Table of contents
Criteria for bearing selection
Features of individual bearing types
Schaeffler offers an extensive range of bearing types
Schaeffler produces a multitude of bearing types, from which the designer can choose the one that best suits his needs. Due to the variety of applications and the different influences on the bearing position, it is not, however, possible to give generally applicable rules here for the selection of the bearing type. When deciding on a specific bearing type, it is often necessary to weigh up several criteria.
Factors that are usually important in the selection of bearings
The aim of the following descriptions is to assist the designer. They make reference to factors that play a primary role in the selection of the bearing type. More precise data on the individual bearing types, their characteristics and the available designs can be found in the specific product chapters.
Criteria for selection of bearing type
Product characteristics that can play a role in selection of the bearing type are, for example:
- the available design envelope ➤ link
- the load values, load directions and the bearing arrangement concept (arrangement of the bearings) ➤ link
- the speeds of the bearings ➤ link
- the accuracy of the bearings ➤ link
- the axial displacement capability of the bearing rings (non-locating bearing function) ➤ link
- the compensation of misalignments ➤ link
- quiet running ➤ link
- the rigidity of the bearing arrangement ➤ link
- the friction in the bearings ➤ link
- the mounting and dismounting of the bearings ➤ link
- the sealing of the bearings and of the bearing position
- the lubrication of the bearings
Comparison of design envelope for particular bearing types
In many applications, it is frequently the case that one of the main dimensions of the rolling bearing is fixed by the overall design of the machine or piece of equipment. Due to the strength specifications of the shaft, this is generally the bore diameter of the bearing.
Bearings for small and large shaft diameters
For small shaft diameters, suitable bearings are ball bearings – especially deep groove ball bearings – and needle roller bearings. Where large diameters are present, the bearings available are cylindrical, tapered, spherical and toroidal roller bearings, as well as deep groove ball bearings, but not excluding needle roller bearings.
Comparison of cylindrical roller bearings
A load carrying capacity of approximately the same magnitude can be achieved both with bearings that are narrow and have a large outside diameter and also with bearings that are wide and have a smaller outside diameter.
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Cylindrical roller bearings with approximately comparable basic load rating C0 SL183026 has a full complement roller set |
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Bearings with small cross-sectional height
Low cross-sectional height and high load carrying capacity
If the design envelope is restricted in a radial direction – for example in high performance gearboxes for vehicles – bearings with a small cross-sectional height such as roller and needle roller and cage assemblies, drawn cup needle roller bearings with or without an inner ring, are particularly suitable ➤ Figure. These bearings have high radial load carrying capacity due to the line contact with low section height.
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Bearings with small cross-sectional height – comparison of radial section height
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Bearings with small axial design envelope and combined load
Bearings for small axial design envelope and combined load
If the design envelope is small in an axial direction, the bearings suitable for bearing positions subjected to radial and axial load are certain series of cylindrical roller bearings – e. g. semi-locating bearings NJ in conjunction with deep groove ball bearings or combined needle roller bearings ➤ Figure and ➤ Figure.
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Semi-locating bearing for supporting axial forces in one direction in conjunction with a deep groove ball bearing
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Combined needle roller bearings for supporting high radial forces and axial forces in one or both directions
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Bearings with pure axial load carrying capacity
Bearings for bearing arrangements under axial load only
If the bearing arrangement is under axial load only, suitable bearings are axial needle roller and axial cylindrical roller and cage assemblies, axial needle roller and axial cylindrical roller bearings and axial deep groove ball bearings with a small axial section height ➤ Figure.
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Axial bearings – comparison of cross-sections
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Load value and load directions
Influence of load on bearing type and bearing size
The load determines the type and size of the bearing
The bearing size is normally determined by the external load ➤ Figure. In selection of the bearing, it must be noted that roller bearings can be subjected to higher loads due to the line contact than ball bearings of the same size.
