Radial location of bearings
The inner and outer rings of spherical plain bearings must be radially secured (located) to the shaft and in the housing so that sliding movements occur in the bearing and do not result in ring creep. Ring creep occurs when a ring turns on its seat in the circumferential direction under load. To locate a bearing in the radial direction usually requires an interference fit. However, an interference fit cannot always be applied, e.g. if easy mounting and dismounting are required, or if the bearing must be able to be displaced axially without restraint. The appropriate fit is always determined by the operating conditions.
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).
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Under heavy loads, spherical plain bearings deform elastically, which may affect the interference fit and lead to ring creep.
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The strength of the associated components must be adequate to accommodate the loads and fully support the bearing.
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If the associated components deform, there is a risk that through-hardened bearing rings crack.
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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 clearance
An 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.
The initial radial internal clearance of bearings differs, depending on the type and size of the bearing. The clearance has been selected so that if the recommended tolerances for the shaft and housing seats are applied, an appropriate operating clearance (or preload) remains in the bearing under normal operating conditions.
If a tight interference fit is used for both bearing rings, or if the operating temperatures are unusual, it may be necessary to use a larger initial internal clearance than “Normal” for steel/steel bearings.
3. Temperature conditions
In operation, the bearing rings normally have a higher temperature than their seats. This means that
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the inner ring fit gets loosen (fig. 3)
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the outer ring fit becomes tighter and may restrict any required axial displacement in the housing.
If there is a considerable temperature difference between the inner ring and outer ring, there is a change in the operating clearance. This condition must be considered when selecting the fit or the bearing could seize, making it difficult or impossible for the shaft to turn.
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).
Therefore:
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Split housings are not suitable for interference fits.
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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.
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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:
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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.
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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.
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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.
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for shaft seats Rz ≤ 10 μm
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for housing bore seats Rz ≤ 16 μm
Recommended fits
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
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the shaft seat are provided in table 1
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the housing bore are provided in table 2
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
To facilitate the calculation of the minimum and maximum values of the theoretical interference or clearance, the standardized bearing bore diameter deviations (Δ
dmp) and the bearing outside diameter deviations (Δ
Dmp) are listed in
table 3 and
table 4.
