A rotating machine element, e.g. the shaft, generally requires two bearings to support and locate it radially and axially relative to the stationary part of the machine, e.g. the housing. Normally, only one of the bearings (the locating bearing) is used to fix the position of the shaft axially, whilst the other bearing (the non-locating bearing) is free to move axially.
Axial location of the shaft is necessary in both directions and the locating bearing must be axially secured on the shaft and in the housing to limit lateral movement. In addition to locating the shaft axially the locating bearing is also generally required to provide radial support and bearings which are able to carry combined loads are then necessary, e.g. deep groove ball bearings, spherical roller bearings and double row or paired single row angular contact ball bearings. A combined bearing arrangement, with radial and axial location provided by separate bearings can also be used, e.g. a cylindrical roller bearing mounted alongside a four-point contact ball bearing or a thrust bearing having radial freedom in the housing.
To avoid cross location of the bearings the non locating bearing, which provides only radial support, must be capable of accommodating the axial displacements which arise from the differential thermal expansion of the shaft and housings. The axial displacements must be compensated for either within the bearing itself, or between the bearing and its seating on the shaft, or in the housing.
Typical examples of locating and non-locating bearings are shown on the applications in Fig. 28.20.
To prevent roll or creep it is important to maintain the correct fits between the bearings and seatings. Inadequate fits can result in damage to both the bearings and associated components. Normally, the only way to prevent movement at the bearing seatings is to provide a sufficient degree of interference for the bearing rings. Interference fits provide a further advantage in that relatively thin section bearing rings are properly supported around their circumference to give a correct load distribution and allow the load carrying ability of the bearing to be fully utilized. However, where there is a requirement for easy mounting and dismounting of the bearing, or where a non-locating bearing must have freedom of movement axially on its seating, interference fits may not be possible.
Bearings with cylindrical bore
The most important factors to be considered when selecting bearing fits are as follows:
Conditions of rotation - The conditions of rotation refer to the direction of the load in relation to the bearing rings.
If the bearing ring rotates and the load is stationary, or if the ring is stationary and the load rotates so that all points on the raceway are loaded in the course of one revolution, the load on the ring is defined as a rotating load. Heavy oscillating loads such as apply to the outer rings of connecting rod bearings are generally considered as rotating loads.
If the bearing ring is stationary and the load is also stationary, or if the ring and load rotate at the same speed so that the load is always directed towards the same point on the raceway, the load on the ring is defined as a 'stationary load'.
Variable external loading, shock loading, vibrations
and out of balance forces in high speed machines, giving rise to changes in the direction of the load which cannot be accurately established, are classified under the term 'direction of load indeterminate'.
A bearing ring subjected to a rotating load will creep on its seating if mounted with a clearance fit, and wear of the contacting surfaces will occur (fretting corrosion). To prevent this an interference fit should be used. The degree of interference required is dictated by the operating conditions referred to below in the notes on internal clearance and temperature conditions.
A bearing ring subjected to a stationary load will not normally creep on its seating and an interference fit is not therefore necessary unless dictated by other requirements of the application.
When the direction of loading is indeterminate, and particularly where heavy loading is involved, it is desirable that both rings have an interference fit. For the inner ring the fit recommended for a rotating inner ring is normally used. However, when the outer ring must be axially free in its housing or if the loading is not heavy a somewhat looser fit than that recommended for rotating loads may be used.
Magnitude of the load - The load on a bearing inner ring causes it to expand resulting in an easing of the fit on the seating; under the influence of a rotating load, creep may then develop. The amount of interference between the ring and its seating must therefore be related to the magnitude of the load: the heavier the load the greater the interference required.
Internal clearance - When bearing rings are mounted with an interference fit the bearing radial internal clearance is reduced because of the expansion of the inner ring and/or contraction of the outer ring. A certain minimum clearance should however remain. The initial clearance and permissible reduction depend on the type and size of bearing. The reduction in clearance due to the interference fit can be such that bearings with radial internal clearance greater than normal may be necessary.
Temperature conditions - In service, the bearing rings normally reach a higher temperature than the component parts to which they are fitted. This can result in an easing of the fit of the inner ring on its seating or alternatively the outer ring may expand and take up its clearance in the housing thereby limiting its axial freedom. Temperature differentials and the direction of heat flow must therefore be carefully considered in selecting fits.
Requirements regarding running accuracy - Where bearings are required to have a high degree of running accuracy, elastic deformation and vibration must be minimized and clearance fits avoided. Bearing seatings on shafts should be at least to tolerance IT5 and housing seatings to tolerance IT6. Accuracy of form (ovality and taper) is also very important and deviations from true form should be as small as possible.
