Focus on Mechanical Couplings: The Spider Insider
By Mark McCullough
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 protrusio...
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 moulded 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 can 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 1 degree and tolerate up to 0.015 in. of parallel misalignment. Jaw designs usually are not recommended for engine-driven or frequent start-stop-reverse applications because of backlash (the amount of free hub movement allowed by the spacing between the jaws and spider legs).
Elastomeric couplings can be classified 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 force by being stretched between them through twisting.
Both compression and shear types offer advantages that rightfully guide them into different applications. Where compression couplings are preferred, it’s generally because of four main advantages. The first is that elastomers–especially synthetic rubber–have higher load capacity in compression than they do in shear. Therefore, compression types can transmit higher torque, and tolerate greater overload, than shear types. The greater the surface area of the elastomer in compression, the higher the torque rating of the coupling. Heavy-duty jaw models with up to seven jaws can accommodate nominal torque ranges up to 170,000 in.-lb.
Second, compression types offer a greater degree of torsional stiffness (less “twist” between hubs) than shear types, with some designs coming fairly close to the very stiff positive-displacement transmission of torque that is characteristic of metallic couplings, offering near-equal movement of the driven shaft for each incremental movement of the driving shaft. With most jaw couplings, however, even small amounts of backlash can make them inappropriate for true positive-displacement applications.
Third, jaw designs are considered fail-safe because the coupling is not necessarily destroyed or rendered inoperable if the spider breaks away. The driving jaws simply advance to contact the driven jaws directly, and the coupling continues to function (albeit with considerable noise and accelerated wear), preventing critical system downtime until the spider can be replaced at a more convenient moment. For this reason, in a jaw coupling of good quality, the jaws are designed to withstand at least 10 times their coupling’s torque rating.
Fourth, simple design with only three parts–a spider housed between two metal hubs–allows easy installation, disassembly and visual inspection. The specially contoured spider usually allows “blind fit” even in the most confined spaces.
Compared to couplings made entirely of metal, jaw and other elastomeric types generally offer additional advantages of: greater radial softness (this allows more misalignment between hubs and/or less reactionary load on bearings); no metal-to-metal contact between driving/driven parts (so there is less internal wear and no need for lubrication); lighter weight and lower cost, when comparing torque capacity to maximum bore capacity; quieter operation; and easy field-replacement of the torque-transmitting element (element failure does not cause loss of the entire coupling).
When elastomeric coupling elements break down, it’s often due to cyclic loading when hysteresis (internal heat build-up in the elastomer) exceeds the material’s limits. Some elastomerics also are more vulnerable to high ambient temperatures and some types of oil, chemical or atmospheric exposure. For this reason, elastomeric couplings offer a selection of element materials to suit specific operating conditions:
– NBR (Nitrile Butadiene Rubber)–sometimes called Buna N–the most economical and widely used standard coupling element material, resembles natural rubber in resilience and elasticity; resistant to oil, hydraulic fluid and most chemicals. Operating temperature ranges from -40F to +212F (-40C to +100C). With a hardness of 80 Shore A, NBR provides the best damping capability among elastomeric elements.
– Urethane–has 1.5 times the torque capacity of NBR with very good chemical and oil resistance, but less damping capability (90 Shore A hardness) and narrower operating temperature range of -30F to +160F (-39C to +71C). Urethane spiders are good choices when the application calls for greater torque in a confined space, or for resistance to atmospheric effects such as ozone, sunlight, and hydrolysis in tropical conditions.
– Hytrel–designed for high operating temperatures of -60F to +250F (-51C to +121C), with excellent resistance to oils and chemicals; it can carry three times the torque of standard NBR. Hytrel also provides resistance to ozone, sunlight, and hydrolysis in tropical conditions. With hardness of 55 Shore D, however, it cuts angular misalignment ratings in half, and damping capacity is low. (Hytrel is a trademark of E.I. DuPont de Nemours & Co.)
– Bronze–Not exactly elastomeric, these rigid, porous, oil-impregnated metal inserts are used only for slow speed (250 rpm maximum) applications requiring high torque or high temperature resistance. Bronze spiders can withstand virtually all chemicals and temperatures from -40 to +450F (-40 to +232C), but their rigidity offers zero damping capacity.
In addition to the available variety of materials, four basic mechanical designs allow further choices to suit specific applications:
– Standard Solid-Centre Spider–This is the most commonly used design in general power transmission applications where the BE dimension (distance Between Ends of driving and driven shafts) affords a suitable gap and will remain fairly constant.
– Open-Centre Type (OCT)–This design often suits close BE situations where equipment must be positioned as closely together as possible. However, because the spider’s legs are joined only by a thin segment of material, this design has no full-diameter support. Accordingly, it has maximum speed limitations of 1,750 rpm for NBR and 3,600 rpm for Urethane/Hytrel, Also, keep in mind that
the hole in the centre is not as large as the maximum bore of the hub, because while the maximum bore of the hub approaches the inner edge of the jaws, a certain amount of elastomeric material must extend around the inner edge of the jaws to connect the legs of the spider.
