What is Coupling?- Definition, Types, and Uses

A coupling is a mechanical element part that connects two shafts together to accurately transmit the power from the drive side to the driven side while absorbing the mounting error, misalignment, etc. of the two shafts.

Coupling in the machine industry is interpreted as “a part that connects two shafts together”, and is generally called “coupling”, “shaft coupling” or “joint”. Let’s discuss in detail what is Coupling and their types.

What is a Coupling?

A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power.

The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both.

In a more general context, a coupling can also be a mechanical device that serves to connect the ends of adjacent parts or objects.

Couplings do not normally allow disconnection of shafts during operation, however, there are torque-limiting couplings that can slip or disconnect when some torque limit is exceeded.

The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both.

Selection, installation, and maintenance of couplings can lead to reduced maintenance time and maintenance costs.

The role of a coupling (shaft fitting)

• Transmit power
• Absorb misalignment
• Absorb vibrations to protect surrounding products
• Do not transfer the heat of the motor, etc., to the driven side.

What is Shaft Coupling?

A shaft coupling is a mechanical component that connects the driveshaft and driven shaft of a motor, etc., in order to transmit power.

Shaft couplings introduce mechanical flexibility, providing tolerance for shaft misalignment. The former is called a coupling and the latter is called a shaft coupling.

As a result, this coupling flexibility can reduce uneven wear on the bearing, equipment vibration, and other mechanical troubles due to misalignment.

Flexible Shaft Couplings can help prevent these issues by transmitting torque while compensating for parallel, angular, and axial misalignment between drive components.

When installed correctly, flexible shaft couplings can also reduce vibration, minimize noise, and protect driveshaft components.

Shaft couplings are used for power and torque transmission between two rotating shafts such as motors and pumps, compressors, and generators.

Shaft couplings are available in a small type mainly for FA (factory automation) and a large casting type used for large power transmissions such as in wind and hydraulic power machinery.

Types Of Shaft Coupling

Different types of shaft Couplings are:

• Rigid Coupling: They are used to connect two perfectly aligned shafts.
• Flexible Coupling: They are used to connect two shafts having lateral and angular misalignment.
• Fluid Coupling or Hydraulic Coupling: They transmit power from one shaft to another shaft, acceleration, and deceleration of hydraulic fluid.

MORE: What is Fluid Coupling?

Types of Coupling

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 
• Disc coupling
• Bushed Coupling
• Grid Couplings
• Roller Chain Coupling
• Tyre Couplings
• Bellows Coupling

#1. Rigid coupling.

Rigid couplings, which are in some cases called sleeve or muff couplings, have been historically imprecise, economical, and frequently homemade segments for a straightforward shaft to shaft associations.

Rigid couplings are designed to connect two shafts together so that no relative motion occurs between them.

Rigid couplings are suitable when precise alignment of two shafts is required. If significant radial or axial misalignment occurs, high stresses may result, which can lead to early failure.

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.

#2. Flexible coupling.

The purpose of a flexible coupling is to transmit torque from one piece of rotating equipment to another, while accepting at the same time a small amount of misalignment.

Flexible coupling misalignment is expressed, as an order of magnitude, in thousandths of an inch.

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. 

#3. Sleeve or Muff Coupling.

Muff coupling is also called sleeve coupling or box coupling. It is a type of rigid coupling. It consists of a sleeve or a hollow cylinder, which is fitted over the ends of the input and output of the shaft by means of a sunk key. It is the simplest form of coupling with only two parts sleeve and key.

A Sleeve coupling is a basic type of coupling. This consists of a pipe whose bore is finished to the required tolerance based on the shaft size.

Based on the usage of the coupling a keyway is made in the bore in order to transmit the torque by means of the key. Two threaded holes are provided in order to lock the coupling in position.

Sleeve couplings are also known as Box Couplings. In this case, shaft ends are coupled together and abutted against each other which are enveloped by muff or sleeve. A gib head sunk keys hold the two shafts and sleeve together

Sleeve coupling is the simplest type of shaft coupling, and it is used when transmitting light to medium torques.