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Radial load carrying capacity – comparison of deep groove ball bearing/cylindrical roller bearing
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Bearings for predominantly radial loads
In the case of radial loads, the forces act perpendicular to the bearing axis
If radial loads (so-called transverse forces) are present – these are forces perpendicular to the longitudinal axis of the shaft – bearings are used that support exclusively or predominantly radial forces, which are therefore radial ball bearings and radial roller bearings.
Cylindrical roller bearings N and NU, needle roller and cage assemblies, drawn cup needle roller bearings with open ends, needle roller bearings and toroidal roller bearings can support radial forces only ➤ Figure.
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Radial bearings, bearings for predominantly radial load
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Bearings for predominantly axial loads
If mainly axial forces are present – these are forces in the direction of the longitudinal axis of the shaft – bearings are used that support exclusively or predominantly axial forces.
Low to moderate axial loads
If low to moderate pure axial loads are present, suitable bearings are axial deep groove ball bearings and four point contact bearings ➤ Figure. If axial load in one direction is present, single direction axial deep groove ball bearings can be used.
Moderate to high axial loads
If moderate to high axial loads in one direction are present, the bearings available are axial needle roller bearings, axial cylindrical roller bearings, axial tapered roller bearings and axial spherical roller bearings ➤ Figure. Axial cylindrical roller bearings or axial spherical roller bearings arranged adjacent to each other in pairs are suitable for high axial loads of alternating direction.
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Bearings for predominantly axial load
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Bearings for combined loads
Certain rolling bearings can be subjected to combined loads (radial and axial).
The axial load carrying capacity of a bearing is determined by the nominal contact angle α. The greater this angle, the higher the axial load carrying capacity of the bearing. An indication of its axial load carrying capacity is also provided by the bearing-specific axial load factor Y in the product tables; the smaller this factor, the higher the axial load capacity.
Suitable bearings for combined load
Suitable bearings are deep groove ball bearings, four point contact bearings, single and double row angular contact ball bearings, spherical roller bearings and single row tapered roller bearings ➤ Figure. Self-aligning ball bearings and cylindrical roller bearings NJ (semi-locating bearing) and NU (locating bearing) + L-section ring HJ (= semi-locating bearing unit) ➤ Figure can also be used.
Axial loads present in one direction only can be supported by single row angular contact ball bearings and tapered roller bearings, cylindrical roller bearings NJ as well as NU + HJand axial spherical roller bearings. For alternating load directions, these bearings must always be mounted with a second bearing (the second bearing must provide support in the opposing direction). For mounting in sets, single row angular contact ball bearings as universal bearings and matched tapered roller bearing sets comprising two single bearings are available.
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Bearings for combined load
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Bearings for combined load
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The axial load can also be supported by means of a separate bearing
If the axial load component is too high, the axial load can also be supported by means of a separate bearing. In addition to a pure axial bearing, radial bearings – e. g. deep groove ball bearings and cylindrical roller bearings ➤ Figure or four point contact bearings capable of supporting axial forces in both directions – can be used.
If the four point contact bearing is to be used as an axial bearing only, the outer ring must have radial clearance in the housing.
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Cylindrical roller bearing and four point contact bearing for supporting combined load
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Bearings for supporting tilting moments
Moment load under eccentric force application
If a load acts eccentrically, the bearing is subjected to tilting moments. In addition to double row deep groove ball bearings and angular contact ball bearings, suitable bearings in this case are in particular the single row angular contact ball bearings or tapered roller bearings matched in and X or O arrangement ➤ Figure.
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Bearings for supporting tilting moments M = tilting moment
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Double row deep groove ball bearing
In ➤ Figure, the tilting moment produced by the eccentrically acting force F is supported by a double row deep groove ball bearing.
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Unilateral bearing arrangement of shaft M = tilting moment F = eccentrically acting radial force
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Speed suitability
Bearings for high and very high speeds
The permissible speed is restricted by the operating temperature
In general, the highest speed achievable by rolling bearings is determined by the permissible operating temperature. This is dependent on the frictional heat generated in the bearing, the heat introduced from external sources and the heat dissipated from the bearing arrangement. When conditions are in equilibrium, the bearing temperature is constant.
