Tire in
Tire designs involve shear of an elastomer (often reinforced with cords) to transmit torque. This inverted tire design has the advantage of not growing under high speeds, which can be an adverse, speed-limiting effect with regular tire couplings.

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.

Jaws of the coupling
This jaw coupling subjects the element to compression between the hub teeth as it transmits torque. Elastomers tend to be strongest under compression.

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.

Slip it around
A combination coupling transmits torque through shear and compression. Axially splitting the element allows easier installation or removal, a concept also used with some tire couplings.

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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