The Coupling Handbook: Part VI

Popular Non-Lubricated Metallic Coupling Types - Disc, Diaphragm, Spring, 3 Link, & Beam

Standard Couplings and Special Purpose Couplings

All metal couplings such as the gear, disc or diaphragm types are available in standard cataloged units and special or made to order (MTO) units. The catalog standard units are designed and manufactured to meet competitive market requirements. MTO couplings can be totally new designs, but most often for economy, simply add modifications to standard units. 

Catalog data usually provides a guide to available capabilities. In addition there are special or made to order units for which the catalog data is a guide to available capabilities.

In the case of gear couplings there is an AGMA standard for mating dimensions from size 1½ to 9 that is followed by coupling suppliers all over the world. Disc couplings are not quite as standard as the gear units, but market forces have driven the various manufacturers to similar designs for catalog standard units. Diaphragm couplings have the least standardization of the three, and in fact more often fit into the special purpose category.

Special purpose couplings are used to meet unusual OEM requirements, process industry requirements, military requirements, or extreme horsepower/speed applications. The special purpose units are found in the gear, disc, and diaphragm design of couplings. Each manufacturer makes claims for their design under competing circumstances. The competing designs could be two disc couplings, disc vs. diaphragm or gear vs. disc. There is not necessarily a complete overlap of the designs so the coupling user or system designer must pick the coupling design that has the most favorable attributes.

Metallic Flexible Element Couplings

This group of power transmission couplings, consisting of disc, diaphragm, bellows, link and spiral wound springs, uses the flexibility of thin metal elements to handle misalignment requirements while the strength of metal is used to transmit torque. Each type will be discussed later in this chapter, but first we offer an overview to put them in perspective.

All five types are designated as infinite-life, low-maintenance and nonlubricated. Infinite life indicates they are designed to operate without breaking or wearing out over time. This usually means the flexing and load carrying methods do not cause wear and are within the endurance limit of the flexing materials. These couplings require a sophisticated stress analysis to determine the loads on the flexing element under varying combinations of load from torque, misalignment and displacement, plus temperature and the loads associated with rotation over a wide range of speeds. In addition, the method of attaching the flexing element to a shaft hub requires a close analysis, as that junction could be the weakest point under some conditions.

The disc and the diaphragm coupling have enjoyed increased reliability from the advancements made in metals technology in analyzing the stresses in flex elements. One analysis tool that is most often used is the Finite Element Analysis (FEA). While this method has been available for many years, the advent of cheap fast personal computers has made it much more practical as a tool for the design engineer.

Three of these types are found in general purpose applications; they are the disc, the link and the spiral wound spring. The disc and the diaphragm are used for high-energy applications and the bellows and the disc are used for motion control applications. Although disc couplings have the broadest popularity, users should be aware that each type of coupling has its limits. 

Sometimes the limit is an economic one, where the coupling can be suitable but is economically at a big disadvantage to another coupling type that works equally well.
This group of couplings has been around for many years, in the case of disc couplings, since 1914. The advent of high-speed machinery made them more popular. They offer the solution to many of the problems associated with high speed, such as balance and the need for lubrication. To make sure the couplings were suitable for high speed machines it was necessary for the engineering community to develop computerized stress analysis methods, and new materials to make the couplings more practical and economical. Even with the advances of modern engineering, these couplings remain more expensive than many of the alternative gear and elastomeric units. These couplings are more sensitive to environmental conditions and rough handling than other types of couplings. While they have a theoretical infinite life, their life can be shortened to a finite one if conditions of the application change, or if they are not carefully accounted for at the time of coupling selection.

With careful selection, the metallic flexible element coupling can be the best choice for high speed, high power and high accuracy applications. In some cases, such as those that require no-backlash or torsionally very stiff couplings, no other couplings are suitable.

The disc coupling became popular as high-speed machinery created demand for non-lubricated, low maintenance, long life couplings. That machinery includes boiler feed pumps, gas and steam turbines, compressors, high-speed test stands and marine propulsion systems. Now industrial disc couplings are used by a wide range of rotating machinery applications.

