Flexible couplings have been around for years. Yet, as rotary machinery calls for higher torque, speed, and flexibility, as well as simplicity and accuracy, this mature component is continually put to the test.

There are many well-established flexible coupling designs, falling into both the mechanical and elastomeric categories. Elastomeric couplings are known for their high flexibility and vibration damping rather than for outstanding torsional rigidity, and fill the transmission needs of a great many imperfectly aligned operations. These couplings are available in quite a few different designs, built around a handful of principles, and they are evolving.

The basics

Elastomeric couplings generally use either natural rubber or a synthetic such as polyurethane for the flexible element. The elastomeric element can be configured to transmit shaft torque by undergoing shear or compression; furthermore, in some designs a combination of both shear and compression is used to transfer torque.

Shear couplings use a tube-like elastomeric element that will wind up a certain degree as it transmits the torque. These elements can connect to the shaft hubs through various means, including clamping, intermeshing teeth, or bonding to metallic members that are in turn bolted to the hubs. The element handles misalignment by flexing and distorting, or, if intermeshing teeth are used, by sliding action between the teeth.

Often, elastomeric shear couplings are shaped like tires. Corded tire designs use elastomers containing reinforcing strands, and the tire elements are usually clamped to each hub. The cross section is U-shaped, and some designs involve an inverted tire, with the bottom of the U pointing at the shaft. One of the primary advantages of the inverted tire design is the resistance to tire diameter growth under centrifugal force, which can cause the hubs to pull together and impart an axial load. And since the point of clamping is at a larger diameter, less clamping force is required to withstand the load; but the larger hubs add weight to the system.

Urethane tires are not reinforced, since the urethane (or similar material) is stronger than natural rubber. Urethane elements are typically bonded to metal rings that are bolted to the hubs.

Compression-style elastomer couplings take advantage of the fact that elastomers in compression are stronger than in shear or tension. However, compression designs are usually stiffer than shear-type couplings. These designs often position the element between axially protruding, intermeshing teeth on the two hubs. Parallel misalignment between shaft centerlines is handled through a clearance fit and compression of the element in a direction perpendicular to the shafts. Angular misalignment is accepted through sliding between teeth as well as compressive distortion. Such a design is called a jaw coupling.

Another compressive design, the block coupling, has elastomeric cylinders (called “blocks”) that are set in pockets formed by the axial protrusions of each hub. By changing the hardness and design of the block, torsional stiffness can easily be altered.

The donut coupling is a compressionstyle arrangement where the elastic element is pre-compressed into a smaller diameter before being bolted to the hubs. Such a preload is meant to ensure that the coupling element is not subjected to tension.

Combination shear and compression designs generally involve shear of a short cylindrical element to provide torque transmission; but the element has teeth as well, and compression occurs to the extent that the element teeth are pinched between the teeth of the two hubs under a torque load. The arrangement is similar to a jaw coupling, except the hub teeth do not overlap, and hence there is a space bridged by the elastomer only, and it undergoes shear. This type of coupling accommodates misalignment partly by the freedom of movement between element teeth and hub teeth (similar to a gear coupling) and also by the elasticity of the element material as it conforms to the relative movement of the hub teeth.

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Coupling tweaks

There are additional devices and design adjustments that can be applied to the coupling variations explained above to improve their performance and simplify their installation.

For example, in one modification, the element of a combination coupling has a compound root radius at the base of the teeth on both flanks. (The tooth then resembles an undercut gear tooth.) This radius increases tooth flexibility by smoothing the transition from the tooth base to the outer connecting web of the element. The added flexibility helps the elastomer teeth better conform to the hub teeth, and this improved contact helps distribute the load more evenly, allowing higher maximum torque and greater overload protection; and the greater flexibility lessens the severity of cracks tending to propagate under cyclic loading. Also, since the teeth bend more readily, the element is easier to assemble onto the hub.

Contact between toothed elastomer couplings can also be improved by refining the shape of the hub teeth; one design has a smooth, curved nose rather than the usual flat nose. This too helps distribute the load more evenly by providing a gradual, rather large surface for the element to press against. (A flat-nose tooth tends to produce line contact rather than surface contact.)

Installation simplification

Some schemes take advantage of material flexibility to ease coupling assembly and element replacement. As mentioned, element installation is facilitated by tooth geometry that increases flexibility. And there are other devices, such as axially splitting the element; in this way it can be looped around the coupling hubs without disassembling other components or rearranging the machinery. This feature is used in tire-in-shear and in combination couplings.

Coupling covers are another consideration. In a combination shear and compression coupling, the cover keeps the element properly positioned between the hubs.

Another function of the cover is to provide a safety in the event of failure; pieces of a broken coupling element can fly outward if left exposed, causing a hazard.

One covering device, called a positive retention cover, is intended to minimize such discharge. The cover is actually fastened to the element with two screws, to prevent axial and rotational movement between the cover and the elastomer. This way the cover won’t slip off under dynamic conditions, and essentially acts as one with the rest of the coupling.

To facilitate cover assembly, locating keys are sometimes used; for instance, two built-in keys placed along the inside cover diameter will slide into matching keyways around the element diameter. The fastening holes will now be lined up correctly. This is particularly useful in blind or inconvenient areas.

Mike Feely is a Product Manager and Sean Ash is a Design Engineer with the Falk Corp., Milwaukee.