Couplings Guide – Types, Applications and Selection

Couplings are very important mechanical elements in machinery. We use couplings in connecting two shafts. So in transmission systems, the use of couplings is very common. For different kinds of connection types, we use different kinds of them.

Here we prepared a guide to select them professionally. It is very important to consider the types and parameters to select a proper type of them.

What are the Couplings?

They are the machine elements to connect the different shafts in different orientations. You know that shafts transfer the power by rotating. Also, there are need to transfer this power to another shaft. In this case, we can use gears. But we use gears between the parallel shafts. But also, we can use the couplings systems to attach the end of the shaft to the end.

In general, we prefer to use straight types of couplings between the aligned shafts. But sometimes, we need to use them in the systems where we need to transfer torque in different orientations of shafts. In this case, we need to use flexible couplings solutions.

Types of Couplings

Rigid Couplings	Flexible Couplings
Rigid Flange Couplings	Elastomeric Couplings
Sleeve Couplings	Flexible Metal
Compression Sleeve Couplings	Falk
Hirth Joints	Bellows
	Schmidt Couplings
	Universal Couplings
	Fluid Couplings
	Gear Couplings
	Geislinger
	Highly Flexible Couplings
	Magnetic Couplings
	Oldham
	Rag Joints

It is very important to know the general types of couplings. Because you need to be aware of all of the types if you are thinking about the use of these systems. Different kinds of applications, there are different types of them available.

Rigid Couplings

They are the types of couplings where the attachment of two aligned shafts takes place. Because of this, they are very important types of them.

They have very basic structures that do not have any complexities. The most important disadvantage of the rigid ones, we can not use them in the misaligned shafts. The alignments of the shafts must be tight intolerances.

Also, their most important advantage is, that they do not backlash. Because two shafts will act as one shaft with the rigid types. Also, they are good for high-precision machines such as servo machines, etc.

Unlike flexible types, the energy loss is zero in rigid couplings. So, it is a very useful feature that the attached shafts are acting as one shaft.

We need to state that, rigid couplings do not possess any vibration damping property because of their rigid structures.

Rigid Flange Couplings

They are the first type of rigid couplings where the connection of two shafts are take place with a flange attachment. There are two flanges attached to the shafts with keys or splines, and these flanges are attached with bolts and nuts.

The attachment phase is very important for the even load distribution. Because it is very important to have even load distribution on the bolts and nuts to obtain a safe design. Otherwise, mechanical failure can occur for the rigid flange systems.

To obtain the even load distribution, we generally attach the nut and bolts in opposite directions respectively. This provides an even distribution of the loads.

For heavy-duty applications, these types of flange ones applications are very common.

Another important advantage of these types, they are very useful for frequent attachment and detachments. In maintenance applications for bigger systems, the use of flange systems is very common.

Sleeve Couplings

Sleeve types are the second type of these applications that we use. In these applications, we use the sleeve structures that are bolted together. There are two halves of sleeves where the two ends of shafts are attached.

Also, we use keys or splines to attach the shafts to the sleeves. Sleeve coupling applications are very common in low loads and middle-load applications.

In sleeve coupling applications, very high-strength bolts that we need. Also, very tight indexing of the connecting shafts is another requirement.

Compression Sleeve Couplings

They are quite the same as the sleeve couplings. But the connection of the sleeve couplings takes place with the compression action between the sleeves instead of the keys or splines. The compression provides the ability to transfer more torques and power if we compare it with the sleeve ones.

But their costs are higher than the ordinary ones. Because we need better surface treatment to obtain the required system.

In large shaft applications, compression sleeve types are not suitable and common.

Hirth Joints

This is a very basic construction to connect the shaft ends. They possess a teethed structure that is connected. And each half has a shaft that is connected with these teeth.

The Hirth’s joints do not allow any misalignment. And also, they have very basic construction. Also, they can provide much higher torques if we compare them with other kinds of applications.

Because of the teeth’ structure, they do not have any backlash problems.

Flexible Couplings

The other important class of couplings is the flexible ones. We use these flexible coupling applications where there is a misalignment between the shafts. They provide torque and power transfer between the shafts in different orientations.