With careful design and analysis the disc coupling can provide a near infinite life unit. It consists of two flanged hubs and one or more disc packs and a center member. There is also a version that uses two hubs and one disc pack. The disc pack is a flat ring with boltholes on a fixed pitch circle. The disc pack is attached to flanges on either side via alternate bolts. The leg of the disc is loaded in tension, and transmits torque in a tangential direction to the disc. Disc packs are often unitized by the coupling manufacturer to make assembly easier.

Single disc pack units (one flex point) will accommodate angular misalignment and axial displacement only. The design configuration that uses two disc packs separated by a center member can accommodate parallel misalignment, axial displacement and angular misalignment. The amount of radial misalignment depends on the combination of disc pack angular capability and the distance between the two flexing points. Sometimes, a long floating shaft separates two single disc pack couplings. These are used to span long gaps as well as provide radial misalignment.

The discs can be circular in shape, flatsided on the OD, or can be scalloped on the OD. As we go from circular to flat to scalloped ODs, the cost of the disc is theoretically higher as the shape is more complex to form.
The disc performance improves with the same progression in shape. Circular discs act like a curved beam, which means the stresses can be very high on the extreme fibers. The flat-sided disc eliminates the curve beam problems and puts the leg in tension. It is more flexible than the circular one since it is slimmer across the width of the leg. The scalloped disc is the most flexible as it is the slimmest. In the cases of the flat- sided and scalloped disc, the metal retains an ample cross-section at the area of the greatest bending stress, which is the area surrounding the bolthole. The slimmest cross-section requires the least force to flex the disc pack when misaligned. That means lower bending forces are reflected back as lower reactionary loads on the nearby bearings.

Disc couplings can have up to 12 bolts alternately fastening the disc pack from one side to the other at the bolt circle. The motion control couplings and light industrial couplings have 4 bolts, industrial couplings 6 or 8, with 10 and 12 reserved for extremely high torque applications. The diameter of the bolt circle, thickness of discs, the number of discs, and the number of bolts are indications of the coupling's torque capability.

The thickness of the discs and the thickness of the disc pack indicate the coupling's flexibility. Short legs with many bolts reduce the lateral (angular) flexibility of the leg.

Thin discs chosen for flexibility can be stacked to an optimum height. A stack of thin discs provides parallel paths for the torque transfer, offering more flexibility than the same thickness of a single disc.

Stacked discs are clamped together using the disc attachment bolts and nuts plus bushings and washers. The clamping is critical in the coupling design and manufacture. Force must be transferred via friction from the shaft hub flange to the disc pack and from the disc pack to the next flange. Otherwise, the bolt would be loaded in shear, and the cross-sectional area around the hole could be overly stressed. It is normal to have a slight bending moment on the bolt from the torque transmission. The washers serve to spread the clamping pressure over a specified area and are beveled on the disc side to prevent stress risers when the disc is deflected. The bolts may be "body fit" to the bolthole or may be bushed. Bushings and "body fits" serve to equalize the loads and prevent high stress areas. A second type of bushing called an overload bushing can be inserted between the nut and the washer, in the flange counterbore. It serves to prevent overloads of torque from stretching the disc, or if the disc were to fail, it would prevent flailing. Bolts and nuts must be torqued to a specified value to obtain the proper clamping force. It is accomplished by holding the bolt while applying torque to the nut. The bolts are high strength, special pieces, machined to be used on disc couplings. Standard bolts or commercial bolts should not be substituted.

