November 04,2026
Selecting the correct bearing internal clearance is essential for maintaining rotational stability, controlling friction, and achieving reliable performance. For OEM engineers and industrial equipment manufacturers, choosing between C2, CN, and C3 clearance requires more than checking a standard bearing catalog. Shaft and housing fits, operating temperature, rotational speed, and applied loads all influence the clearance available after installation.
Insufficient operating clearance can create excessive internal contact and unwanted preload, while excessive clearance may affect running accuracy and load distribution. Both conditions can compromise bearing performance.
For specialized equipment, bearing internal clearance selection should therefore be based on the complete operating environment, not simply the initial clearance designation.
This guide explains how common clearance classes differ, what changes bearing operating clearance, and how to establish suitable specifications for custom bearing applications.
Bearing internal clearance refers to the total relative movement between the inner and outer rings before the bearing is installed or subjected to external loading.
For radial bearings, radial internal clearance is measured as the total radial movement between the rings. Axial internal clearance describes the corresponding movement in the axial direction, where applicable.
Three conditions should be distinguished:
Initial clearance: The clearance measured before mounting under specified measurement conditions.
Mounted clearance: The clearance remaining after the bearing is installed in the shaft and housing.
Operating clearance: The clearance under actual operating temperatures, speeds, and loading conditions.
These values can differ significantly. Mounting interference may change ring dimensions, while temperature differences during operation can further alter the available clearance.
Understanding this distinction is the starting point for selecting a suitable bearing clearance specification.
Standard radial internal clearance is commonly divided into several groups: C2, CN, C3, C4, and C5.
| Clearance Class | Relative Clearance | Typical Consideration |
|---|---|---|
| C2 | Smaller than normal | Applications requiring reduced initial clearance |
| CN | Normal | General-purpose applications |
| C3 | Greater than normal | Applications with expected clearance reduction |
| C4 | Greater than C3 | Applications requiring greater initial clearance |
| C5 | Greater than C4 | Specialized clearance requirements |
The actual numerical clearance range depends on bearing type, bore diameter, and the applicable dimensional standard. These classes should not be treated as fixed clearance values across different bearing sizes.
C2 clearance may be suitable when mounting interference and thermal expansion are limited or carefully controlled. However, a smaller initial clearance requires particular attention to the risk of excessive clearance reduction after installation.
CN clearance is a common starting point for conventional industrial applications where mounting fits and operating conditions fall within normal design requirements.
C3 clearance provides more initial clearance than CN. It is frequently considered when interference fits or differential thermal expansion are expected to reduce clearance during operation.
C4 and C5 may be considered when the application requires greater initial clearance, but their suitability must also be confirmed through engineering evaluation.
One important distinction: clearance class is not the same as precision class. C3 does not automatically mean higher accuracy, better quality, or greater speed capability.
The correct specification depends on the operating condition after assembly.
Three factors deserve particular attention when evaluating bearing operating clearance.
Temperature directly affects bearing ring dimensions through thermal expansion.
When the inner ring becomes hotter than the outer ring, it generally expands more, reducing radial internal clearance. If the outer ring operates at a higher temperature, the effect may be different.
The actual change depends on:
Inner and outer ring temperatures
Ring dimensions and material properties
Housing thermal conductivity
Heat dissipation and operating duration
For example, a continuously running electric motor may develop a different temperature distribution from equipment operating intermittently.
For high-temperature or continuous-duty applications, engineers should evaluate the expected temperature of both rings rather than relying only on ambient temperature.
Mounting fits also influence internal clearance.
An interference fit between the inner ring and shaft generally expands the inner ring. An interference fit between the outer ring and housing generally compresses the outer ring.
Both effects can reduce radial internal clearance.
The amount of reduction depends on the interference, ring geometry, shaft and housing stiffness, and material properties.
This is why clearance should not be selected independently of assembly tolerances. For more detailed information, see MTWB's bearing shaft and housing interference fit calculation guide.
Rotational speed influences bearing behavior through centrifugal effects, rolling element dynamics, ring deformation, and heat generation.
Applied loads also affect elastic deformation and the distribution of contact between rolling elements and raceways.
