Selecting fits

When selecting fits, the information provided in this section should be considered, together with the general guidelines in the section thereafter.

1. Conditions of rotation

Conditions of rotation refer to the bearing ring being considered relative to the direction of the load (table 1). Essentially, there are three different conditions:
  • rotating load
  • stationary load
  • direction of load indeterminate
Rotating loads pertain if either the ring or the direction of the applied load is stationary while the other rotates. Heavy loads that do not rotate but oscillate, such as loads acting on connecting rod bearings, are generally considered to be rotating loads. A bearing ring subjected to a rotating load creeps on its seat if mounted with a too loose fit, and leads to wear and/or fretting corrosion of the contact surfaces. To prevent this, an adequate interference fit between the rotating ring and its seat must be used. The degree of interference is dictated by the operating conditions (→ points 2 and 4 below).
Stationary loads pertain if either both the bearing ring and the direction of the applied load are stationary or both are rotating at the same speed. Under these conditions, a bearing ring normally does not turn on its seat. Therefore, the ring does not need to have an interference fit, unless it is required for other reasons.
Direction of load indeterminate refers to variable external loads, shock loads, vibrations and unbalanced loads in high-speed applications. These give rise to changes in the direction of load, which cannot be accurately described. When the direction of load is indeterminate and particularly where heavy loads are involved, SKF recommends an interference fit for both rings. For the inner ring, the recommended fit for a rotating load is normally used. However, when the outer ring must be free to move axially in the housing, and the load is not heavy, a somewhat looser fit than that recommended for a rotating load can be used.

2. Magnitude of the load

The degree of interference between the inner ring and the shaft seat must be selected based on the magnitude of the load on the bearing. Typically, the inner ring of a bearing deforms proportionately to the load. This deformation can loosen the interference fit between the inner ring and shaft, causing the ring to creep (turn) on its shaft seat. The heavier the load, the tighter the interference fit required (fig. 1). An interference fit has an influence on the bearing clearance or preload. Shock loads and vibration also need to be considered, as a tighter fit might be necessary under these conditions.
Magnitude of bearing load is defined as:
  • light load: P ≤ 0,05 C
  • normal load: 0,05 C < P ≤ 0,1 C
  • heavy load: 0,1 C < P ≤ 0,15 C
  • very heavy load: P > 0,15 C

3. Bearing internal clearance

Bearings with an interference fit on a shaft or in a housing elastically deform (expand or compress) the ring to reduce the bearing internal clearance. However, a certain minimum clearance should remain (→ Bearing clearance). The interference fit can be so tight that bearings with an initial clearance that is greater than Normal have to be used to prevent unwanted preload (fig. 2 ).

4. Temperature differences

In many applications, the inner ring temperature is higher than the outer ring temperature. This can reduce internal clearance (→ fig. 3 and Bearing clearance) or increase preload (→ Bearing preload).
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 of the inner ring on its seat, while outer ring expansion can prevent the desired axial displacement of the ring in its housing. Fast start-ups can also loosen the inner ring fit when frictional heat generated by the bearing is not dissipated quickly enough. In some cases, friction from bearing seals can generate enough heat to loosen the inner ring fit.
Temperature differences and the direction of heat flow in the bearing arrangement must be taken into consideration.

5. Running accuracy

For applications requiring a high degree of running accuracy, interference fits are recommended. Loose fits can reduce stiffness and contribute to vibration. Bearing seats should conform at least to IT5 tolerance grade for the shaft and IT6 tolerance grade for the housing. Tight total run-out tolerances should also be applied (table 2).

6. Design and material of the shaft and housing

The fit of a bearing ring on its seat must not distort the ring (out-of-round). This can be caused, for example, by discontinuities in the seat surface. Therefore, SKF generally does not recommend split housings where outer rings require a tight, M7 or even tighter, interference fit.
The selected tolerance class for a split housing should not result in a fit tighter than that obtained with tolerance group H (or at most, tolerance group K).
To provide adequate support for bearing rings mounted in thin-walled housings, light alloy housings or on hollow shafts, tighter interference fits than those normally recommended for thick-walled steel or cast iron housings or for solid shafts should be used (→ Fits for hollow shafts). Also, sometimes interference fits that are not so tight may be required if the shaft material has a higher coefficient of thermal expansion than standard steel.

7. Ease of mounting and dismounting

Bearings with a loose fit are usually easier to mount and dismount than those with interference fits. In applications that require interference fits and relatively easy mounting and dismounting, separable bearings or bearings with a tapered bore should be considered (→ Bearings with a tapered bore). Bearings with a tapered bore can be mounted on adapter or withdrawal sleeves on plain or stepped shafts, or mounted directly on a tapered shaft seat (fig. 4, fig. 5 and fig. 6).

8. Displacement of the bearing in the non-locating position

If bearings in the non-locating position cannot accommodate axial displacement internally (within the bearing), the outer ring must be free to move axially on its seat at all times. To do this, the ring that carries a stationary load can have a loose fit (fig. 7). For some particular applications, where the outer ring is under stationary load and the bearing must move axially in the housing seat to accommodate displacement, a hardened intermediate bushing or sleeve can be fitted in the housing bore to prevent the bearing from damaging its seat. Any damage to the housing seat can restrict axial movement or prohibit it entirely over time. This is particularly important if the housing is made of a light alloy.
If needle roller bearings, CARB toroidal roller bearings or cylindrical roller bearings without flanges on one ring are used, both bearing rings can be mounted with an interference fit, because axial displacement can take place internally, within the bearing.
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