The Coupling Handbook - Part III
Popular Elastomeric Coupling Types - Compression Loaded, Shear Loaded, Combination, and Torsional
General Elastomeric Capabilities and Types
Elastomeric flexible couplings transmit torque between the two shafts by means of an elastomeric material (rubber, urethane, etc.) positioned between the driving and driven hubs.
The resiliency of the elastomeric material gives these couplings varying degrees of torsional softness not available in all-metal couplings, and generally greater misalignment capability than all-metal couplings. It also allows a single flex plane to accommodate both angular and parallel misalignment. Couplings made as metal flexing element or metal sliding element couplings require two flex planes to achieve parallel misalignment.
Power intensity (torque-carrying capacity vs. coupling size) of elastomeric couplings is lower than that of all-metal couplings. With no (or little) friction wear between components, however, elastomeric couplings are considered low maintenance, although elastomer breakdown in some coupling configurations is a maintenance issue.
Elastomeric couplings are quieter than some all-metal types. The softness of the elastomer cushions the vibration and cyclic torque noises that result from backlash. Noise reduction can be an advantage in certain applications, such as HVAC systems.
Because the elastomeric element handles misalignment by distorting, that action produces reactionary loads on the adjacent shaft bearings. The reactionary loads vary in inverse proportion to the softness of the elastomeric element. In all cases, greater misalignment will mean higher reactionary loads. Combined angular, parallel (radial) and axial misalignments will result in the greatest reactionary load. Speed is a problem for elastomeric couplings. The deflection of an elastomeric coupling is large for the load applied. Large centrifugal forces may cause the element to protrude out of the coupling and hit the coupling guard.
Temperature is a restriction for elastomeric couplings. The material loses its strength as the temperature rises. Eventually the strength reduces to zero. Temperature limits vary by type of elastomer, but generally 200 to 250 °F (110 °C) is the top end. Some elastomeric couplings may be used to dampen torsional vibration energy. Hystersis, a characteristic exhibited by rubber with binders, allows the elastomeric material to absorb dynamic energy. The energy is in turn lost in heat generation. If the material is able to radiate or otherwise conduct the heat to a sink, damping will occur without damage to the coupling elastomer. If the heat builds up in the elastomeric element it will fail or melt down. Elastomeric couplings of both the compression type and shear type are used to control torsional vibration by damping the torsional vibration energy. The amount of hystersis is a function of the elastomeric material as well as the stress level.
Damping of torsional energy in a power transmission system can also be accomplished by means other than the flexible coupling. Frictional dampers, viscous dampers and torque converters are all used. The characteristic of damping exhibited by these couplings is different from torsional tuning of a system. Torsional tuning uses the dynamic torsional stiffness of the coupling to establish a low torsional critical speed.
Torsional stiffness of a coupling is a mechanical property of the coupling materials, modulus of elasticity, and the geometry of the coupling element. Metal couplings usually depend on the spacer piece or floating shaft to lower the resilience or torsional stiffness. Torsional stiffness is described as the torque necessary to deflect a coupling in the circular direction. When dealing with power transmission couplings, it is usually measured in inch-pounds per radian (Newton Meters per radian in metric). Rubber in shear and rubber in compression provide the lowest torsional stiffness. Note that the geometric configuration of the coupling will determine the loading. The unit may not be acting like a torsional spring just because we are applying a torque load. Other elastomers in the plastic range are progressively stiffer. Coupling materials like urethane, and Zytel® make for stiff couplings. They have little resilience, but carry more compressive load. Choice of materials in designing elastomeric couplings is a balance between resilience and load carrying capability. Resilience is helpful for both cyclic loading and misalignment capabilities.
Types of Elastomeric Couplings
Elastomeric couplings classify into three main types by the way their elastomeric element transmits torque - i.e. the element is either "in compression", "in shear", or a combination of the two.
Compression Types. This type of elastomeric coupling is characterized by a design in which the driving and driven hubs rotate in the same plane, with parts of the driving hub pushing parts of the driven hub through elastomeric elements positioned as cushions between them, but not attached to either hub. As torque is transmitted, the elastomeric elements are being compressed. Parallel offset misalignment is accepted via compressive distortion of the elastomer material. Angular misalignment is accepted via sliding or distortion of the elastomer material depending on the method of securing to the hubs.
Compression type couplings generally offer two advantages over shear types. First, because elastomers have higher load capacity in compression than in shear, compression types can transmit higher torque and tolerate greater overload. Second, they offer a greater degree of torsional stiffness, with some designs approaching the positive-displacement stiffness of metallic couplings. However, greater torsional stiffness generally produces higher reactionary shaft loads when the coupling is subject to parallel misalignment.
Shear Types This type of elastomeric coupling is characterized by a design in which all parts of driving and driven hubs rotate in different planes, with the driving hub pulling the driven hub through an elastomeric element attached to both hubs by various methods. These can include clamping, intermeshing teeth, or by bonding to metallic brackets that are bolted to the hubs. As torque is transmitted, the elastomeric element absorbs some of the torque force by being stretched through twisting. The design accepts misalignment through the deflection and distortion of the elastomeric member and also through sliding, if the elastomeric member is attached to the hubs through the use of intermeshing teeth.
Shear type couplings generally offer two advantages over compression types. First, they accommodate more parallel and angular offset while inducing less reactionary load to the bearing. This makes them especially appropriate where shafts may be relatively thin and susceptible to bending. Second, they offer a greater degree of torsional softness, which in some cases provides greater protection against the destructive effects of torsional vibration. Greater torsional softness generally produces lower reactionary shaft loads when the coupling is subjected to misalignment.
The in-shear design also allows the coupling to act as a "fuse" to protect the driver and driven equipment from torque spikes or system overloads which might cause damage elsewhere.
Combination Shear and Compression Type This type of elastomeric coupling transmits torque between hubs through an elastomeric element in-shear, but transmits torque from hub to element (and back again) by compression between hub teeth and intermeshing teeth formed into both ends of the element. Misalignment is accommodated primarily by the sliding of the elastomer against the hub teeth (similar to a gear coupling).
1. Compression Loaded Designs
A classic example of compression-type couplings, first patented in 1927, is the jaw coupling. It is still one of the most widely used flexible couplings in the world and one of the lowest cost couplings available.
