Factors to consider when selecting a bearing type

1. Bearing Loads

The magnitude, direction and nature of the loads acting on a bearing are the primary factors in selecting the appropriate bearing type.

When selecting a bearing type based on load magnitude, roller bearings are generally more suitable for withstanding heavy loads, as the main components make line contact, and they exhibit minimal deformation under load. Ball bearings, on the other hand, involve point contact and are therefore suitable for withstanding light to moderate loads; consequently, ball bearings should be the preferred choice when the load is relatively small.

When selecting a bearing type based on the direction of the load, thrust bearings are generally chosen for purely axial loads. For smaller purely axial loads, thrust ball bearings may be selected; for larger purely axial loads, thrust roller bearings may be selected. For purely radial loads, deep groove ball bearings, cylindrical roller bearings or needle roller bearings are generally selected. When the bearing is subjected to a radial load R alongside a small axial load A, deep groove ball bearings, angular contact ball bearings with a small contact angle, or tapered roller bearings may be selected; where the axial load is substantial, angular contact ball bearings with a larger contact angle or tapered roller bearings may be selected, or a combination of radial and thrust bearings may be employed, with each bearing type handling the radial and axial loads respectively.

2. Bearing Speed

At normal speeds, the speed level has little effect on the selection of bearing type; only at higher speeds does it have a significant influence. Bearing catalogues list the limiting speeds for various types and sizes of bearings. This limiting speed refers to the maximum permissible speed under conditions of moderate load (P ≤ 0.1C, where C is the basic dynamic load rating), normal cooling conditions, and for bearings with Class 0 tolerances. However, as the limiting speed is primarily constrained by the temperature rise during operation, the limiting speed stated in the catalogue should not be regarded as an absolute limit that must not be exceeded. If the operating speed of the bearing exceeds the limiting speed, the measures outlined in point 5 below may be adopted.

Based on the requirements for bearings regarding speed, the following points can be established:

1) Compared to roller bearings, ball bearings have a higher limiting speed; therefore, ball bearings should be the preferred choice for high-speed applications;

2) For bearings with the same inner diameter, the smaller the outer diameter, the lighter and smaller the rolling elements. Consequently, the centrifugal inertial force exerted by the rolling elements on the outer ring raceway during operation is reduced, making them more suitable for operation at higher speeds. Consequently, at high speeds, it is advisable to select bearings from the ultra-light, extra-light and light series. Bearings from the heavy and extra-heavy series are only used in low-speed, heavy-load applications. If a single light-series bearing does not meet the load capacity requirements, one may consider using a wide-series bearing, or mounting two light-series bearings in tandem.

3) The material and structure of the cage have a significant impact on the bearing’s operating speed. Solid cages permit slightly higher speeds than stamped cages.

4) The maximum operating speeds of thrust bearings are generally low. When operating at high speeds and the axial load is not particularly high, angular contact ball bearings may be used to support purely axial forces.

5) If the operating speed slightly exceeds the limit speed specified in the catalogue, measures such as increasing the bearing tolerance class, appropriately increasing the radial clearance, selecting recirculating lubrication or oil mist lubrication, and enhancing the cooling of the circulating oil can be taken to improve the bearing’s high-speed performance. If the operating speed significantly exceeds the limit speed, specially designed high-speed rolling bearings should be selected.

3. Bearing Self-Aligning Capabilities

When the centre line of the shaft does not coincide with that of the bearing housing, resulting in angular misalignment, or when the shaft bends or tilts due to applied forces, this causes the axes of the bearing’s inner and outer rings to become misaligned. In such cases, self-aligning ball bearings or self-aligning roller bearings with a certain degree of self-aligning capability should be used. These bearings can continue to function normally even when there is a slight relative misalignment between the axes of the inner and outer rings.

Cylindrical roller bearings and needle roller bearings are particularly sensitive to misalignment; their load-carrying capacity under misaligned conditions may be lower than that of ball bearings. Therefore, the use of these bearings should be avoided wherever possible when the stiffness of the shaft and the bearing housing bore is low.

4. Bearing Installation and Removal

Ease of installation and removal is another factor to consider when selecting a bearing type. Where the bearing housing has no split face and the bearing assembly must be installed and removed axially, preference should be given to bearings with separable inner and outer rings (such as N0000, NA0000, 30000, etc.). When installing bearings on long shafts, bearings with an inner ring bore of 1:12 taper (for mounting on a locking sleeve) may be selected to facilitate disassembly.