What to look for in a servocoupling

William Hewitson
Product Engineering Manager
Ruland Manufacturing Co. Inc.
Marlborough, Mass.

Servocouplings are typified by their ability to transmit torque with zero backlash. There are several different coupling designs with this property, so which one should you choose? Here are some basic guidelines to help decide.

Beam-type couplings come in single and multiple-beam styles and are typically made from a single piece of aluminum. Single-beam couplings have one continuous helical cut about their OD spanning several rotations. They are highly flexible, transmit low bearing loads, and tolerate angular misalignment or axial motion. Parallel misalignment makes the coupling beam bend simultaneously in two directions, which develops high stresses and shortens service life. Single-beam couplers also tend to windup under torsional load and so are best for such lowtorque uses as connections to encoders and other light instrumentation.

Multiple-beam couplings, as the name implies, use two or three overlapping short beams. The arrangement retains much of the misalignment capability of single-beam types yet boosts torsional stiffness and torque capacity. Of course the added stiffness also raises bearing loads when couplings are misaligned. On the upside, multiple beams better handle parallel misalignment than single-beam types.

Stainless-steel units have improved corrosion resistance and double the torque capacity and stiffness of comparable aluminum parts. However, small drive motors can have trouble accelerating the added mass and inertia. Multiple-beam couplings work best for such light-duty applications as connecting a servomotor to a leadscrew.

OLDHAM COUPLINGS
Oldham couplings consist of two hubs and a plastic or metal center insert. Drive tenons on the hubs transmit torque to mating slots in the insert located 90° apart on opposite sides. The three parts join with a slight press fit so the coupling can operate with zero backlash. Bearing loads are independent of misalignment levels because only frictional force between the hubs and insert resists misalignment. In contrast, couplings with bending members act as springs, so bearing loads scale with misalignment levels.

Oldham couplings work with parallel misalignments of 0.100 in. or more depending on coupling size. However, large angular misalignments compromise constant-velocity properties. The coupling's three-piece design handles small amounts of axial motion (0.005 in.) but prohibits use in push-pull applications. Speeds are limited to about 4,000 rpm. Over time, relative sliding between the parts wears the insert and increases backlash. Replacing the insert restores original performance.

In general, use metallic inserts when zero backlash, high torsional stiffness, and torque capacity are important. Nonmetallic inserts make sense for applications needing less precision. Nonmetallic inserts also lower vibration and noise and provide electrical isolation. And they can act as a mechanical “fuse” to prevent damage to more expensive components.

JAW COUPLINGS
Zero-backlash, curved-jaw couplings are a variation on conventional straight-jaw types. They consist of two metallic hubs and a multilobed, elastomer insert or “spider.” The spider fits between curved drive jaws on the coupling hubs. The curved jaws help reduce spider deformation and limit the effects of centrifugal force at high speeds. As with Oldham couplings, a light press fit between the three parts retains zero backlash. Spiders operate in compression so be careful not to exceed the maximum torque rating for zero backlash, which can be significantly below the mechanical limits of the spider itself. Doing so will cause a loss of preload and permit backlash.

Curved-jaw couplings can operate at speeds to 40,000 rpm or more. They handle small amounts of axial motion, though excessive parallel and angular misalignment raises bearing loads beyond that of most other types of servocouplings. Unlike the insert in Oldham couplers that can act as a mechanical fuse, a failed spider will not prevent a jaw coupling from turning the load. Instead, the hub jaws engage one another and make metal-to-metal contact.

DISK COUPLINGS
Disk couplings contain two hubs and a thin-metallic or composite disk that transmits torque. A tight-fitting pin fastens the hubs to the disk and prevents relative play or backlash.

Single-disk couplings handle only small levels of parallel misalignment because of the complex bending that takes place on the disk. Two-disk couplings overcome this limitation. They use two disks separated by a rigid center member that attaches to a hub at each end. The two-disk style lets each disk bend in opposite directions under parallel offsets up to 5° with minimal bearing loads. The rigid center member is typically metallic, but plastic versions are available to provide electrical isolation.

A downside to disk couplings is they are extremely delicate and prone to damage when misused or installed improperly. Be sure to keep misalignment within rated levels for proper operation.

BELLOWS COUPLINGS
Bellows couplings are an assembly of two hubs and a thin-walled stainlesssteelor nickel bellows. Nickel bellows are electrodeposited onto a bellowsshaped mandrel. The mandrel is then chemically dissolved, leaving behind the finished bellows. The technique precisely controls bellows wall thickness and permits thinner walls than possible with other methods. Thin walls lower torque capacity but boost sensitivity and responsiveness, important metrics for small, precision instrumentation applications. Stainless-steel bellows are typically made by a process called hydroforming and are stronger than nickel types. Here, a thin-walled tube goes into a machine and hydraulic pressure forms the bellows convolutions around specialized tooling.

Bellows couplings impart low bearing loads that remain constant at all points of rotation — without the damaging cyclical high and low loading of some other coupler types — yet remain rigid under torsional loads. A high stiffness makes bellows couplings ideal for applications needing a high degree of accuracy and repeatability.

Bellows couplings accommodate up to 2° of angular misalignment and 0.010 to 0.020-in. parallel misalignment and axial motion. The use of stainless-steel hubs improves corrosion resistance, though the trade-off is higher mass and inertia. Couplings with aluminum hubs lower rotational inertia, important in modern, highly responsive systems. Bellows couplings can be balanced at the factory for operation to 10,000 rpm or higher.

RIGID COUPLINGS
Small, rigid couplings (especially in aluminum) excel in servo applications. They have zero backlash, high torque capacity, and virtually zero windup. However, any misalignment forces fully transmit to shafts, bearings, and the coupling itself, which can lead to premature failure of connected components. Also, avoid using rigid couplings for high-speed applications where heat buildup is an issue, because they are not good at compensating for thermal growth in shafting.