A shaft coupling is one of the most common machine elements because it is just so important in power transmission systems. Thus, they find use in a variety of applications and service environments.

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As a result, designers and engineers have designed many variations of couplings for specific service conditions and environments over the years.

This article will familiarise you with the different types of couplings and discuss choosing the right option for your application.

What is a Coupling?

A coupling is a mechanical device that connects similar or dissimilar shafts in machines to transmit power and movement. It is usually a temporary connection (but can be permanent in some cases) and capable of removal for service or replacement. A coupling may be rigid or flexible.

What is a Coupling?

Due to the availability of many designs, there can be stark differences in the construction and function of two types of mechanical couplings. Some couplings can connect to shafts without moving the shaft, while most will require shaft movement for fitting.

In most cases, a coupling does not change the direction of motion or angular velocity, unlike gears. It cannot be connected or disconnected mid-operation, unlike clutches. Couplings can only transfer torque over short distances, for longer distances chain drives and belt drives are better alternatives. Couplings are often paired with lead screw assemblies to connect the screw shaft in-line to a motor.

The coupling works by maintaining a strong but flexible connection at all times between two shafts to transfer motion from one shaft to another. It does so at all values of loads and misalignment without permitting any relative motion between the two shafts.

The Purpose of Couplings

A shaft coupling can perform multiple functions in a machine. The design may incorporate more than one of these coupling features into the product&#;s function in advanced applications.

Let us take a brief look at what these are:

• Power transmission
• Shock and vibration absorption
• Misalignment accommodation
• Heat flow interruption
• Overload protection

Power transmission

The primary purpose in most cases is power and torque transmission from a driving shaft to a driven shaft &#; for example, a coupling connecting a motor to a pump or a compressor.

Absorb shock and vibration

A shaft coupling can smooth out any shocks or vibrations from the driving element to the driven element. This feature reduces the wear on the components and increases the service life of the setup.

Misalignments between shafts can result from initial mounting errors or may develop over time due to other reasons. Most couplings can accommodate some degree of misalignment (axial, angular and parallel) between shafts.

Interrupt heat flow

A shaft coupling can also interrupt the flow of heat between the connected shafts. If the prime mover tends to heat up during operation, the machinery on the drive side is protected from being exposed to this heat.

Overload protection

Special couplings known as Overload Safety Mechanical Coupling are designed with the intention of overload protection. On sensing an overload condition, these torque-limiting couplings sever the connection between the two shafts. They either slip or disconnect to protect sensitive machines.

Types of Couplings

Couplings come in a host of different shapes and sizes. Some of them work great for generic applications, while some others are custom-designed for really specific scenarios.

To make an informed choice, it is important to be aware of the capabilities and differences between the different types of couplings. This section presents information about the following types of couplings and how they work:

• Rigid coupling
• Flexible coupling
• Sleeve or muff coupling
• Split muff coupling
• Flange coupling
• Gear coupling
• Universal joint (Hooke&#;s joint)
• Oldham coupling
• Diaphragm coupling
• Jaw coupling
• Beam coupling
• Fluid coupling

Rigid coupling

As the name suggests, a rigid coupling permits little to no relative movement between the shafts. Engineers prefer rigid couplings when precise alignment is necessary. 

Any shaft coupling that can restrict any undesired shaft movement is known as a rigid coupling, and thus, it is an umbrella term that includes different specific couplings. Some examples of this type of shaft coupling are sleeve, compression and flange coupling.

Once a rigid coupling is used to connect two equipment shafts, they act as a single shaft. Rigid couplings find use in vertical applications, such as vertical pumps.

They are also used to transmit torque in high-torque applications such as large turbines. They cannot employ flexible couplings, and hence, more and more turbines now use rigid couplings between turbine cylinders. This arrangement ensures that the turbine shaft acts as a continuous rotor.

Flexible coupling

Any shaft coupling that can permit some degree of relative motion between the constituent shafts and provide vibration isolation is known as a flexible coupling. If shafts were aligned all the time perfectly and the machines did not move or vibrate during operation, there would be no need for a flexible coupling.

