Motion System Design

Bridging the gaps: coupling selection for servo systems

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 couplings

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 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.

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 lower-torque 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.

Oldham couplings

An oldham coupling is a three-piece coupling comprised of two hubs and a center member. The center disk, which is usually made of plastic (or metal) is the torque-transmitting element. Torque transmission is accomplished by mating slots in the center disk, located on opposite sides of the disk and oriented 90° apart, with drive tenons on the hubs. The disk's slots are slightly press fit on the tenons of the hub. This press fit allows the coupling to operate with zero backlash, though with time, sliding of the disk over the tenons can create wear — until the coupling exhibits backlash. However, the disks are inexpensive items that are easily replaced, and a new insert quickly 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 hub and disk, oldham-coupling bearing loads do not increase as misalignment increases. Unlike other couplings, there are no bending members to act as detrimental springs that increase bearing loads as shafts become increasingly misaligned.

This type of coupling allows a small amount of angular misalignment (less than 0.5°) and axial motion (less than 0.005 in.) and is limited to speeds of 4,000 rpm. Larger amounts of angular misalignment cause the coupling to lose its constant velocity characteristic. Also, axial motion is limited by the three-piece design of the coupling, which does not allow for push-pull motion. Further, because the center disk is a floating member, both shafts must be supported to keep the coupling from falling apart.

As far as parallel misalignment goes, this design is particularly well suited for handling relatively large amounts — from 0.025 to 0.100 in. or more depending on coupling size. Coupling manufacturers generally provide conservative misalignment ratings so users can obtain maximum life. However, these ratings may be surpassed at the expense of coupling longevity.

The ability to choose different disk materials is an advantage of this type of coupling. Several manufacturers offer material choices to meet application needs. Generally, using one material is best where zero backlash with high torsional stiffness and torque are required. Other materials are useful in applications that have less precise positioning requirements, can tolerate some backlash, and also benefit from a quieter coupling that absorbs vibration. When nonmetallic, inserts are electrically isolating and can act as a mechanical fuse. If the plastic insert fails, it breaks cleanly and does not allow any transmission of power, preventing damage from occurring to other more expensive machinery components.

Jaw couplings

Conventional straight-jaw couplings are not appropriate for servo applications where accuracy of torque transmission is required. However, curved-jaw couplings — a variation on the jaw design — are well suited to servo applications. Curved jaws help reduce coupling deformation and limit the effects of centrifugal forces during high-speed operation.

These zero backlash curved-jaw couplings consist of two metallic hubs and a star-shaped elastomer insert, commonly called a spider. This multi-lobed insert fits between the drive jaws on the coupling hubs. (In other words, jaws from each hub alternate with the lobes of the spider.) As in the oldham coupling, there is a press fit between the jaws and the spider so there's no backlash. However, in contrast to the oldham coupling (in which torque disks are in shear under torsional loads) a jaw coupling's spider operates in compression.

When using a zero-backlash jaw coupling, users must be careful not to exceed the manufacturer's rating for maximum torque with zero-backlash, which can be significantly below the physical limitations of the spider. If torque does go higher, the spider can be compressed until there's no longer a preload; then backlash occurs, sometimes going unnoticed until a larger problem results. Jaw couplings are well balanced and can easily handle high-rpm applications (to speeds of 40,000-plus rpm) but are not able to handle big misalignments — especially axial motion. Large parallel and angular misalignments cause bearing loads higher than those produced by most other servo coupling types.

End users must understand how jaw couplings behave upon failure. If its spider fails, this kind of coupling doesn't disengage. Instead, the jaws from the two hubs pair (mating like teeth on two gears) and continue to transmit torque with metal-to-metal contact. Depending on the application this may be desirable; if it isn't, mating can cause problems in the larger, overall system.

One advantage to jaw couplings is the ability to mix and match spiders based on the application. Manufacturers of zero-backlash jaw couplings offer multiple materials with different values of hardness and temperature capabilities, so users can specify inserts that meet the application's exact performance criteria.

Disk couplings

At the very least, a disk coupling includes two hubs and a thin metallic (or composite) disk that transmits torque. (Usually this disk is fastened to the hubs with a tight-fitting pin that prevents play or backlash between the parts.) However, some manufacturers offer disk couplings with two disks that are

  • Separated by a rigid center member and

  • Attached to a hub at each end.

This rigid center member is usually metallic, but plastic versions offer electrical isolation. (Insulating property gains do come at a loss of torque capacity and torsional stiffness.) Interestingly, the difference between one and two-disk variations is similar to the difference between single and multiple-beam couplings. Single-disk couplings don't accommodate parallel misalignment due to the complex bending of the disk that is required. On the other hand, in two-disk styles, the disks bend in opposite directions to harness any parallel offset.

Other disk-coupling properties are comparable to those of bellows couplings, another type we'll explore shortly. In particular, the way they transmit torque is very similar. Torsionally, the disks are very stiff, with stiffness ratings slightly lower than those of bellows varieties. The disks are also very thin, so they bend easily under misalignment loading. This means the coupling handles large amounts of misalignment (up to 5°) with some of the lowest bearing loads available in a servo coupling. One drawback is that these couplings are very delicate and prone to damage if misused or installed improperly. So, for proper operation, special care must be taken to ensure that misalignment is within coupling ratings.

