Magnitude of load
The magnitude of the load is one of the factors that usually determines the size of the bearing to be used. Generally, roller bearings are able to support heavier loads than similar sized ball bearings (fig 1) and bearings having a full complement of rolling elements can accommodate heavier loads than the corresponding caged bearings. Ball bearings are mostly used where loads are light or moderate. For heavy loads and where shaft diameters are large, roller bearings are usually the more appropriate choice.
Direction of load
NU and N design cylindrical roller bearings, needle roller bearings and toroidal roller bearings can only support pure radial loads (fig 2). All other radial bearings can accommodate some axial loads in addition to radial loads; see Combined load.
Thrust ball bearings and four-point contact ball bearings (fig 3) are suitable for light or moderate loads that are purely axial. Single direction thrust ball bearings can only accommodate axial loads acting in one direction; for axial loads acting in both directions, double direction thrust ball bearings are needed.
Angular contact thrust ball bearings can support moderate axial loads at high speeds; here the single direction bearings can also accommodate simultaneously acting radial loads, while double direction bearings are normally used only for purely axial loads (fig 4).
For moderate and heavy axial loads acting in one direction, needle roller thrust bearings, cylindrical and tapered roller thrust bearings are suitable, as are spherical roller thrust bearings (fig 5). Spherical roller thrust bearings can also accommodate simultaneously acting radial loads. For heavy alternating axial loads, two cylindrical roller thrust bearings or two spherical roller thrust bearings can be mounted adjacent to each other.
A combined load comprises a radial and an axial load acting simultaneously. The ability of a bearing to carry an axial load is determined by the angle of contact a - the greater the angle, the more suitable the bearing for axial loads. An indication of this is given by the calculation factor Y, which becomes smaller as the contact angle α increases. The values of this factor for a bearing type or for individual bearings can be found in the introductory text of the product table sections, or in the actual product tables. The axial load carrying capacity of a deep groove ball bearing depends on its internal design and the internal clearance in the bearing, see the section Single row deep groove ball bearings.
For combined loads, single and double row angular contact ball bearings and single row tapered roller bearings are most commonly used, although deep groove ball bearings and spherical roller bearings are suitable (fig 6). In addition, self-aligning ball bearings and NJ and NUP design cylindrical roller bearings as well as NJ and NU design cylindrical roller bearings with HJ angle rings can be used for combined loads where the axial component is relatively small (fig 7).
Single row angular contact ball bearings, tapered roller bearings, NJ design cylindrical roller bearings and NU design cylindrical roller bearings + HJ angle ring and spherical roller thrust bearings can accommodate axial loads acting in one direction only. For axial loads of alternating direction these bearings must be combined with a second bearing. For this reason, single row angular contact ball bearings are available as bearings for universal matching and single row tapered roller bearings can be supplied as matched sets comprising two single row bearings, see sections Single row angular contact ball bearings and Paired single row tapered roller bearings.
When the axial component of combined loads is large, it may be supported independently from the radial load by a separate bearing. In addition to thrust bearings, some radial bearings, e.g. deep groove ball bearings or four-point contact ball bearings (fig 8) are suitable for this task. To make sure that the bearing is only subjected to the axial load in such cases, the bearing outer ring must be mounted with radial clearance (fig 9).
When a load acts eccentrically on a bearing, a tilting moment will occur. Double row bearings, e.g. deep groove or angular contact ball bearings, can accommodate tilting moments, but paired single row angular contact ball bearings or tapered roller bearings arranged face-to-face, or better still back-to-back, are more suitable (fig 10).