Unfortunately, this is not how machines operate in reality, and designers have to deal with all the above issues in machine design. For example, CNC machining lathes have high accuracy and speed requirements in order to perform high-speed processing operations. Flexible couplings can improve performance and accuracy by reducing the vibration and compensating for misalignment.

These couplings can reduce the amount of wear and tear on the machines by the flaws and dynamics that are a part of almost every system. As an added bonus they&#;re generally rather easy to install and have a long working life.

&#;Flexible coupling&#; is also an umbrella term and houses many specific couplings under its name. These couplings form the majority of the types of couplings in use today. Some popular examples of flexible couplings are gear coupling, universal joint and Oldham coupling.

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Sleeve or muff coupling

Sleeve coupling is the simplest example of a rigid style coupling. It consists of a cast-iron sleeve (hollow cylinder) or muff. It has an internal diameter equal to the external diameter of the shafts being connected. A gib head key is used to restrict the relative motion and prevent slippage between the shafts and the sleeves.

Some sleeve couplings and shafts have threaded holes that match up on assembly to prevent any axial movement of the shafts. The power transmission from one shaft to the other occurs through the sleeve, the keyway and the key. This shaft coupling is used for light to medium-duty torques.

The sleeve coupling has few moving parts, making it a sturdy choice as long as all the parts are designed keeping in mind the expected torque values.

Split muff coupling

For easier assembly, the sleeve in a sleeve coupling can be divided into two parts. By doing this, the technician no longer needs to move the connected shafts for assembly or disassembly of a coupling.

This is what a split muff coupling or a compression coupling is. The two halves of the sleeve are held in place using studs or bolts.

Similar to sleeve coupling, these couplings transmit power through the key. Split muff couplings are used in heavy-duty applications.

Flange coupling

In flange couplings, a flange is slipped onto each of the shafts to be connected. The flanges are secured to each other through studs or bolts and onto the shaft by a key. Using set screws or a tapered key ensures that the flange hub will not slip backwards and expose the shaft interfaces.

One of the flanges has a protruding ring on its face, while the other has an equivalent recess to accommodate it. This type of construction helps the flanges (and, in turn, shafts) maintain alignment without creating any undue stress on the shafts.

Flange coupling is used in medium to heavy-duty applications. They can create effective seals between two tubes, and hence, in addition to power transmission, they are used in pressurised fluid systems. Flange couplings are of three major types:

• Unprotected type flange coupling
• Protected type flange coupling
• Marine type flange coupling

Gear coupling

A gear coupling is very similar to a flange coupling. However, it is a flexible type of coupling and can be used for non-collinear shafts. Gear couplings accommodate angular misalignment of about 2 degrees and parallel misalignment of 0.25&#;0.5 mm. 

The setup for gear couplings consists of two hubs (with external gear teeth), two flange sleeves (with internal gear teeth), seals (O-rings and a gasket) and the furnished fasteners.

The power transmission between the two ends of the coupling occurs through the internal and external gears in the gear coupling.

Gear couplings are capable of high torque transmission. As a result, they find use in heavy-duty applications. They require periodic lubrication (grease) for optimum performance.

Universal joint (Hooke&#;s joint)

When two shafts aren&#;t parallel and intersect at a small angle, we use a universal joint. This joint can accommodate small angular misalignment while providing high torque transmission capacity.

The universal joint consists of a pair of hinges connected through a cross-shaft. The two hinges are positioned at 90 degrees to each other. The cross-shaft maintains this orientation and is also responsible for the power transfer. The universal joint is not a constant velocity coupling, i.e., the driving and driven shafts rotate at different speeds.

They find use in a variety of different applications, hence the name. The most popular uses of universal joints are in car gearboxes and differentials.

Oldham coupling

Oldham Coupling

Oldham coupling is a special shaft coupling used exclusively for lateral shaft misalignment. When two shafts are parallel but not collinear, an Oldham coupling is most suitable.

