Selecting Bearing Arrangement
In part 2 of our bearing selection series with Koyo Bearings, we review key points in selecting the proper bearing arrangement. You can find the original post on Koyo Bearings’ website. If you missed Part 1, you can read it here.
There are two key points to decide on the arrangement of bearings.
- Thermal expansion should be considered
- In consideration of the applied load conditions, the fixed side and the free side should be separated
Thermal Expansion
When a machine is operated and the bearings rotate, heat will be generated inside the bearings. This heat is transferred to surrounding components, causing the shaft and the housing to expand.
Separate the Fixed Side and Free Side
To prevent shaft expansion and contraction, bearings should be arranged so that the fixed side and free side are separated. Bearings provide load by being inserted between the rotating and fixed parts of a machine. From among two bearings on a single shaft, one is designated as the fixed side bearing, and it fixes the shaft and housing in the axial direction. This is called “locating.” The other bearing is designated as the free side bearing, and it allows the shaft to move freely in the axial direction, preventing thermal expansion and contraction of the shaft.
Fixed Side Bearing
A radial load and an axial load are applied to the fixed side bearing in order to fix the shaft and housing in the axial direction. For this reason, you should select a type of bearing that can support both the radial and axial loads and is appropriate to the magnitude of these loads.
It is also possible for separate fixed side bearings to support a radial load and an axial load, respectively. As shown in Figure 4, there is also an arrangement in which a deep groove ball bearing supports the axial load, and a cylindrical roller bearing supports only the radial load, by providing a clearance between the outside diameter of the deep groove ball bearing and the bore diameter of the housing.
Which of the two bearings is used as the fixed bearing is determined by considering the balance of the machine structure and bearing service life.
As shown in Figure 5, the arrangement can also be such that the fixed side bearing is a large size of deep groove ball bearing that can support heavier radial load and axial load, while the free side bearing is a small size of deep groove ball bearing that supports only lighter radial load.
Free Side Bearing
The free side bearing prevents thermal expansion and contraction of the shaft. In the case of a separable bearing (where the inner ring and outer ring can be separated), the expansion and contraction of the shaft is prevented between the rollers and the raceway surface.
For non-separable bearings (where the inner ring and outer ring cannot be separated), a clearance is provided between the outer diameter of the bearing and the inner diameter of the housing to prevent the thermal expansion and contraction of both the shaft and the bearing.
Check for Misalignment
A large misalignment may occur between the inner and outer rings of the bearing due to load-induced shaft bending or mounting errors between the fixed side and free side bearings. If there is a large misalignment, an abnormal load will be put on the bearing, which can cause damage.
Check the bearing type according to the allowable misalignment angle of the bearing (see Table 4).
There are also bearing arrangements in which the fixed and free sides are not distinguished. Small machines that support small loads with short shafts are less affected by shaft expansion/contraction, so two bearings may be arranged without distinction between the fixed and free sides (see Figure 9).
Giving Rigidity to the Shaft
When the preload is applied to enhance rigidity to the shaft, there is no distinction between the fixed side and free side bearings. In this case, two angular contact ball bearings or tapered roller bearings are mounted with a back-to-back or face-to-face arrangement. (See Figure 10.)
Table 5 shows a comparison of face-to-face and back-to-back bearing arrangements.
When deciding between face-to-face or back-to-back bearing arrangements, it is necessary to take the “moment” into account.
The “moment” is obtained by multiplying the magnitude of the applied load and the distance to the point where the load is applied, and either face-to-face or back-to-back bearing arrangement is selected according to this “magnitude of the moment.”
As indicated in Table 5, select face-to-face bearing arrangement when the moment is small, and back-to-back bearing arrangement when the moment is large.
Conclusion
We explained how to decide on the bearing arrangement in order to determine whether the bearing type you chose was appropriate.
- Select a free side bearing to prevent expansion and contraction of the shaft due to thermal expansion.
- Select a bearing with an allowable misalignment angle.
- When giving preload to make the shaft rigid, it is not necessary to distinguish bearings for fixed and free sides but use two angular contact ball bearings or tapered roller bearings that are arranged face-to-face or back-to-back with each other.
To view the full selection of Koyo Bearings products, visit their page on the MISUMI website here.
Works Cited
Koyo Bearings. (2020, February 26). Bearing Trivia. Retrieved from Koyo Bearings: https://koyo.jtekt.co.jp/en/2020/02/column02-02.html
