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Selecting fits

Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

Fits can be selected by following the recommendations for bearing seat diameter tolerances (→ Fits for standard conditions). These recommendations will provide adequate solutions for the majority of applications. However, they do not cover all details of a specific application and so you may find that adjustments may be necessary. When selecting fits, you should consider the following topics.

Conditions of rotation

Conditions of rotation refer to the relative motion between a bearing ring and the load acting upon it (table 1). Essentially, there are three different conditions:

  • rotating load
  • stationary load
  • direction of load indeterminate
Rotating loads occur where either the bearing ring or the applied load is stationary while the other rotates. A bearing ring mounted with a loose fit will creep on its seat when subjected to a rotating load, and this can lead to fretting corrosion and eventually wear. To prevent this from happening, an adequate interference fit, between the ring subjected to rotating load and its seat, is required.
For the purpose of selecting fits, loads that oscillate (such as loads acting on connecting rod bearings) are considered to be rotating loads.

Stationary loads occur where both the bearing ring and the applied load are stationary or both are rotating at the same speed. Under these conditions, a bearing ring normally does not creep and there is no risk of fretting corrosion or wear. In this case, the ring does not need to have an interference fit.

Direction of load indeterminate refers to variable or alternating external loads, sudden load peaks, vibration or unbalanced loads in high-speed applications. These give rise to changes in the direction of load, which cannot be accurately described. Where the direction of load is indeterminate and particularly where heavy loads are involved, there is a risk of fretting corrosion or wear. You should use an interference fit for both rings. The same fit as for a rotating load is normally suitable.
Where the outer ring should be able to move axially in its housing, a loose fit must be used. However, a loose fit can result in housing wear. Where this cannot be tolerated, either protect the bearing seat surface or select a bearing that accommodates the axial displacement within itself (cylindrical roller, needle roller or CARB bearing). These bearings can be mounted with an interference fit for both rings.

Magnitude of load
The ring of a bearing deforms proportionately to the load. For rotating inner ring loads, this deformation can loosen the interference fit between the inner ring and shaft, causing the ring to creep on its shaft seat. The heavier the load, the tighter the interference fit required. The required interference can be estimated using:



where
Δrequired interference [µm]
dbearing bore diameter [mm]
Bbearing width [mm]
Frradial load [kN]

Where sudden load peaks or vibration occurs, a tighter fit can be required.

Temperature differences
  • In operation, bearing rings normally reach a temperature that is higher than that of the components to which they are fitted. This can loosen the fit on the shaft seat, while outer ring expansion can prevent the desired axial displacement in the housing.
  • Rapid start-up can loosen the inner ring fit when the frictional heat generated by the bearing is not dissipated quickly enough. In some cases, friction from seals can generate enough heat to loosen the inner ring fit.
  • External heat and the direction of heat flow can have an effect on fits. Steady-state as well as transient conditions must be considered. For additional information about temperature differences, refer to Selecting internal clearance or preload.
Precision requirements
To minimize deflections and vibration in precision or high-speed applications, interference or transition fits are recommended.

Design and material of the shaft and housing
  • Distortion of the bearing rings caused by shaft or housing design, for example by discontinuities of the seat or uneven wall thickness, should be avoided.
  • For split housings, SKF generally recommends loose fits. The tighter (less loose) the fit is in a split housing, the higher are the requirements for the geometrical tolerances of the seat. Split housings machined to tight tolerances, like SKF plummer block housings, can be used for transition fits up to K7.
  • Bearings mounted in thin­-walled housings or on hollow shafts require tighter interference fits than those recommended for robust cast iron housings or solid shafts (→ Tolerances for seats on hollow shafts).
  • Shafts or housings made of materials other than steel or cast iron may require different fits depending on material strength and thermal properties.
Ease of mounting and dismounting
Loose fits are beneficial for easy mounting and dismounting. In applications where interference fits are required for both the shaft and housing seat, separable bearings or bearings with a tapered bore should be considered. Bearings with a tapered bore can be mounted on tapered sleeves (fig. 1) or on a tapered shaft seat (fig. 2).

Axial displacement of the bearing in the non-locating position
Where a non-­locating bearing needs to be able to move axially on its seat, the ring subjected to the stationary load should have a loose fit. For additional information about bearings in the non-locating position, refer to Arrangements and their bearing types.

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