Since elastomeric technology was not what it is today, the spiders were originally made from materials such as leather. Now a wide array of materials are available. Typical applications include pumps, gearboxes, compressors, fans/blowers, mixers, conveyors, and generators, usually driven by an electric motor. Jaw couplings usually are not recommended for engine-driven, frequent stop-start or reciprocating loads because they are not designed to dampen torsional vibration. However, they might be able to serve such applications if the proper service factors are used in sizing the coupling. Damping capability depends largely on the geometry, type and amount of elastomer used.
Its design is simple, usually involving only three parts. Both driving and driven hubs have two to seven jaws (thick, stubby protrusions) formed around their circumferences, pointing towards the opposing hub. When the hubs are brought together, jaws from both hubs mesh loosely with each other. Gaps between them, and sometimes the central inner space between the hubs, are filled with an elastomeric material, usually molded into a single asterisk-shaped element called a "spider". The legs of the spider protrude radially to become the cushions between the jaws. Some designs of Jaw couplings use blocks or tubes of rubber that are placed in between the opposing jaw faces and must be held in place through the use of a retaining collar, or the hubs have enclosed cavities into which the elastomer is placed.
In general, the greater the surface area (and volume) of the elastomer in compression, the higher the torque rating of the coupling. Exploded view of Jaw coupling
Torque is transmitted from one shaft to the other through the compression of the elastomer between the driver hub jaws and the driven hub jaws. Since the jaws between the two hubs rotate intermeshed in the same plane, this design is called "fail-safe". If the elastomer should fail, the coupling will still transmit the torque, albeit quite noisily given the metal-to-metal contact. This is still the preferred alternative for some applications, where the equipment is critical to a production process and cannot be allowed to stop.
Some degree of permanent compressive set is normal as elastomeric elements age in service. This is a helpful feature for Jaw couplings; when permanent set reduces the element's original thickness by 25% or more, it provides a visual sign 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 builtin 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 torque ratings are primarily limited by the elastomer material's compression strength, not the jaw/hub strength. Thus, a jaw coupling can handle brief or infrequent torque spikes above the nominal rating far better than the elastomer in-shear designs. It would take a torque of 6 or 7 times the nominal rating of rubber elastomers to break off the hub jaws. If you change the spider from natural rubber to Hytrel® which has much greater compression strength, the torque rating for the coupling is magnified 2 to 3 times. By contrast, an elastomer that transmits torque through a shearing action cannot absorb torque any greater than 3 or 4 times its nominal rating without tearing.
Other features of jaw couplings include; no metal-to-metal contact for quiet operation, resistance to oil/grease/dirt/moisture in many tough environments, simple to install and align, and low maintenance requirements. Many variations of jaw couplings are possible, ranging from flywheel designs, spacer couplings, special hub materials as well as a variety of elastomeric materials to choose from. In addition, jaw couplings are one of the lowest cost couplings.
There are some limitations to jaw couplings. Their angular and parallel misalignment is more limited than with in-shear designs. When misaligned they introduce fairly significant reactionary loads on the shafts. Maximum bore is limited by two factors: the inside diameter of the jaws and the length through bore of the hub. Generally, the bore (shaft diameter) should be no greater than the length of shaft engagement in the hub. Maximum axial float accommodated by jaw couplings is limited to about 10% of the axial thickness of the spider. Most designs have backlash or free play between the fit of the elastomer/jaws and are not suited to motion control applications. Temperature capacity is usually no greater than 250°F (121°C), that keeps them out of some applications. Vertical applications are difficult since standard hubs are clearance fit bores with only one set screw, thus hubs must be modified in order to grip the shaft tightly enough.
Jaw Coupling Types
Several different types of jaw couplings are available to serve different application requirements. Most of them fall into two general categories: a "straight side" type, in which the jaw side faces are flat and straight; and a "curved jaw" type, in which the jaw side faces have a cupped shape.
Straight-side jaw couplings are available in sizes with bore capacities from 1/8" (4mm) up to 2-7/8" (73mm). This coupling type is used for light to medium duty applications with a maximum torque capacity of 6,228 in-lbs. (704 Nm).
Small size hubs are made from sintered iron, while larger sizes are cast iron hubs. Neither sintered iron nor cast iron can be welded to by normal methods.
• Angular misalignment will vary from ½ to 1° maximum depending on the material used. (Materials discussed later.)
• The same goes for parallel misalignment capability, which will vary from .010" to .015" with different spider materials.
Standard straight-side jaw couplings offer several alternatives in spider constructions in addition to the basic asterisk-shaped solid spider or open-center spider spiders, both of which are held captive naturally within the assembled coupling. (Open-center spiders simply allow greater axial freedom for installation on shafts with a close BE dimension.) Alternatives include collar, ring-in-groove, block, and in-shear.
Collar Types are those fitted with elastomeric elements that are installed and removed externally. Such elements usually take the form of a linear spider in which the legs are molded into a single strip of elastomeric material that is wrapped around the assembled coupling so that the legs drop into the spaces between intermeshed jaws. These wrap-around spiders require a circular collar around the coupling's circumference to prevent the elastomeric strip from being flung off by centrifugal force. Typically, the collar is a stamped steel ring held in place by three retaining screws to one of the hubs.
Ring-in-groove types, sometimes called "Snap-Wrap", are similar to collar types except that wrap-around spider is held in place with a Spiralox retaining ring that snaps into a groove that is molded into the spider's perimeter. This version is only available in NBR spider material and the maximum speed is 1750 RPM. Standard hubs are used. The ring is removed easily with needle-nose pliers.
These features are ideal for those situations where the shaft ends must be positioned closely together, yet the shaft diameters are greater than what can be accommodated in the open center type spider.
The compression block types serve heavy-duty applications that require shaft size and/or torque ratings beyond the capability of standard Jaw couplings. Usually of larger diameters, these designs transmit torque through independent blocks of elastomeric material, in cube, oval, or wedge shapes. Sometimes called load cushions or elastomer cylinders, these blocks are individually inserted into the spaces between the assembled coupling's intermeshed jaws, and held in place by a steel collar. This design offers the advantage of easily changing its torsional stiffness by varying the hardness and design of the blocks. The compression block jaw coupling is available with a maximum bore capacity of 12.0" (300 mm), and torque up to 1,000,000 in-lbs. (113,000 N m). Common applications include compressors, large fans, blowers, mixers, and municipal or irrigation pumps.