The allowable number and thickness of the individual discs in the pack is a function of how well the clamping force can be transmitted through the pack. The center members of very thick disc packs would not have sufficient clamping. Also important are the true positions of the boltholes in the discs, the dimensions of the discs and the true position of the boltholes in the flanges. Those details serve to make sure loads are equalized on each leg of the disc pack.
The loads on the disc pack of the coupling consist of two types. There are steady state loads associated with the transmitted torque, the rotational load (centrifugal force), thermal load, and the axial displacement. Then there are reversing loads associated with the angular misalignment. The force that is bending the links when the coupling is misaligned is a reversing fatigue loading on the flex link. 
Designers use a fatigue analysis to determine the stress levels of the disc elements. They can use a Modified Goodman Diagram or Constant Life Curve to establish coupling ratings. The objective is to limit the stresses so that the disc pack has a theoretical infinite life at the "catalog" rated values of torque and misalignment. In the case of special purpose couplings, the rated values are determined by the needs of the application and the job specifications. Misalignment and torque capabilities are interrelated on the disc coupling.

The reactionary force generated by a disc coupling is equal and opposite to the force bending the link when the coupling is misaligned or displaced. Forces for axial and radial misalignment are predictable and available from the coupling manufacturer as a chart of load vs. deflection. Axial forces are usually low and the coupling tends to be self-centering. Friction between the discs creates critical damping and a non-linear stiffness prevents an axial natural frequency from being a problem for this style of flexible element coupling. It has a built in limited end float capability.

The application engineer or user of disc couplings must account for the misalignment and the torque when selecting the coupling. An unaccounted for extra amount of misalignment is detrimental to coupling life.

The coupling is an infinite life unit as long as the discs do not receive an overload in excess of the fatigue capability. The construction of the disc coupling, with thin metal disc stacked very closely together makes it subject to varying types of corrosion. Fretting corrosion between discs is common, but can be prevented by coating the discs or by using corrosion resistant metals. Another problem is atmospheric corrosion such as salt in the air or chemicals in the operating environment. The thin discs would corrode quickly in the stressed areas. Once the metal is reduced in cross-section its life is reduced to a finite value.

The disc coupling is backlash free and torsionally very stiff. Because it is all metal and has no sliding or rubbing parts, the coupling is very suitable for high-speed applications.

Because the disc coupling is often used on high-speed applications, it is designed and built to be balanced. Unbalance is minimized through concentricity of the parts and by piloted fits. That would include piloted fits of all the connected parts, and tight tolerances on individual parts.

Types of Disc Couplings

Disc couplings can be made as a close-coupled design, or built as spacer type couplings. Close-coupled units that are designed to drop in the place of standard gear couplings have split centerpieces for replacing the disc pack. Spacer couplings, the most popular configuration, are either the marine type or reduced moment type. The marine type, with the flex element positioned outside the shaft hub, is easier to disassemble, and the ID of the disc pack is not controlled by the shaft hub diameter. Reduced moment couplings place the flex point and therefore flexing element directly over the shaft hubs nearest to a support bearing. The moment arm is reduced, reducing bearing loads and changing the lateral critical speed. (See Balancing Chapter for information on lateral critical speeds.) Reduced moment couplings can increase the diameter of the flexing element or limit the hub bore.

Disc couplings have almost as many configurations as are described in the gear coupling section of this handbook. This often allows disc couplings to be used in place of gear couplings, especially on high-speed applications where the higher cost of the disc coupling is justified by the reduction in maintenance. Disc couplings are capable of carrying loads as great as the gear coupling while using just a little more space. In fact, a variation of the disc coupling will match up to the flange drilling of a rigid gear coupling. That variation meets AGMA 516 Flange Dimensions (future AGMA 9008-AXX) and can be built as a reduced moment or floating shaft type. It is often used in paper mills or other markets where a disc coupling is used to replace a floating shaft gear coupling.

The basic industrial disc coupling consists of two hubs, two disc packs, and a center or spacer piece. The disc packs are bolted to the spacer and then to the shaft hub. Torque capabilities range from 1.0 inch-lb. in the 4-bolt miniature coupling sizes, to 2.0 million inch-lbs. or greater in the large 8-or 10-bolt industrial units. The lowest torque units may have only one disc at each flex point.

