Bearings
All about bearing clearance
What is it, and why is it important?
Bearing clearance is a critical factor that affects the performance and lifespan of a bearing. Proper clearance helps minimize wear and reduce noise. In this article, we discuss:
- The different types of bearing clearance
- How clearance changes after assembly and during operation
- The influence of shaft and housing adjustments on bearing clearance
Types of bearing clearance
Bearing clearance is classified into two main types:
Radial clearance
Radial clearance refers to the movement of bearing rings perpendicular to the center line of the shaft. This determines how smoothly a bearing rotates and is important for applications where precision and minimal friction are essential.
Axial clearance
Axial clearance is the displacement of bearing rings parallel to the shaft centerline. This is important in applications where axial forces occur, such as in gear systems or screw drives.
Clearance change after mounting and during operation
The internal clearance of a bearing is measured without load. However, once a bearing is mounted, the clearance changes due to:
- The tolerances of shaft and housing adjustments
- Stretching the inner ring and pressing the outer ring together
- Heat expansion during operation
During operation a bearing should have virtually no clearance, especially in ball bearings, spherical roller bearings, and cylindrical roller bearings. A slight positive clearance of a few microns is often ideal.
Clearance classes according to ISO standards
Radial clearance classes for unmounted bearings are defined in ISO standards. Commonly used clearance classes are:
| Clearance class | Description |
|---|---|
| C2 | Clearance smaller than normal |
| C0 | Standard clearance for normal application |
| C3 | Larger clearance than C0 |
| C4 | Larger clearance than C3 |
| C5 | Larger clearance than C4 |
When choosing the right clearance, factors such as temperature, load, and rotational speed play an important role.
The influence of shaft and housing applications
Even the best bearing will not function properly if the shaft and housing application are not correct. The ISO tolerances, along with the bore and outer diameter tolerances of a bearing, determine the final fit.
Rotational ratio and load
The choice of the right fit is determined by the rotation ratio:
- Rotating load: When the ring is stationary and the load is rotating, or vice versa.
- Stationary load: When both the ring and the load are not moving or rotating at the same speed.
- Undetermined load: When the load varies due to vibration or impact.
For optimum bearing life, the fit must be correctly matched to the load condition.
Other influences on the fit
In addition to load and rotation, the following factors also play a role:
- Shaft and hosing material: Aluminum, for example, expands more than steel when heated.
- Thin-walled houses or hollow shafts: These may result in an abnormal fit and should be checked extra
- High vibrations: May affect bearing operation and stability.