Flexible ones are more complex than rigid ones in structure. So in general, they are more expensive. This is a major disadvantage of these systems in general.

And also, they provide vibration damping and electrically insulative characteristic between the connecting shafts. So, this is a very important advantage of them over rigid structures.

Furthermore, they are not good for precision applications. Because they have backlash problems which provide an extra problem in the precision.

There are different types of flexible types. You can select the proper one for your application in different mechanisms.

Elastomeric Couplings

They are the most common types of flexible coupling systems. In these systems, there is an elastomer hard rubber disc that provides angular displacement between the shafts. The most important advantage of the elastomeric ones is the vibration damping characteristics. So, with this characteristic, we use it in the high vibrational system where we need to transfer loads. Also, electrical isolation is a very important parameter for these types.

There are two main types of elastomeric couplings. They are jaw and flexible disc ones.

• Flexible Disc: In these types, there is a rubber flexible disc between the flanges of them. This flexible disc provides an angular displacement. And we attach these flanges and discs with bolt and nut connections.
• Jaws and Spider: In these types of them, two jaws are attached to a rubber spider. This rubber spider provides the flexible structure of these coupling systems. The most important problem in these systems is the backlash problem. We can adjust the hardness of the spider rubber according to the application.

Flexible Metal Couplings

This is another type of flexible coupling. The only difference is, that there is no rubber material between the discs. There is a metal part instead of rubber. This provides extra torque transmission and extra power capabilities between the shafts. But the vibration damping characteristics are lower than the rubber ones.

Falk Couplings

Falk couplings are also a very different design from these systems. Because they have a spring in the threaded sections of the hub. This spring system provides a flexible connection between the shafts.

Also with the increasing loads, the spring system shortened and provide much more rigid structures.

Bellows Couplings

These are the type of spring-like structures that allows moderate misalignment between the shafts. And also, the most important disadvantage of these couplings, they tend to more fatigue damage. And also, the important advantage of these, they have no backlash which provides better precision operation.

Schmidt Couplings

They are the types of them where the connection between the shafts take place with the connection of 6 links which are bearings. So, they allow moderate misalignment between the shafts. Also, the most important advantage of these couplings is they allow very good parallel offset between the shafts thanks to the structures of links.

Also, Schmidt types are used between the shafts up to 50mm diameters with moderate torque transmissions. Furthermore, they need proper lubrication for their operation of them.

Universal Couplings

These joints are very common in automotive applications where they allow for very big angular misalignments between the shafts. Also, we know them as Cardan joints or CV joints. They transfer the torque with an intermediate element between the edges of the joints.

Also, they possess parallel misalignments. The most important disadvantage of these coıplings, the output torque, and rotational speed is lower than the input one. This leads a wear and vibration damping problems. Also, they need very good lubrication.

Fluid Couplings

They are also very different designs of couplings. The fluid types are ideal for where very big vibrations. Because there is no contact between the two shafts. One shaft has connected to a rotor and another one is connected to a stator. Once the rotor starts to rotate, a hydrodynamic effect occurs. And the fluid at the rotor is pumped to the stator. And this effect makes them rotate.

Fluid couplings are very common in truck and bus applications.

Gear Couplings

At the two ends of shafts, they connected with internal gear structures. So, they possess a parallel misalignment between the shafts. And with the special design of the gears, they also possess angular displacements between the shafts.

They can transfer very high torque values. But the lubrication can be a problematic issue for the gear couplings.

Geislinger coupling

They are all metal ones that connect the shafts and obtain a very good connection. They possess very good misalignment and vibration damping characteristics.

Highly Flexible Couplings

Highly flexible types are the types of couplings where the only purpose is to prevent the vibrations and torsional vibrations coming from a shaft. If there is a very big vibration cvoming from machinery, these ones are good to transfer this high vibration of torque to the other shafts.

The backlash of these couplings can be a problematic issue.

Magnetic Couplings

Magnetic couplings are the types of couplings where the torque transmission takes place with the magnetic connection of both ends of the shafts. So, there is no physical connection between the shafts.

They possess a level of misalignment. But the torque capacities of these couplings can be lower than the physical ones. Also, the vibration damping characteristic is very good because there is no physical contact between the shaft edges.

