The bearing arrangement of a rotating machine component, e.g. a shaft, generally requires two bearings to support and locate the component radially and axially relative to the stationary part of the machine, such as a housing. Depending on the application, load, requisite running accuracy and cost considerations the arrangement may consist of
- locating and non-locating bearing arrangements
- adjusted bearing arrangements
- "floating" bearing arrangements.
Bearing arrangements consisting of a single bearing that can support radial, axial and moment loads, e.g. for an articulated joint, are not dealt with in this catalogue. If such arrangements are required it is advisable to contact the SKF application engineering service.
Locating and non-locating bearing arrangementsThe locating bearing at one end of the shaft provides radial support and at the same time locates the shaft axially in both directions. It must, therefore, be fixed in position both on the shaft and in the housing. Suitable bearings are radial bearings which can accommodate combined loads, e.g. deep groove ball bearings, double row or paired single row angular contact ball bearings, self-aligning ball bearings, spherical roller bearings or matched tapered roller bearings. Combinations of a radial bearing that can accommodate purely radial load, e.g. a cylindrical roller bearing having one ring without flanges, with a deep groove ball bearing, four-point contact ball bearing or a double direction thrust bearing can also be used as the locating bearing. The second bearing then provides axial location in both directions but must be mounted with radial freedom (i.e. have a clearance fit) in the housing.
The non-locating bearing at the other end of the shaft provides radial support only. It must also enable axial displacement so that the bearings do not mutually stress each other, e.g. when the shaft length changes as a result of thermal expansion. Axial displacement can take place within the bearing in the case of needle roller bearings, NU and N design cylindrical roller bearings and CARB toroidal roller bearings, or between one of the bearing rings and its seat, preferably between the outer ring and its seat in the housing bore.
From the large number of locating/non-locating bearing combinations the popular combinations are described in the following.
For stiff bearing arrangements where "frictionless" axial displacements should take place within the bearing the following combinations may be used
- deep groove ball bearing/cylindrical roller bearing (fig 1)
- double row angular contact ball bearing/ cylindrical roller bearing (fig 2)
- matched single row tapered roller bearings/ cylindrical roller bearing (fig 3)
- NUP design cylindrical roller bearing/NU design cylindrical roller bearing (fig 4)
- NU design cylindrical roller bearing and four-point contact ball bearing/NU design cylindrical roller bearing (fig 5).
For the above combinations, angular misalignment of the shaft must be kept to a minimum. If this is not possible it is advisable to use combinations of self-aligning bearings to enable for misalignment, viz.
The ability of these arrangements to accommodate angular misalignments as well as axial displacements avoids generating internal axial forces in the bearing system.
For bearing arrangements with rotating inner ring load where, changes in the shaft length are to be accommodated between the bearing and its seat, axial displacement should take place between the outer ring of the bearing and the housing. The most usual combinations are
Adjusted bearing arrangementsIn adjusted bearing arrangements the shaft is axially located in one direction by the one bearing and in the opposite direction by the other bearing. This type of arrangement is referred to as "cross located" and is generally used for short shafts. Suitable bearings include all types of radial bearings that can accommodate axial loads in at least one direction, including
In certain cases where single row angular contact ball bearings or tapered roller bearings are used for cross-located arrangements, preload may be necessary, see section Bearing preload.
"Floating" bearing arrangementsFloating bearing arrangements are also cross located and are suitable where demands regarding axial location are moderate or where other components on the shaft serve to locate it axially.
Suitable bearings for this type of arrangement are
- deep groove ball bearings (fig 13)
- self-aligning ball bearings
- spherical roller bearings.
In these types of arrangements it is important that one ring of each bearing should be able to move on or in its seat, preferably the outer ring in the housing. A floating bearing arrangement can also be obtained with two NJ design cylindrical roller bearings, with offset inner rings (fig 14). In this case the axial movement can take place within the bearing.