It is composed of a thick and hollow cylindrical tube called a sleeve or muff whose inner diameter is the same as the shaft. The sleeve transmits the torque across the shafts.

#4. Split Muff coupling.

In split muff coupling, the sleeve or muff isn’t a single different part instead it is split into 2. The muffs are semi-cylindrical in shape which then fits over the shaft. Threaded holes are provided on the muffs so that both the shafts can be joined with steel bolts or studs.

The split muff coupling is also called compression coupling or clamp coupling. It is a rigid type of coupling. In this coupling, the sleeve is made of two halves.

The halves of the muff are made of cast iron. The two halves of the sleeve are clamped together by means of mild steel studs or bolts and nuts.

The split muff coupling is also called compression coupling or clamp coupling. It is a rigid type of coupling. In this coupling, the sleeve is made of two halves.

The halves of the muff are made of cast iron. One-half of the muff is fixed from below and the other half is placed from above. The two halves of the sleeve are clamped together by means of mild steel studs or bolts and nuts.

The number of bolts can be four or eight. They are always in multiples of four. The bolts are placed in recesses formed in the sleeve halves.

The advantage of this coupling is that the position of the shafts need not be changed for assembling or disassembling of the coupling. This coupling may be used for heavy-duty and moderate speeds.

#5. Flange Coupling.

Flange coupling is a sort of connector between turning chutes that have two arrangements of flanges. Flanges are fitted or provided at the end of shafts.

The flanges are tightened together by means of a number of nuts and bolts. One of these flanges or chutes is fixed at the end of each shaft.

Flange Coupling is a driving coupling between rotating shafts that consists of flanges one of which is fixed at the end of each shaft, the two Flanges being bolted together with a ring of bolts to complete the drive.

This type of coupling is meant to bring two tube ends together in a flush, sealed manner. This two-piece coupling unit consists of a keyed receiving side for the flanged end to be fastened to, so it may be married to the opposing tube end, which also has a flanged end.

Each flange has either a male or female coupler opening so that when the two ends are brought together, they are aligned without causing resistance or drag in the material being passed through them.

This male or female coupling method also creates a stable connection that is resistant to shifting, keeping the flange coupling sturdily in place.

Flange couplings are typically used in pressurized piping systems where two pipe or tubing ends have to come together.

The connecting methods for flange couplings are usually very strong because of either the pressure of the material or the sometimes-hazardous nature of materials passed through many industrial piping systems.

High thread count nut and bolt connections are used to secure the flange couplings in place.

These nuts and bolts are usually made from tempered steel or alloys to provide enduring strength and the ability to be tightened to the utmost level to ensure the piping system doesn’t leak at any flanged junction. Most flange couplings utilize four, six, or up to 12 bolt assemblies.

#6. Gear Coupling.

Gear couplings are designed to transmit torque between two shafts that are not collinear. They typically consist of two flexible joints one fixed to each shaft which are connected by a spindle, or third shaft.

The gear coupling connects the drive motor to the gearbox in hoist mechanisms, but it can also connect the gearbox directly to smaller wire rope drums using a flanged half.

In terms of their design, gear couplings transmit torque via hubs with crowned gear teeth that are in permanent mesh with the straight gear teeth of the sleeves a design that provides the highest torque transmission for the smallest size.

They also run at high speeds, conform to the AGMA bolting pattern and compensate for angular, radial, and axial shaft misalignment.

#7. Oldham Coupling.

Oldham couplings are a three-piece assembly comprised of two lightweight aluminum or corrosion-resistant stainless-steel hubs and a center disk.

The tenons on the hubs mate to the slots in the disk with a slight press fit, allowing the coupling to operate with zero backlashes.

Oldham couplings are commonly used in servo-driven systems that require precise motion control and low inertia, balanced design.

The Oldham coupling is a form of flexible coupling designed for applications that must be free from backlash.

They are also increasingly being used as a replacement for straight jaw couplings. The Oldham coupling consists of three discs.

Two of the discs are connected to either side of the drive, while the third, made from one of several different plastics, is sandwiched in between with a tongue and groove design.

The tongue and groove on one side is perpendicular to the tongue and the groove on the other. Springs are often used to reduce the coupling’s backlash.

