Jaw couplings are a general, all-purpose industrial power transmission device that operates in a range of temperatures, handles angular misalignment and the accompanying reactionary loads, resists chemicals and has good speed and dampening capabilities. A basic jaw coupling is made of three parts: two hubs, generally bored to a clearance fit tolerance, and an elastomeric insert. These inserts, sometimes referred to as spiders, are designed to last a long time, but do eventually need to be replaced.
When the inserts fail, the coupling’s fail-safe design comes into play. The hubs will interlock and continue to carry the load, but without the dampening capability that the insert provided, affording an operator the opportunity to quickly shut down a system. Once an insert fails, replace the insert and the two hubs as soon as possible, as the hubs will have worn on each other and no longer be useable. Hub on hub operation without an insert is sub-optimal, and not recommended for continued system use.
These couplings also operate without lubrication. Unlike gear, grid, or chain couplings that require periodic reapplication of grease, as long as the insert remains intact, a jaw coupling will continue to work without additional maintenance.
Jaw couplings, in their traditional form, transmit torque through the elastomeric flexing element in compression. From an elastomeric element perspective, this is preferable for torque transmission because elastomers can generally take more punishment and load (inclusive of handling peak loads) in compression than during in-shear motion.
The standard jaw-coupling insert is constructed of solid Nitrile Butadiene (Buna N) rubber. This flexible elastomer is oil resistant, resembles natural rubber in resilience and elasticity, and operates in a temperature range of -40° to 212°F (-40° to 100°C). This rubber is suited for cyclic loads. Broadly speaking, this insert generally has a shore hardness of 80A, high dampening capacity, and good chemical resistance.
Another variation of the Buna N rubber insert is the open center option. While the closed center version is a more robust design, and suited for higher speed applications, the closed center does not allow for extremely close (near touching) shaft separation. Simply put, the open center spider is designed for applications where the distance between shaft ends is very tight, and there is no room for a center section.
A third variation is for the insert to wrap around the outside of the coupling, and then be held in place with a retaining ring. This variation allows easy insert replacement without the need for the hubs to be slid back or moved on the shaft. The primary disadvantage of this variation is that it is typically restricted to lower RPM speeds, sometimes as low as 1750 RPM based on design.
In addition to the standard Buna N inserts, urethane, Hytrel, and Bronze are also popular materials. Urethane has a greater torque capability relative to Buna N (approximately 1.5 times), but provides less dampening effect. Its operational temperature range is also reduced on both ends of the spectrum (-30° to 160°F, -34° to 71°C). It does have good resistance to oil and chemicals, but is not recommended for cyclic or start/stop applications.
Hytrel inserts are for high torque and high temperature operations (-60° to 250°F, -51° to 121°C) and have excellent resistance to oil and chemicals. They are not recommended for cyclic or start/stop applications, and are generally limited to a half degree of angular misalignment (versus generally 1 degree for Buna N and Urethane). The ability to handle parallel misalignment does remain similar to Buna N & Urethane inserts at approximately 0.015 in.
Rigid, porous oil-impregnated bronze inserts are for slow speed applications (max 250 RPM) that require high torque capabilities. These inserts are largely unaffected by extreme temperatures (-40° to 450°F, -40° to 232°C), water, oil, or dirt. Angular misalignment capability is similar to Hytrel (generally at a half degree), but parallel misalignment handling is reduced to 0.010 in.
Jaw couplings are limited in operating temperature to what can be handled by the elastomeric insert. These couplings do offer moderate misalignment handling capabilities (both in angular and parallel), but some applications may require more. Radial stiffness may introduce high reactionary loads in these couplings, and vertical applications may require special hubs (if setscrew proves insufficient in keeping hubs from sliding). Lastly, traditional jaw couplings are not suited for motion control applications because there is some looseness between the jaws and elastomer, which will become more compressed over time.
Curved jaw couplings are different than straight jaw in that their teeth have a curved profile. This design style is popular in Europe, but it is also broadly popular in motion control applications, because using a very stiff urethane insert can create a backlash free coupling (sometimes known as a GS or Spidex). One draw back of the curved jaw design is that, because of the jaw curvature, it cannot be converted to an in-shear design.