Adjustment means setting the bearing internal clearance, see section Mounting - Bearings with a cylindrical bore, or setting the preload of a bearing arrangement.
The radial preload usually used for cylindrical roller bearings, double row angular contact ball bearings and sometimes for deep groove ball bearings, for example, is achieved by using a sufficient degree of interference for one or both of the bearing rings to reduce the initial internal clearance of the bearing to zero so that in operation there will be a negative clearance, i.e. preload.
Bearings with a tapered bore are particularly suitable for radial preloading since, by driving the bearing up on to its tapered seat, the preload can be applied to within narrow limits.
Axial preload in single row angular contact ball bearings, tapered roller bearings and also deep groove ball bearings is produced by displacing one bearing ring axially in relation to the other by an amount corresponding to the desired preload force. There are two main groups of adjustment methods that differ in the principle on which they are based: individual adjustment and collective adjustment.
With individual adjustment, each bearing arrangement is adjusted separately using nuts, shims, spacer sleeves, deformable sleeves etc. Measuring and inspection procedures provide that the established nominal preload force is obtained with the least possible deviation. There are different methods depending on the quantity of bearings to be measured
- adjustment using preload path
- adjustment using friction torque
- adjustment using direct force measurement.
Individual adjustment has the advantage that individual components can be produced to Normal tolerances and the desired preload can be achieved with a reasonably high degree of accuracy.
Adjustment using preload path
This method of adjustment is frequently used when the components of a bearing arrangement are pre-assembled. The preload is achieved, for example, for a pinion bearing arrangement by
- fitting intermediate rings between the outer and inner rings of the two bearings (fig 1)
- inserting shims between a housing shoulder and a bearing outer ring or between the casing and the housing (fig 2), the housing being in this case the flanged angled insert
- fitting a spacer ring between a shaft shoulder and one of the bearing inner rings (fig 3) or between the inner rings of the two bearings.
The width of the shims, intermediate rings or spacer rings is determined by
- the distance between the shaft and housing shoulders
- the total width of both bearings
- the preload path (axial displacement) corresponding to the desired preload force
- a correction factor for the preload path to take account of the thermal expansion in operation
- the manufacturing tolerances of all related components, established by measuring the actual dimensions before mounting
- a correction factor to take account of a certain loss of preload force after a certain period of operation.
This method of adjustment is based on the relationship between the preload force and the elastic deformations within the preloaded system. The requisite preload can be determined from a preload force/preload path diagram (diagram 1).
Adjustment using the frictional moment
This method is popular in series production because of the short time required and because considerable automation is possible. Since there is a definite relationship between bearing preload and the frictional moment in the bearing, it is possible to stop adjustment when a frictional moment corresponding to the desired preload has been reached if the frictional moment is continuously monitored. However, it should be remembered that the frictional moment can vary from bearing to bearing, and that it also depends on the preservative used, or on the lubrication conditions and the speed.
Adjustment using direct force measurement
As the purpose of bearing adjustment is to produce a given preload in the bearings, it would seem sensible to use a method either to produce the force directly, or to measure the force directly. However, in practice the indirect methods of adjustment by preload path or friction torque are preferred as they are simple and can be achieved easily and more cost efficiently.
With this method of adjustment, which may also be termed "random statistical adjustment", the bearings, shaft and housing, spacer rings or sleeves etc. are produced in normal quantities and randomly assembled, the components being fully interchangeable. Where tapered roller bearings are concerned, this interchangeability also extends to the outer rings and inner ring assemblies. In order not to have to resort to the uneconomic production of very accurate bearings and associated components, it is assumed that the limiting values of the tolerances – statistically – seldom occur together. If, however, the preload force is to be obtained with as little scatter as possible, manufacturing tolerances must be reduced. The advantage of collective adjustment is that no inspection is required and no extra equipment needed when mounting the bearings.