COUPLINGS: Definition & Types of Couplings

WHAT IS COUPLINGS?

Couplings are a device that connects both ends of two shafts to transmit power.  

The main purpose of the couplings is to connect the two rotating devices, allowing some misalignment and / or edge movement.  

In more general situations, couplings are also mechanical devices used to connect the ends of adjacent parts or objects.  

Couplings usually cannot separate the shaft during operation, but there are torque limiting clutches that can slip or separate if certain torque limits are exceeded.  

Couplings selection, installation, and maintenance can reduce maintenance time and costs. 

TYPES OF COUPLINGS  

  1. Beam Coupling  

Beam couplings, also known as helical couplings, are flexible couplings used to transfer torque between two shafts, with respect to angular deviations, parallel deviations, and even to the other shaft of one shaft.  

It even allows for axial movement. This structure uses a single material and is made flexible by removing the material along a spiral path, resulting in a spiral curved flexible beam.  

Made of a single material, beam-type couplings do not have the backlash found in some multipart couplings.  

Another advantage of fully machined couplings is the ability to incorporate functionality into the final product while maintaining the integrity of a single part.  

As the pitch of the spiral beam changes, not only does the misalignment function change, but other performance characteristics such as torque capacity and torsional stiffness also change. 

It is also possible to have multiple starts within the same spiral.  

The materials which are utilized in the production of beam couplings also decrease their performance and their suitability for the limited applications such as food, pharmaceuticals and aerospace.  

The materials used for this are typically aluminum alloys and stainless steel, but they can also be made from acetal, maraging steel and titanium.  

The most common applications are attaching rotary encoders on shafts and robotics motion control.  

  1. Bush pin flange coupling 

 Bush pin flange couplings are used for slightly inaccurate alignment of the two shafts.   

This is a modified form of protected type flange coupling.  

This type of coupling has pins and works with coupling bolts.  

Rubber or leather bushings are used on the pins.  

The coupling has two halves of different structures.  

The pin is firmly attached to one of the flanges with a nut and held loosely by the other flange.  

This coupling is used to connect shafts with little parallel, angular, or axial misalignment.  

In this coupling, the rubber bush absorbs shocks and vibrations during operation.  

This type of coupling is mainly used to couple electric motors and machines.   

  1. Clamp or split-muff coupling 

In this coupling, the sleeve or sleeves consists of two halves made of cast iron and are interconnected using bolts or structural steel bolts.  

Without changing the position of the shaft this type of coupling can be assembled or disassembled.  

This coupling is used for high power transmission at medium speeds.  

  1. Elastic Coupling  

Elastic couplings carry torque or other loads through flexible components.  

An example is the coupling used to connect windsurfing rig (sails, masts, components) to sailboard. In windsurfing terminology, commonly referred to as “universal joints”, modern designs are usually based on strong and flexible materials and are technically well known as elastic couplings. 

They can take the form of tendons or hourglasses and are made of strong and durable elastic material.  

In this application, the coupling does not transmit torque, but it does transmit sailing force to the board and generate thrust (part of the sailing force is also transmitted to the rider’s body).  

  1. Gear coupling 

A tooth coupling is a mechanical device used to transfer torque between two shafts that are not in the same straight line.  

It consists of flexible joints attached to each shaft.  

The two joints are connected by a spindle, which is the third shaft. Each joint consists of an internal / external gear pair with a gear ratio of 1:1.  

The outer diameter of the tooth surface and the external gear is spherical to allow an angular offset between the two gears.  

Mechanically, the gear corresponds to a modified rotary spline profile.  

It is called a gear because its teeth are relatively large.  

Gear couplings and universal joints are used in similar applications.  

Gear couplings have higher torque density and less vibration in universal joints than universal joints designed for specific spaces.  

The limit of torque density of universal joints is due to the limited cross-sectional area of ​​the cloth and yoke.  

