6113 Structural rigidity and bearing preloading

The universal practice of preloading bearing assembly supporting a shaft or hub (Fig. 6.24) raises the rigidity of the bearing assembly so that its deflection under operating conditions is minimized. Insufficient structural rigidity of a shaft or hub assembly may be due to a number of factors which can largely be overcome by preloading the bearings. Bearing

Fig. 6.24 Final drive pinion bearing spring preload analogy

preload goes a long way towards compensating for the following inherent side effects which occur during service.

1 The actual elasticity of the roller elements and their respective tracks cause the bearings to deflect both radially and axially in proportion to the applied load and could amount to a considerable increase in shaft movement under working conditions.

2 As the rolling elements become compressed between the inner and outer tracks, the minute surface irregularities tend to deform under the loaded half of the bearing so that the inner raceway ring or cone member centre axis becomes eccentric to that of the outer ring or cup member.

3 If the structure of the housing which contains the bearing is not sufficiently substantial or is made from soft or low strength aluminium alloy, it may yield under heavy loads so that the bearing roller elements become loose in their tracks.

4 Temperature changes may cause the inner or outer track members to become slack in their housings once they have reached operating temperature conditions even though they may have had an interference fit when originally assembled.

5 Over the working life of a bearing the metal to metal contact of the rolling elements and their raceways will planish the rolling surfaces so that bearing slackness may develop.

6.1.14 Bearing selection (Fig. 6.25) The rigidity of a rolling contact bearing to withstand both radial and axial loads simultaneously is a major factor in the type of bearing chosen for a particular application. With straight cut gear teeth, pairs of meshing gears are forced apart due to the leverage action when torque is applied so that radial loads alone are imposed onto the bearings. However the

Fig. 6.25 Load-deflection characteristics of several rolling contact bearings (70 mm inner ring shaft diameter)

majority of transmission gear trains have either helical cut teeth or are bevel gears. In either case, end thrust is generated which must be absorbed by the bearings to prevent the gears separating in an axial direction. Bearings are therefore designed not only to carry radial loads but also to support various amounts of axial thrust. As can be seen in Fig. 6.25 the various types of rolling contact bearings offer a range of axial load-deflection characteristics. The least rigid bearing constructions are the deep groove ball and the self-alignment ball bearings, whereas the roller type bearings, with the exception of the angular contact ball and pure thrust ball bearings, provide considerably more axial stiffness. Furthermore, the ability for taper roller bearings to increase their axial load capacity depends to some extent on the angle of bearing contact. The larger the angle, the greater the axial load carrying capacity for a given axial deflection will be. The radial load-deflection characteristics follow a very similar relationship as the previous axial ones with the exception of the pure thrust ball bearing which cannot support radial loads.

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