It is possible to build disc couplings as vertical units, electrically insulating units, shear pin units or to include a floating (torsion) shaft. The spacer units can be built with lightweight composite fiber spacers between two single flex element couplings, a design often-used on cooling tower applications. Couplings with axial movement limitations are available, as are split spacer close-coupled units. The industrial units can have flange adapters rather than hubs or hubs can be adapted to use either internal or external clamping devices (shrink discs).

The petroleum industry is a major user of disc couplings because of their high-speed rotating equipment. Many variations of the disc coupling are built to meet specific requirements of API 610 (Pumps) or API 671 (Unspared and Critical Equipment). One variation is the "drop out" type. It is a spacer-type disc coupling that utilizes a prepackaged center section that drops into or out of place and fastens to the shaft hubs.This factory-assembled center section includes two unitized disc packs, a center section (spacer) and two "guard rings" all bolted together with the bolts properly torqued.

The "drop-out" style of disc coupling is available with safety devices either built into the guard rings or as separate pieces. The safety device keeps the disc and spacer from flying out if a failure occurs. A shim pack is used to properly place the center section on high performance drop-out special purpose disc couplings. The shim pack allows for a variation in distance between shaft ends without prestretching the discs in an axial direction. Prestretching the discs is reserved for couplings that have thermal movement when in operation. Oversize hubs are available for the drop out type of disc couplings, but not available for the industrial type. The oversize hub allows the coupling to have torque and bore capabilities that are comparable with each other.

All varieties of the disc coupling can be inspected for damage without disassembly as long as you can see the discs. It is also possible to check the coupling with a strobe light while in operation. Under a strobe light the discs can be seen flexing and moving and a damaged disc would show up.

Disc couplings should not be used when rugged environmental conditions exist or when axial movement is required and must be carefully applied when transient loads are encountered. It is more expensive than a gear coupling.

Diaphragm Couplings

The diaphragm coupling was developed in the mid-fifties as an infinite life, non-lubricated, low maintenance design for aircraft applications. It was introduced into industrial applications in the late sixties, initially to meet the demands of high-speed high horsepower service in petrochemical processing.
In diaphragm couplings the flexing element is a metal plate that is loaded in shear by introducing the torque at the OD and transferring it to the ID. The process is reversed at the opposite flex point. Each flex point of the coupling can utilize one plate or a stack of plates operating in parallel. This coupling is used for the extreme applications of torque and speed and in those applications where reliability is paramount. Applications range from helicopter drives to high horsepower high-speed gas turbine drivers for generators and compressors.

Diaphragm couplings will have the largest OD of the three; disc, gear or diaphragm. The large OD reduces the forces at the periphery where the coupling is attached to the flange. Attachment is via clamping bolts at the OD and welding or bolting or major diameter fit splines at the ID. Where low moment versions are needed the diaphragm couplings can be welded at the OD of the flexible element, machined integral at the ID and bolted at the removal joint.
There are probably more types and styles of flexing elements available in diaphragm couplings than in any other style of coupling. Each style has its adherents and all work to some level of satisfaction. The user or specifier of the diaphragm coupling must have confidence in their chosen supplier of diaphragm couplings. This is more true of this style than the other types of couplings because no common designs exist. The couplings are used in a more sophisticated application than the gear or the disc.

Single diaphragms are contoured or shaped to have a constant stress across the diaphragm. The stress combines shear loading for torque transmittal with a bending moment for angular misalignment and stretching for axial displacement. As we found on disc couplings, ratings are established by a stress analysis using a Modified Goodman Diagram or a Constant Life Curve and augmented with sufficient factors of safety.

Diaphragm materials are high strength alloy steels perhaps in corrosion resistant versions. Single element diaphragm couplings when made from corrosion resistant materials can be considered the true infinite life device since fretting corrosion between a stack of fretting elements is eliminated. Or, multiple flex elements can be assembled to have space between the plates to eliminate the fretting corrosion that otherwise would reduce coupling life to a finite value.