In-shear spiders, the newest improvement in spider design, completely change the way the jaw coupling functions. These spiders are axially twice as wide as standard spiders for straight sided jaw hubs, so instead of allowing the jaws of both hubs to intermesh in the same plane, they push the hubs apart so the jaws rotate in separate planes, and in axial alignment hub-to-hub. This arrangement causes the radially removable elastomer to transmit torque through a combination of shear and compression method. The spider is held in place with a floating stainless steel ring, which locks into special grooves in the OD of the spider.
As with the collar and snap-wrap designs, the jaw in-shear allows easy removal and replacement of the spider without disturbing the hubs. There are no fasteners to worry about either since the retaining ring slides into grooves in the spider. One available design fits standard straight-sided jaw coupling hubs on the market which makes it an easy retrofit design. Another version uses special hubs with many shorter, stubbier jaws and a special elastomer, but achieves the same concept in features/benefits.
The primary benefits are (1) simplified maintenance (2) non-failsafe operation (3) greater angular misalignment capacity of 2°, and (4) greater torsional softness.
This coupling should only be used for electric motor driven applications, most commonly centrifugal pumps, fans, mixers, gear boxes, and plastic extruding machines.
Special Hub Materials and Designs
Jaw coupling hubs are typically made of sintered iron or, for larger sizes, cast iron. Neither can be welded to by normal methods. In some applications customers will desire to weld the connection of the hub to the shaft, or weld another component such as a shaft collar, sprocket or pulley to the diameter of the hub. Special materials such as 1018 steel, 303/316 stainless or 660/464 bronze are possible to meet those and other unique application requirements. The torque and misalignment ratings do not change based upon the hub material. The elastomer spider determines those ratings.
Light Hubs: This category of the standard jaw coupling uses hubs made from aluminum or other light metals. It provides for a significantly lighter coupling if lower inertia is important. When an application calls for better corrosion resistance than sintered or cast iron, but not the expense of stainless steel, aluminum is a good alternative. Light material hubs use the same spiders as the standard straight sided jaw.
Special modifications such as clamped hubs, bushed hubs, extra long or shorter than standard hubs, and pinholes are possible. The use of clamped hubs or bushings with couplings is common. Generally these are advantageous when the application requires a firmer grip on the shaft than is provided with clearance (slip fit) bores and one or two set screws. These include vertical drives, motion control, or equipment with high levels of vibration and shock loads.
When jaw couplings were invented, elastomeric technology was not what it is today, and spiders were originally made from natural materials such as leather. A wide array of materials are now available, including several non-rubber-based elastomers that offer light weight, chemical resistance with the ability to be molded into complex shapes. They are also economical to manufacture and use. Generally, rubber-based spiders are more resilient and better for cyclic loading and misalignment capabilities, while synthetic spiders make for torsionally stiffer couplings that can carry a more compressive load.
1. NBR (Nitrile Butadiene Rubber) a.k.a. Buna-N -- is the standard and most economical material for jaw coupling spiders. It offers the best combination of temperature and chemical resistance, misalignment, and damping ability. Rubber has the best resiliency in bouncing back from deformations that occur in cyclic or heavy shock loads. This is the only material suitable for reciprocating engine applications.
Most sizes of NBR spiders are 80A-shore hardness and are black in color. Temperature range is -40°F (-40°C) to 212°F (100°C). Also referred to as "SOX" by some manufacturers. This material will allow the Jaw coupling to experience a torsional wind-up at full torque load of 4°-10°, depending on the coupling size.
Another attribute of natural rubber products used in compression is that they take a permanent "set" or loss of volume after just a short time in operation. This does not become a performance problem until the spider thickness is anything less than 75% of its original size, at which point it should be replaced. This limits their selection in motion control/precision applications since increased free-play in the coupling results from the "set". Shelf life of natural rubber elastomers is 5 years.
2. URETHANE has a 1.5 times greater torque capacity than NBR due to its greater compressive strength (either 40D or 55D shore hard ness is used) as well as better abrasion/wear characteristics. It holds up better to environmental conditions such as ozone, ultravio let, and some oils-chemicals versus the NBR. It is limited to -30°F (-34°C) to 160°F (71°C) temperatures however, and should not be used in heavy cyclic or start/stop applications since the damping ability is limited. The in-shear spider is a slightly different type of urethane and is rated for -30°F (-34°C) to 200°F (93°C). Urethane spiders typically are blue color and offer a shelf life of 5 years.
3. HYTREL® increases the torque capacity of the jaw coupling approximately 2½ times versus the NBR with its higher compressiveload carrying ability. These spiders are a tan or cream color with a 55D shore hardness. This material provides the best chemical resistance as well as a temperature range of -60°F (-51°C) to 250°F (121°C). However, as with Urethane, it should not be used in appli cations where cyclic loads, frequent starts/stops, or regular shocks and vibrations occur. The shelf life is 10 years. Angular misalign ment is only 1/2° versus the NBR and Urethane that are both 1°.
4. BRONZE is not an elastomer, but is one of the options available for those high temperature requirements (up to 450°F) which most other materials are not capable of. Most commonly, bronze is selected in salt water/marine applications. It is only to be used for slow speeds, less than 250 RPM, since the coupling will prematurely wear from metal-to-metal contact otherwise.
5. NYLON is a good electrical insulator, holds up well under heavy continuous loading, and may be substituted where bronze is too noisy. The torque rating is the same as for Hytrel®.
6. VITON® is a synthetic rubber that has a temperature range of -65°F to 450°F with a durometer of 75-85A scale. It provides the high temperature capability of bronze with excellent chemical resistance. The torque rating is the same as for NBR and may be slightly derated depending on the application conditions.
7. ZYTEL® is a fiberglass reinforced compound with excellent resistance to most chemicals and corrosion. It is three times more torsionally stiff than Hytrel® and can operate in temperatures ranging from -40°F (-40°C) to 300°F (149°C).