The design consists of two flanges that slip onto the shaft and a middle part known as the centre disc. The centre disc has a lug on each face. The two lugs are actually rectangular projections that are perpendicular to each other and fit into the grooves cut out into the flanges on each side.

The flanges are fixed to the shaft through keys. Thus, the power transmission takes place from the driving shaft to the key to the flange to the centre disc and then through the second flange to the driven shaft.

Oldham coupling is ideal for scenarios where there is a parallel offset between two shafts. Such parallel misalignment can happen in cases where power transmission is needed between shafts at different elevations. When the shafts are in motion, the centre disc goes back and forth and adjusts for the lateral variation.

Diaphragm coupling

Diaphragm couplings are great all-rounder shaft couplings. They can accommodate parallel misalignment as well as high angular and axial misalignment. They also have high torque capabilities and can transmit torque at high speeds without the need for lubrication.

Diaphragm couplings are available in various styles and sizes. The structure consists of two diaphragms with an intermediate member between them. The diaphragm is basically one or more flexible plates or metallic membranes that connect the drive flanges on the shafts to the intermediate member through bolts on both sides.

Diaphragm couplings were initially developed for helicopter drive shafts. But over the years, they have found much use in other rotating equipment as well. They are most commonly used in turbomachinery due to their high-speed function. Applications today include turbines, compressors, generators, aircraft, etc.

Jaw coupling

Jaw coupling is a material flexing coupling. It finds use in general low power transmission and motion control applications. It can accommodate any angular misalignment. Similar to diaphragm couplings, jaw couplings do not need lubrication.

This coupling consists of two hubs with intermeshing jaws that fit into an elastomeric spider. The spider is usually made of copper alloys, polyurethane, Hyrtel or NBR and is responsible for torque transmission.

Due to the elastic nature of the spider, it is suitable for the transmission of shock loads. It can also dampen reactionary forces and vibration pretty well.

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Engineers use jaw couplings in applications such as compressors, blowers, mixers and pumps. 

Beam coupling

A beam coupling is a machined coupling that offers high flexibility in terms of parallel, axial and angular misalignment. It is one of the best low-power transmission couplings.

A beam-style coupling has a cylindrical structure with helical cuts. The attributes of these cuts, such as their lead and the number of starts, can be modified to provide misalignment capabilities of varying degrees. In fact, engineers can make these changes without sacrificing the structure&#;s integrity as it is made of a single piece. Thus, a second name for beam coupling is helical coupling.

In essence, beam couplings are actually curved flexible beams. They are available in single-beam and multi-beam versions. Multi-beam couplings can handle greater parallel misalignment than single-beam couplings.

A beam coupling is more suitable for low-load applications as torsional windup can be a real issue. Thus, it is used in servo motors and motion control in robotics.

Fluid coupling

Fluid Coupling Working Principle

A fluid coupling is a special type that uses hydraulic fluid to transmit torque from one shaft to another. 

The shaft coupling consists of an impeller connected to the driving shaft and a runner connected to the driven shaft. The whole setup is fixed in a housing, also known as a shell.

When the driving shaft rotates, the impeller accelerates the fluid, which then comes into contact with the runner blades. The fluid then transfers its mechanical energy to the runner and exits the blades at a low velocity. 

A fluid coupling is used in automobile transmission, marine propulsion, locomotive and some industrial applications with constant cyclic loading.

Parameters for Choosing

Shaft couplings are an integral component of motion control and power transmission systems. They provide incredible advantages and combat many assembly and service environment issues when applied correctly.

To do this, designers must consider many factors to make the right choice. Being aware of them helps reduce instances of coupling failure and improve system capabilities. These factors are:

• Torque levels
• Alignment limits
• Rotational speeds
• Lubrication constraints

Torque levels

Most manufacturers use rated torque as a basis for classifying coupling. The value of torque depends on whether a coupling is used for motion control or power transmission applications. The former has lower torque and loads compared to the latter. Knowing the expected torque levels in an application will narrow down the selection of the right coupling.

