The correct application of preload in bearings

Choosing a method of preloading bearings is an important decision. Trevor Morris explains the three main types and their differences.

Preloading and axial adjustment is the operation of 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 also fatigue life.

When applied correctly, preloading reduces or overcomes the causes of these problems; it controls radial and axial play, and gives predictable system rigidity, reduces nonrepetitive runout, 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 judged to be 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.

In addition, it is important to choose the most appropriate method of preloading, there being three main types: springs, axial adjustment and preload ground duplex bearings.

Of the three, springs are usually regarded as the simplest method and should be considered first.

Spring preloading provides a relatively constant preload because it is less sensitive to differential thermal expansion than rigid preloading, and accommodates minor misalignment better.

Also, it is possible to use bearings, which have not had the added cost of preload grinding.

A disadvantage is that spring preloading cannot generally accept reversing thrust loads.

Space must also be provided to accommodate both the springs and spring travel, and springs may tend to misalign the ring being loaded due to required clearance fits.

Despite the disadvantages, this is a popular method of applying preload, as evidenced by the many types of springs, coil springs and Belleville, wave or finger spring washers among them, that are available to achieve it.

In fitment terms, the spring is usually applied to the nonrotating part of the bearing - typically the outer ring.

This ring must have a clearance fit in the housing at all temperatures.

The second major method of achieving internal clearance control, axial adjustment, calls for the mounting of at least two bearings in opposition, so that the inner and outer rings of each bearing are offset axially.

Threaded members, shims and spacers are typical means of providing rigid preloads through axial adjustment.

This technique requires great care and accuracy to avoid excessive take up of internal clearance, which might occur during setup by overloading the bearings, or during operation due to thermal expansion.

Precision lapped shims are usually preferable to threaded members, because 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 one to two micrometers 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.

In contrast to spring and axial adjustment methods, using duplex bearings offers the advantage that the means of achieving preloading is built-in.

Duplex bearings are matched pairs of bearings which 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 meet, establishing a permanent preload in the bearing set.

Duplex bearings are usually 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 bidirectional thrust loads or heavy unidirectional thrust loads.

Other advantages include their ease of assembly and minimum runout.

When using duplex bearings consideration should be given to the following points: increased torque, reduced speed capacity, sensitivity to differential thermal expansion, susceptibility to gross torque variations due to misalignment, and poor adaptability to interference fitting.

Most Barden deep groove and angular contact bearings are available in universal duplex sets or can be furnished in specific DB (back-to-back), DF (face-to-face) or DT (tandem) con urations.

DB mounting is suited for most applications having good alignment of bearing housings and shafts.

It is also preferable where high moment rigidity is required, and where the shaft runs warmer than the housing.

In contrast, DF mounting is used in only a few applications, mainly where misalignment must be accommodated.

Speed capability is usually lower than a DB pair of identical preload.

The final method, DT mounting, employs tandem pairs that offer greater capacity without increasing bearing size, through load sharing.

The DT pairs can counter heavy thrust loads from one direction, but they cannot take reversing loads as DB and DF pairs can.

DT sets do not have inbuilt preloading and should be used in conjunction with spring preloading.

(Updated by CR, May 2007).

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