The elastomeric jaw coupling, Figure 1, is one of the most widely applied types of flexible couplings. It has two hubs, each having two or more stubby protrusions around its perimeter, called jaws, pointing toward the opposing hub. Filling the gaps between the jaws are blocks of elastomeric material, usually molded into a single asterisk-shaped element called a spider, Figure 2.

Just as coupling designs differ to satisfy different application criteria, so do the spiders in jaw couplings. The spider determines the coupling’s torque rating. It also affects the coupling’s response to vibration, temperature, chemicals, misalignment, and high speed, as well as its ease of installation.

Selecting the right type of spider is just as important as selecting the right type and size of coupling. For that reason, understanding the coupling operation, as well as the different spider designs and materials, will help you in specifying new couplings or maintaining existing ones.

Elastomeric materials

When elastomeric coupling elements break down, it’s often due to cyclic loading that causes excessive heat build-up (hysteresis) in the elastomer. Some elastomers are vulnerable to high temperatures, and they have poor resistance to oil, hydraulic fluids, and other chemicals, plus atmospheric contaminants. For this reason, coupling manufacturers offer a choice of elastomeric materials to suit specific operating conditions. Here are the four most commonly used materials:

• Nitrile butadiene rubber (NBR). Sometimes called Buna N, this is the most economical and widely used coupling element material. It resembles natural rubber in resilience and elasticity, plus resistance to oil, hydraulic fluid, and most chemicals. Its operating temperature ranges from 240 to 212 F. NBR provides the best damping capability among elastomeric elements.

• Urethane. This material has 1.5 times the torque capacity of NBR with very good chemical and oil resistance, but less damping capability and a narrower operating range of 230 to 160 F. Urethane is a good choice where an application calls for high torque in a confined space, or resistance to atmospheric effects such as ozone, sunlight, and hydrolysis in tropical conditions. Its cost is 1.5 to 2 times that of NBR.

• Hytrel. Designed for high operating temperature (260 to 250 F), with excellent resistance to oils and chemicals, Hytrel carries 3 times the torque of NBR. It also resists ozone, sunlight, and hydrolysis. With Hytrel, angular misalignment ratings are cut in half, and damping capacity is low. Cost is 3 times that of NBR.

• Bronze. Not really elastomeric, these rigid, oil-impregnated metal inserts are used only for slow speed (up to 250 rpm) applications requiring high torque or high-temperature resistance. Bronze inserts withstand virtually all chemicals and temperatures from 240 to 450 F, but their rigidity gives zero damping capacity. They do offer a small amount of misalignment capability, via clearances between parts. Cost is at least 3 times that of NBR.

Besides these four, some companies offer neoprene, Viton, nylon, and EPDM spiders in limited quantities and sizes.

Spider designs

In addition to the variety of materials, four basic designs of elastomeric elements offer further choices to suit specific applications, Figure 3:

• Solid-center spider. This is the most commonly used design for applications where the distance between ends of driving and driven shafts is large enough to accommodate the spider thickness.

• Open-center type (OCT) spider. This type is used in situations where shaft ends must be positioned closer together than the solid center design allows. A thin segment of elastomeric material connects the spider legs, and fits in a small space between the jaw inner edges and the hub bore. The spider ID is slightly smaller than the hub bore, so that it overlaps the bore. Because the spider’s legs are joined only by a thin segment of material, they have limited support. Accordingly, speed is limited to 1,750 rpm for NBR and 3,600 rpm for urethane and Hytrel. Cost of the open center type is about the same as the solid center type.

Continue on page 2