A four-arm turnstile pivots on a turntable bearing to move steel coils in a strip metal manufacturing plant. Made by Pro/Eco Ltd., such turnstiles handle coils weighing up to 50,000 lb. Later the coils are slit, packaged, and transferred to shipping conveyors by turntables mounted on similar bearings. The inset shows a typical bearing with teeth on the outer race to help drive a turntable assembly.

To supply rotary motion, you're most likely to design a rotating assembly with standard rolling-element bearings mounted on each end of a shaft. Depending on the type, these bearings accommodate radial or thrust loads. However, such designs typically require mounting hardware such as spacers and clamp rings to secure the bearings, as well as gearing to drive the assembly. Equipment builders often have to adjust the bearing clearance or preload at assembly to attain the desired performance.

On the other hand, large diameter turntable bearings may be a better way to accomplish the rotating motion. These devices are best suited to rotary applications that require a slewing motion (clockwise or counter-clockwise) or continuous rotation at low speeds (bearing raceway velocities less than 700 ft/min).

Turning to an alternative

Large turntable bearings differ from conventional shaft-mounted types mainly in how they mount to equipment and how they carry loads. Available in 12-in. bore or larger sizes, they generally let you design a simpler configuration with savings in assembly and material costs. These bearings eliminate mounting components such as spacers and clamp rings. They usually come with mounting holes for bolting to a machine structure, which precludes the need for a shaft. Some of them incorporate an integral gear, eliminating the need for a separate one. Most assemblies operate in a horizontal position to support heavy machinery. However, they can accommodate vertical or inclined mountings as well.

For a slewing application, a conventional ball bearing with its own races must be secured between inner and outer race housings of the rotating assembly by clamp rings. Angular contact ball bearings also require spacers (not shown). Moreover, separate gear sets must be added to drive the assembly.
Select figure to enlarge.

A single turntable bearing can handle both radial and thrust loads, as well as overturning moments resulting from off-center loads. This eliminates the need for two or more conventional bearings to accomplish the same task. Most versions carry more load than a shaft-mounted bearing because they have larger diameter balls or rollers within a given design envelope.

These large bearings are usually custom designed for an application. Tolerances vary with the configuration, in contrast to tolerances for conventional bearings, which are established by the American Bearing Manufacturers Association (ABMA). The bearing clearance or preload, as required by the application, is built in during manufacture. This eliminates adjustment at final assembly. Unlike conventional bearings, spacer rings and "match grinding" to produce the required clearance or preload are not required.

The basic types include fourpoint- contact ball bearings, crossed-roller bearings, and three-row roller bearings. Each one is tailored to fit different applications.

Four-point-contact ball

Most turntable bearings contain balls as the rolling elements, sandwiched between inner and outer races. Each ball contacts the raceway at two points rather than over a full contact radius. This arrangement maximizes load capacity and minimizes ball skidding under heavy load. The contact angle between ball and race, measured from the radial (horizontal) centerline of the ball, ranges from 35 to 60 deg, depending on the application and its particular combination of loads. For example, an indexing table with high overturning moment loads normally uses a bearing with a 60-deg contact angle. Conversely, a tool holder with a significant radial loading and minimal overturning moment loads most likely would use a 35-deg angle.

Common uses for the ball bearings include machines in what are called static applications -- repeated slewing movements of less than 360 deg. Examples include welding positioners, fork lift rotators, steering gears, indexing tables, steel coil turnstiles, conveyors, medical equipment, and overhead cranes.

Crossed roller

Other bearings depend on cylindrical rolling elements to transmit loads. In the crossed-roller type, the rollers are inclined 45 deg to the bearing centerline in alternating fashion (first roller inclined to the left, second one inclined to the right, etc.). In this arrangement, called a one-to-one configuration, half of the rollers transmit thrust forces in one direction and half in the other. Applications with thrust loads primarily in one direction call for rollers oriented in a two-to-one or three-to-one configuration (more rollers on one side) to increase thrust load capacity.

For a given bore size, a crossedroller bearing has slightly less static capacity than a four-point contact ball, yet more dynamic capacity. A crossed-roller bearing has more stiffness, especially vital in dynamic applications (continuous 360-deg rotation) where precise positioning calls for minimal deflection under load. Examples include machine tools, polishing machines, indexing tables, robots, telescopes, radar, and optical equipment.

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