During operation, the center disk slides on the tongues, or tenons, of each hub (which are orientated 90° apart) to transmit torque.

While the couplings accommodate a small amount of angular and axial misalignment, they are especially useful in applications with parallel misalignment.

The Oldham coupling features several other advantages including their compact size and potential for electrical isolation through the plastic center disk. The couplings may also act as a sort of fuse for a machine.

If torque limits are exceeded the center disc of the coupling will break apart first, preventing torque transmission and potential damage to more costly machine components.

#8. Universal Coupling.

A universal or hook coupling is used to connect two shafts whose axes intersect at a small angle. The bending of the two shafts may be constant, but in actual practice, it changes when the momentum is transferred from one shaft to another.

The main application of universal or hook coupling is found in transmission from the gearbox to automobiles’ differential or back axle.

In such a case, we use a coupling of two hooks, connecting the gearbox at one end and the differential at the other end at each end of the propeller shaft.

The coupling of a hook is also used to transmit electricity to the various spindles of several drilling machines. It is used as a knee joint in a milling machine.

#9. Diaphragm Coupling.

A diaphragm coupling consists of one or more metallic membranes which are attached at the outside diameter of a drive flange and transfer torque radially through the diaphragm to an inside diameter attachment. The other type of metallic membrane coupling is disk coupling.

Diaphragm couplings utilize a single or a series of plates or diaphragms for flexible members. It transmits torque from the outside diameter of a flexible plate to the inside diameter, across the spool or spacer piece, and then from the inside to the outside diameter.

• Allows for angular, parallel, and high axial misalignments
• High torque, used in high-speed applications

#10. Jaw Coupling.

A jaw coupling is a type of general-purpose power transmission coupling that also can be used in motion control (servo) applications.

It is designed to transmit torque (by connecting two shafts) while damping system vibrations and accommodating misalignment, which protects other components from damage.

These types of coupling are composed of three parts: two metallic hubs and an elastomer insert called an element, but commonly referred to as a “spider”.

The three-part press fit together with a jaw from each hub fitted alternately with the lobes of the spider. Jaw coupling torque is transmitted through the elastomer lobes in compression.

• Flex element is commonly made of NBR, polyurethane, Hytrel, or Bronze
• Accommodates misalignment
• Transmits torque
• Used for torsional dampening (vibration)
• Low torque, general-purpose applications

#11. Beam coupling.

A beam coupling, also known as helical coupling, is a flexible coupling for transmitting torque between two shafts while allowing for angular misalignment, parallel offset, and even axial motion, of one shaft relative to the other.

A beam coupling consists of a single piece of material made flexible by the removal of material in a helical pattern along its length.

As with all couplings, the purpose of a beam coupling is to transmit torque between two shafts, but unlike a rigid coupling, a beam coupling can accommodate angular misalignment, parallel offset, and even axial motion, of one shaft relative to the other.

The beam coupling also differs from other coupling types in that its one-piece construction prevents the backlash usually encountered by couplings made of multiple parts.

Beam couplings can be found in a variety of materials including titanium and acetal with stainless steel and aluminum being the two most common.

The light weight of an aluminum beam coupling means they are suited for applications where a high level of responsiveness is needed.

Stainless steel, on the other hand, while providing greater strength and torsional stiffness, has a greater mass and thus does not have the same level of responsiveness.

#12. Fluid coupling.

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.

#13. Disc Coupling.

A disc coupling, by definition, transmits torque from a driving to a driven bolt or shaft tangentially on a common bolt circle.

Torque is transmitted between the bolts through a series of thin, stainless steel discs assembled in a pack. Misalignment is accomplished by deforming the material between the bolts.

This type of coupling is a high-performance motion control coupling designed to be the torque transmitting element (by connecting two shafts together) while accommodating shaft misalignment.

It is designed to be flexible while remaining torsionally strong under high torque loads. Typically, disc couplings can handle speeds up to 10,000 r/min.

