Considerations for preload
Depending on the bearing type, preload may be either radial or axial. Cylindrical roller bearings, for example, can only be preloaded radially because of their design, while thrust ball and cylindrical roller thrust bearings can only be preloaded axially. Single row angular contact ball bearings and tapered roller bearings (fig. 1 and fig. 2), which are normally subjected to axial preload, are generally mounted together with a second bearing of the same type and size in a back-to-back (load lines diverge) or face-to-face (load lines converge) arrangement. Deep groove ball bearings can also be preloaded axially. If so, the bearings should have a greater radial internal clearance than Normal (such as C3) so that, as with angular contact ball bearings, a contact angle that is greater than zero is obtained.
For both tapered roller and angular contact ball bearings, the distance L between the pressure centres is longer when the bearings are arranged back-to-back ( fig. 3 and fig. 4) compared to bearings that are arranged face-to-face (fig. 5 and fig. 6). This means that bearings arranged back-to-back can accommodate relatively large tilting moments even if the distance between the bearing centres is relatively short. The radial loads and bearing deformation resulting from a moment load is smaller for bearings arranged back-to-back than for bearings arranged face-to-face.
If in operation the shaft temperature is higher than the housing temperature, the preload, which was adjusted at ambient temperature during mounting, increases. The increase is greater for bearings arranged face-to-face than for bearings arranged back-to-back. In both cases, however, thermal expansion of the inner ring in the radial direction serves to reduce clearance or increase preload. This tendency is increased by thermal expansion of the rings in the axial direction when the bearings are face-to-face, but is reduced for back-to-back arrangements.
For back-to-back arrangements only: Depending on the distance between the bearings, when the coefficient of thermal expansion is the same for the bearings and associated components, thermal expansion in both the radial and axial directions can cancel each other out so that preload remains unchanged.