Spiders are key to jaw coupling performance
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.
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• Snap-wrap spider. This flat-strip, open-end version connects the spider legs around the perimeter of the coupling rather than at the center, so they can be installed without disturbing the hub alignment. With no center connections, the spider does not overlap into the hub bore, thereby allowing shaft ends to extend fully into the bore (to a minimal gap). Wrapped around the jaws, this type of spider is held in place by either a ring or collar. When retained by a ring, it has a speed limit of 1,750 rpm. The collar configuration, on the other hand, achieves the same speed rating as a standard coupling because the collar is attached to one hub. Cost is about twice that of the previous two types.
• Load cushions. As separate blocks, these cushions are easy to install and remove radially, which is especially helpful in heavy-duty applications. Typically available in NBR and Hytrel, but only for certain coupling models, load cushions are held in place by a collar.
The first three types of elastomeric elements are generally available in 3/8 to 7/8-in. bore diameter, the forth type in 3 to 7-in. bore.
Selecting replacements
When replacing a failed elastomeric element in a jaw coupling, it’s easiest to find one similar to the original part (if not identical), perhaps applying a fudge factor based on torque, just to be conservative. Too often, however, this procedure invites a repeat failure or equally short service life.
A better approach is to first determine why the previous element failed or wore out prematurely. Either the material or design may have been the wrong choice initially.
The following application criteria will help you choose the correct element:
• Actual torque needed at the driven shaft, including variable torque caused by cyclical or erratic loading.
• Vibration, both linear and torsional. Experienced vendors can assist you with vibration analysis.
• Shaft-to-shaft alignment, both angular and parallel. Note whether driving and driven units are, or can be, mounted on a common base plate.
• Ambient conditions, such as temperature and exposure to chemicals and oils.
• Start-stop-reversing requirements.
• Axial movement, space between shaft ends, or other space limitations.
• Installation or maintenance restrictions.
Resist the temptation to overstate coupling service factors. These factors are intended to compensate for torque or load variations in different drive systems; if chosen too conservatively, they can misguide the selection of both coupling types and their elastomeric materials. Aside from raising coupling costs unnecessarily, selection based on overly high service factors often causes damage elsewhere in the system.
After reviewing these criteria, select a coupling and spider from the manufacturer’s catalog. If you’re not sure, review the intended operating conditions with the manufacturer or vendor. Seek not only the vendor’s recommendations for the type of spider, but also the reasons behind those recommendations.
Maintenance tips
Some permanent compressive set normally occurs as elastomeric elements age in service. When permanent set reduces the element’s original thickness by 25%, replace the element.
Compression is applied only to the spider legs or cushions forward of the driving jaws — trailing legs or cushions behind the driving jaws remain relaxed, Figure 2. Accordingly, when compressive set reaches a maximum in the driving legs, advance the trailing legs or load cushions into the driving position. In effect, jaw couplings carry built-in replacement elements, which reduces replacement costs in most applications. Couplings applied in reversing drives or those with frequently varying torque usually relinquish this benefit.
The jaws of one hub should be prevented from contacting the face of the opposing hub, which would cause a noisy, grinding action. For this reason, spiders and load cushions often incorporate spacer dots that ensure separation between the hubs, Figure 3. When no dots are provided, make sure the spider is thick enough to ensure that the two opposing halves of the coupling do not touch each other.
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Coupling basics
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 deg 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 element 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 towards 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 hubs by being stretched between them.
Capabilities. 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 higher torque and tolerate more overload than shear types. Heavyduty 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.
Jaw couplings are not suitable for most engine-driven or frequent start-stop-reversing applications because of backlash (hub rotation allowed by the spacing between jaws and spider legs). Also, this backlash makes most jaw couplings unsuitable for positive-displacement (pump) and precision motion control applications.
Mark McCullough is a product manager, Lovejoy Inc., Downers Grove, Ill.