Alignment limits

Different applications have different alignment needs. Similarly, some shaft couplings can only accommodate one type of misalignment, while others can handle multiple types.

Manufacturers also mention the misalignment limits for different types of misalignment for every coupling. This consideration helps further narrow down the search and pair the right coupling with the right machine.

Maximum rotational speed

Every coupling also has a maximum allowable RPM. This limit is also published with shaft couplings. General-purpose couplings cannot be used as-is for high RPM applications. High RPM couplings need static and dynamic balancing to ensure safe, smooth and noise-free service.

Such balanced designs are created by precise machining and appropriate fastener distribution. Using the expected RPM as a yardstick can help with the correct coupling selection. 

Lubrication constraints

Sometimes, service conditions may prevent frequent relubrication of shaft couplings that need it. On the other hand, some shaft couplings are designed without the need for any lubrication over their entire life.

If the torque requirements are low, modified versions of conventional couplings are also available. These versions come with metal-on-metal lubrication or metal and plastic combinations to eliminate lubrication altogether. Designers must make the right coupling choice by evaluating the service conditions and application needs.

Sorting Out Flexible Couplings

When the time comes to specify replacements for mechanical power transmission couplings, it&#;s human nature to take the easy path&#;simply find something similar (if not identical) to the coupling that failed, maybe apply a few oversizing fudge factors just to be conservative. Too often, however, this practice only invites a repeat failure&#;or more costly system damage.

The wiser approach is to start with the assumption (or at least the suspicion) that the previous coupling failed because it was the wrong type of coupling for that application. Taking time to determine the right type of coupling is worthwhile even if it only verifies the previous design. But it might lead you to something totally different that will work better and last longer.

If so, that result also will reward you by extending the life of bearings, bushings and seals, preventing fretted spline shafts, minimizing noise and vibration, and cutting long-term maintenance costs.

In most cases, industrial power transmission calls for flexible rather than rigid couplings in order to forgive minor shaft misalignment. For that reason, this article will focus solely on the selection of flexible couplings.

Determining the right type of flexible coupling starts with profiling the application as follows:

• type of prime mover (electric motor, diesel engine, other);
• real horsepower and/or torque requirements of the driven side of the system, rather than the rated horsepower of the prime mover (note the range of variable torque resulting from cyclical or erratic loading, &#;worst-case&#; startup loading, and the amount of start-stop-reversing activity common during normal operation);
• driven-system inertia values in relation to prime-mover inertia (equipment vendors can supply data);
• vibration, both linear and torsional (experienced vendors or consultants can help you evaluate vibration);
• shaft-to-shaft misalignment &#; note degree of angular offset (where shafts are not parallel) and amount of parallel offset (distance between shaft centers if shafts are parallel but not axially aligned); also note whether driving/driven units are or could be sharing the same base-plate;
• axial (in/out) shaft movement, BE distance (Between Ends of driving and driven shafts), and any other space-related limitations.
• ambient conditions (mainly temperature range and chemical/oil exposure)

The next step is to review available types of flexible couplings to see which type best suits your application profile.

Initially, flexible couplings divide into two primary groups&#; metallic and elastomeric. Metallic types are all-metal designs that gain their flexibility from movement of loosely fitted parts that roll or slide against each other, or from bending of non-moving metallic parts. Elastomeric types gain flexibility from using resilient, non-moving, rubber or plastic elements to transmit torque between usually metallic driving/driven hubs.

Metallic types are best suited to applications that require or permit:

• torsional stiffness (very little &#;twist&#; between hubs &#; in some cases providing positive displacement of the driven shaft for each incremental movement of the driving shaft);
• operation in relatively high ambient temperatures and/or presence of certain oils or chemicals;
• electric motor drive (metallics generally are not recommended for gas/diesel engine drive);
• relatively constant, low-inertia loads (generally not recommended for driving reciprocal pumps, compressors, other pulsating machinery)

Elastomeric types are best suited to applications that require or permit:

• torsional softness (allows &#;twist&#; between hubs &#; absorbs shock and vibration, can better tolerate engine drive and pulsating or relatively high-inertia loads);
• greater radial softness (allows more angular misalignment between shafts, puts less reactionary or side load on bearings and bushings);
• lighter weight/lower cost, in terms of torque capacity relative to maximum bore capacity; quieter operation.

