When selecting SKF super-precision bearings for high-speed applications, calculated rating life (and therefore basic load rating) is typically not a limiting factor. Other criteria such as stiffness, size of the required bore in a hollow shaft, machining speed and accuracy are normally the decisive factors.
When selecting the bearing type, the magnitude and direction of the load play an important role.
Super-precision cylindrical roller bearings can accommodate heavier radial loads than samesize ball bearings. They are incapable of supporting axial loads but can accommodate a limited amount of axial displacement between their inner and outer rings because there are no flanges on either the inner or outer ring, depending on the specific design.
Double direction angular contact thrust ball bearings in the BTW and BTM series are designed to support axial loads only, acting in either direction. Sets of angular contact ball bearings are also a viable solution, particularly in high-speed applications.
For large size bearing arrangements or those subjected to very heavy axial loads, special single direction thrust ball bearings or cylindrical roller thrust bearings are recommended. For detailed information about these special bearings, contact the SKF application engineering service.
To make sure that the axial bearing is only subjected to axial load, the housing washer should be mounted with radial clearance.
A combined load consists of a radial and axial load acting simultaneously (fig. 1). A very effective way to accommodate combined loads is by using bearing types that can accommodate both radial and axial loads.
Super-precision bearings with these characteristics include:
angular contact ball bearings in the 718, 719, 70 and 72 series
single direction angular contact thrust ball bearings in the BSA and BSD series
double direction angular contact thrust ball bearings in the BEAS and BEAM series
axial-radial cylindrical roller bearings in the NRT series
The ability of a bearing to accommodate an axial or radial load is determined by the contact angle α (fig. 1). A bearing with a 0° contact angle can accommodate pure radial loads only. As the contact angle increases, the axial load carrying capacity increases proportionately. When the contact angle reaches 90°, the bearing becomes a full thrust bearing, capable of accommodating only axial loads. Speed capability, however, is inversely proportional to the contact angle, meaning that as the contact angle increases, speed capability decreases.
Axial-radial cylindrical roller bearings accommodate the axial and radial components of a combined load with separate rows of rollers perpendicular to each other.
In applications where there are combined loads with a very heavy axial load component, the radial and axial loads can be supported by separate bearings.