Types of preload
Depending on the type of bearing the preload may be either radial or axial. Cylindrical roller bearings, for example, because of their design, can only be preloaded radially, and thrust ball and cylindrical roller thrust bearings bearings can only be preloaded axially. Single row angular contact ball bearings and tapered roller bearings (fig 1), which are normally subjected to axial preload, are generally mounted together with a second bearing of the same type in a back-to-back (a) or face-to-face (b) arrangement. Deep groove ball bearings are also generally preloaded axially, to do so, the bearings should have a greater radial internal clearance than Normal (e.g. C3) so that, as with angular contact ball bearings, a contact angle which is greater than zero will be produced.
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 2) and shorter when they are arranged face-to-face (fig 3) than the distance l between the bearing centres. 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 forces resulting from the moment load and the deformation caused by these in the bearings are smaller than for bearings arranged face-to-face.
If in operation the shaft becomes warmer than the housing, the preload which was adjusted (set) at ambient temperature during mounting will increase, the increase being greater for face-to-face than for back-to-back arrangements. In both cases the thermal expansion in the radial direction serves to reduce clearance or increase preload. This tendency is increased by the thermal expansion 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, for a given distance between the bearings and when the coefficient of thermal expansion is the same for the bearings and associated components, the radial and axial thermal expansions will cancel each other out so that the preload will not change.