Machinedesign 1539 Four Arm Turnstile Pivots1098 0 0

Turn the tables on ROTARY MOTION

Oct. 1, 2000
Large diameter turntable bearings offer a simpler way to deliver low-speed reversing motion

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.

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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Three-row roller

This type of bearing has two rows of rollers oriented perpendicular to the bearing centerline to handle thrust and moment loads, and one row parallel to the centerline to transmit radial forces. With this arrangement, the bearing has a higher static and dynamic capacity than the other two types. Because the rollers are perpendicular to each other, it offers higher stiffness as well. Three-row bearings work well in applications that require precise positioning such as machine tools, precision turntables, and communication antennae.

Common features

Virtually all turntable bearings are made from high carbon steel such as AISI 4150, which can be case hardened to a substantial depth. After machining, the bearing raceways are induction hardened to a range of about Rc 58 to 62 to obtain the strength needed to handle high compressive loads between the rolling elements and raceways. Then, they are ground to achieve the internal clearance or preload required by the application.

The inner and outer races require mounting to flat machine structures with at least Grade 8 bolts.

Unlike conventional bearings, a turntable version often incorporates integral gear teeth (in various tooth forms) on either the inner or outer race. Most gear teeth meet a precision level between AGMA class 6 and 10. However, optional grinding achieves higher precision (AGMA class 12 to 14). To increase wear resistance and strength, gearing can be induction hardened, usually in the Rc 55 to 60 range.

Most of these bearings come with lip seals to keep out contaminants. However, dynamic face seals can be substituted to increase protection against leakage and contaminants or to reduce torsional drag. Grease lubrication is most common. However, higher speed assemblies usually operate in an oil bath, which offers more consistent lubrication and easier changeout.

More inducements

Turntable bearings offer several other reasons to use them. Here's a sampling.

• Save space. The traditional way to mount bearings that must transmit combination loads (including those due to overturning moments) is to fit two angularcontact bearings on opposite ends of a vertical spindle shaft. This is commonly called a kingpost arrangement. On the other hand, a turntable bearing assembly with a hollow shaft takes less axial space and provides clearance through the shaft for hardware such as wiring and hydraulic hoses. Even a gear can be cut into the inner race, protecting it against a bad environment.

• Increase precision. Due to its large diameter, a turntable bearing has inherently higher stiffness than a conventional shaftmounted bearing. This enables more accurate machine positioning.

• Design flexibility. Most assemblies are custom designed for specific applications. This permits modifications to mounting configurations, sealing, gearing, and degree of precision.

• Lower cost. A turntable bearing usually costs more than a conventional shaft-mounted bearing. However, an entire rotating assembly using a turntable bearing is usually less expensive than with shaftmounted bearings. Only one bearing is required, and you can attach it simply by bolting its races to two flat and rigid mounting structures.

Design considerations

Normally, you can select conventional rolling-element bearings from a catalog to meet size and capacity requirements. The catalog usually specifies static and dynamic ratings, as well as mounting criteria, which seldom change.

On the other hand, you rarely can select large diameter turntable bearings from a catalog because of special loading and mounting configurations that require custom design.

Combination loading (thrust, radial, and overturning moment) is very common with such devices. When these loads are to be applied simultaneously, which is the usual situation, the bearing manufacturer performs a complex analysis to ensure the design is adequate.

Deflection under load is an important design criteria in some applications. Such cases require calculating the deflection under a specific loading situation.

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These bearings virtually always bolt to a large diameter mounting structure. Such structures tend to distort, so make sure they are flat and rigid to get equal load distribution.

Loose or tight

Each turntable bearing is manufactured with a specific amount of diametral clearance or preload between the races, depending on the application requirements. This diametral clearance or preload is calculated from measurements in the radial and axial directions.

In one example, a turntable in a conveying system acts as a carousel and doesn't require precise positioning. Additionally, it isn't practical to machine the mounting structure for the bearing as flat as needed. Therefore, a generous clearance in the bearing will compensate for the mounting structure limitations.

Conversely, an assembly line robot needs to consistently position a heavy item very precisely. This application requires minimal deflection under load, so a stiff preloaded bearing is appropriate.

Rick L. Shaw is a vice president at Avon Bearings, Avon, Ohio.

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