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
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
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:
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.
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
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
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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.
The purpose of the Design phase in the Software Development Life Cycle is to produce a solution to a problem given in the SRS(Software Requirement Specification) document. The output of the design phase is a Software Design Document (SDD).
Coupling and Cohesion are two key concepts in software engineering that are used to measure the quality of a software system&#;s design.
What is Coupling and Cohesion?
Coupling refers to the degree of interdependence between software modules. High coupling means that modules are closely connected and changes in one module may affect other modules. Low coupling means that modules are independent, and changes in one module have little impact on other modules.
Cohesion refers to the degree to which elements within a module work together to fulfill a single, well-defined purpose. High cohesion means that elements are closely related and focused on a single purpose, while low cohesion means that elements are loosely related and serve multiple purposes.
Both coupling and cohesion are important factors in determining the maintainability, scalability, and reliability of a software system. High coupling and low cohesion can make a system difficult to change and test, while low coupling and high cohesion make a system easier to maintain and improve.
Basically, design is a two-part iterative process. The first part is Conceptual Design which tells the customer what the system will do. Second is Technical Design which allows the system builders to understand the actual hardware and software needed to solve a customer&#;s problem.
Conceptual design of the system:
Written in simple language i.e. customer understandable language.
Detailed explanation about system characteristics.
Describes the functionality of the system.
It is independent of implementation.
Linked with requirement document.
Technical Design of the System:
Hardware component and design.
Functionality and hierarchy of software components.
Software architecture
Network architecture
Data structure and flow of data.
I/O component of the system.
Shows interface.
Modularization is the process of dividing a software system into multiple independent modules where each module works independently. There are many advantages of Modularization in software engineering. Some of these are given below:
Easy to understand the system.
System maintenance is easy.
A module can be used many times as their requirements. No need to write it again and again.
Coupling is the measure of the degree of interdependence between the modules. A good software will have low coupling.
Following are the types of Coupling:
If the dependency between the modules is based on the fact that they communicate by passing only data, then the modules are said to be data coupled. In data coupling, the components are independent of each other and communicate through data. Module communications don&#;t contain tramp data. Example-customer billing system.
In stamp coupling, the complete data structure is passed from one module to another module. Therefore, it involves tramp data. It may be necessary due to efficiency factors- this choice was made by the insightful designer, not a lazy programmer.
If the modules communicate by passing control information, then they are said to be control coupled. It can be bad if parameters indicate completely different behavior and good if parameters allow factoring and reuse of functionality. Example- sort function that takes comparison function as an argument.
In external coupling, the modules depend on other modules, external to the software being developed or to a particular type of hardware. Ex- protocol, external file, device format, etc.
The modules have shared data such as global data structures. The changes in global data mean tracing back to all modules which access that data to evaluate the effect of the change. So it has got disadvantages like difficulty in reusing modules, reduced ability to control data accesses, and reduced maintainability.
In a content coupling, one module can modify the data of another module, or control flow is passed from one module to the other module. This is the worst form of coupling and should be avoided.
Temporal coupling occurs when two modules depend on the timing or order of events, such as one module needing to execute before another. This type of coupling can result in design issues and difficulties in testing and maintenance.
Sequential coupling occurs when the output of one module is used as the input of another module, creating a chain or sequence of dependencies. This type of coupling can be difficult to maintain and modify.
Communicational coupling occurs when two or more modules share a common communication mechanism, such as a shared message queue or database. This type of coupling can lead to performance issues and difficulty in debugging.
Functional coupling occurs when two modules depend on each other&#;s functionality, such as one module calling a function from another module. This type of coupling can result in tightly-coupled code that is difficult to modify and maintain.
Data-structured coupling occurs when two or more modules share a common data structure, such as a database table or data file. This type of coupling can lead to difficulty in maintaining the integrity of the data structure and can result in performance issues.
Interaction coupling occurs due to the methods of a class invoking methods of other classes. Like with functions, the worst form of coupling here is if methods directly access internal parts of other methods. Coupling is lowest if methods communicate directly through parameters.
Component coupling refers to the interaction between two classes where a class has variables of the other class. Three clear situations exist as to how this can happen. A class C can be component coupled with another class C1, if C has an instance variable of type C1, or C has a method whose parameter is of type C1,or if C has a method which has a local variable of type C1. It should be clear that whenever there is component coupling, there is likely to be interaction coupling.
