Often the boundary dimensions of a bearing are predetermined by the machine’s design. Typically, the shaft diameter determines the bearing bore diameter. For the same bore diameter, different outside diameters and widths may be available (fig. 1). The availability of bearings in a certain ISO dimension series depends on bearing type and bore diameter.
Other space-related criteria that influence the selection of bearing type include:
shafts with small diameter
(approx. d < 10 mm)
- deep groove ball bearings
- needle roller bearings
- self-aligning ball bearings
thrust ball bearings
shafts with normal diameter
- all bearing types
very limited radial space
- needle roller bearings
- deep groove ball bearings in the 618 or 619 series
CARB toroidal roller bearings in the C49, C59 or C69 series
- bearings without inner or outer ring and raceways machined directly on the shaft or in the housing
When selecting bearing type based on load criteria, you should bear in mind that:
- Roller bearings accommodate heavier loads than same-sized ball bearings.
- Full complement bearings accommodate heavier loads than the corresponding bearing with a cage.
Matrix 1 [PDF] provides an overview of the radial, axial and moment load capability of various bearing types.
Combined radial and axial loads
The direction of load is a primary factor in bearing type selection. Where the load on a bearing is a combination of radial and axial load, the ratio of the components determines the direction of the combined load (fig. 2).
The suitability of a bearing for a certain direction of load corresponds to its contact angle α (diagram 1) – the greater the contact angle, the higher the axial load carrying capacity of the bearing. You can see this indicated in the value of the calculation factor Y (→ individual product sections), which decreases as the contact angle increases. ISO defines bearings with contact angles ≤ 45° as radial bearings, and the others as thrust bearings, independent of their actual use.
To accommodate combined loads with a light axial component, bearings with a small contact angle can be used. Deep groove ball bearings are a common choice for light to moderate axial loads. With increasing axial load, a larger deep groove ball bearing (with higher axial load carrying capacity) can be used. For even higher axial load, bearings with a larger contact angle may be required, like angular contact ball bearings or tapered roller bearings. These bearing types can be arranged in tandem to accommodate high axial loads.
When combined loads have a large alternating axial load component, suitable solutions include:
- a pair of universally matchable angular contact ball bearings
- matched sets of tapered roller bearings
- double-row tapered roller bearings
Where a four-point contact ball bearing is used to accommodate the axial component of a radial load (fig. 3), the bearing outer ring must be mounted radially free and should not be clamped axially. Otherwise, the bearing may be subjected to unintended radial load.
The permissible operating temperature of rolling bearings imposes limits on the speed at which they can be operated. The operating temperature is determined, to a great extent, on the frictional heat generated in the bearing, except in machines where process heat is dominant.
Matrix 1 [PDF] provides an overview of the speed capability of various bearing types.
When selecting bearing type on the basis of operating speed, you should consider the following:
- Ball bearings have a lower frictional moment than same-sized roller bearings.
- Thrust bearings cannot accommodate speeds as high as same-sized radial bearings.
- Single row bearing types typically generate low frictional heat and are therefore more suitable for high-speed operation than double or multi-row bearings.
- Bearings with rolling elements made of ceramics (hybrid bearings) accommodate higher speeds than their all-steel equivalents.
Matrix 1 [PDF] provides an overview of the capability of various bearing types to accommodate misalignment. The different types of misalignment are explained in table 1.
Bearing types vary in their ability to compensate for misalignment between the shaft and housing:
Self-aligning bearings (fig. 4)
Self-aligning bearings can compensate for misalignment within the bearing. Values for the permissible misalignment are listed in the relevant product section.
Alignment bearings (fig. 5)
Alignment bearings can accommodate initial static misalignment because of their sphered outside surface. Values for the permissible misalignment are listed in the relevant product section.
Rigid bearings (deep groove ball bearings, angular contact ball bearings, cylindrical, needle and tapered roller bearings) accommodate misalignment within the limits of their internal clearance. Values for the permissible misalignment are listed in the relevant product section. For rigid bearings, any misalignment may reduce service life.
Precision requirements typically do not influence bearing type selection. Most SKF bearings are available in various tolerance classes. Details are provided in the product sections.
For very high precision requirements (e.g. machine tool applications), use SKF super-precision bearings.
The stiffness of a rolling bearing is characterized by the magnitude of the elastic deformation in the bearing under load and depends not only on bearing type, but also on bearing size and operating clearance.
When selecting bearing type on the basis of stiffness requirements you should consider, for bearings with the same size, that:
- stiffness is higher for roller than ball bearings
- stiffness is higher for full complement bearings than for the corresponding bearing with a cage
- stiffness is higher for hybrid bearings than for the corresponding all-steel bearing
- stiffness can be enhanced by applying a preload
When selecting bearing type, you should consider the mounting and dismounting requirements:
Is it required or beneficial to mount the inner and outer ring independently?
→ Select a separable bearing.
Is it required or beneficial to mount the bearing on a tapered seat or with a tapered sleeve?
→ Select a bearing with a tapered bore.
→ Consider using SKF ConCentra ball or roller bearing units.
(Ball bearing units, Roller bearing units)
Separable bearings are easier to mount and dismount, particularly if interference fits are required for both rings.
See matrix 1 [PDF] for separable bearing types.
Tapered boreBearings with a tapered bore can be mounted on a tapered shaft seat or mounted on a cylindrical shaft seat using an adapter or withdrawal sleeve (fig. 6). See matrix 1 [PDF] for bearing types available with tapered bore.
There are two reasons for sealing bearings or bearing arrangements:
- keeping the lubricant in the bearing, and avoiding pollution of adjacent components
- protecting the bearing from contamination, and prolonging bearing service life
Capped bearings (sealed bearings or bearings with shields) can provide cost-effective and space-saving solutions for many applications. Bearing types, for which integral sealing is available, are indicated in matrix 1 [PDF].
After determining your required bearing type, you may find it beneficial to select an appropriate bearing from our assortment of popular items, because they have a high level of availability and generally provide a cost-effective solution. Popular items are marked in the product tables with the symbol ►.
If a required bearing has an outside diameter D ≥ 420 mm, and is not marked as popular, then check its availability with SKF.
Capped (sealed bearings or bearings with shields) typically provide more cost-effective solutions than using external sealing. In addition to providing good sealing performance, these ready-greased bearings do not require initial grease fill.
Availability of standard housings and sleeves
Using standard housings and sleeves generally leads to more cost-effective bearing arrangements. Bearing types for which these standard components are available are indicated in matrix 1 [PDF].