Flexible Jaw couplings are generally used to connect continuous-duty electric motors with driven machinery, pumps, and gearboxes. They typically accommodate angular shaft misalignment up to 1 degree and parallel misalignment up to 0.015 in.
All elastomeric couplings are classified according to how their elastomeric elements transmit torque between driving and driven hubs — the elements is either in compression or shear.
In jaw couplings, the element is loaded in compression between the jaws of mating hubs. These jaws operate in the same plane, with the driving hub jaws pushing toward the driven hub jaws. Legs of the elastomeric spider transmit and cushion the force between the driving and driven jaws by being compressed between them.
This contrasts to shear-type couplings, in which driving and driven hubs operate in separate planes, with the driving hub pulling the driven hub through an elastomeric element suspended between them. Here, the element transmits and cushions the force between the hubs by being stretched between them.
Compression type couplings offer advantages in four areas: load capacity, torsional stiffness, safety, and easy installation. First, all elastomers — especially synthetic rubber — have higher load capacity in compression than they do in shear. Therefore, compression types transmit torque and tolerate more overload than shear types. Heavy-duty jaw models with up to seven jaws handle torque ranges up to 170,000 lb-in.
Second, compression types offer more torsional stiffness (less twist between hubs) than shear types, with some versions closely approaching the very stiff characteristic of metallic couplings. This high torsional stiffness minimizes backlash between motor and driven machine, offering near-equal movement of the driven shaft for each incremental movement of the driving shaft, an important factor in certain conveyor and pump applications.
Third, jaw couplings are fail-safe: the coupling can still operate even if the spider breaks. The driving jaws simply rotate until they contact the driven jaws directly, and the coupling continues to function (albeit with considerable noise and accelerated wear). Maintenance personnel can thus replace the spider at a convenient time, which may prevent critical system downtime. For this reason, the jaws in a well-designed and manufactured coupling withstand several times the coupling’s torque rating.
Fourth, the simple three-piece assembly — a spider sandwiched between metal hubs — makes these units easy to install and inspect. The contoured spider usually allows “blind fit” even in the most confined spaces.