Bearings for very high speeds
The highest speeds are achieved by single row bearings with particularly low friction. Under pure radial load, these are open deep groove ball bearings, while under combined load they are angular contact ball bearings ➤ Figure.
Due to their design, the permissible speeds for axial bearings are generally lower than those for radial bearings.
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Bearings for high and very high speeds
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Accuracy
Bearings with increased accuracy
For most applications, the normal dimensional and running accuracy of rolling bearings is sufficient (tolerance class Normal). Where there are increased demands on running accuracy and in bearing arrangements with very high speeds, such as in the case of main spindles for machine tools, bearings with increased accuracy are necessary.
Axial displacement facility
Compensation within the bearing
Axial displacement occurs within the bearing
A shaft is normally supported using a locating bearing and a non-locating bearing. The locating bearing gives tight axial guidance of the shaft in both directions. Non-locating bearings can be displaced in an axial direction and thus prevent the locating bearing and non-locating bearing from bracing against each other. In this way, non-locating bearings compensate for changes in axial length and thermal elongation ➤ Figure.
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Cylindrical roller bearing NU, non‑locating bearing, length compensation within bearing s = axial displacement distance |
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Suitable non-locating bearings for compensation within the bearing
Bearings suitable as non-locating bearings for compensations with the bearing include, in particular, cylindrical roller bearings NU and N with one ribless ring (inner or outer ring), cylindrical roller bearings NJ, some designs of full complement cylindrical roller bearings (SI bearings), needle roller bearings and toroidal roller bearings ➤ Figure.
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Non-locating bearings for compensation within bearing
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Compensation by means of sliding seat on a bearing ring
Non-separable bearings such as deep groove ball bearings and spherical roller bearings can also be used as non-locating bearings. However, one of the two bearing rings must then have a loose fit and must not be in axial contact, so that the relevant ring can be displaced on the seating surface ➤ Figure.
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Length compensation by means of sliding seat s = axial displacement distance (loosely fitted bearing ring)
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Compensation of misalignments
Skewing has a negative effect on bearing function and reduces the bearing operating life
If skewing occurs between the shaft and housing – for example if bearing seats are not aligned, the shaft deforms under load or the bearing positions are a considerable distance apart – this must be compensated by means of suitable bearings (self-aligning bearings). Misalignments should also be expected if individual housings, such as plummer block or flanged housings, are used. Similar effects (alignment inaccuracies of the bearing positions) are caused by angular defects between the radial seating surface and the lateral contact surface of a rolling bearing ring.
Self-aligning rolling bearings
Dynamic and static compensation of misalignments and angular defects
Misalignments and angular defects can be compensated within certain limits by means of self-aligning rolling bearings (see product chapter). These include bearings such as barrel roller bearings, spherical roller bearings, self-aligning ball bearings, toroidal roller bearings and axial spherical roller bearings ➤ Figure. These bearings have a concave outer ring raceway in which the inner ring together with the rolling element set can swivel.
The rings may only be swivelled while being simultaneously rotated, otherwise the raceways will be damaged.
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Bearings for static and dynamic adjustment motions
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Bearings for compensation of static angular defects
Radial insert ball bearings and aligning needle roller bearings are suitable for static adjustment motions
Radial insert ball bearings and aligning needle roller bearings have a crowned (spherical) outer ring outside surface and can align themselves on the concave mating surface after mounting ➤ Figure. These bearings are suitable for static adjustment motions. They must not, however, be used for dynamic adjustment motions, oscillating motions etc.
Axial deep groove ball bearings
Axial deep groove ball bearings cannot support tumbling motion of the shaft and therefore react with high sensitivity to angular defects.
Bearings with spherical housing locating washer
If the contact surface in the housing is not perpendicular to the rotational axis of the bearing, the angular defect can be compensated by means of axial deep groove ball bearings with a spherical housing locating washer and support washer ➤ Figure.