Design and material of shaft and housing - The fit of the bearing ring on its seating must not lead to uneven distortion (out of round) of the bearing ring, which may for example be caused by surface irregularities of the seatings. Split housings are not suitable when outer rings are to have an interference fit and the limits of tolerance selected should not give a tighter fit than that obtained when tolerance groups H or J apply. To ensure adequate support for bearing rings mounted in thin walled housings, light alloy housings or on hollow shafts, heavier interference fits must be used than would normally be selected for think walled steel or cast iron housings or solid shafts.
Ease of mounting and dismounting - Bearings having clearance fits are preferred for many applications to facilitate installation and removal. When operating conditions necessitate the use of interference fits and ease of mounting and dismounting is also essential, separate bearings or bearings having a tapered bore and an adapter or withdrawal sleeve can often provide a solution.
Displacement of a non-locating bearing - When a non separable bearing is used at the non-locating position, it is necessary that under all conditions of operation one of the rings is free to move axially. This is ensured by using a clearance fit for that ring which carries a stationary load. Where for example, light alloy housings are used, it may sometimes be necessary to fit a hardened intermediate bush between the outer ring and the housing. If certain types of cylindrical roller bearings, or needle roller bearings are used at the non-locating position, then both inner and outer rings can be mounted with an interference fit.
Bearings with a tapered bore are often used to facilitate mounting and dismounting and in some cases this type of bearing may be considered essential to the application. They can be mounted either directly on to a tapered shaft, or by means of an externally tapered sleeve on to a cylindrical shaft.
The axial displacement of a bearing on its tapered seating determines the fit of the inner ring and special instructions relating to the reduction of clearance of bearings with a tapered bore must be observed. The fit of the outer ring in the housing is the same as that for bearings having a cylindrical bore. Adapter and withdrawal sleeves allow greater shaft tolerances to be used (h9 or h10). Errors of form (ovality and taper) of the shaft seating must, however, still be closely controlled (tolerance IT5 or IT7).
Tolerances for the bore and outside diameter of metric rolling bearings are internationally standardized. The desired fits are achieved by selecting suitable tolerances for the shaft and housing using the ISO tolerance system incorporated in BS 4500.
For any particular bearing, the manufacturer's catalogue should be consulted with regard to recommended fits because these must be related to the actual size of the bearings supplied.
Interference fits in general only provide sufficient resistance to axial movement of a bearing on its seating when no axial forces are to be transmitted and the only requirement is that lateral movement of the ring should be prevented. Positive axial location or locking is necessary in all other cases. To prevent axial movement in either direction of a locating bearing it must be located at both sides. When non separable bearings are used as non locating bearings, only one ring, that having the tighter fit, is axially located, the other ring must be free to move axially in relation to the shaft or housing.
Where the bearings are arranged so that axial location of the shaft is given by each bearing in one direction only it is sufficient for the rings to be located at one side only.
Methods of location (Fig. 28.21)
Bearings having interference fits are generally mounted against a shoulder on the shaft or in the housing. The inner ring is normally secured in place by means of a locknut and locking washer (a), or by an end plate
(j) (k) Fig. 28.21 Bearing location methods
(j) (k) Fig. 28.21 Bearing location methods attached by set screws to the shaft end (b). The outer ring is normally retained by the housing end cover (c), but a threaded ring screwed into the housing bore is sometimes used (d).
Instead of shaft or housing abutment shoulders, it is frequently convenient to use spacing sleeves or collars between the bearing rings (e), or a bearing ring and the adjacent component, e.g. a gear (f). On shafts, location can also be achieved using a split collar which seats in a groove in the shaft and is retained by either a solid outer ring which can be slid over it, or by the inner ring of the bearing itself.
Axial location of rolling bearings by means of snap rings can save space, assist rapid mounting and dismounting and simplify machining of shaft and housings. An abutment collar should be inserted between the snap ring and the bearing if heavy loads have to be carried, in order that the snap ring is not subjected to large bending moments across its section. If required, the axial clearance, which is generally present between the snap ring and the snap ring groove can be reduced by selecting an abutment collar of suitable width or by using shims. Deep groove ball bearings with a snap ring groove in the outer ring and fitted with a snap ring sometimes provide a simplified and compact housing arrangement.
Bearings with a tapered bore mounted directly on tapered shafts are usually retained by a locknut on the shaft (g), or the locknut may be screwed on to an externally threaded split ring inserted into a groove in the shaft (h). With adapter sleeve mounting, the locknut positions the bearing relative to the sleeve ( j). When bearings with an adaptor sleeve are mounted on shafts without an abutment shoulder, the axial load which can be applied depends on the resulting friction between shaft and sleeve. When bearings with a tapered bore are mounted on withdrawal sleeves the inner ring of the bearing must be mounted against an abutment (k). A suitable abutment can be provided by a collar which can frequently serve as part of a labyrinth seal. The withdrawal sleeve must be secured in position either by means of a locknut or an end plate and set screws.
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