The second step of SKF bearing selection is to select bearing type and arrangement. Each SKF bearing type has characteristic properties that make it more or less suitable for use in a given application. This section provides information on what to consider when selecting a bearing arrangement and the types of bearing to use with it. It also provides guidelines on choosing bearing types to satisfy specific demands of an application, such as accommodating available space, loads, misalignment, and more.
Arrangement and SKF bearing types
A SKF bearing arrangement supports and locates a shaft, radially and axially, relative to other components such as housings. Typically, two bearing supports are required to position a shaft. Depending on certain requirements, such as stiffness or load directions, a bearing support may consist of one or more bearings.
SKF bearing arrangements comprising two bearing supports are:
- locating/non-locating bearing arrangements
- adjusted bearing arrangements
- floating bearing arrangements
"Suitability of SKF rolling bearings for industrial applications" provides an overview of the suitability of various bearing types for different bearing arrangements.
A SKF single bearing arrangement consists of just one bearing that supports radial, axial and moment loads.
Locating/non-locating SKF bearing arrangements
In locating/non-locating bearing arrangements:
- The locating support provides axial location of the shaft relative to the housing.
- The non-locating support accommodates axial displacements that occur when thermal expansion of the shaft relative to the housing changes the distance between the two SKF bearings. Additionally, it compensates for the accumulation of tolerances of the components, which affects the distance between the two SKF bearings.
SKF bearings for the locating support
Radial bearings that can accommodate combined (radial and axial) loads are used for the locating bearing support. These include:
- deep groove ball bearings
- two universally matchable single row angular contact ball bearings, arranged back-to-back or face-to-face
- double row angular contact ball bearings
- self-aligning ball bearings
- spherical roller bearings
- matched tapered roller bearings, arranged back-to-back or face-to-face
- cylindrical roller bearings with flanges on both rings or cylindrical roller bearings mounted with an angle ring (thrust collar)
SKF bearing combinations for the locating support
The locating bearing support can consist of a combination of bearings.
- To accommodate the radial load, a cylindrical roller bearing that has one ring without flanges may be used.
- To provide the axial location, deep groove ball bearing, a four-point contact ball bearing, or a pair of angular contact ball bearings may be used.
The outer ring of the axial locating bearing must be mounted radially free and should not be clamped. Otherwise, this SKF bearing can be subjected to unintended radial loads.
SKF bearings for the non-locating support
There are two ways to accommodate axial displacements at the non-locating bearing support:
1, Use a bearing type that enables axial displacement within the bearing:
cylindrical roller bearings with flanges on one ring only
needle roller bearings
CARB toroidal roller bearings
When these bearings are rotating, they accommodate axial displacement and induce almost no axial load on the bearing arrangement. You should use this solution where an interference fit is required for both rings.
2, Use a loose fit between one bearing ring and its seat. Suitable bearing types include:
deep groove ball bearings
self-aligning ball bearings
spherical roller bearings
pairs of angular contact ball bearings or tapered roller bearings
Axial movements of a bearing on its seat cause axial loads, which might have an impact on the bearing service life.
When using other bearing types, you may need to take additional design considerations into account.
Typical combinations of SKF bearing supports
From the large number of possible locating/non-locating bearing combinations, the following are the most popular.
Conventional bearing arrangements in which limited angular misalignment occurs include:
- deep groove ball bearing / cylindrical roller bearing
- double row angular contact ball bearing / NU or N design cylindrical roller bearing
- matched single row tapered roller bearings / NU or N design cylindrical roller bearing
- NUP design cylindrical roller bearing / NU design cylindrical roller bearing
- NU design cylindrical roller bearing and a four-point contact ball bearing / NU design cylindrical roller bearing
SKF self-aligning bearing systems, which can compensate for more misalignment, are:
- spherical roller bearing / CARB toroidal roller bearing
- self-aligning ball bearing / CARB toroidal roller bearing
For SKF bearing arrangements where the axial displacement is accommodated between a bearing ring and its seat
- deep groove ball bearing / deep groove ball bearing
- self-aligning ball bearings or spherical roller bearings for both bearing positions
- matched single row angular contact ball bearings / deep groove ball bearing
Adjusted bearing arrangements
In adjusted bearing arrangements, the shaft is located axially in one direction by one bearing support and in the opposite direction by the other (cross-located). Adjusted SKF bearing arrangements require proper adjustment of clearance or preload during mounting.
These bearing arrangements are generally used for short shafts, where thermal expansion has only a little effect. The most suitable bearings are:
- angular contact ball bearings
- tapered roller bearings
Floating bearing arrangements
In floating bearing arrangements the shaft is cross-located, but is able to move axially a certain distance between the two end positions (i.e. “float”). When determining the required "float" distance , consider thermal expansion of the shaft relative to the housing and tolerances of the components, which affect the distance between the two bearings.
With this arrangement, the shaft can also be axially located by other components on the shaft (e.g. a double helical gear). Most common bearings are:
- deep groove ball bearings
- self-aligning ball bearings
- spherical roller bearings
- NJ design cylindrical roller bearings, mirrored, with offset rings
Selection criteria
Available space
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. 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
- all bearing types
- 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
Loads
When selecting SKF 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.
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.
- Cylindrical roller thrust bearing
- Needle roller thrust bearing
- Thrust ball bearing
- Angular contact thrust ball bearing
- Spherical roller thrust bearing
- Single row angular contact ball bearing
- Four-point contact ball bearing
- Double row angular contact ball bearing
- Self-aligning ball bearing
-
Deep groove ball bearing
The contact angle depends on load and clearance. - Tapered roller bearing
- Spherical roller bearing
- Needle roller bearing
- CARB toroidal roller bearing
- Cylindrical roller bearing
The suitability of a bearing for a certain direction of load corresponds to its contact angle α – 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, 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, 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.
Speed and friction
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.
When selecting bearing type on the basis of operating speed, you should consider the following:
- SKF 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.
Misalignment
SKF bearing types vary in their ability to compensate for misalignment between the shaft and housing:
Self-aligning bearings
SKF Self-aligning bearings can compensate for misalignment within the bearing. Values for the permissible misalignment are listed in the relevant product section.
Alignment bearings
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
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.
Temperature
The permissible operating temperature of rolling bearings can be limited by:
- the dimensional stability of the bearing rings and rolling elements
- the cage
- the seals
- the lubricant
Precision
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.
Stiffness
The stiffness of a SKF 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 SKF 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
Mounting and dismounting
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.
Separable bearings
Separable bearings are easier to mount and dismount, particularly if interference fits are required for both rings.
Integral sealing
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.
Cost and availability
Popular items
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.
Capped bearings
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.