The importance of bearing preload and axial adjustment

Paul Boughton

Gary Hughes outlines the benefits of preloading and axial adjustment of bearings

Preloading and axial adjustment is a method of removing or controlling the internal clearance of a bearing. This is important because the degree of internal clearance within a bearing can influence noise, vibration, heat build-up and fatigue life.

When applied correctly, preloading reduces or overcomes the causes of these problems: it controls radial and axial play; provides predictable system rigidity; reduces non-repetitive run-out; reduces the difference in contact angles between the inner and outer rings at very high speeds; and controls ball skidding under very high acceleration.

In all bearing arrangements where preload is deemed necessary, it should be applied as lightly as possible to achieve the desired result, avoiding excessive heat generation, which reduces speed capability and bearing life.

It is also important to select the most appropriate method of preloading, of which there are three types.

Spring preloading provides a relatively constant preload because it is less sensitive to differential thermal expansion than rigid preloading, and better accommodates minor misalignments. Also, it is possible to use bearings that have not had the added cost of preload grinding. One disadvantage is that springs cannot accept reversing thrust loads. Space must also be provided to accommodate the springs and spring travel.

However, despite its disadvantages, spring preloading is still popular. Numerous types of springs are available for preloading, including coil springs, Belleville, wave or finger spring washers. For mounting, the spring is normally applied to the non-rotating part of the bearing, typically the outer ring. This ring must have a clearance fit in the housing at all temperatures to ensure the preload force from the spring is effective.

Axial adjustment calls for the mounting of at least two bearings in opposition, so that inner and outer rings of each bearing are offset axially. Threaded members, shims and spacers are typical methods of providing rigid preloads via axial adjustment.

Great care and accuracy are required in order to avoid excessive take up of internal clearance, which may occur during setup by overloading the bearings, or during operation due to thermal expansion.

Precision lapped shims are normally preferred to threaded members, as helical threads can lead to misalignment. The shims should be manufactured to parallelism tolerances equal to those of the bearings, because they must be capable of spacing the bearings to accuracies of 1-2 micrometres or better. Bearing ring faces must be well aligned and solidly seated and there must be extreme cleanliness during assembly. Axial adjustment does not increase bearing friction and is therefore preferred for very low torque applications.

Ready-to-mount bearings

With Duplex bearings, the means of achieving preload is built-in. This method can be the simplest for the customer, who receives the bearings ready-to-mount and with the confidence that those bearings are preloaded to the precise requirement of the application.

Duplex bearings are matched pairs of bearings that have their inner or outer ring faces selectively relieved by a precise amount known as the preload offset. When the bearings are clamped together during installation, the offset faces align, establishing a permanent preload in the bearing set. Duplex bearings are normally speed-limited due to heat generated by this rigid preload.

Duplexing is used mainly where the requirement is for predictable radial and axial rigidity. Duplex bearings can withstand bi-directional thrust loads or heavy uni-directional thrust loads. Other advantages include their ease of assembly and minimum runout.l

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Gary Hughes is Product Engineering Manager, Barden Corporation UK Ltd., Estover, Plymouth, Devon, UK.

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