Curved Jaw Couplings
While the straight jaw coupling is known around the world, there is also another design that has wide acceptance, primarily in Europe and Asia. It is generically referred to as the curved jaw coupling. This jaw coupling product is available in sizes covering bores from 5/32" up through 5-11/16" (145mm) and torque from 35 in-lbs. up to 66,375 in-lbs. (7,500 Nm).
While the coupling still consists of two hubs and a spider in the center that is under compression, the main difference is in the geometry of the jaws and the corresponding spider legs. The intermeshing faces of a radial curvature, giving them a concave or cupped shape. This provides a built-in encapsulation of the spider legs by the hubs. The corresponding spider legs are crowned, or curved both axially and radially to follow the jaw face shape, making them similar to a gear tooth in geometry.
The jaw and spider curvature has two important benefits. First, by encapsulating the spider legs, it permits higher speed ratings compared with similar size straight-sided jaw couplings. It also extends angular misalignment capacity to 1.3° for some sizes.
Most curved jaw hubs have four jaws vs. three for similar sizes of straight-sided, with the jaws pushed farther out toward the perimeter of the hub. This enables the spiders to have large open centers. The design characteristic's combine to allow larger maximum bores in most cases and to accommodate close "BSE" dimensions.
The curved jaw design also results in some special limitations. Due to encapsulation, radially removable spiders (wrap around, block) cannot be used. The damping capacity of the design is lessened under greater loads. The overall length of the coupling is usually greater than the similar straight-sided jaw coupling. The type of sintered iron commonly used in the smaller sizes is much denser, translating into heavier couplings. And finally, spacer couplings can only be achieved by using extended hub lengths, this adds a lot of weight and still does not allow for a true drop-out section.
The standard material is urethane for all curved jaw spiders. There are simply three different shore hardness which yield differing levels of torque capacity. Each of the shore hardness numbers are color-coded for easy identification, blue for 80-shore, white/yellow for 92-shore, and red for the 98/95-shore. All of the spiders are an Open Center Type (OCT). The urethane composition allows for a maximum temperature rating of 212°F (100°C) versus the 160°F upper limit for the L-type urethane. Some manufacturers also offer Hytrel® as an alternate material as well.
Also available for curvedjaw applications is the No Backlash (NBL) spider. This is simply a special, thicker spider that can be used with the standard hubs to provide a snugger fit for those low backlash requirements. It only provides a true zero backlash up to 10% of the rated torque of the spider. It is available in two-shore hardness (92-yellow and 98-red). Some manufacturers also offer a special hub, often referred to as the "GS" style, for use with the NBL spiders. The GS hubs are of similar geometry to the standard curved jaw hub except the jaws are slightly oversized to make the intermeshing of the three components a true interference fit. This style can either be pre-assembled at the factory or assembled by the user with the aid of a lubricant since the components are so tightly fitted. It provides full zero backlash performance for motion control applications up to 10-25% of their rated torque, depending on the size.
Special Considerations for Selection
Because this coupling was designed in Europe, it uses the DIN 740 rating methodology, which gives you a Nominal (Tkn) as well as Maximum (Tkmax) rating. Nominal torque Tkn is the steady state design torque for the coupling. Maximum torque Tkmax is a cyclic torque capability for 100,000 cycles or 50,000 reversing cycles.
In terms of the selection process, it means that the Service Factors are unique for the curved jaw coupling. There are independent factors which must be multiplied by the nominal torque of the application to arrive at the design torque. Only when the coupling/spider Tkn and Tkmax ratings are both greater than the respective nominal and design torque (calculated for the application) do you have the proper size coupling. The urethane spiders, while rated for a maximum temperature of 212°F, have a de-rating factor that must be applied to their misalignment capability. This takes effect at any condition above 86°F.
Donut Shaped Elastomeric Couplings
This style of coupling was developed in 1970 for use with diesel engines. The donut shaped elastomeric coupling consists of a rubber donut fastened with cap screws to hubs. The hubs provide the shaft connection. The elastomer mounts in between the hubs to transmit the torque and allow misalignment. Metal inserts (either aluminum or steel) are bonded into the elastomer and provide a durable material through which the fasteners attach to the hubs. The elastomer donut is precompressed between the fasteners to make certain that the torque is always transferred in a compression mode. The elastomer is stronger in compression than in tension. By preloading the donut any tensile forces merely relieve the compression and do not put the unit into a tensile load-carrying situation. Donuts can have a square, rectangular, octagonal or other cross-section design. They do not have to be round.
Donut couplings can have one hub that is smaller than the other to fit inside the donut. It is called the cylindrical hub. The donut is fastened to the inner or cylindrical hub by radial fasteners. The other hub is a flanged hub to which the donut is attached by axial fasteners. The elastomer uses metal inserts that transfer torque by friction between the metal inserts and the metal hubs then through the elastomer to the next set of fasteners attached to the other hub. The torque path alternates from one leg of the donut to the next. The fasteners are tightened to make a high friction joint and avoid loading the bolts in shear. Donut couplings that use the cylinder and flange hub system have bore limits on the cylindrical hubs compared to other couplings of similar torque capabilities.
One way to eliminate the cylindrical hub limitations is to use a wraparound type of elastomer. The inserts are all devised to use radial cap screws to fasten the elastomeric element to alternating hubs. This one does not have the flywheel plate option or material options for the element.
Another design has spider shaped hubs with arms that are at the same diameter as the bolt circle within the donut. Once again the attachment alternates from one hub to the other, but the fasteners are all axial. Torque is carried by an elastomer in compression and is transferred to the hub via metal tabs inserted in the rubber. Torque is also transmitted by the friction between the bolt sleeve inserts and the hubs. The torque path is from hub to insert to elastomer to insert to hub. The elastomer carries the load in compression on alternate legs. This design is not available with flywheel plates or stiff elastomer materials.
Donut type couplings can handle a load in either direction as the load shifts to alternate legs still in compression. Even more importantly the donut can accommodate alternating loads and cyclic loads without backlash. There is windup in elastomeric couplings. These couplings when constructed of rubber exhibit a quality of hystersis. That quality enables the coupling to dampen the vibration energy that passes through the coupling.