Multiple flex element construction reduces the reactionary forces from misalignment. Although the flex elements must have uniform cross-section in the flexing plane, they offer a variety of styles, including convoluted (wavy), perforated, or spoke designs. The reason for the various shapes is to allow flexing with the least amount of force while still transmitting the torque.

Diaphragm couplings have more flexibility than disc couplings, therefore reducing cyclic loads associated with angular misalignment. This results in a stronger coupling with less susceptibility to fatigue failure, greater reliability and higher torque capability in a general sense. 

Diaphragm couplings with one flex point have only angular and axial flexing capability. To attain radial (parallel) misalignment capability, they must have two flex points. In this configuration, the coupling can be described as a spacer unit. As with the gear and disc couplings, diaphragm constructions can be "marine type", "reduced moment type" or a combination of the two. As on the other couplings, the reduced moment coupling can restrict the hub diameter and the shaft capability, or can make the coupling larger in overall diameter.

Comparing the flexing element couplings illustrates that the disc and the diaphragm are close in capabilities. The disc unit can have a lower cost, but the diaphragm can be considered more reliable and suitable for higher torque and speed.

Here are some comparative comments applicable to this group of couplings:

• The diaphragm coupling is considered to be stronger than the disc coupling and can be made in larger diameters than the disc coupling. (Note diameter is an indicator of torque capability.)
• The diaphragm coupling is considered to have more axial displacement capability at lower forces than the disc coupling.
• Single straight contoured diaphragm couplings eliminate the fretting corrosion associated with multiple elements. Single wavy types have more axial capability.
• Diaphragm couplings disconnect upon failure, which can be beneficial to the connected machinery. It becomes a fusible link.
• In contrast with gear couplings, both the disc and the diaphragm are non-lubricated, low maintenance, infinite life couplings.
• They both produce lower reactionary loads than gear couplings when misaligned.
• They both accept greater misalignment at higher speeds than gear couplings.
• Both the disc and the diaphragm coupling are easier to balance than the gear coupling.
• The gear coupling has the lowest first cost and the least sensitivity to rough circumstances.
• The gear coupling has the most axial displacement capability.
• The gear coupling has the highest power intensity.
• Gear coupling life is determined by wear, and overload generally results in reduced wear life rather than abrupt failure. This is subject to existing wear and the size of the overload.
• The disc coupling is usually smaller in diameter than the diaphragm coupling.
• The disc coupling is usually less expensive in first cost than the diaphragm coupling.
• The disc coupling has a built in overload capability. Disc failure, while it may be abrupt, will not disconnect the coupling if the bolts are capable of carrying the load in shear. This feature often can allow the coupling to continue operating through an orderly shut down. 

Triple-Wound Spring Couplings

The spring type coupling uses a tension spring fitted with rounded caplike hubs on both ends to facilitate shaft mounting. Normally the spring should be loaded so as to tighten the wrap of the spring because it can carry more torque that way. In this coupling, the triple wound construction has the middle coil wound in a direction opposite the inner and outer coils, so it can transmit torque effectively in either direction.

Flexing the spring as a tension spring accommodates angular misalignment. However, each end of the spring can flex independently, creating two flex planes that accommodate radial (parallel) misalignment. The amount of parallel offset allowed is primarily a function of the coupling (spring) length.

Generally these are small couplings with torque capabilities up to 1800 inch-pounds (213 Nm) in the largest size. Bores range up to 1.5 inches (38 mm). Angular misalignment can reach 3° for the triple wound spring. Speeds can run up to 30,000 RPM in the smaller sizes. They are capable of some endplay.

The triple wound spring coupling has both backlash and windup. The backlash amounts to 1/3 of the windup at full torque. Windup, depending on size, can range from .85° to 1.8° at maximum torque. Because the windup and backlash are known values these couplings are sometimes used for indexing, positioning or robotic applications.