Another way to look at it: wrong applications for each type are those characterized by the conditions that most readily shorten their life. In metallic couplings, premature failure of the torque-transmitting element most often results from metal fatigue, usually due to flexing caused by excessive shaft misalignment or erratic/pulsating/high-inertia loads. In elastomeric couplings, breakdown of the torque-transmitting element most often results from excessive heat &#; either from ambient temperatures or from hysteresis (internal buildup in the elastomer) &#; or from deterioration due to contact with certain oils or chemicals.

The preceding overview should help establish which group generally looks best for a given application; the following discussion presents the basic alternatives available in both groups to further guide your selection.

Metallic Coupling Alternatives

Metallic flexible couplings group into three basic families: Mated Parts, Membrane and Specialty.

Mated Parts Couplings

Mated-Parts designs include gear, grid and chain types, in which torque is transmitted across separate metal elements that push against each other. Generally, these designs offer high torque-to-outside diameter ratios, accommodate misalignment up to 2 degrees, but allow little parallel misalignment. They provide relatively high torsional stiffness, but due to moderate backlash, they usually are not recommended for pulsating or frequent stop- start applications. All require routine lubrication and maintenance of seals.

Gear couplings consist of two shaft-mounted hubs with gear teeth around their external circumferences. Both hubs are enclosed within a common connecting sleeve that has gear teeth around its internal circumference, which mate with the hubs&#; teeth. Continuous teeth along the length of the sleeve allow generous tolerance for axial (in/out) shaft movement. In lower torque ranges, nylon sleeves can eliminate need for lubrication and provide quieter operation; higher torque and/or RPM ranges can be achieved with special models having heat-treated teeth.

Grid couplings are the only metallic type to offer moderate torsional shock/vibration damping capacity. This design employs a spring steel grid pre-formed to snake back-and-forth between two shaft-mounted hubs, nesting in slots formed around the external circumference of each hub. The beam effect of this grid as it spans the gap between the two hubs gives this design its resilience. The grid also forgives minor axial shaft movement, but movement that significantly shortens the gap between hubs reduces grid resilience. A common sleeve encloses both hubs and grid.

Chain couplings consist of two sprocket-like shaft hubs linked around their circumference by a continuous length of double roller chain, which is enclosed in sleeve-type cover. Low-torque applications can opt for a low-noise plastic chain; high-torque applications can be accommodated by special heat-treated chains and sprockets.

Membrane Couplings

Membrane coupling designs comprise laminated disc, flexible link and diaphragm types, in which torque is transmitted through single, tightly fitted metal elements rather than across separate, loose metal elements pushing against each other. This assures positive displacement with zero backlash and no routine maintenance requirements. Membrane types cover a broad range of horsepower and torque capacities, with varying degrees of angular flexibility achieved by deflection of the metal elements. They generally do not allow parallel misalignment.

Laminated Disc couplings transmit torque through a stack of thin, O-shaped metal discs suspended between two flange-type, shaft-mounted hubs. The disc stack is bolt-attached alternately to driving and driven hub flanges along a common bolt-circle diameter.

The beam effect of the disc stack&#;s thin laminate construction, in free span between driving and driven bolts, allows an angular flexibility of up to 1 degrees, but will not permit axial shaft movement or parallel offset.

Flexible Link couplings are a variation of the disc design that uses three or more flat strip springs called &#;flexlinks&#; in place of a laminated disc pack. The ends of the flexlinks are attached (usually riveted) to carriers mounted on driving and driven shaft hubs, enabling the driving carrier to pull the driven carrier in rotation. The carriers are shaped with radial arms that position their flexlink attachments near the circumference of the coupling to maximize flexlink length.

