FEATURE BEARINGS
ROLLER BEARINGS GO BACK TO BASICS
Phil Burge, country communications manager at SKF, explains the basic functions and advantages of roller bearings and the applications in which they are typically used
touching one another. While this arrangement tolerates high radial loads, it does not have a high axial load capacity. So-called ‘full complement’ cylindrical roller bearings, which carry the maximum number of rollers for a given size of bearing, are designed without a cage. Most roller bearings can work at high-speed. However, because of their higher load capacities, larger bearings tend to be used in low-speed, high load applications, such as wind turbines and steel rolling mills. A number of factors contribute to a
roller bearing’s performance, such as the geometry of the rollers, raceways and cages, the materials of construction, their heat treatment, and the surface finish of all contact surfaces.
R
oller bearings, or something like them, were probably first used
thousands of years ago – rolling logs were used to transport heavy objects over great distances. The first known instance of a modern roller bearing was invented by British clockmaker John Harrison in the 18th century, for his H3 marine timekeeper. Nowadays, the rolling element of a roller bearing takes one of four distinct forms: A cylinder, a barrel (spherically profiled element), a tapered cylinder (cone-shaped element), or a very thin cylinder (needle).
LOAD DISTRIBUTION An important aspect of the roller bearing design is that the contact between the inner and outer raceways is not a point - as it is with a ball bearing - but a line. This distributes the load more evenly over the raceways, allowing the bearing to handle greater loads than a ball bearing of similar size. The configuration normally includes a
specially shaped cage between the inner and outer raceways. This retains the rolling elements and prevents them from
12 FEBRUARY 2017 | DESIGN SOLUTIONS
Cylindrical roller bearings run in raceways with a rectangular cross-section
DESIGN CHOICE The type of rolling element chosen will be determined by the application, its tolerances (particularly misalignment) and the static and dynamic forces acting on the bearing. Cylindrical roller bearings run in
raceways with a rectangular cross- section. The ratio of roller length to diameter is limited, in order to avoid any tendency to skew in the raceways. Also, the bearing can be designed to allow a degree of axial displacement. Bearings with a cage can accommodate heavy radial loads, as well as rapid accelerations and high speeds. Full complement bearings, that incorporate the maximum number of rollers, are suitable for very heavy radial loads at moderate speeds. High-capacity cylindrical roller bearings combine the high load carrying capacity of a full complement bearing, with the high-speed capability of a bearing with a cage. They are often used in railway axleboxes and in railway traction motors, as well as indexing roller units for continuous furnaces. Needle roller bearings have cylindrical
rollers that are small in diameter relative to their length. The ends of the rollers
are relieved slightly to modify the line contact between the rollers and raceways. This prevents stress peaks at the roller ends to prolong service life. In spite of their small cross section, needle roller bearings can carry high loads and are suitable for bearing arrangements where the radial space is limited.
COMBINED LOADS Tapered roller bearings have tapered inner and outer raceways, and tapered rollers. They are designed for combined loads such as simultaneously acting radial and axial loads. Their axial load- carrying capacity increases in proportion to the angle of contact of the rolling element, with respect to the bearing’s rotational axis. The contact angle for tapered roller bearings is typically 10- 16°, but if a higher axial load capacity is required, it can be as much as 30°. Single row tapered roller bearings are
separable, in that the inner ring with roller and cage assembly (the cone) can be mounted separately from the outer ring (the cup). 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 these bearings are self-aligning and can withstand misalignment of the shaft relative to the bearing housing. Spherical roller bearings can take heavy radial loads, as well as heavy axial loads in both directions. They are used in many heavy- duty applications that experience misalignment, such as wind turbines, off-road construction equipment and large industrial pumps. All in all, we have come a very long way from rolling logs and from an 18th century clockmaker.
SKF T: 01582 490049
www.skf.co.uk
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