Bellows couplings

A bellows coupling is a conclave of two hubs and a thin-walled metallic bellows. In most cases the hubs/bellows assembly is either welded or glued together. Although other materials are used, the two most common for the bellows are stainless steel and nickel. Nickel bellows are manufactured using electrodeposition. This involves machining a solid mandrel in the shape of the finished bellows. Nickel is electrodeposited onto this mandrel; the mandrel is then chemically dissolved to leave behind the finished bellows. This method allows precise control over wall thickness and produces thinner walls than other forming methods. Thinner walls give these couplings greater sensitivity and responsiveness, making them suitable for extremely precise, small instrumentation applications. However, the thinner walls also reduce torque capacity, putting a limit on useful applications. Stainless steel bellows are stronger than nickel versions and are usually manufactured with a process called hydroforming. (In this process, a thin-walled tube is placed into a machine and hydraulic pressure is used to form the convolutions of the bellows around specialized tooling.)

The characteristics of bellows make them ideal for transmitting torque in motion control applications. Thin, uniform bellows walls allow bending under loads caused by the three basic types of misalignment between shafts — angular, parallel, and axial. Generally, bellows allow for up to 1° to 2° of angular misalignment and 0.010 to 0.020 in. of parallel misalignment and axial motion. The thin walls result in low bearing loads that remain constant at all points of rotation, without the damaging cyclical high and low loading points found in other coupling types.

Perhaps most importantly, bellows couplings are the one of the most rigid styles under torsional loads. The stiffer the coupling, the more accurately motion is translated from the motor to the driven component — very beneficial to applications that require higher accuracy and repeatability.

The use of aluminum hubs with a bellows results in a coupling with very low inertia, a useful feature in highly responsive systems. Some bellows manufacturers also balance their couplings so they can be used in faster applications (10,000-plus rpm) as well. In addition, bellows couplings are available with stainless steel hubs, which can be useful in applications where corrosion resistance is important. However, the extra mass of stainless steel does detract from the main benefit of this otherwise lightweight component.

Rigid couplings

Rigid couplings are known for being imprecise, inexpensive, and often homemade components for simple shaft-to-shaft connections — so many designers wouldn't consider using this coupling type in motion control applications. But now, smaller-sized rigid couplings (especially in aluminum) are increasingly common due to their high torque capacity, stiffness, and zero backlash.

As the name implies, rigid couplings are torsionally rigid with virtually zero windup under torque loads. The drawback is that they are also rigid under loads caused by misalignment. If any misalignment is present in the system, the forces cause shafts, bearings, or the coupling itself to fail prematurely. This means that these couplings cannot be run at extremely high rpms, because they don't compensate for any thermal changes in shafting. However, in situations where misalignment can be tightly controlled, rigid couplings offer excellent performance characteristics.

Most important in motion control applications is that the rigid coupling itself does not introduce misalignment into a system where it cannot be absorbed without damage to bearings, seals, or system performance. Frequently, the coupling itself is used to establish the needed alignment. The motor and other component mounts are loosened so that there is free play. Then, the shafts are connected to the rigid coupling that, if precisely made, will align the shafts. Finally, the components are centered on any remaining free play and the mounts tightened.

Shaft alignment is best when its bores are honed, since honing assures that both bores are collinear. Honing also corrects any residual distortions caused by stresses introduced during the manufacturing process, resulting in a round, precisely sized bore. Proper sizing and geometry assures a large percentage of shaft contact and greater torque transmission ability.

Rigid couplings lack a mechanism to absorb the vibration inherent in many mechanical systems. Consequently, vibration can loosen hardware and degrade torque transmission during normal use. Placing a nylon treatment on the screw threads can reduce these effects on the hardware for increased reliability. Also, as a dissimilar material nylon reduces galling of the screw threads in stainless steel couplings.

Setscrews fix the simplest rigid couplings to shafts through impingement. A superior alternative is clamp-style rigid couplings, because they wrap around the shaft to provide high torsional holding power without shaft damage and the fretting that is unavoidable with setscrews. Two-piece styles also allow for disassembly and maintenance without removal of other components. When the hardware on a two-piece rigid coupling is opposing, it dynamically balances and the coupling can operate at higher speeds. (As a general guideline, one-piece rigid couplings are evaluated for applications up to 3,000 rpm. This can increase to 4,000 rpm when a two-piece style with opposing hardware is used.)

Most clamp-style rigid couplings have cap screws close together and arranged in pairs. This design, especially in combination with a crosscut, facilitates greater holding power and accommodates slight deviations in the shaft sizes being connected. One warning: It is recommended that this coupling style be installed by tightening the paired screws alternately in several steps. The close proximity of the screws results in a mutuality of the hoop stress developed in the coupling by each screw in a pair. As each screw is tightened, it tends to relax any tension developed by its companion.

Acknowledgements to Fred F. Ruland and thanks to Robert G. Ruland. For more information, call (800)225-4234 or visit

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.

Sandwiched for safety

An oldham coupling is a three-piece coupling comprised of two hubs and a center member. Unlike other couplings, there are no bending members to increase bearing loads as shafts become increasingly misaligned. If the center insert fails, it breaks cleanly and prevents damage to more expensive components.

A quick bite

Curved-jaw couplings — a variation on the jaw design — are well suited to servo applications. Curved jaws help reduce coupling deformation and limit the effects of centrifugal forces during high-speed operation.

A quick bite

Curved-jaw couplings — a variation on the jaw design — are well suited to servo applications. Curved jaws help reduce coupling deformation and limit the effects of centrifugal forces during high-speed operation.

Delicate disks

At the very least, a disk coupling includes two hubs and a thin metallic (or composite) disk that transmits torque. However, some manufacturers offer disk couplings with two disks.

Bellows for bending

In most cases the hubs/bellows assembly is either welded or glued together. Thinner bellows walls allow bending under loads caused by angular, parallel, and axial misalignment between shafts.

Simplicity offers advantages

Rigid couplings are stiff under loads caused by misalignment. However, in situations where misalignment can be tightly controlled, rigid couplings offer excellent performance characteristics.

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