These effects are influenced by bearing geometry, material, lubrication, load direction, and operating duration. Therefore, higher speed does not automatically mean that a larger clearance is required.
Engineers should consider maximum RPM, continuous operating speed, load, lubrication, and thermal equilibrium together.
For applications with strict noise and vibration requirements, clearance should also be evaluated alongside raceway geometry and lubrication. See MTWB's guide to bearing noise and vibration factors related to internal clearance.
Standard clearance classes cover many conventional applications, but specialized equipment may operate outside typical design conditions.
Examples include:
High-speed motors with demanding thermal requirements
Precision automation equipment with strict running accuracy
Medical devices requiring stable rotational performance
Industrial machinery operating under continuous heavy loads
Compact assemblies with unusual shaft and housing fits
In these situations, selecting C3 simply because the application operates at high speed, or C2 because tighter clearance appears preferable, may lead to an unsuitable result.
For example, a C3 bearing may work effectively in an assembly with substantial interference fits but retain excessive clearance in another assembly with loose fits and relatively low operating temperatures.
Likewise, a CN bearing may be suitable under conventional conditions but become unsuitable when differential thermal expansion significantly reduces its operating clearance.
The important question is not which clearance class is generally better, but which specification produces the required operating condition in the actual assembly.
For non-standard OEM applications, bearing internal clearance selection should follow a structured engineering process.
Step 1: Define the operating requirements
Identify bearing type, dimensions, radial and axial loads, maximum RPM, continuous operating speed, temperature range, lubrication method, and duty cycle.
Step 2: Confirm mounting conditions
Specify shaft and housing materials, dimensional tolerances, and intended fits. These parameters help estimate the clearance change after installation.
Step 3: Evaluate thermal expansion
Estimate the operating temperatures of both bearing rings. Consider continuous operation, heat transfer from surrounding components, and startup conditions.
Step 4: Establish the target operating condition
Determine whether the application requires positive operating clearance or a deliberately controlled preload arrangement.
For example, precision positioning systems may require a specific stiffness and preload condition, while conventional radial bearing arrangements may require sufficient positive operating clearance.
Step 5: Validate the design
Where necessary, manufacture and test a prototype under representative operating conditions.
Evaluation may include operating temperature, vibration, rotational torque, dimensional consistency, and performance under the specified load and speed.
The final clearance specification should be documented and controlled during production.
Providing complete technical information helps a supplier evaluate the application without relying on assumptions.
| Information | Recommended Details |
|---|---|
| Bearing type | Deep groove ball, cylindrical roller, etc. |
| Dimensions | Bore, outside diameter, and width |
| Existing clearance | C2, CN, C3, or measured value |
| Rotational speed | Normal and maximum RPM |
| Applied load | Radial and axial loads |
| Temperature | Ambient and estimated operating temperatures |
| Mounting fits | Shaft and housing tolerances |
| Lubrication | Grease, oil, or other method |
| Performance requirements | Noise, vibration, torque, and accuracy |
| Order quantity | Prototype, trial batch, and production volume |
If the original bearing specification is unavailable, an existing sample, equipment drawing, or dimensional measurements can provide a useful starting point.
For projects involving incomplete drawings or non-standard requirements, the custom bearing development process from drawing review to engineering evaluation provides additional guidance.
When standard clearance classes cannot fully address an application's requirements, a customized bearing specification may be necessary.
The evaluation may involve internal geometry, ring materials, dimensional tolerances, mounting fits, thermal conditions, and required operating performance.
MTWB supports OEM bearing development based on customer drawings, existing samples, and application requirements. The engineering evaluation connects the bearing specification with actual operating conditions before prototype manufacturing.
For projects requiring design verification, custom bearing prototyping from the first sample to OEM production explains how prototype evaluation can support the transition to repeatable production.
Are you developing equipment with specific temperature, speed, load, or mounting requirements?
Share your bearing drawing, existing sample, or application specifications with MTWB. Include available operating parameters such as RPM, temperature range, and shaft and housing fits.
Our engineering team can review your requirements and discuss suitable bearing clearance specifications, prototype evaluation, and OEM production options.
Contact MTWB to Discuss Your Custom Bearing Project.