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Bearings for static angular defects (adjustment motions)
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Self-alignment facility of deep groove ball bearings
Single row deep groove ball bearings have only a small self-alignment facility
In deep groove ball bearings, misalignments lead to unfavourable running of the balls and induce additional loads in the bearing that shorten the operating life of the bearing. In order to keep these loads at a low level, only small adjustment angles are therefore permitted as a function of the load for single row deep groove ball bearings.
Double row deep groove ball bearings cannot undergo angular motion. When using these bearings, therefore, misalignments are not permissible.
Self-alignment facility of cylindrical roller bearings, tapered roller bearings, needle roller bearings
The self-alignment facility is smaller than in the case of deep groove ball bearings
The self-alignment facility of cylindrical, tapered and needle roller bearings is smaller than that of deep groove ball bearings. The transverse form of the rolling elements and raceways of these roller bearings is designed such that, at the stated adjustment angles, the load on the material at the rolling contact is still uniform enough that the basic rating life is not impaired.
Where angular defects are larger than those indicated in the product chapters, rollers and needle rollers are no longer subjected uniformly to load over their full length. As a result, unacceptably high edge pressures may occur.
Quiet running
Deep groove ball bearings have very low running noise
In small electrical devices, such as office equipment or household appliances, low running noise is often required. Deep groove ball bearings are particularly suitable for such requirements ➤ Figure. A noise evaluation of the series is permitted by means of the Schaeffler Noise Index ➤ link. It is advantageous to apply axial adjustment to the bearings, for example by means of disc springs.
Rigidity
Roller bearings are more rigid than ball bearings
The rigidity of a rolling bearing is determined by the type, bearing size and operating clearance. The rigidity increases with the number of rolling elements supporting the load. A particularly high level of system rigidity is the objective, for example in main spindle bearing arrangements and rotary table bearing arrangements in machine tools and in pinion bearing arrangements.
The rigidity of roller bearings is higher, due to the contact conditions between the rolling elements and raceways (line contact), than the rigidity of ball bearings.
Friction
Bearings with low bearing friction
In addition to the introduction and dissipation of heat, the decisive factor for the operating temperature of a bearing arrangement is bearing friction. Bearings with particularly low friction include open deep groove ball bearings, single row angular contact ball bearings and cylindrical roller bearings with cage under radial load ➤ Figure and ➤ Figure. Comparatively high friction is generated in bearings with contact seals, full complement cylindrical roller bearings and axial roller bearings.
Mounting and dismounting
Separable (non self-retaining) and non-separable bearings
In the case of separable bearings, the bearing rings can be mounted independently of each other
Bearings that are not self-retaining simplify the mounting and dismounting of bearings, since the bearing parts can be mounted individually. This is also an advantage if both rings have a tight fit. Separable bearings include four point contact bearings, double row angular contact ball bearings with a split inner ring, certain cylindrical roller bearings, tapered roller bearings, axial deep groove ball bearings, axial spherical roller bearings, axial cylindrical roller bearings and certain needle roller bearings ➤ Figure.
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Separable bearings
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Non-separable bearings
Deep groove ball bearings, single row angular contact ball bearings, self‑aligning ball bearings, barrel roller bearings and spherical roller bearings are not generally separable.
Bearings with tapered bore
With a tapered shaft seat, the radial internal clearance in the bearing can be set to a defined value
Bearings with a tapered bore can be mounted directly on a tapered shaft seat, for example in the case of single row and double row cylindrical roller bearings of a high precision variant. During mounting of these bearings, the radial internal clearance can be set to a defined value.
Mounting and dismounting of bearings can be aided by means of adapter and withdrawal sleeves
Where the requirements for running accuracy are low, adapter and extraction sleeves can be used to locate self-aligning ball bearings, barrel roller bearings and spherical roller bearings with a tapered bore on a cylindrical shaft seat in particular ➤ Figure. The mounting and dismounting of such bearing arrangements is particularly simple.
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Bearings with tapered bore, adapter and withdrawal sleeve
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