The beam effect of the flexlinks, in free span between driving and driven carrier arms, gives the three-link design high angular flexibility of up to 6 degrees, with low reactionary load on bearings. Designs using four or six flexlinks can accommodate greater torque, but reduce angular flexibility. Flexlink designs do not allow axial shaft movement, but will tolerate slight parallel offset.

Diaphragm couplings transmit torque through a stack of thin metal diaphragms (full but typically perforated discs). The stack is attached to one shaft-mounted hub near its OD, and attached to the other shaft-mounted hub near its ID, so torque flows between OD and ID rather than around the OD. The free span of the diaphragm between OD and ID deflects to accommodate moderate angular misalignment of to 1ø and allow minor axial shaft movement.

Specialty Couplings

Specialty metallic couplings encompass a variety of designs such as wrapped spring, helically formed beam, bellows and offset types.

Wrapped Spring couplings allow up to 4.5 degrees of angular and up to .045&#; of parallel misalignment, plus high RPM ranges. These designs consist of three concentric, tightly wound, square-wire springs, with the inner and outer coils wrapped in the same direction opposing the direction of the center coil in order to enable coupling rotation in either direction. The spring pack is brazed to hubs at both ends, making a single-piece coupling that is very easy to install.

The spring coupling has no backlash, but it is not torsionally rigid and therefore may not be suitable for some positioning applications.

Curved Beam couplings include two single-piece designs that feature high torsional stiffness and zero backlash, making them well suited for servo-motor, encoder and other precise-positioning applications. They accommodate high angular misalignment with low reactionary loads on bearings, good for applications with small-diameter shafts that could easily bend.

One curved beam design, called the Helically Formed coupling, is machined from solid bar stock with spiral patterns cut through to its core, creating a long, curved beam. Its torsional stiffness varies in a linear fashion &#; i.e. the amount of &#;twist&#; is directly proportional to the torque load. In special high-speed designs, RPM can range up to 50,000 RPM.

The other curved beam design, called the Bellows coupling, is made from a single piece of tubular stock axially compressed into a series of rounded &#;accordion&#; folds. This design offers extremely high torsional stiffness, measured in arc. sec./in. oz.

Offset couplings are unique in their ability to accommodate extremely large parallel misalignment between shafts &#; up to 17&#; offset in the largest coupling size &#; although maximum angular misalignment is limited to 0.5 degrees. An alternative design allows up to 3ø angular misalignment, but will accept only up to 0.5&#; of parallel offset. These highly specialized and complex designs have many moving parts and must be very carefully specified.

Elastomeric Coupling Alternatives

Elastomeric couplings classify into two main categories by the way their elastomeric element transmits torque &#; i.e. the element is either &#;in compression&#; or &#;in shear&#;.

When the element is in compression, parts of the driving hub push parts of the driven hub. The element separates driving from driven parts like a cushion, absorbing some of the torque force by being compressed between them.

When the element is in shear, the driving hub pulls the driven hub through their mutual connection to the element, which absorbs some of the torque force by being stretched through twisting.

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

Shear-type couplings in turn offer two general advantages over compression types. First, they accommodate more parallel and angular offset while inducing less reactionary bearing load. 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.

Compression-Type Designs

Elastomeric compression-type couplings comprise three main designs: jaw; donut; and pin & bushing.

Jaw couplings are distinguished by hubs that have two to seven axially oriented jaws (thick, stubby protrusions) arranged around their circumferences. Jaws of driving and driven hubs mesh loosely; filling the gaps between them are cushions of elastomeric material, usually molded into a single asterisk-shaped element called a &#;spider&#;.

Permanent compressive set occurs as the element ages in service; a 25% reduction from original thickness signals replacement. In most applications, compression is applied only to the spider cushions forward of the driving jaws, so spider life can be doubled by advancing the unused trailing cushion into the driving position.

Jaw designs are considered &#;failsafe&#; because if the spider breaks away, the driving jaws can contact the driven jaws directly, maintaining operation until the spider can be replaced.