There are two different styles of disc coupling:

• Single disc style couplings are composed of two hubs (the ends of the coupling, which are typically made from aluminum, but stainless steel is used as well) and a single, flat, stainless steel disc spring.
• Double-disc style couplings are also composed of two hubs but have an additional center spacer sandwiching two-disc springs. The center spacer can be made out of the same material as the hubs but is sometimes available in insulating acetal, which makes the coupling electrically isolating.

Torsion ally stiff and still flexible, disc couplings are a great solution for high-speed applications. The downside is that they are more delicate than the average coupling and can be damaged if misused. Special care should be taken to ensure that misalignment is within the ratings of the coupling.

#14. Bushed Coupling.

Bush couplings are mainly used as flexible links in applications where reliable link transfer is required under severe operating conditions.

A bush coupling consists of two hubs that can be made of different materials and are fitted with pins where rubber bushes are attached.

These types of coupling are flexible couplings that are reliable and for this reason, they are widely applied to hoisting applications.

The coupling bolts are known as pins. Rubber or leather bushes are used on top of pins. Also, there is a variation in the construction of two parts of the coupling.

There is a 5 mm clearance remaining between the faces of the two halves of the coupling. And there is no rigid connection between them, and the drive is through compressed rubber or leather bushes.

#15. Bellow Couplings.

Bellows couplings are one form of flexible coupling with twin coupling ends called hubs capping a precision-engineered corrugated tube that serves as the coupling body.

Bellows couplings are known for their exceptional torsional rigidity to accurately transmit velocity, angular position, and torque.

Their slight flexibility (at the corrugated bellows) serves to address limited amounts of axial, angular, and parallel misalignment between the shafts or other components being joined.

Bellows couplings are typically made from a stainless-steel tube that is hydroformed (or in some cases welded) to create deep corrugations. Such hydroformed bellows begin as a sheet of stainless steel or other metal.

This sheet is drawn into a tube which is then pressurized from within against a ribbed die to form a corrugated shape. Then the end hubs are welded or bonded in some manner to this coupling bellows.

How Does a Coupling Work?

Couplings work by securely clamping two shafts together, allowing one shaft to transfer energy to the other.

At the same time, the coupling absorbs shock, vibration, and heat generated by the first shaft, protecting the surrounding components while still effectively transferring torque.

Installing a rigid coupling allows two separate shafts to operate as one. Their high torsional stiffness allows no relative motion between the shafts of the driving and driven units.

Eliminating relative movement maximizes the amount of torque that can be transmitted across the shafts

Use of coupling

Shaft couplings are used in machinery for many purposes, the most common of which are the following:

• For connection to shafts of units manufactured separately as a motor and generator and provide for repair or disconnection for option.
• To provide shaft misalignment or to introduce mechanical flexibility.
• To reduce the transmission of shock loads from one shaft to another.
• To introduce protection against overload.
• It should not have any projecting parts.

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: 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.

Accommodate any misalignment: 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.

Requirements of a good coupling

A good shaft coupling should have the following requirements:

• It should be simple to connect or disconnect.
• It must transmit full power from one shaft to another shaft without damage.
• It should hold the shaft in the correct alignment.
• It should decrease the transmission of shock loads from one shaft to another.
• It should not have any projecting parts.

Parameters for Choosing Couplings

Shaft couplings are an integral component of motion control and power transmission systems. They provide incredible advantages and combat many assemblies 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

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.

Coupling maintenance and failure

Coupling maintenance requires a regularly scheduled inspection of each coupling. It consists of:

• Performing visual inspections,
• Checking for signs of wear or fatigue
• Cleaning couplings regularly
• Checking and changing lubricant regularly if the coupling is lubricated. This maintenance is required annually for most couplings and more frequently for couplings in adverse environments or demanding operating conditions.
• Documenting the maintenance performed on each coupling, along with the date.

Even with proper maintenance, however, couplings can fail. Underlying reasons for failure, other than maintenance, include:

• Improper installation
• Poor coupling selection
• Operation beyond design capabilities.

The only way to improve coupling life is to understand what caused the failure and to correct it prior to installing a new coupling. Some external signs that indicate potential coupling failure include:

• Abnormal noise, such as screeching, squealing, or chattering
• Excessive vibration or wobble
• Failed seals are indicated by lubricant leakage or contamination.

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