William Hewitson
Director of manufacturing and engineering
Ruland Manufacturing Co., Inc.
Marlborough, Mass.

Choosing the proper servo coupling for an application is a critical part of total system design and greatly affects its overall performance capabilities. For this reason, considering the coupling early in the design process and aligning the coupling performance attributes with the functionality goals of the system can eliminate many problems that typically occur in motion control applications. Each of the couplings we'll discuss here has individual characteristics that make them ideal for many different uses. A single type of coupling, however, cannot be applied to every application in the field. This leads to the wide variety of couplings currently available and gives the design engineer the ability to select the best possible coupling to maximize system performance and durability.

Shaft misalignment, stiffness, torque capacity, rpm, inertia, bearing loads, and space requirements must be satisfied for couplings to work properly. It is therefore important to know application parameters beforehand. Because each servo coupling type has different performance characteristics, these application parameters often eliminate some types and suggest the use of others.

Beam-type couplings are manufactured from a single piece of material (usually aluminum) and utilize a system of spiral cuts to accommodate misalignment and transmit torque. For many applications, beam couplings are a good place to start. They have good performance characteristics and are economical. The single-piece designs of these couplings allows for the transmission of torque with zero backlash and no maintenance.

Beam couplings

Beam couplings are available with a single beam or multiple beams. Single-beam styles have one long, continuous cut that usually consists of multiple complete rotations. This results in a flexible coupling that yields light bearing loads. It's able to accommodate all types of misalignment, but works especially well with angular misalignment or axial motion. One limitation: Parallel misalignment capabilities are reduced because the single beam must bend in two different directions at the same time, creating larger stresses — stress that can cause premature failure.

Tailored cut
Beam-type couplings are manufactured from a single piece of material (usually aluminum) and utilize a system of spiral cuts to accommodate misalignment.

Although long single beams allow easier bending under misalignment, they have the same affect on coupling rigidity under torsional loads. What does this mean? The relatively large amount of windup under torsional loads adversely affects the accuracy of the coupling. For this reason, single-beam couplings are best utilized in lowertorque applications: connecting encoders and other light instrumentation, for example.

Multiple-beam couplings usually consist of two or three nested (overlapping) beams. The multiple cut design allows both the beams and the coupling to be shorter without sacrificing misalignment capabilities. In turn, shorter beams make the coupling stiffer torsionally and overlapping beams work in parallel to increase the allowable maximum torque. Multiple beam couplings are suitable for use in light duty applications — for example, connecting servomotors to leadscrews. Bearing loads are increased by a sizeable amount over the single beam variety, but in most cases they remain low enough to protect bearings effectively. Further, some manufacturers take the multiple-beam concept to another level. Instead of having a single set of multiple cuts, two sets of multiple cuts are utilized.

The use of multiple sets of cuts gives the coupling additional flexibility and misalignment capability. It also increases misalignment capabilities, making the coupling more forgiving of parallel misalignment. In contrast to couplings with one beam (or even a single set of beams) under parallel misalignment, in these multiple beam couplings one set of beams bends in one direction and the second set bends in the other.

Most commonly, aluminum versions of these couplings are used. However, several manufacturers offer designs available in stainless steel. In addition to corrosion protection, stainless steel increases the torque capacity and stiffness of the coupling — sometimes to double that of aluminum parts of the same design. But as before, the increase in torque and stiffness comes at a cost; it is accompanied by an increase in mass and inertia. This is important to consider with applications using smaller motors, since in these situations a large percentage of motor torque is used to overcome the coupling's inertia.