Jaw couplings generally are recommended for electric motor-driven machinery, pumps, gearboxes, etc. Most jaw designs typically are limited to angular shaft misalignment of 1 degrees and tolerate very little parallel offset. Backlash due to spacing between jaws and element cushions usually makes jaw couplings inappropriate for true positive-displacement applications.

Donut couplings use a donut-shaped ring of elastomeric material installed with a set of bolts or pins alternately engaging the ring from the driving and driven hub. Torque is transmitted through the donut material via compression between driving and driven bolts. But, while the &#;leading&#; portion of the donut is in compression, the &#;trailing&#; portion may be in tension, depending on the donut/hub design. This feature eliminates backlash and allows the coupling to absorb torsional vibration.

Standard donut designs may vary in torsional stiffness and are rated for medium to heavy duty service, with angular misalignment allowance as much as 3 degrees in some cases, and good parallel misalignment allowance.

Precompressed natural rubber donut designs are torsionally softer than most compression couplings, and widely favored for high-shock, start/stop applications such as engine-driven systems, compressors, violent pounding or crushing equipment, marine and off-road equipment.

Pin & bushing couplings transmit torque through driving pins that project from both driving and driven hubs; each pin engages an elastomeric bushing or &#;biscuit&#; suspended in a rigid disk between the hubs. Similar in concept to the donut design, this coupling is torsionally softer than other compression types and does a better job of absorbing torsional shock. It allows angular misalignment up to 2 degrees, but not much parallel offset.

Shear-Type Designs

Elastomeric shear-type couplings include three main designs: sleeve; tire; and molded-element.

Sleeve couplings are characterized by a tubular elastomeric element molded with serrated flanges at both ends. These flanges mate with serrated sockets molded into the coupling&#;s hubs.

Sleeve types in some cases may twist as much as 15 degrees between hubs, providing excellent protection against torsional shock and vibration. They accommodate angular misalignment up to 2ø and parallel offset up to approximately .05&#; without imposing much reactionary load on bearings.

Because of their open-center construction, sleeve type couplings allow shaft-to-shaft applications with very little clearance between shaft ends.

Tire couplings, named for their resemblance to an auto tire, consist 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.

Design variations are available, including an inverted tire coupling in which the tire element arcs inward toward the axis, designed for higher RPM service.

The tire coupling is torsionally soft and can damp vibration. High radial softness accommodates angular misalignment up to 4 degrees 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 tire designs a wide variety of applications, including those using internal combustion engines.

Molded-element couplings feature an elastomeric element that is molded into the metallic hub of the coupling, usually in a socket having a serrated perimeter. These designs are most often recommended for connecting internal combustion engine flywheels to pumps, transmissions, blowers, generators and compressors, especially where close coupling is desired.

A very broad range of element materials, from torsionally soft to stiff, allows wide latitude in adjusting natural frequencies of engine-driven systems to avoid inducing destructive resonance at critical RPM ranges. Angular misalignment ranges from 0.5 degrees to 2 degrees depending on coupling construction and element hardness, and parallel offset is generally limited to .05&#;.

In general, the torsionally softer alternatives are used with high-inertia loads and where good coupling alignment is difficult to attain. Torsionally stiff alternatives are favored for low-inertia loads, but demand careful attention to alignment.

Flexible couplings have evolved into a rich variety of types, providing a wide range of performance tradeoffs. When selecting among them, resist the temptation to overstate service factors. Coupling service factors are intended to compensate for the variation of torque loads typical of different kinds of driven systems, and to provide for reasonable service life of the coupling; if chosen too conservatively, they can misguide selection, raise coupling costs to unnecessary levels, perhaps even invite damage elsewhere in the system. Remember that properly selected couplings are supposed to serve as a fuse&#;i.e. if the system is overloaded, improperly operated or somehow drifts out of spec, the coupling should break before something more expensive does.

Thoroughly review the suggested application profile with your coupling vendors and seek not only their recommendations for the right type of coupling, but also the reasons behind those recommendations. With the variety of couplings available today, careful selection usually leads to a long lasting match between coupling characteristics and the demands of the application.

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