that perform reliably under actual operating conditions.
What Is Radial Internal Clearance?
Radial internal clearance is measured before the bearing is mounted and without an external load. It refers to the total radial movement of one ring relative to the other, taking the rolling elements into account.
Once a bearing is installed, its internal clearance usually becomes smaller. Two factors have a particularly significant effect:
When a bearing inner ring is mounted with an interference fit, it expands as it is pressed onto the shaft. This reduces the internal clearance. The amount of reduction depends on the fit, bearing design and shaft material.
The outer ring can also be affected when it is fitted tightly into a housing. Both fits should be considered when evaluating the remaining operating clearance.
During operation, the inner ring often runs hotter than the outer ring, particularly in applications involving high speeds or heavy loads. As the inner ring expands, the radial clearance decreases.
The temperature difference between the two rings is an important consideration, especially in large industrial bearings and equipment designed for continuous operation.
The combined effect of mounting fits and temperature can significantly reduce the original clearance. If the remaining clearance becomes too small or turns negative, the bearing may experience excessive friction, rising temperatures and increased vibration. In severe cases, this can lead to premature bearing failure.
C2, CN, C3 and C4: Understanding the Differences
Radial internal clearance is divided into several groups. The main difference is the amount of clearance available before mounting.
| Group | Designation | Clearance Compared with Normal | Typical Applications |
|---|---|---|---|
| Group 2 | C2 | Smaller than Normal | Applications requiring tighter internal clearance and controlled operating conditions |
| Group N | CN (Normal) | Standard clearance | General industrial applications with conventional fits and operating temperatures |
| Group 3 | C3 | Larger than Normal | Applications involving interference fits, temperature differences or heavier operating conditions |
| Group 4 | C4 | Larger than C3 | Applications requiring additional clearance due to demanding fits and thermal conditions |
These clearance groups are covered by ISO 5753-1. The actual clearance values vary according to bearing type and bore diameter. For example, a C3 clearance value for a deep groove ball bearing should not be directly compared with that of a large cylindrical roller bearing.
It is also worth remembering that C3 and C4 are not quality grades. They simply indicate larger initial clearance ranges. A bearing with a higher clearance designation is not automatically better for every application.
How to Select the Right Bearing Clearance
The purpose of clearance selection is to ensure that the bearing retains suitable internal clearance under actual operating conditions. Rather than choosing C3 or C4 by habit, it is better to evaluate the mounting arrangement and operating environment first.
Start with the Normal Clearance Group
CN is suitable for many standard applications where shaft and housing fits are not excessively tight and operating temperatures remain within normal limits. If no clearance suffix is specified in the bearing designation, Normal clearance is generally assumed.
Consider Shaft and Housing Fits
An interference fit reduces internal clearance, particularly when the inner ring is mounted tightly on a solid shaft. Applications using tighter fits, such as k6 or m6, may require C3 clearance or another suitable group. The actual reduction should be evaluated based on the fit tolerance and bearing dimensions.
Evaluate Operating Temperature
Consider the temperature difference between the inner and outer rings during normal operation. Continuous running, heavy loads and high-speed conditions can increase this difference. Where thermal expansion significantly reduces clearance, a larger initial clearance may be necessary.
Check the Remaining Operating Clearance
The effects of mounting fits and temperature should be considered together. The selected clearance group needs to provide sufficient remaining clearance after installation and during operation. For demanding applications, engineering calculations and operating data are preferable to relying on general rules of thumb.
Typical Applications of C2, C3 and C4 Bearings
- Applications requiring relatively tight internal clearance
- Light shaft and housing fits
- Stable operating temperatures
- Equipment where controlled running accuracy is important
- Interference fits on shafts or in housings
- Moderate temperature differences
- Electric motors and industrial pumps
- Gearboxes and general industrial machinery
- Applications where additional mounting clearance is required
- Applications involving demanding interference fits
- Significant temperature differences
- Large industrial bearings
- Vibrating screens and heavy-duty mining equipment
- Equipment operating under demanding thermal conditions
These are typical application examples rather than fixed selection rules. In mining operations, bearings used in crushers, conveyors and vibrating screens may require different clearance groups depending on their bearing arrangement, rotational speed, load and operating temperature. You can also learn more about our bearing solutions for mining equipment and the demands of heavy-duty mining applications.
For industrial gearboxes, bearing clearance should be considered together with shaft fits, gear loads and operating temperatures. Read more about industrial gearbox bearings and their application requirements.
Radial Clearance in Cylindrical Roller Bearings
The same basic clearance principles apply to cylindrical roller bearings, but their clearance values are different from those of ball bearings. ISO 5753-1 provides separate clearance limits according to bearing design and bore diameter.
Cylindrical roller bearings are commonly used in applications involving heavy radial loads, such as rolling mills, industrial gearboxes, electric motors and other large machines. Depending on the mounting arrangement, interference fits and operating temperature can have a considerable effect on their internal clearance.
For floating arrangements using NU or N-type cylindrical roller bearings, axial displacement is accommodated by the bearing design. However, this does not eliminate the need to select the correct radial clearance. Internal clearance still affects roller load distribution, friction and operating temperature.
When specifying a cylindrical roller bearing, the clearance group should therefore be considered together with the bearing type, shaft and housing tolerances, load conditions and expected operating temperature.
Frequently Asked Questions
Does a C3 bearing always have more clearance during operation?
Not necessarily. C3 indicates a larger initial clearance before mounting. Once installed, the clearance is reduced by interference fits and thermal expansion. A C3 bearing may have a normal operating clearance after installation, depending on the application.
Can I use C2 clearance with an interference fit?
It depends on the actual fit and operating conditions, but extra care is needed. Since C2 has less initial clearance than CN, a tight interference fit can leave insufficient clearance after mounting. The fit and expected temperature difference should be checked before selecting C2.
Is C4 clearance only used for high-temperature applications?
No. C4 may also be selected where demanding interference fits or other operating conditions cause a substantial reduction in internal clearance. Some large industrial and mining applications require a higher initial clearance, but the correct group must be determined from the actual bearing arrangement and operating conditions.
How do I decide between C3 and C4?
Compare the expected clearance reduction from mounting fits and temperature with the available clearance range. C3 is sufficient for many applications involving interference fits and moderate temperature differences. C4 may be necessary when the reduction is greater. For large bearings, it is advisable to check the actual dimensions and operating conditions before making a decision.
Does radial internal clearance affect bearing service life?
Yes. Insufficient operating clearance can increase friction, heat generation and contact stress. Excessive clearance may lead to uneven load distribution, increased vibration or unstable running conditions. Selecting a suitable clearance helps the bearing operate reliably under its intended load and speed.
Conclusion
Selecting C2, CN, C3 or C4 clearance is not simply a matter of choosing a higher or lower number. The right choice depends on how the bearing is mounted and how it performs under actual operating conditions.
Shaft and housing fits, temperature differences, bearing dimensions and operating loads all influence the clearance remaining after installation. For standard machinery, CN may be sufficient. Where tighter fits or greater thermal expansion are expected, C3 or C4 may be more suitable.
Taking these factors into account during bearing selection can help reduce unnecessary heat generation, improve running stability and avoid clearance-related failures.