Cohesion is a measure of the degree to which the elements of the module are functionally related. It is the degree to which all elements directed towards performing a single task are contained in the component. Basically, cohesion is the internal glue that keeps the module together. A good software design will have high cohesion.
Following are the types of Cohesion:
Every essential element for a single computation is contained in the component. A functional cohesion performs the task and functions. It is an ideal situation.
An element outputs some data that becomes the input for other element, i.e., data flow between the parts. It occurs naturally in functional programming languages.
Two elements operate on the same input data or contribute towards the same output data. Example- update record in the database and send it to the printer.
Elements of procedural cohesion ensure the order of execution. Actions are still weakly connected and unlikely to be reusable. Ex- calculate student GPA, print student record, calculate cumulative GPA, print cumulative GPA.
The elements are related by their timing involved. A module connected with temporal cohesion all the tasks must be executed in the same time span. This cohesion contains the code for initializing all the parts of the system. Lots of different activities occur, all at unit time.
The elements are logically related and not functionally. Ex- A component reads inputs from tape, disk, and network. All the code for these functions is in the same component. Operations are related, but the functions are significantly different.
The elements are not related(unrelated). The elements have no conceptual relationship other than location in source code. It is accidental and the worst form of cohesion. Ex- print next line and reverse the characters of a string in a single component.
This type of cohesion occurs when elements or tasks are grouped together in a module based on their sequence of execution, such as a module that performs a set of related procedures in a specific order. Procedural cohesion can be found in structured programming languages.
Communicational cohesion occurs when elements or tasks are grouped together in a module based on their interactions with each other, such as a module that handles all interactions with a specific external system or module. This type of cohesion can be found in object-oriented programming languages.
Temporal cohesion occurs when elements or tasks are grouped together in a module based on their timing or frequency of execution, such as a module that handles all periodic or scheduled tasks in a system. Temporal cohesion is commonly used in real-time and embedded systems.
Informational cohesion occurs when elements or tasks are grouped together in a module based on their relationship to a specific data structure or object, such as a module that operates on a specific data type or object. Informational cohesion is commonly used in object-oriented programming.
This type of cohesion occurs when all elements or tasks in a module contribute to a single well-defined function or purpose, and there is little or no coupling between the elements. Functional cohesion is considered the most desirable type of cohesion as it leads to more maintainable and reusable code.
Layer cohesion occurs when elements or tasks in a module are grouped together based on their level of abstraction or responsibility, such as a module that handles only low-level hardware interactions or a module that handles only high-level business logic. Layer cohesion is commonly used in large-scale software systems to organize code into manageable layers.
Advantages of low coupling
Improved maintainability: Low coupling reduces the impact of changes in one module on other modules, making it easier to modify or replace individual components without affecting the entire system.
Enhanced modularity: Low coupling allows modules to be developed and tested in isolation, improving the modularity and reusability of code.
Better scalability: Low coupling facilitates the addition of new modules and the removal of existing ones, making it easier to scale the system as needed.
Advantages of high cohesion
Improved readability and understandability: High cohesion results in clear, focused modules with a single, well-defined purpose, making it easier for developers to understand the code and make changes.
Better error isolation: High cohesion reduces the likelihood that a change in one part of a module will affect other parts, making it easier to
Improved reliability: High cohesion leads to modules that are less prone to errors and that function more consistently,
leading to an overall improvement in the reliability of the system.
Disadvantages of high coupling
Increased complexity: High coupling increases the interdependence between modules, making the system more complex and difficult to understand.
Reduced flexibility: High coupling makes it more difficult to modify or replace individual components without affecting the entire system.
Decreased modularity: High coupling makes it more difficult to develop and test modules in isolation, reducing the modularity and reusability of code.
Disadvantages of low cohesion
Increased code duplication: Low cohesion can lead to the duplication of code, as elements that belong together are split into separate modules.
Reduced functionality: Low cohesion can result in modules that lack a clear purpose and contain elements that don&#;t belong together, reducing their functionality and making them harder to maintain.
Difficulty in understanding the module: Low cohesion can make it harder for developers to understand the purpose and behavior of a module, leading to errors and a lack of clarity.
Conclusion
In conclusion, it&#;s good for software to have low coupling and high cohesion. Low coupling means the different parts of the software don&#;t rely too much on each other, which makes it safer to make changes without causing unexpected problems. High cohesion means each part of the software has a clear purpose and sticks to it, making the code easier to work with and reuse. Following these principles helps make software stronger, more adaptable, and easier to grow.
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