Flange ends could be substituted for the round hubs or a combination of hub on one end and flange on the other. A bolted-in combination of flange and hub on both ends can be used to make a drop out version.

When a spring coupling operates in the misaligned system it is subject to both a nominal torque load and a cyclic flexing load. Overload on infinite life couplings will result in fatigue failure. The overload can be continuous torque, cyclic misalignment forces or a combination of both. Spring couplings must not be operated in excess of the maximum capability or the failure is likely to be sudden and dramatic.

Link-type couplings are a variation of the disc design. One variation uses three flat strip springs called "flexlinks" in place of a laminated disc pack. The single thickness corrosion resistant spring steel flat strips are proportioned to transmit torque in compressive or tensile beam end loading. These flexlinks are attached to a high strength steel triangular plate at the inner diameter and a circular flange at the outer side with rivets.

The riveting of the flexlinks to their connected parts is a factory assembly. The triangular plate bolts to a delta shaped hub or a round hub that can be mounted on the shaft. The round flange is formed with three axial arms that bolt to another round flange that has identical flexlinks, triangular plates, and hubs. The two pieces form a double flex coupling with a torque tube between the flex planes. The bolting is connected when the unit is assembled to the equipment shafts.

The links function the same as a disc leg. The difference is that the links are thick enough to be loaded in compression on one side and tension on the other. The link thickness is sufficient to prevent buckling under maximum rated loading, yet not too thick so as to bend near the rivet and accommodate angular misalignment. The flexlinks are sized to keep stresses well within the infinite-life fatigue limit of the steel. Spacer washers position the flexlinks so that each can flex in a prescribed path without interfering with adjacent flexlinks. Angular misalignment is in the order of 5°, and because there are two flex planes (one at each flange), their combined angular misalignment capability provides radial (parallel) misalignment as well. However, axial displacement adds an additional tensile load to the link.

When the coupling is operating under misalignment the fatigue points are located at the rivets and at the bolts in the center of the torque tube. Additional tensile loads on the flexlinks that come from axial movement will detract from the nominal torque capability. One could identify this via the Modified Goodman Diagram. One variation of the flex link coupling eliminates the center bolting. It is the spacer version or the floating shaft version. In that case a hollow tube with triangular plates at each end is inserted between the flanges. The flanges are modified to eliminate the axial arms. The flange on the hollow tube bolts directly to the circular flange and the misalignment forces that result in fatigue loading are taken at the link to rivet connection.

The normal configuration of the flexlink coupling is that of a reduced moment coupling. The delta shaped hubs will fit inside the flange torque tube for reduced moment or they can be mounted externally for marine style. The reduced moment coupling is shorter, but is not available as a spacer or a floating shaft version. Round shaped hubs are always mounted externally as they won't fit into the inside of the torque tube. Round shaped hubs are the only types that are available in stainless steel.

The "flexlink" coupling was developed for highly misaligned, but lightly loaded applications. It is most suitable for smooth torque loads without transients. It works best when the load is not cyclic. The reason for the load capability is that most of the fatigue or cyclic capability is used up in the misalignment cyclic loading.

Motion Control Couplings

Typical operational requirements for Motion Control couplings include torsional rigidity, low inertia, constant velocity, low radial stiffness, zero backlash, corrosion resistance and the capability of cyclic (repeated start/stop/reverse) activity. Motion Control couplings can be used on applications such as shaft encoders, resolvers, all forms of servo devices, linear and ball screw actuators, robots, stepmotors, light duty pumps and metering devices, plotters, medical equipment, positioning tables, computers and radar.

Although there are many alternatives for Motion Control Couplings, the most popular include the bellows, beam, disc and curved jaw. Covered in this section are the bellows and beam couplings. The disc type coupling has been covered separately in the earlier portion of this section, and curved jaw couplings (including the low backlash version for motion control) are covered in the Elastomeric Coupling section.

