Not all linear motion applications consist of straight lines. Some applications require an occasional curve or even circular motion. Spray painting, pick and place, rotary assembly, and multi-station assembly machines are typical applications that may require motion executed in the round, Figure 1.

One type of curved linear-motion system uses V-ball bearings. It has opposing female bearings with V-shaped outer rollers in a two-and-two arrangement. The bearings ride on a track with matching zone-hardened V-shaped rails. A carriage plate on top of the two-and-two bearings is the mounting platform, Figure 2. Thus, the carriage assembly effectively runs on eight line-contact points on a track with varying circumferential diameters.

Two types of carriages are available. When using a fixed segment of a ring, fixed center carriage plates are the most popular. A bogie carriage, Figure 3, is best for use around S-bends, slideways with differing bend radii, and curves where looseness in the movement between straight and curved sections is not desirable. The bogie carriage runs on swivel bearings, which operate on a principal similar to that used in train and tram bogies to negotiate bends in the track. The two types or carriages facilitate the flexibility of V-ball-bearing track systems. It is not necessary to mount these systems to a machined flat surface unless high accuracy is required.

V-ball bearing track systems are best suited to light loads — direct loads from 120 to 3,800 N (26.98 to 854.38 lb) and moment loads from 0.6 to 220 Nm (0.53 to 1,946.90 lb-in.)in a lubricated system. Refer to manufacturers’ tables for precise load handling capabilities.

Rings and segments

Often when the application calls for a curved linear system, an engineer is looking for pure radial movement, such as that found in tool changing mechanisms, measurement of turbine blades, rotating manipulators, movement of prisms in laser measurement machines, and rotation of photographic cameras. These applications require a segment of a ring, typically available in 90 or 180 deg, or a complete ring with 360 degrees of rotation, Figure 1.

Rings offer stability with support as near to the load as possible. In spraying applications, for example, a paint nozzle mounts on a fixed center carriage plate that tracks the curvature of the sprayed product to ensure even coating. A gearcut rack on the outside diameter of the register face of the ring serves as the drive mechanism. (The gearcut rack can also be placed on the internal register face of the ring).

A ring is typically mounted to a base with the carriage assembly as the sliding member. However, the slide ring can be the rotating element by locating its center with two concentric V-ball bearings and at least one adjustable eccentric bearing. Additional eccentric bearings can be added to achieve higher load capacity and stability.

Track systems

Once the ring selection is made, engineers need the ability to link any linear and curved segments to make a track system. Some linear slideways are available in lengths to 4 m; for longer lengths, slide segments are matched and butted together. Using standard tracks and ring or segments of rings, hundreds of arrangements are possible.

The most popular track system is the oval, with two equal length linear slides and two 180 deg segments. Several carriages may run on the track simultaneously, (also referred to as a continuous motion system) using a toothed timing belt with location lugs to drive the carriages. This track arrangement is used in micro-chip X-ray machines, automotivecomponent assembly, biscuit cutting, food packaging, and box-filling machines.

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