Beam couplings have beams formed by a long helical cut or series of shorter overlapping cuts. The coupling shown has two separate sets of multiple overlapping beams that better accommodate misalignment. Having two beam groups helps the coupling bend in two directions at once, as required by a parallel offset.

Servo systems must be totally predictable where positioning is concerned, and this calls for limited play and high mechanical stiffness, from shafting and gears right down to the mountings. The servo-grade coupling, therefore, must have zero backlash and significant torsional rigidity while conforming to parameters such as misalignment, speed, and torque. Several distinct forms of servo couplings meet the many possible application needs.

Beam

Beam-type couplings are made from a single piece of material, usually aluminum. They’re cut spirally, forming platelike beams that bend to give lateral flexibility while remaining torsionally rigid and strong. This coupling style is a solid all-purpose choice, offering good performance at a low cost. The single-piece design, with no joints or mating parts, promotes zero backlash and requires no maintenance.

There are two basic variations of beam couplings – single-beam and multiplebeam arrangements. The single-beam style has one long continuous cut that usually encompasses several complete turns. This results in a coupling that’s very flexible and can minimize bearing loads. It accommodates all types of misalignment, but works best with angular misalignment or axial displacement. Parallel misalignment capabilities are smaller because the single beam has to bend in two different directions at the same time, undergoing high stress that could lead to premature failure. Although a longer beam will bend more easily, it is also more flexible in the torsional direction. The large amount of windup adversely affects the accuracy of the coupling and reduces its overall performance. Singlebeam couplings, while relatively inexpensive, are best for low-torque applications, especially in connections to encoders, tachometers, and other light instrumentation that might be installed away from the primary load path.

Multiple-beam couplings consist of two or three overlapped beams. Instead of a continuous helix, repeated helical cuts are overlapped, such as in multi-start threads on a screw, although each cut may be less than a full rotation. Overlapping allows shorter beams without losing much of the coupling’s misalignment capabilities. Shortening the beams and overlapping them to work in parallel increases the coupling’s torsional rigidity and capacity. Nevertheless, the loss of flexibility is enough to increase bearing loads (reactions from misalignment) a good deal over the single-beam variety, but these loads still tend to be low enough to keep the bearings safe. Multi-beam couplings are suitable for use in semilight applications, such as between a servomotor and a lead screw.

Oldham couplings have a disc, usually non-metallic, held between two hubs. Slots on either side of the disc (perpendicular to each other) fit tightly onto the hub projections. These couplings are great for parallel misalignment but can’t handle much angular misalignment or axial motion.

The beam concept can be further modified. Instead of one long cut or set of adjacent multiple cuts, two separate groupings of cuts can be made. This gives additional flexibility with better acceptance of parallel misalignment, as one set of beams bends in one direction while the second set bends in the other direction, with a rigid section between them. Such an arrangement is most often used in conjunction with multiple-beam construction rather than single-beam.

While beam couplings are most commonly aluminum, stainless steel is usually an option. In addition to corrosion protection, stainless steel raises the torsional strength and stiffness of the coupling; these can be close to double that of a comparable aluminum component. However, the higher rigidity and torque capacity is offset by a dramatic increase in mass and inertia. A smaller motor will spend a large percentage of its torque to overcome the coupling’s inertia.

Oldham

This three-piece coupling is comprised of two hubs and a center disc. The disc is the torque-transmitting element and is made of plastic or, less commonly, metal. There are slots in the center disc, located on opposite faces of the disc and oriented 90° apart. Drive tenons (load-bearing projections) on the hubs are fitted to the disc slots with a slight press fit, and torque is thus transferred between disc and hubs. The press fit provides zero backlash, but over time the sliding of the disc over the tenons creates wear to the point that the coupling will have play. However, the discs are inexpensive and easily replaced, and inserting a new one restores the coupling’s original performance.

In operation, the center element slides on the tenon of the hub to accommodate misalignment. Because the only resistance to misalignment is the frictional force between the hub and disc, oldham couplings allow bearing loads to remain the same regardless of the level of misalignment. Othertypes of servo couplings flex to fit the misalignment, with greater force required for larger deflections, and the load is ultimately taken up by the bearings.

Oldham couplings shift to accept significant parallel misalignment (from 0.025 in. to 0.1 in. or more depending on coupling size) but they allow less than 0.5° of angular misalignment and less than 0.005 in. of axial motion and are limited to speeds of 4,000 rpm. With larger degrees of angular misalignment the coupling loses its constant-velocity characteristic. As for axial movement, the three-piece design (with the center disc a floating member) prohibits use in pushpull applications. Furthermore, both shafts must be supported to keep the coupling from falling apart. Manufacturers generally provide smaller misalignment ratings in order to improve coupling life, but these ratings can be surpassed at the expense of endurance.

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