Oldham Couplings

They are physical types that have very different structures. They have a jaw connection to each other. And there is a tooth that the jaws are connected to it. This jaw assembly provides very good parallel misalignment of the input and output shafts.

The backlash can be a problematic issue. But with the use of the springs, the backlash is tolerated in these systems.

Rag Joints

Rag joints are the most common joints in automotive steering applications. They provide good angular misalignment which is very good for steering applications.

As you see above, there are various types of couplings available in the market. Selecting the right one can be very tough. But comparing with the other kinds of applications can give an insight for you. For example, you know that the Cardan joints and rag joints are common systems in steering applications. So, you can use them if you design steering applications.

Parameters in the Selection of a Coupling

Around the various kinds of types, selecting the right one for your application may not seem easy. You need to consider lots of kinds of parameters. Here we explain these general parameters that you need to consider in the selection of proper couplings for your applications.

1 – Flexible or Rigid?

This is the first important consideration that you need to have. Do you need to use flexible couplings or rigid couplings? If there is a misalignment between the connected shafts, you need to select one of the flexible couplings available above. And also, you need to select rigid ones if there is no mşsalignment.

For example, if you just need to extend the power transmission line by adding extra shafts, you can consider the rigid coupling structures. They just add the end of the shaft to the end.

And also, for example, there is an angular misalignment between the shafts and the power must be transferred between non-parallel shafts, you can consider the Cardan’s joints, etc.

2 – Type of Misalignment

If there is a misalignment in your design, you need to consider the types of misalignment. There are generally two types of misalignment. The first one is the angular misalignment and the second one is the parallel misalignment of the shafts.

For both types, you need to use flexible couplings. But also, the type of misalignment will decrease the available selections. For example, if there is only parallel misalignment, you can use a jaw type of couplings.

Also if you need an angular misalignment, you can use a Cardan joint again.

3 – Total Load

All types are couplings are not able to transfer all the loads. All types of couplings have different capacities to transfer loads. So, you need to know the general idea about which level loads you will transfer through the couplings. And you need to be aware of their load carrying capacities of them. So, you can acquire this information by checking the coupling catalogs.

4 – Shaft Size

The diameter of the shafts is also very important in the design of the couplings. Because all the coupling types are not available in all the shaft sizes. They have optimum size limitations for proper working.

You need to consider these size limitations in the selection of them. We need to use catalogs of the different available types of them. You may probably be given the choices of different coupling applications.

5 – Vibrational Damping Characteristics

As you noticed above, the vibration damping characteristics of the couplings are a very important thing. If your system requires vibration damping while you are transferring power through the shafts, you need to select the best coupling system to overcome this situation.

For example, the general flexible couplings are very good at vibration damping. So, you need to consider them.

Also, machinery that produces power which has a very big amount of vibrations such as internal combustion engines. And you need to use this power in a precision system. So, you need to consider the vibration damping characteristics of the couplings.

6 – Rotational Speed

The rotational speed is also a very important parameter that you need to consider. Because, with the increasing rotational speeds, some of the couplings may not work efficiently. Also in the catalogs about them, there is the available rotational speed for the different kinds of them.

You need to select a proper coupling system that suits your applications best.

7 – Cost

For example, you can stay between two of the coupling selections which are available for your applications. You need to consider the cost in that case. We will not say to buy the cheaper one… You need to consider the long-term cost of these coupling systems.

For example, you need to calculate the maintenance costs and initial investment costs. Also, some of these systems need strict lubrication. So, you need to consider the lubrication costs of these systems.

By considering all the long-term calculations, you need to select the one with the lowest price.

These are the general parameters that you need to consider before deciding which coupling system you need to use.

Applications that We Use Couplings

In power transmission systems, the use of couplings is very common. Because they are very important elements to transfer the rotational power between the distances. These are the general applications that we use in coupling systems.

Power Transmission Systems

Power transmission systems are very common in most machinery. There is the main power generator which can be an electric motor or an internal combustion engine. And the rotational power is transferred to the sections of the machinery that required the power through the gear systems, shafts, or pulley systems.

If you need to transfer the power to long distances, you need to use multiple shafts. So, the use of coupling systems is inevitable.

