FEATURE BEARINGS Cylindrical roller bearings run in
raceways that are rectangular in cross-section. The roller length to diameter ratio is limited in order to avoid any tendency to skew in the raceways, and the bearing geometry can be designed to allow a degree of axial displacement. Bearings with a cage can accommodate heavy radial loads, rapid accelerations and high speeds, and are often used in railway axleboxes and traction motors, as well as indexing units for continuous furnaces. Spherical roller bearings have two rows
of rollers, a common spherically profiled outer raceway and two inner raceways inclined at an angle to the bearing axis. The centre point of the sphere formed by the outer raceway is at the bearing axis. This geometry ensures that the bearings are self-aligning and insensitive to misalignment of the shaft relative to the bearing housing. Spherical roller bearings are designed to accommodate heavy radial loads, as well as heavy axial loads in both directions, and can be found in many heavy-duty applications that are subject to misalignment, such as wind turbines, off-road construction equipment and large industrial pumps. Tapered roller bearings have
tapered inner and outer raceways and tapered rollers. They are designed to accommodate combined loads such as simultaneously acting radial and axial loads. The axial load carrying capacity of tapered roller bearings increases in proportion to the angle of contact of the rolling element with respect to the bearing’s rotational axis. The contact angle for most tapered roller bearings is between 10 and 16˚; however, if a higher axial load capacity is required, this can be as much as 30˚. Needle roller bearings have cylindrical
rollers that are small in diameter relative to their length, the ends of the rollers being relieved slightly to modify the line contact between the rollers and raceways. This prevents stress peaks at the roller ends, thus prolonging bearing service life. In spite of their small cross
section, needle roller bearings have a high load carrying capacity and are suitable for bearing arrangements where the radial space is limited. Novel hybrid geometries that
combine the self-aligning capability of spherical roller bearings with the axial displacement ability of cylindrical roller bearings – so-called ‘toroidal’ bearings - are also available. SKF’s toroidal roller bearings are marketed under the name CARB, and this type is typically used to reduce noise and vibration in paper machines and fans.
PLAIN BEARINGS A plain bearing has no rolling elements. Instead, it encloses the supported shaft within a cylindrical sleeve or bushing made from a suitable low-friction composite or special construction that delivers a slow-release lubricant. While most designs are intolerant of misalignment, a more interesting configuration is the spherical plain bearing. This offers an alternative to spherical roller bearings which are generally designed for continuous rotation and can only accommodate limited misalignment. Spherical plain bearings are suitable for oscillating movements and can accommodate more extremes of misalignment. Available in a variety of sizes and load
carrying capacities, smaller spherical plain bearings are often incorporated
Spherical plain bearings are often incorporated into rod ends – articulating joints frequently used in automotive and aerospace applications as well as a host of industrial machinery
rollers) whose inner and outer raceway diameters are many times greater than its sectional height or profile. Slewing bearings can accommodate axial, radial and moment loads acting either singly or in combination, and in any direction. They can perform both slewing as well as rotational movements and, as such, were originally developed to support horizontally mounted rotating turret type structures such as
“While slewing bearings are frequently used in heavy machinery such as cranes and tunnel boring machines, they are
now increasingly to be found supporting the yaw mechanisms of wind turbines, in solar tracking systems (solar mirror arrays, for example) and large industrial indexing platens”
into rod ends – articulating joints frequently used in automotive and aerospace applications as well as a host of industrial machinery requiring rotation in combination with a smooth oscillating motion. While this type of bearing is often
referred to as being ‘maintenance-free’, it should be noted that the self-lubricating, dry sliding materials of plain bearings are not as stiff as steel and consequently are subject to greater deformation under load. They are also more sensitive than steel to alternating or shock loads.
SLEWING BEARINGS A slewing (oscillating)
bearing is essentially a rolling element bearing (equipped with balls or
12 SEPTEMBER 2018 | DESIGN SOLUTIONS
Left: INSOCOAT bearings are designed to prevent electric current from passing through the bearing
naval artillery. Nowadays, however, they can also be deployed in vertically orientated applications. While slewing bearings are frequently
used in heavy machinery such as cranes and tunnel boring machines, they are now increasingly to be found supporting the yaw mechanisms of wind turbines, in solar tracking systems (solar mirror arrays, for example) and large industrial indexing platens. Readers wishing to learn more about
these bearing types and their numerous applications are reminded that all of them are to be found within SKF’s extensive bearings portfolio, and supported by SKF experts who can offer advice on selection, mounting and maintenance.
SKF T: 01582 490049
www.skf.co.uk
/ DESIGNSOLUTIONS
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