SKF bearing interfaces selection

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SKF bearing interfaces selection
Bearing seats on shafts and in housings, and components which locate a bearing axially, have a significant impact on SKF bearing performance. To fully exploit the load carrying ability of a SKF bearing, its rings or washers should be fully supported around their complete circumference and across the entire width of the raceway. Bearing seats should be manufactured to adequate geometrical and dimensional tolerances and be uninterrupted by grooves, holes or other features. Please refer to SKF bearing selection.

In this section you can find recommendations and requirements for designing bearing interfaces, including:

criteria when selecting bearing fits
recommended fits for standard conditions
tables to help determine minimum, maximum and probable values of clearance or interference between the bearing and its seat
recommendations for specifying geometrical tolerances of bearing seats
recommendations for the axial support of bearing rings
further design considerations for bearing interfaces
The ISO tolerance system

selection of the tolerance class
Fits for rolling bearings are typically specified with standard tolerance classes for holes and shafts as described in ISO 286-2. As SKF bearings are typically manufactured to ISO tolerances, the selection of the tolerance class for the bearing seat determines the fit. illustrates the position and width of the tolerance intervals of commonly used tolerance classes relative to the bearing bore and outside diameter tolerances. The figure above is valid for bearings with Normal tolerances and of medium size. It is important to note that the ISO tolerance classes for rolling bearings and for holes and shafts are different. The tolerances for each size vary over the full range of actual sizes. You should therefore select the respective tolerance classes for bearing seats based on the actual bearing size for your application.

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 and transient conditions must be considered. For additional information about temperature differences.

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, such as 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.
  • 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.

Tolerances for bearing seats and abutments

Dimensional tolerances for bearing seats are dictated by the required fit. Precision requirements of the application will direct you to which bearing tolerance class to use, and consequently, what run-out tolerance of the seat is needed. The run-out of the seat is specified by the total radial run-out of the seat surface and the total axial run-out of the abutment (ISO 1101, 18.16).

For bearings with Normal tolerances in general industrial applications, seats are typically machined to the following tolerances:

  • shaft seats to grade IT6 dimensional tolerances and grade IT5 total run-out tolerances
  • housing seats to grade IT7 dimensional tolerances and grade IT6 total run-out tolerances

Seat tolerances for standard conditions

The following tables provide recommendations for tolerances of shaft and housing seats. They are valid for standard applications but do not cover all details of a specific application. The information under Selecting fits and Tolerances for bearing seats and abutments should be additionally considered.

These recommendations are valid for bearings with Normal dimensional tolerances. They can also be used for bearings to P6 dimensional tolerances. The tighter P6 tolerance zone changes the resulting fit only slightly.

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