– Snap-Wrap–This flat-strip, open-end design connects the spider legs around the perimeter of the coupling rather than at the centre, allowing easy removal or installation without disturbing the alignment of either coupling hub. With no centre connections, this design does not overlap into the bore, and therefore allows shaft ends to extend at maximum bore diameter to a minimal BE. Radially “wrapped” around the jaws, this type of spider must be held in place by either a ring or a collar. When retained by ring, it has a maximum rpm limit of 1,750. The collar configuration, on the other hand, achieves the same rpm rating as the standard coupling because the collar is attached to one hub.
– Load Cushions–As small, separate blocks, these cushions may easily be installed and removed radially, which can be very helpful for maintenance in heavy-duty applications. Typically available in NBR and Hytrel materials, but only for certain models of coupling, load cushions must be held in place by a collar.
In the past few years, jaw couplings have evolved two more variations. First came the curved jaw design, so named because jaws and spider legs have both radial and axial curvature (crowning). This hub and spider configuration offers more torsional softness at lower torque levels than the standard jaw coupling and extends angular misalignment capability up to 1.5 degrees.
The second and newest development is the jaw in-shear design. Here, driving and driven hubs are backed away from each other so their jaws rotate in separate planes, connected by a wider spider that transmits torque in-shear. This design provides torsionally softer performance, permits angular misalignment up to 2 degrees, and allows greater axial float. The spider is a wrap-around type held in place by a retaining ring. One design is available in which the ring twist-locks into grooves moulded into the spider to eliminate any need for fasteners or tools.
Although in-shear transmission abdicates the fail-safe benefit of standard jaw designs, it allows this coupling to serve as a fuse in case of torque overloads, letting the spider fail first in order to prevent damage to something more costly elsewhere in the system. The spider fits many existing standard jaw couplings, allowing users to easily and economically convert standard units to in-shear service.
In-shear designs use 50D Shore hardness urethane spiders. These have torsionally softer performance than standard in-compression elastomers, plus five degree windup capability at full load, which provides greater protection against the destructive effects of torsional vibration. The material withstands temperatures up to 200F (93C) and handles angular misalignment up to 2 degrees, while inducing less reactionary bearing load. The in-shear spider’s extra width also gives the coupling greater axial float, ranging from 0.031-in. (0.78 mm) up to 0.063-in. (1.6 mm).
Some degree of permanent compressive set is normal as elastomeric elements age in service. This is a helpful feature for jaw couplings; as when permanent set reduces the element’s original thickness by 25 per cent, it provides a visual signal that the element should be replaced.
Another helpful feature unique to jaw couplings is that compression is applied only to the spider legs or load cushions forward of the driving jaws–trailing legs or cushions behind the driving jaws remain relaxed. Accordingly, when compressive set reaches maximum in the driving cushions, the spider’s trailing legs or cushions can be advanced into the driving position. Thus, in most applications, jaw couplings carry a built-in set of replacement elastomers, which can be used to reduce replacement costs. Note that couplings applied in reversing drives or those with frequently varying torque usually relinquish this benefit.
Jaw coupling installations never should allow the jaws of one hub to contact the face of the opposing hub, which would cause a noisy, “grinding” action. For that reason, spiders and load cushions often are equipped with spacer dots designed to enforce a suitable separation between metal components. When no dots are provided, extra care should be taken to assure that the two opposing halves of the coupling do not touch each other.
When replacing the elastomeric element in jaw couplings, it’s always easiest to simply find something similar to what was there before (if not identical) and perhaps apply a fudge factor based on torque, just to be conservative. Too often, however, this process only invites a repeat failure or equally short service life.
A better approach is to first give some thought to why the previous spider failed or wore out sooner than expected. Always consider that the material or design that failed might have been the wrong choice in the first place.
The following application criteria are helpful in determining the correct choice of spider:
– actual torque needed at the driven shaft (note variable torque resulting from cyclical or erratic loading);
– vibration (linear and torsional) –experienced vendors can assist you with vibration analysis;
– shaft-to-shaft alignment (angular and parallel)–and note whether driving/driven units are or can be mounted on a common base plate;
– ambient conditions (temperature range, exposure to chemicals, oils, etc.)
– start-stop-reversing requirements;
– axial movement, BE or other space limitations; and
– installation or maintenance restrictions.
As with all couplings, it’s important to resist the temptation to overstate service factors. Coupling service factors are intended to compensate for the variation of torque loads typical of different kinds of driven systems; if chosen too conservatively, they can misguide the selection of both coupling types and their elastomeric materials. Aside from raising costs to unnecessary levels, “over service-factor” selection very often causes damage elsewhere in the system.
Thoroughly review the above operating conditions with your coupling vendors and seek not only their recommendations for the right type of spider, but also the reasons behind those recommendations. With the rich variety of elastomeric materials available today, careful selection usually leads to a long-lasting match between coupling characteristics and the demands of the application.
Mark McCullough is Director, Marketing & Application Engineering, for Lovejoy, Inc., Downers Grove, IL