Elastomers for Donut Type Couplings
The base elastomer is a natural rubber with binders. It is suitable to about 190°F temperature before it loses strength. When the temperature increases the coupling must be derated. The formulations of this elastomer are identified by the shore hardness. Each successively harder rubber carries more torque, but is torsionally less resilient.
Alternate elastomers include Hytrel® and Zytel®. Each is considerably more stiff than rubber. The change in materials will mean an increase in normal torque capability. The change in material may require the coupling design to change in order to accommodate the fastening of the Donut coupling with all bolts axial elastomer to the metal hub.
Pin & Bushing Type Couplings
Pin & Bushing couplings transmit torque through cylindrical or barrel shaped metal pins that are enclosed in elastomeric bushings. The elastomeric bushing covers one half the pin while the other half has stepped diameters with a threaded end. The shaft connections are flanged hubs drilled to hold the bushing or the threaded end. It can be done with the bushings all in one flanged hub or they can be alternated from side to side. The bushing is inserted into a cylindrical hole while the threaded and stepped end is inserted into a stepped hole with counter bores on either side. A nut is attached to the threads to hold the bushing in place during operation.
The elastomers are compressed into the holes and may have a shape that permits easy installation. Elastomers can be rubber, the original bushing type, Viton®, or urethane type materials. Hubs are cast iron, steel or stainless steel. Pins are steel or stainless steel. The elastomer cushions shock loads and compensates for misalignment. Pin and bushing couplings are inherently fail-safe with the pins continuing to transmit torque when the bushing is worn. The bushing can both wear and fatigue from usage. Pin and bushing couplings are non-lubricated.
The pin and bushing coupling are high capacity vs. their size. The capacity or torque capability is directly related to the bolt circle diameter, number of pins, and the type of elastomer. They are designed to make it easy to replace the pins and bushings which are the wearing components.
2. Shear Loaded Designs
Shear-Type Donut (Sleeve)
The original patent on this design was issued in the late 1950's. The shear type donut coupling (sometimes called a sleeve type) was marketed heavily to the pump industry, in particular the ANSI chemical process pump segment. A strong following was built up which continues to this day. Much like a jaw coupling, the shear type donut is simple in design. A standard coupling is composed of three components, 2 flanges and 1 sleeve. The sleeve (a short, spool-shaped, tubular element) has serrations molded around the perimeter of each end, which mate with corresponding serrations molded into both hub flanges. This puts the element in-shear between the two flanges, so the torque is transmitted through the twisting of the elastomeric sleeve. There are several features to this coupling which translate into tangible benefits to the user:
• Because this design is double-engagement, it is radially very soft and produces very little reactionary load on bearings and shafts when misaligned. However, misalignment will shorten sleeve life.
• The torsionally soft design of an in-shear elastomer helps to dampen out most peak overloads and prevent vibratory torque from going back to the driver.
• The sleeve has a large open center, which allows close positioning of the shafts.
• The torque overload capacity of this coupling is only 3 or 4 times the rated torque (the point at which the sleeve will tear, round-off the teeth, or "pop out"), versus the 6 or 7 times for a jaw coupling. Thus it provides the "fusible link" protection characteristic of in-shear couplings.
The shear type donut style coupling is best suited in the following applications:
• Where system alignment may be hard to maintain over a period of time, and the coupling needs to tolerate the drift.
• Where the motor and pump are on a common base plate but there is no pump mounting bracket involved, i.e. a "non-piloted" pump application.
• Where shafts are closely coupled (i.e. minimal BE dimension).
• Where shafts are relatively small for the torque loads, or the bearings are light duty.
In general, the shear type donut coupling will work well on electric motor driven applications with uniform loads such as; centrifugal pumps, blowers and fans, screw compressors, some conveyors, line shafts, and vacuum pumps. Care should be taken however, that shear type donut couplings are not used under the following conditions:
• Where loads have high-inertia, especially if they produce variable torque loads, or where overloads/spikes are expected to be greater than 2X nominal ratings.
• Where reciprocating engines, compressors or pumps are involved. Shear type donut couplings do not respond well to torsional vibrations. • Where the coupling will operate regularly at less than 25% of its rated torque. The sleeve teeth will wear prematurely due to the rubbing action against the flange if too lightly loaded. This can be a concern particularly with the Hytrel® sleeves since they have such high ratings.
There are five manufacturers of this design. All produce their product to be fully interchangeable. However, serrations in sleeve ends and hub flanges must mate, so components from different manufacturers may not always fit together properly. This is due to the tolerance that is built into each company's initial design criterion (i.e. how tight or loose they want the fit between components to be), and the state of wear of the tooling that produces the sleeves and flanges. Mixing of components from different manufacturers must be avoided if at all possible.
A basic, economical flange, the J-type is available only in four smaller sizes 3 through 6, with smaller bore models cast in zinc alloy and larger bore models in cast iron, all limited to the lower torque sleeve materials (discussed later).
Provides a greater variety of sizes, from 5S to 16S, with all flanges made of cast iron. Characterized by extra cast-in thickness projecting from the inner face of the hub, which allows greater through-the-bore shaft engagement, S-type flanges allow larger bores than available with J-type flanges, and can be used with all sleeve materials.
This flange is modified to accept an industry-standard bushing. Offered in sizes 6B through 16B. The use of a bushing limits the bore capacity of the coupling, but provides a better grip on the shaft. It can also simplify the stock room of many users, if they use bushings on other P.T. components. Due to the torque limits of the bushing, Btype flanges cannot be used with higher torque sleeves.
Similar to the B-type for on industry standard bushing, the T-type is a standard flange modified to accept another industry style of bushing. There are two ways to mount the bushing to the flange. The first way is from the serration side (rear) or from the same side of the flange as the shaft is inserted initially (front). As with the B-flanges, the T-type cannot be used with high torque sleeves due to the limits of the bushing ratings.
Intended primarily for pump applications, these flanges are separable from their shaft-mounted hubs by removal of four hex-head cap screws axially installed through each hub. This enables the flange-and-sleeve assembly to drop out so routine pump maintenance can be performed without disturbing pump or motor mounting and alignment. Various sizes for Spacer Flanges and Spacer Hubs can be mixed/matched to provide ANSI standard separations of 3-1/2", 5" and 7", and dozens of other non-standard shaft separations as well, in coupling sizes 5 through 14. SC type flanges and hubs can also be used in combination with other flanged hubs to create a half-spacer coupling. Any of the available sleeve materials can be used.
