Mechanical Design Bearing Selection Guide (2): Bearing Arrangement Configuration
Published on:
2024-12-19 17:19
In mechanical design, we first roughly determine the type of bearing to be used, and then consider the arrangement of the bearings in the shaft system. In general machinery, two or more bearings are usually assembled on one shaft to bear the load. How to determine the arrangement of bearings is mainly based on the following two points:
• Consider thermal expansion of the shaft
• Differentiate between fixed and free sides taking into account the load conditions
1. Consider bearing arrangement based on thermal expansion
When the machine is running and the bearing is rotating, the inside of the bearing will heat up, and the heat will be transferred to the surrounding parts, causing thermal expansion of the shaft and the housing. In most cases, there is a temperature difference between the shaft and the housing, so the expansion and contraction of the shaft caused by the thermal expansion difference must be absorbed. If the bearing system cannot absorb the expansion and contraction of the shaft, the bearing will be subjected to excessive axial force and cause damage. Therefore, the expansion and contraction of the shaft must be taken into account when arranging the bearings.
The bearing is installed between the rotating part and the fixed part of the machine to bear the force. Usually two bearings are installed on one shaft. One is used as a fixed side bearing to fix the shaft and the bearing seat in the axial direction, which is called a fixed bearing. The other is used as a free side bearing to allow the shaft to move freely in the axial direction and absorb the shaft expansion caused by the difference in thermal expansion.
2. Fixed side bearing
The fixed side bearing fixes the shaft and the bearing seat in the axial direction, so it bears radial load and axial load. Therefore, the type of bearing that bears radial load and axial load is mainly selected according to the load size.
3. Free-side bearing
The expansion and contraction of the shaft is absorbed by the free-side bearing. Separable bearings (separable inner and outer rings) absorb the expansion and contraction of the shaft between the rollers and the raceway surface. Non-separable (inner and outer rings cannot be separated) bearings have a clearance between the outer diameter of the bearing and the inner diameter of the housing to absorb the expansion and contraction of the shaft together with the bearing.
4. Confirm the relative tilt of the inner and outer rings of the bearing
The deflection of the shaft caused by the load or the installation error between the fixed and free side bearings will cause a large relative tilt between the inner and outer rings of the bearing. If the relative tilt is too large, the bearing will be subjected to abnormal loads, causing damage. In addition to the configuration of the fixed side and the free side, the self-aligning ability of the bearing system must also be considered.
5. Bearing arrangement without distinction between fixed and free sides
1) Small equipment with little influence of shaft expansion and contraction
In small machinery with short shafts bearing small loads, the influence of shaft expansion and contraction is small, so sometimes two bearings are arranged without distinction between fixed and free sides.
2) When the equipment needs to ensure the rigidity of the shaft
When the shaft is rigidified by applying preload, the bearing does not have a fixed side and a free side. In this case, two angular contact ball bearings or tapered roller bearings are installed face to face and back to back.
6. Calculation of thermal expansion and contraction of the shaft
Calculate the thermal expansion of the shaft between the two support bearings. If a cylindrical roller bearing is selected, it is necessary to confirm the tolerable offset of the bearing and whether the spacing between the corresponding end cover and the bearing meets the requirements.
If deep groove ball bearings or spherical roller bearings are selected, the corresponding clearance fit and the movable distance between the end cover and the bearing must meet the requirements.
7. Summary
• Select a free-side bearing that absorbs shaft expansion caused by thermal expansion.
• Select the appropriate type of self-adjustable bearing according to the inclination angle of the inner and outer rings.
• When the shaft is made rigid by applying preload, there is no distinction between fixed-side and free-side bearings.
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