Steering Systems

In steering systems of automotive applications, the use of Cardan couplings is very common. Because we need to transfer the rotational motion of the steering wheel to the rotational motion of the wheels. So, there is a great transfer of the load between the steering wheel and the wheels.

So, in the linkage of the steering wheel and steering wheel shaft, there are Cardan couplings to change the angular direction of the rotation.

Rear Axles of Cars

In rear-wheel drive cars, the engine is generally at the front line. After the engine, the transmission gear system takes the rotational power. And this rotational power must be transferred to the rear axle which is a very long distance.

So we need to use a shaft system to transmit this power. The shaft is connected to the transmission with a coupling. Also, there is a differential at the rear axle. The shaft must connect to this differential with a coupling. And if you take a closer look, there is a small angular misalignment in these both coupling applications.

And the use of rag joints in these applications is the most common application.

Industrial Machinery

In industrial machinery systems, the applications of the different types of them are very common. This is because industrial machinery requires power transmission in different systems. For example, a hydraulic system requires a power transmission from an electrical engineer. So, the use of couplings is very common in lots of industrial applications.

There are different types of them according to the different requirements of the power transmission systems.

Pumps and Compressors

Pumps and compressors are very important devices that we use in different types of engineering applications such as pneumatic and hydraulic systems. They are the devices that compress liquids by increasing their pressure values. And there is rotating machinery inside the pumps that create pumping action.

This rotational motion takes place with the action of electrical motors or internal combustion engines. And there must be a shaft between the pumps and compressors. Compressors are also the equivalents of pumps for compressible fluids and gases.

So shafts are connected to the pump systems and compressors with the proper couplings. There are 2 applications in pump and compressor systems that we use in general.

All the Shaft Connections

As you understand from the application examples, we use couplings in the connection of different systems and shafts. In general, motors are manufactured with an output shaft. And you need to connect this output shaft to a system that creates mechanical work. So, you need a shaft connection which you will probably use coupling between them.

So at the output sections of the motors, there is nearly always the use of a coupling system.

Advantages and Disadvantages of Couplings

Like all mechanical systems, there are advantages and disadvantages to coupling systems. So, you need to consider the advantages and disadvantages of these systems before applying your applications.

Advantages of Couplings

• They are generally low-cost systems to transfer power between long distances. If we compare with the pulley systems and gear systems, they are low cost.
• Also, the construction of these elements is very basic. If we think about the maintenance costs, the replacements and maintenance instructions are much easier than the other systems.
• There are lots of kinds of types that you can select the most proper one for your application. For different kinds of applications and different kinds of requirements, you can use different types of couplings.
• In different angular and parallel alignments of shafts, these systems can transfer power between these shafts.
• The vibrational damping characteristics of these systems are also superior. Some of the couplings applications are very good in vibration damping. Especially the flexible types are very good in vibrational damping.

Disadvantages of Couplings

• Among the disadvantages of couplings, they can cause mechanical losses and efficiency losses in the transmission of power. Especially in flexible systems, these losses can be problematic.
• The precision of the general machinery drops because of the use of these systems. Because backlashes can be a problem in attaining these precisions. But there are coupling solutions to solve these backlash problems.
• Use of them adds extra costs to the general machinery which can be a problematic issue.

So, these are the general disadvantages and advantages of couplings. But most of the time, the use of coupling systems is mandatory. Because we need to transfer the torque to other sections of machinery in general.

Conclusion on Couplings

These are the general aspects of coupling systems that we use in machinery. They are very important machine elements that we use in power transmission. Also, the direction of the power changes with these systems.

There are different kinds of these systems that we use. We can choose one of them according to the type of application. They differ according to their flexibility. For example, we use flexible coupling systems where there is an angular or parallel misalignment of the shafts.

Also, the general applications are these systems are very common. In power transmission lines, we use different kinds of these systems. For example in the power transmission of the engine systems, we use couplings.

We need to consider the general advantages and disadvantages of couplings if we need to apply our applications. They are general principles. If you are aware of the general merits and demerits of these systems, you would make better design considerations.

Furthermore, there are different kinds of parameters that are affecting the selection of couplings. We need to consider these parameters while we are selecting the proper system for our applications.

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FAQs About Couplings