Elastomer (Sleeve) Types
EPDM is the standard material used. It is a rubber-like compound that allows the sleeve to twist as much as 15° at full torque. It has the highest temperature rating (275°F/135°C) of the sleeves available. It provides good resistance to most commonly found chemicals and is not affected by dirt or moisture. This sleeve is a dull black color. Sleeves have angular misalignment capability of 1° and parallel misalignment ranging from .010" (size 3 coupling) to .062" (size 16). Neoprene® sleeves, which also can twist as much as 15° at full torque, offer better chemical resistance than EPDM, especially to oil, but is rated only for a max. temperature of 200°F(93°C). The color of the sleeve is black with a shiny finish and a green dot for easy identification. As with EPDM, Neoprene® sleeves have angular misalignment of 1° with parallel misalignment ranging from .010" up to .062" .
HYTREL® is a polyester elastomer designed for high torque and excellent chemical resistance. It carries four times the torque of the EPDM/Neoprene® materials but is limited to ¼° angular misalignment and parallel misalignment from .010" (size 6) up to .035" for size 14 couplings. It only twists to about 7° at full torque. The Hytrel® material is orange in color.
B. Sleeve Designs
One-Piece Solid sleeves are identified by material as JE (EPDM), JN (Neoprene®), and H (Hytrel®) types. They are the least expensive of the rubber sleeves, available in sizes 3 to 10 for JE, and 3 to 8 for JN. For Hytrel®, they are available in sizes 6 to 12.
One-Piece Split sleeves are identified by material as the JES (EPDM) and JNS (Neoprene®) types. They are used for applications where the shafts are positioned closely together and the sleeve must be "peeled away" for replacement. They are available in the same sizes as the JE and JN sleeves.
Two-Piece Split sleeve are made up of two completely separated halves. For the E (EPDM) and N (Neoprene®) styles, a retaining ring is used to prevent the sleeve from bowing outward or being flung off under speed. The HS (Hytrel®) is such a rigid material that the ring is not necessary. The E sleeve is available for size 5 - 16 couplings, the N sleeve for sizes 5 - 14, HS for sizes 6 - 14. This design provides the greatest ease of installation and replacement.
Clamped Elastomer in Shear Couplings
Corded tire types
This design came about in the late 1950's as a solution for dealing with transient torque peaks and shock loads in diesel-driven pumps. Named for their resemblance to an auto tire, this design consists of two flanged hubs equipped with clamping plates, which grip the coupling's hollow, ring-shaped element, by its inner rims. Furthering the similarity, tire coupling elements usually are rubber derivative elastomers with layers of cord, such as nylon, vulcanized into the tire shape. The coupling transmits torque through the friction of the clamp applied to the inner rims of the tire and a shearing of the element. Slippage of the coupling may be expected to occur at about four times the rated torque.
The two significant limitations to the corded tire type coupling are speed and space constraints. As speed increases, the coupling exerts axial forces on the shafts due to the centrifugal forces working on the elastomer. And the geometry of the tire itself makes for a large outside diameter for its torque capability. A design variation includes an inverted tire coupling in which the tire element arcs inward toward the axis, thus overcoming the centrifugal forces at speed. This affords 10-30% higher RPM service, depending on its size.
The corded tire coupling is torsionally soft and can dampen vibration. High radial softness accommodates angular misalignment up to 4° and parallel offset up to 1/8". Rare among elastomeric couplings is its capability to allow a certain amount of axial shaft movement. These properties give corded tire designs a wide variety of applications including those driven by internal combustion engines. This coupling is offered in spacer designs as well as with hubs which can accept bushings.
Bonded Urethane Tire
This design was first marketed in the 1970's and has found success primarily in the process pump industry because of several features that the corded tire lacks. The design utilizes a urethane material that is bonded to two half-circle metal rings (a.k.a. "shoes") which are then bolted to the two hubs. Torque is transmitted from the hubs through the shoes/bonded joint and then the shear-plane of the split urethane tire.
The design offers advantages such as radial removal of the element halves, high angular misalignment capability (4°), and shock load cushioning. In its standard close coupled configuration, it can span greater BSE lengths than most in-compression couplings, and it also has the large opening in the center of the tire to allow complete flexibility in positioning shaft ends. The outside diameter (OD) of this design is also smaller than the Corded Tire type for similar shaft and torque capacities.
Spacer couplings are achieved by using the same shaft hubs and simply extending the lengths of the steel shoes onto which the elastomer is bonded. Hubs can also be reversed in their mounting orientation to further add to the BSE permutations possible. Bushings are also commonly used on this style of coupling. A heavy duty elastomer option (25% more torque) is available, but it reduces the misalignment capacity by 50%.
It has proven to be ideal in applications such as pumps, screw compressors, blowers, mixers, crushers, and general power transmission drives.
Limitations of the urethane tire type include the large number of fasteners required for installation and removal of the elastomer, and the fatigue of the element and the bond between steel and elastomer under torsional vibrations.
3. Combination Shear & Compression Loaded Designs
Jaw with Elastomer In-Shear
Another design of elastomeric jaw coupling completely changes the way the jaw coupling functions. Instead of the jaws of the hubs interlocking, the use of an in-shear spider pushes the hubs apart and aligns the jaws of each hub along the same axial plane. Thein-shear spider then is twice the axial width of a standard spider, and it is loaded in shear rather than in compression. This spider provides certain features different from common jaw couplings.
• Radial removable spider
• In-Shear design for non-failsafe operation
• No metal-to-metal contact should the elastomer fail
• No need for tools to install or replace the elastomer
• Non-lubrication benefit of an elastomeric coupling
• 2° angular misalignment
A floating ring encases the outside of the spider and locks into special grooves on its OD. There are several designs on the market, with only one manufacturer offering the benefit that this special in-shear spider is used with regular jaw coupling hubs, same as for the in compression design. Urethane is the most common elastomer material available. It has a combination of durability, chemical resistance, and torque/load carrying strengths.
