Radial location of bearings
1. Type and magnitude of the load
The degree of interference must suit the type and magnitude of the load, i.e. the heavier the load and the stronger the shock loads, the tighter the interference required (fig. 1).
- Under heavy loads, spherical plain bearings deform elastically, which may affect the interference fit and lead to ring creep.
- The strength of the associated components must be adequate to accommodate the loads and fully support the bearing.
- If the associated components deform, there is a risk that through-hardened bearing rings crack.
Steel/steel radial spherical plain bearings require a tighter fit than comparable maintenance-free bearings, which have a lower coefficient of friction.
2. Bearing internal clearanceAn interference fit on the shaft and in the housing causes the inner ring to expand elastically, and the outer ring to be compressed elastically. This reduces the initial internal clearance in the bearing, prior to operation. The operating clearance (fig. 2) furthermore takes the load and operating temperature into consideration.
3. Temperature conditions
In operation, the bearing rings normally have a higher temperature than their seats. This means that
- the inner ring fit gets loosen (fig. 3)
the outer ring fit becomes tighter and may restrict any required axial displacement in the housing.
4. Design of associated components
The bearing seats on the shaft and in the housing must not lead to uneven distortion
(out-of-round) of the bearing rings (fig. 4).
- Split housings are not suitable for interference fits.
- Thin-walled housings, light alloy housings and hollow shafts require a tighter fit than thick-walled steel or cast iron housings and solid shafts – and must have sufficient strength.
Heavy loads and interference fits require thick-walled one-piece steel or cast iron housings and solid steel shafts.
5. Axial displacement of non-locating bearings
A non-locating bearing provides radial support only and must always be able to be displaced axially (fig. 5). This is normally achieved by selecting a loose fit for one of the bearing rings, generally the inner ring of a spherical plain bearing. Reasons include the following:
- The shaft seat can be easily and economically hardened and ground to facilitate axial displacement. The hardness of the shaft should be at least 50 HRC.
- The outer rings of most spherical plain bearings are axially fractured at one or two positions, or are radially split. This can make axial displacement difficult, if not impossible.
The housing bore should be protected against wear.
Surface finish of seats
The recommended surface roughness for bearing
seats is in accordance with ISO 4288:1997.
- for shaft seats Rz ≤ 10 μm
for housing bore seats Rz ≤ 16 μm
Only a limited number of ISO tolerance classes are appropriate for spherical plain bearings. Fig. 6 shows schematically the relative positions of these in relation to the bore and outside diameter of the bearings. The recommended tolerance classes for
These recommendations are based on the considerations described above and have been confirmed in a wide variety of bearing applications. The ISO tolerance limits are listed in