Jaw Couplings

Staff Report

One of the most widely applied types of flexible couplings is an elastomeric design known as the jaw coupling. This design is characterized by two hubs, each having two or more thick, stubby protrusions around their perimeters, called jaws, pointing toward the opposing hub.

These jaws mesh loosely when the two hubs are brought together. Filling the gaps between the jaws are blocks of elastomeric material, usually molded into a single asterisk-shaped element called a spider.

Just as coupling designs vary to satisfy different application criteria, so do the spiders in jaw-type couplings. The spider is the key determinant of the torque rating of each jaw coupling. It also can make a significant difference in the coupling’s response to vibration, temperature, chemicals, misalignment, high RPM, space limitations and ease of installation or removal.

Selecting the right type of spider is just as important as selecting the right type and size of coupling. For that reason, users will benefit from a fuller understanding of the different spider constructions and materials available, when specifying new or maintaining existing couplings.

First, let’s review some important basics of jaw couplings and the elastomeric design group to which they belong.

Jaw couplings generally are recommended for continuous-duty electric motor-driven machinery, pumps, gearboxes, etc. They typically are limited to angular shaft misalignment of 1º and tolerate up to .015 inches of parallel misalignment. Jaw designs usually are not recommended for engine-driven or frequent start-stop-reversing applications because of backlash (the amount of free hub movement allowed by the spacing between the jaws and spider legs).

Elastomeric couplings classify into one of two categories by the way their elastomeric element transmits torque between driving and driven hubs — i.e. the element is either "in compression" or "in shear".

In jaw couplings, the element is in compression, because the jaws of both the driving and driven hubs operate in the same plane, with the driving jaws pushing the driven jaws. Here, the legs of the elastomeric spider serve as cushions between the torque force of the driving jaws and the resistance of the driven jaws, absorbing that force by being compressed between them.

This contrasts with shear-type designs, in which driving and driven hubs operate wholly in separate planes, with the driving hub pulling the driven hub through their mutual connection to an elastomeric element suspended between them. Here, the element serves as a link between the torque force of the driving hub and the resistance of the driven hub, absorbing that...

...Continued in the pages of Twin Plant News, Subscribe Today!