The bellows coupling uses a thin cross section tubular metal bellows formed with annular corrugations as the flexible element. The bellows material is usually steel or stainless steel and can be single thickness or two layers. Each end of the bellows is attached to a hub. Torque is transmitted in shear or from twisting across the bellows. Since the bellows is thin, it is capable of flexing for misalignment. The metal is subject to fatigue failure because the load from misalignment is cyclic and imposed on top of the nominal torque. In addition, the motion control application often calls for start-stop and reversal of the applied torque. The bellows coupling is one of the most torsionally stiff types of couplings, which give it an advantage in positioning drives and encoders. Currently its main limits are bore sizes to about 2 inches ( 50 mm) and nominal torque to 1750 inch-pounds (200 Nm). The majority of the applications are for shafts under 1 inch.

While the bellows itself is very stiff and strong, the connection of the hubs to the shaft and the method of attaching the hubs to the bellows can be weak points. The hub must attach to the bellows without becoming a fatigue point or a slip point.

The hub must be tightly attached to the shaft to prevent slip and backlash. Often the hubs are split, which allows a clamping action. Other types use tapered bushings attached with keys or a setscrew to the shaft. Shaft sizes are small and the hubs are often made from aluminum to reduce inertia, so the shrink fit option is not used.

There are many similarities between the bellows and beam couplings described in the next section. Although they are used for the same type of applications, the bellows is consistently more stiff than the beam with a few exceptions.

The beam coupling is an all-metal, non-lubricated coupling made from bar stock. It is manufactured by cutting a helix in a hollow bar forming a curved beam spring. The variations in OD, ID, number of helix starts, width of the beams (related to thickness of coils) and number of coils within a specific coupling length all serve to define the characteristics of the coupling.

The coupling is formed from a single piece of aluminum or stainless steel and can be cut with single or double flex planes. Some beam models can far exceed the bellows in angular misalignment, while others have configurations that make them act like a different device than a coupling. The devices include U-joints and springs. The flexure allowed by the curved beam portion of the coupling is capable of accommodating angular, parallel and axial misalignment while continuing to transmit torque between the attached shafts.

The beam coupling has a torsional stiffness that ranges from medium high to very high, which makes it a "constant velocity" coupling i.e. at every point during rotation, the driven half turns exactly the same amount and at the same rate as the driving half. It is highly appropriate for power transmission and motion control applications where extremely accurate positioning, frequent start/stop or reversing, and zero backlash are essential. It operates either clockwise or counter clockwise without sacrificing windup or torque capabilities. It has a very high tolerance to heat, chemicals, and corrosion.

The beam coupling and the bellows coupling have some overlap of attributes that allows them to be used on similar applications. In general the beam coupling, while torsionally stiff, is not as stiff as the bellows coupling. The cross-section of the beam coupling is sufficient to allow it to function, as a single thickness spring whereas the torsional spring coupling mentioned earlier in this section needs to be triple wound.

Beam couplings are offered with either one setscrew or with a split clamping hub for attachment to shafts. The clamping hub is highly recommended for instrumentation type applications due to the reliability of the attachment. It inhibits any movement or slip between the hub and the shaft.

Beam couplings operate with 3°, 4°, or 5° of angular misalignment, depending on the size and type selected. Some special variations of flexure will handle 90°, and can serve as U-Joints. They will also accommodate lateral translation. The maximum angular and parallel offsets cannot occur at the same time. The amount of parallel offset depends on size and configuration of the coupling, but it ranges from 0.004 to 0.035 inches. Beam couplings will allow axial motion up to .010 inches (.254 mm) in the small units and twice that in large units. However, care must be taken at the time of installation not to compress or expand the coupling axially, as the beam should be relaxed when installed.

The beam coupling can be used up to 10,000 RPM while misaligned and produces very low reactionary loads. Consequently the support bearings will be lightly loaded even when the coupling is misaligned within its specified limits. Reactionary loads are proportional to the spring flex constant.

Go To Next Section - Part 7: Selecting the Right Coupling - General Characteristics & Application Considerations
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