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.
Types of Coupling
Different Types of coupling are:
- Beam coupling
- Sleeve or Muff Coupling
- Split Muff coupling
- Flange coupling
- Disc coupling
- Bushed Coupling
- Diaphragm Coupling
- Grid Couplings
- Roller Chain Coupling
- Gear coupling
- Tyre Couplings
- Jaw Couplings
- Oldham Coupling
- Universal Coupling
- Bellows Coupling
1. 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.
2. Sleeve or Muff Coupling
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.
3. Split Muff coupling
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.
4. Flange Coupling
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.
A flange 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.
5. 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.
A disc coupling is a high-performance motion control coupling designed to be the torque transmitting element (by connecting two shafts together) while accommodating for 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 has 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.
6. 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.
They 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.
7. 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 the 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 inside to outside diameter.
- Allows for angular, parallel, and high axial misalignments
- High torque, used in high-speed applications
8. Grid Coupling
Grid couplings, like disc and gear couplings, are designed with high torque density applications in mind. Typically composed of two shaft hubs, a horizontally split cover kit, and a serpentine grid, grid couplings are well suited for shock loading applications.
Torque is transmitted between the two shaft hubs through the grid element, and the shock-absorbing grid dissipates shock loading applications, minimizing the on the connected equipment. With proper installation, maintenance and lubrication, you can deliver years of reliable service for your operation.
Grid couplings are a versatile, proven technology with interchangeable components readily available from several major coupling manufacturers.
Grid couplings have a high-power density (transmit a high amount of torque relative to their size), and are relatively straightforward and simple to install. They also have good resistance to environmental conditions and are available for both inch and metric bores.
9. Roller Chain Coupling
A roller chain coupling is a mechanical device composed of a double-strand roller chain and two modified sprockets. The design is simple and highly effective, despite its small size, and is composed of a robust chain and specially cut, hardened-tooth sprockets that allow a high amount of torque to be transmitted.
Roller chain couplings are extremely simple but provide a compact and flexible coupling that is suitable for an extremely wide range of applications. Because of the design, the torque is distributed throughout the roller chain and sprocket teeth so there is an even amount of torque distributed throughout the coupler when in motion.
Another good thing about a roller chain coupling is that the sprockets and chain produce a slight clearance, which means that absolute shaft alignment is not required for the coupling to function, however, it is highly recommended to have the shafts completely aligned, if possible.
10. 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.
11. Tyre Couplings
Tire or tyre couplings are torsional soft, shaft couplings with a flexible body that compensates misalignment and protects other components in the transmission system.
tire couplings are highly flexible and free of torsional backlash. Because of their low torsional stiffness and damping capacity, couplings are especially suitable for coupling machines with a highly nonuniform torque pattern.
These tire couplings are also suitable for connecting machines with high shaft misalignment. The elastic tire can simply be slipped over the hub parts. The elastic tire is held firmly in place by fitting the clamping ring.
The connection transmits the torque by frictional engagement. Standard tire coupling types are designed as shaft-to-shaft connections. Application-related types can be implemented on request.
The coupling can be fitted with elastic tires made of natural rubber for ambient temperatures of -50°C to +50°C and with elastic tires made of chloroprene rubber from -15°C to +70°C. The chloroprene rubber tire is marked FRAS, “Fire-resistant and Antistatic”.
- Reduces transmission of shock loads or vibration.
- High misalignment capacity
- Easy assembly w/o moving hubs or connected equipment
- Moderate to high-speed operation
- Wide range of torque capacity
12. 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.
Jaw couplings are composed of three parts: two metallic hubs and an elastomer insert called an element, but commonly referred to as a “spider”. The three-parts 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
13. 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.
14. Universal (or Hooke’s) 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.
15. Bellows 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.
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.
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.
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.