This coupling should only be selected for electric motor driven applications. The most common ones include centrifugal pumps, fans, mixers, gearboxes, and plastic extruding machines.
Torque ratings and service factors are unique for this version of a jaw coupling.
Gear with Elastomer
This is a gear coupling like the continuous sleeve gear coupling, except the sleeve is made from a slippery elastomeric material. The advantage of this type of sleeve material is the no lubrication feature. They are limited in torque, speed and size. The limits are imposed because the hub tooth to sleeve tooth friction eventually exceeds the elastomers inherent lubrication capability.
The coupling consists of a molded nylon continuous sleeve with internal gear teeth that match gear teeth on the periphery of a metal hub. The metal hubs are made from steel bar stock or powdered metal. The combination of molded sleeves and powdered metal pressed hubs are a very economical coupling combination. The hubs are held in the sleeve by spiral rings. The hubs are mounted on the shaft with the traditional clearance fit key, set screw or a clamped type split hub. The nylon sleeve has high torsional stiffness and is resistant to chemical attack. The hub tooth is crowned to obtain the misalignment capability. Backlash is designed to be at a minimum in this style of coupling. Misalignment capability varies from 1° to as much as 5° depending on the manufacturer.
Nylon sleeve couplings are used for motor/generator sets, pump sets and other light to medium duty industrial applications. Often they are used on the front power take-off of internal combustion engines because they are small and lightweight. The coupling configuration permits vertical and blind assembly when needed. Speed capabilities under light loads can reach 5000 RPM, misalignment to 5° and ambient temperature to 150 °F. The bore capability is usually less than 2 inches for the popular sizes, however like all clearance fit couplings some versions are available to 4 inches bore. For torque capabilities refer to the manufacturers catalogs as the torque is tied to the speed rating as much as to the physical properties of the coupling.
4. Torsional Couplings
Several designs of couplings were developed to solve the problem of damping torsional vibration. The primary source of those vibrations are diesel engines, but there could be other sources. The torsional vibration travels through the coupling to the connected equipment. The vibrations can damage both the connected equipment and the coupling itself. A discussion of torsional systems and vibrations are included in the Applications section of this handbook.
The primary torsional coupling uses a resilient elastomer as the flexing medium. All of the couplings described in the elastomeric section of the handbook have been used on torsional service with varying degrees of success. The elastomeric types described in the following section are the couplings with the best attributes for torsional service.
The elastomer shape used in the coupling is very important for damping and damping is an important attribute for the torsional coupling. The most successful shapes are radially loaded cylinders, toruses, and spheres. In addition, the thickness may be much larger than found in conventional elastomeric couplings. Sharp corners are usually avoided in the torsional designs to reduce stress concentrations.
There are also non-resilient elastomers used for torsional couplings. Non resilient couplings or stiff torsional couplings are used for low inertia, diesel driven equipment.
Some metallic couplings were designed with the diesel in mind. One notable one is the grid coupling described in its own section of this handbook.
The resiliency and the torsional softness of the coupling are used to judge the coupling's ability in torsional systems. Torsionally soft couplings are those units that have a ratio of dynamic torsional stiffness to nominal torque of less than 30.
Donut Shaped Elastomeric Couplings
The donut shaped elastomeric coupling consists of a rubber donut fastened with cap screws to hubs. The hubs provide the shaft connection, the elastomer mounts in between the hubs to transmit the torque and allow misalignment. Metal inserts (either aluminum or steel) are bonded into the elastomer and provide a durable material through which the fasteners attach to the hubs. The elastomer donut is precompressed between the fasteners to make certain that the torque is always transferred in a compression mode. The elastomer is stronger in compression than in tension. By preloading the donut, any tensile forces merely relieves the compression and does not put the unit into a tensile load-carrying situation. Donuts can have a square, rectangular, octagonal or other cross-section design. They do not have to be round.
Donut couplings have one hub that is smaller than the other to fit inside the donut. It is called the cylindrical hub. The donut is fastened to the inner or cylindrical hub by radial fasteners. The other hub is a flanged hub to which the donut is attached by axial fasteners. The elastomer uses metal inserts that transfer torque by friction between the metal inserts and the metal hubs then through the elastomer to the next set of fasteners attached to the other hub. The torque path alternates from one leg of the donut to the next. The fasteners are tightened so make a high friction joint to avoid loading the bolts in shear. Donut couplings that use the cylinder and flange hub system have bore limits on the cylindrical hubs compared to other couplings of similar torque capabilities.
The donut style elastomeric coupling is primarily used on torsional damping and tuning systems associated with Diesel drivers. In such a case a flywheel plate replaces the flanged hub. The flywheel plate is drilled to match various SAE designated or DIN designated flywheel dimensions. The coupling is configured to dissipate heat that is generated by hysteresis. It is also rated for a maximum torque, a nominal torque, and a vibratory torque. Each of the values are different, the maximum torque is limited to a specific number of cycles.
Donut type couplings can handle a load in either direction as the load shifts to alternate legs still in compression. The donut can accommodate alternating loads and cyclic loads without backlash. There is windup in elastomeric couplings. These couplings when constructed of rubber exhibit a quality of hystersis. That quality enables the coupling to dampen the vibration energy that passes through the coupling.
Elastomers for Donut Type couplings
The base elastomer is a natural rubber with binders. It is suitable to about 190°F temperature before it loses strength. When the temperature increases the coupling must be derated. The formulations of this elastomer are identified by the shore hardness. Each successively harder rubber carries more torque, but is torsionally less resilient. The variations allow the application engineer to tune the system for critical speed as well as torsional vibration damping. Hystersis, a characteristic exhibited by rubber with binders, allows the elastomeric material to adsorb dynamic energy. The energy is in turn is lost in heat generation.
If the material is able to radiate or otherwise conduct the heat to a sink, damping will occur without damage to the coupling elastomer. If the heat builds up in the elastomeric element it will fail or melt down.
