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

Servocouplings come in a variety of designs but all have zero backlash.

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.