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For some applications – for example, high-speed, high-load, external vibration – bearing rings may need to be firmly located to stop movement between the bearing and shaft or housing.

This is achieved by making the shaft slightly larger than the bearing bore, or the housing slightly smaller than the bearing OD, requiring greater force to fit the bearing.

This prevents the bearing rings from creeping in a circumferential direction that gives rise to increased wear.

Interference fits stretch the bearing inner ring or compress the outer ring and reduce the bearing’s radial play.

Imagine an actual shaft diameter of 10.007mm and an actual bearing bore of 9.993mm – this gives an interference fit of 0.014mm (i.e.

the shaft is 0.014mm larger than the bearing bore).

The bearing’s radial play may be reduced by as much as 80 per cent of this figure, or 0.011mm.

This may take up all available play and cause the bearing to fail so interference fits sometimes require looser radial play.

Out of round shafts and housings can distort bearing rings, making the bearing noisy or rough when rotated.

The standards of roundness and surface finish that apply to the bearing should also apply to shaft and housing.

This is very important for electric motor and other quiet-running applications.

Shaft and housing fits depend on the type of ‘rotating load’.

A bearing ring is subjected to a rotating load when the load is applied to all points of that ring during operation.

Take, for example, inner-ring rotating load – this could be a bearing in a vacuum cleaner motor belt driving the roller brush.

The shaft and bearing inner ring are rotating and load is in a constant direction in relation to the bearing so, as the inner ring turns, all parts of it are subjected to the load.

This arrangement requires a tighter or interference shaft fit.

With an outer-ring rotating load – for example a bearing in a pulley – the shaft and inner ring are fixed while the outer ring and housing (the pulley) do rotate.

The load is in a constant direction in relation to the bearing so as the outer ring turns, all parts of it are subjected to the load.

This requires a tighter or interference housing fit.

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