Bearing inner rings that are mounted with an interference fit typically abut a shoulder on the shaft on one side. On the opposite side, they are normally secured by a precision lock nut (fig. 1).
Bearings with a tapered bore, mounted directly on a tapered shaft seat, are generally retained on the shaft by a spacer seated against a fixed abutment at the large end of the taper and a precision lock nut at the small end of the taper. The spacer width is adjusted to limit the drive-up distance of the bearing on its tapered seat.
For detailed information about precision lock nuts, refer to Precision lock nuts.
Another way to locate a bearing axially is to use a stepped sleeve (fig. 3) with a tight interference fit on the shaft. These sleeves are particularly suitable for super-precision bearing arrangements, as they have very small run-out and provide superior accuracy compared to threaded lock nuts. Therefore, stepped sleeves are typically used in very high-speed spindles where the accuracy provided by conventional locking devices may be inadequate.
For detailed information about stepped sleeves, refer to Stepped sleeves.
Bearing outer rings that are mounted with an interference fit typically abut a shoulder in the housing on one side. On the opposite side, they are normally located by a housing cover.
Housing covers and their securing screws can, in some cases, have a negative impact on bearing form and performance. If the wall thickness between the bearing seat and the bolt holes is too small, and/or the bolts are tightened too much, the outer ring raceway may deform. Bearings in the lightest ISO dimension series 18 and 19 are more susceptible to this than those in the ISO dimension series 10 or above.
It is advisable to use a larger number of small diameter bolts. Using only three or four bolts should be avoided because a small number of tightening points may produce lobes in the housing bore. This can result in noise, vibration, unstable preload or premature failure due to load concentrations. For complex spindle designs where space is limited, only thin-section bearings and a limited number of bolts may be possible. In these cases, SKF recommends an FEM (finite element method) analysis to accurately predict deformation.
As a guideline to achieve an appropriate clamping force between the cover spigot end face and the side face of the bearing outer ring, the cover spigot length should be adjusted so that, before the bolts are tightened, the axial gap between the cover and the housing side face is between 15 and 20 μm per 100 mm housing bore diameter (fig. 4).