How The Contact Angle Affects In Angular Contact Ball Bearings

2023 Author : Hannah Pearcy | [email protected] . Last modified: 2023-05-24 11:12

Each rolling bearing has a contact angle, through which the forces are transmitted within the bearing. The simplest case is a deep groove ball bearing: there the contact angle is 0 ° with purely radial loads. In comparison, an angular contact ball bearing always has a contact angle that is greater than 0 °.

The larger this contact angle, the higher the axial load capacity of the rolling bearing. In extreme cases there is a contact angle of 90 °; then it is a pure thrust bearing. Angular contact ball bearings therefore move between pure radial and axial bearings. Accordingly, they can absorb combined radial and axial loads.

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In the field of single-row angular contact ball bearings with brass cages, SKF offers an expanded range that has a smaller contact angle than the usual 40 °. Due to the smaller contact angle, the axial load-bearing capacity drops a little; However, there are other advantages to this: for example, under certain conditions, speeds up to 20% higher than with a 40 ° contact angle can be achieved. This is due, among other things, to the more favorable kinematic conditions in the bearing, which minimize the amount of sliding and therefore generate less heat. In addition, the smaller contact angle ensures greater radial rigidity, which is advantageous in predominantly radially loaded applications.

Angular contact ball bearings with optimized brass cage

Apart from that, SKF has optimized the brass cage for both the 25 ° and the 40 ° variants: thanks to its firmer material and its improved shape, the cage is now even more robust and also allows higher speeds than before. Despite these increases in performance, the cage takes up less space than its predecessor, so that more volume is available for the lubricant - and this enables longer lubrication intervals. In addition, the vibration and noise levels are 15% lower with the new brass cage.

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Frequent areas of application of angular contact ball bearings are pumps, compressors or even electric motors. In such applications, single row angular contact ball bearings are usually installed at least in pairs. Because if an angular contact ball bearing is loaded purely axially or purely radially, the contact angle always creates a corresponding radial or axial force. Due to the design, axial forces can only be absorbed in one direction by a single bearing. If it is loaded in the opposite direction, it can be destroyed.

Since axial forces from both directions occur in most cases, the opposite forces must be absorbed by a counter bearing. The simplest form of a paired bearing is the double row angular contact ball bearing with a fixed "O arrangement". Individual universal angular contact ball bearings can also be combined in different ways - both in an O and X arrangement.

Angular contact ball bearings installed in pairs

If angular contact ball bearings are installed in pairs, there are various options for setting a defined preload or internal clearance. The most common method is the use of so-called "universally pairable" bearings. Such universal bearings offer the advantage that they are already coordinated with one another in the factory so that a defined preload / air is achieved when assembled on a block. The initially existing block gap between the inner or outer rings of the bearings is closed by clamping the bearings.

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In order to achieve the exact preload / internal clearance, a very limited tolerance of the residues and protrusions of a few micrometers is required. Universal bearings therefore considerably simplify assembly. The usual assembly methods are associated with a significantly higher effort. The preload of the bearings can also be adjusted by making special intermediate rings in the housing or on the shaft. To do this, however, the bearings must be measured in a complex manner and intermediate rings must be manufactured individually for each pair of bearings.

In order to reduce the assembly effort, the AC angular contact ball bearings with 25 ° contact angle are directly available as universal bearings according to the SKF Explorer performance class.

Tip: When are asymmetrical bearing sets used?

Especially in applications with dominant axial force from one direction, it is advisable to provide an asymmetrical bearing set in the design in order to prevent problems with the minimum load not being reached and to avoid premature bearing failures. In addition, the bearings with a 25 ° contact angle can also be used in applications where high speeds and / or increased radial rigidity are required.

Avoid bearing failure

In angular contact ball bearing sets, the axial force is often dominant from one direction. This can be the case with fans or pumps that mainly rotate in one direction. In such applications, one angular contact ball bearing absorbs the axial force, while the second angular contact ball bearing, the so-called “backup bearing”, is relieved. However, rolling bearings always require a certain minimum load for smooth operation. On the relieved bearing there is a high risk that this minimum load will not be reached. This can lead to a disrupted rolling behavior of the balls, which causes “smearing”, which leads to an increase in temperature and premature bearing failure due to surface damage and / or cage breakage.

Up to now, bearing sets consisting of two identical 40 ° single-cell bearings have often been used in such applications with dominant one-sided axial loads. However, this arrangement is not optimal since the large 40 ° angle is more susceptible to minimum load problems when unloaded.

Realize asymmetrical bearing sets

With the new series with a 25 ° contact angle, asymmetrical sets of 40 ° and 25 ° bearings can now be realized. This pair of bearings offers great advantages: In the asymmetrical bearing set, the dominant axial force is absorbed over the large contact angle, while the unloaded bearing with a smaller contact angle reduces the risk of greasing - because this increases the threshold for the lifting force. The lifting force is the external axial force from which a backup bearing in a preloaded bearing set is completely relieved and the minimum load required is no longer guaranteed.

This means that with the smaller contact angle of 25 °, the backup bearing is less relieved with the same external load. This reduces the risk of premature bearing failure, which increases operational safety.

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* Achim Schmidt is application engineer "OEM & Distribution" at SKF in Schweinfurt