Alternate elastomers include Hytrel® and Zytel®. Each is considerably more stiff than rubber. The change in material may require the coupling design to change to accommodate the fastening of the elastomer to the metal hub. The increase in stiffness changes the unit from torsionally soft to torsionally stiff, and as a result the tuned critical moves from a value below operating speed to one above operating speed. The change in materials will mean an increase in normal torque capability. Refer to the chapter on torsional applications for more information on critical speeds and damping requirements.
Elastomer Block Compression Couplings
This type of coupling is similar to the jaw coupling in that torque is transferred from one hub to the other by compressing captured rubber blocks. In this case the hubs consist of an external claw hub matched to an internal pocket hub that contains the elastomer. There are several varieties of these couplings.
Variations include the shape of the elastomer blocks and the type of elastomer. The ideal shape for the elastomer is a cylinder, loaded radially. Alternatives use rounded-off rectangular shapes. The coupling is used for both shaft-to-shaft connections as well as shaft-to-flywheel connections. A popular application for this coupling is the diesel driven generator. Another common application is the synchronous motor driven compressor. Both of these applications are very high horsepower units. An example of a lower horsepower application is the electric motor driven reciprocating compressor.
This style of torsional coupling is manufactured with OD's of 3 inches to several feet. Obviously the large size carries a very high torque that is associated with the large generator sets and ship propulsion. The hubs can be a casting of iron or bar stock or a forging of steel. Torsional stiffness ratio for these units is in the medium range, which is consistent with the application requirements. A very soft unit would either not have the torque capability or would have to be dimensionally too big to get the torque capability.
Bonded Elastomer in Shear Couplings
This coupling was developed in 1980 for diesel flywheel applications. There are two basic types and both use an elastomer element in-shear. These couplings have a very low torsional stiffness ratio, in the range of 1.5 to 12. The normal torque capability of these couplings range from 900 inch-pounds to more than 1,000,000 inch-pounds depending on the size and type. They are used with the largest of diesel engines and small ones when extremely low torsional stiffness is needed. The couplings can be configured with elements in series to reduce the torsional stiffness even more, or in parallel to increase the torque capabilities. As with other rubber couplings there are several elastomer variations that are identified by the shore hardness. The higher the "shore" number ithe stiffer the torsionaly coupling is.
The first type of bonded elastomer in-shear coupling is a rubber disk bonded to an inner (or driven) metal ring. The inner ring can be combined with various hub types for fastening to a shaft. The types include tapered OD split hubs, bolted straight bore cylindrical hubs, and special hubs for connecting to U-Joint shafting. The outer diameter of the rubber disk is an external toothed form that slides into a circular metal ring with internal teeth. The circular metal ring is cast aluminum to keep the weight, and therefore the inertia, low. The outer ring OD is configured to bolt to a diesel engine flywheel.
The rubber disk has a designed shape to ensure that equal stress occurs over most of its section, thus providing a large torsional angle and avoiding high stress in these areas. Loading at the inner ring and outer teeth is reduced below normally accepted levels by the design of those two areas. The load is carried in shear from its periphery to its center. This style has a non-linear torsional stiffness. The element could have some backlash in the tooth form at the periphery, although normallyit it is a tight fit. The tooth area becomes a wear point when the coupling is misaligned. Coupling life therefore, is dependant on the wear as well as the torsional loading cycles.
The second type of bonder elastomer in-shear is a four-sided closed ring of elastomer with a special cross sectional shape. The OD, the ID, and one side are flat and perpendicular to each other, the fourth side is tapered from OD to ID in a conical shape. The torque load is carried from one side to the other via shear forces. Both the OD and the thickness from side to side determine torque capability. The elastomer is bonded to metal plates on each side. The plate on the flat side is configured to attach to a flywheel adapter or a shaft hub disk at the OD. The plate on the other side matches the conical shape and is configured to bolt to coupling hubs or half couplings at the ID. The ID may include plain bearings to carry minor radial and axial loads.
There is a wide variety of secondary couplings that are bolted to the side opposite the flywheel. They include gear coupling halves, link couplings, and disc plates. The secondary couplings provide misalignment capabilities not available from the primary torsional coupling. Cardan shaft adapters and clutches have also been attached to the coupling.
The rubber element again has a designed shape to provide for equal stresses across the element. The element has a linear torsional stiffness. There is no backlash in this style of element, however there is torsional windup. This coupling is a non-wear configuration and coupling life is dependent on the torsional damping and maximum load cycles.
Torsionally Stiff Couplings (Flywheel)
While torsional softness can be a benefit for elastomeric couplings, there are some applications that require stiff elastomers. Most of the elastomeric coupling types have an alternative stiff elastomeric material. Jaw coupling, donut shaped compression loaded, and unclamped donut in shear are sometimes supplied with Hytrel® or other stiff elastomers such as Zytel® or urethane. The stiff elastomer is used for greater torque capability without going to a larger size. Stiff elastomers have less resilience and may restrict the angular misalignment capability to much lower values.
Many times the switch to a torsionally stiffer elastomer is to tune the torsional system to a higher natural frequency. This is done on some diesel driven systems with light inertia loading. One example is a diesel driven hydraulic pump for off-highway equipment. Torsionally stiff couplings for these applications are a significant coupling need. The couplings are designed for attachment from a flywheel to a driven shaft. The couplings can be of the compression type as described under the "Donut Shaped Elastomeric Coupling" section or can be a stiff elastomeric disc loaded in shear.
The stiff elastomeric disk, loaded in shear, has a torque capability up to 21,240 inch pounds. The torsional stiffness ratio is above 100. Normally this is a high volume molded disk to make an economical coupling for small diesel production engines.
Shear loaded disks are molded of Zytel® or nylon with strengthening fibers. The disk is designed with boltholes on the periphery to match a flywheel-drilling pattern. The ID is designed to mate with a coupling hub in a sliding fit. The coupling hub can have typical gear coupling teeth with crowning or can have four to six crowned dogs. The crowning accommodates a limited amount of angular misalignment while transferring torque from the element to the hub. Hubs are made from steel bar stock or from powdered metal. The hub bore is usually a spline to match a standard hydraulic pump shaft, but could be a straight bore with key.
The torsionally stiff coupling for flywheel applications is designed for blind assembly. Having the shaft hub slide into the flywheel attached elastomer does this. This coupling type is often supplied with pump mounting plates and flywheel enclosures.