The gear teeth of the gear coupling have a lot of play to allow for angular deviation.  

Excessive play can cause vibration. 

Gear coupling are generally based on angular deviation. 

The angle of the spindle with respect to the axis of the connected shaft is of 4-5°.  

Universal joints can cause higher misalignment. Single-joint gear couplings are also used to connect two nominal coaxial shafts.  

In this application, the device is called flexible gear or flexible coupling.  

The single joint allows for small misalignments such as mounting errors and changes in shaft alignment due to operating conditions.  

These types of gear couplings are typically limited to 1/4–1/2 ° angle deviations. 

  1. Flexible Coupling  

The Flexible Coupling is designed to deliver torque smoothly while allowing axial, radial, and angular deviations.  

Flexibility is like moving part of the coupling  with little or no resistance in the event of a misalignment.  

This means that there are no significant axial or bending stresses on the shaft.  

Many types of flexible couplings are commercially available.  

Each is designed to transmit a specific limit torque.  

The manufacturer’s catalog lists design data that allows you to choose the right coupling.  

Remember that torque is equal to power divided by speed.  

With a given clutch size, the force that the clutch can transmit is not always directly proportional, but it increases with increasing speed.  

Of course, the effect of centrifugal force determines the upper limit of speed.   

The degree of misalignment that can be absorbed by a particular coupling should be taken from the manufacturer’s catalog data, but the value depends on the size and design of the coupling.  

Small couplings can be limited to 0.005″ parallel deviations, while large couplings can be 0.030″ or more.  

A typical allowable angle deviation is 3°. Allowable axial movement, sometimes referred to as end play, is up to 0.030 inches for many coupling types.  

Spacers allow the coupling to be removed without moving the pump or motor, providing access for changing the pump seal.  

A floating shaft type flexible coupling used in large span applications. Both sides of the flexible coupling should be firmly supported by the connecting shaft.  

  1. Highly Flexible Coupling  

Highly Flexible Coupling is used when there is a resonance or torsional vibration problem to eliminate the torsional vibration problem and compensate for the impact load.  

These are used when the system requires high torsional elasticity and displacement capacity.  

This type of coupling provides effective damping of torsional vibrations and high displacement capacitance to protect the drive.  

The flexible elastic coupling structure facilitates assembly.  

These couplings also compensate for shaft misalignment (radial, axial, angle) and torque is transferred to shear.   

Depending on the size and stiffness of the coupling, the flexible parts can be in one row, multiple rows, or both. 

FLEXIBLE DISK COUPLING SELECTION PROCEDURE  

This procedure uses a pump drive as an example and assumes that you know the following data: (US unit is used)  

Power transmission via coupling in horsepower, P 

Normal speed of clutch Rpm, n 

Maximum speed that coupling can be achieved under all operating conditions.  

Desirable distance between pump shaft and drive motor shaft. 

Drive motor shaft and pump shaft diameter  

The application types listed are used to determine the required service factor selection procedure: 

1. Increase power Capacity Per 100 rpm: HP-100 rpm 

2. Determine the service factor SF for your particular application.  

3. Select one or more candidate couplings using the calculated power rating per 100 rpm with the specified service factor.  

4. Make sure that the maximum bore size of the coupling fits the specified shaft diameter. You can order a bore diameter smaller than the maximum value.  

5. Make sure that the maximum speed of the clutch does not exceed the maximum speed of the device.  

6. Check system operating torque and maximum torque due to start-up or impact load  

COUPLINGS MAINTENANCE AND FAILURE  

Regular inspection of each clutch is required for clutch maintenance. The configuration is as follows:  

  1.  Perform a visual inspection. 
  1.  Check for signs of wear or fatigue. 
  1.  Clean the coupling regularly. 
  1.  Check the lubricant regularly and replace it when the coupling is lubricated. This maintenance is required annually for most couplings and more often for couplings in adverse environments or harsh operating conditions.   
  1. A dated document of the maintenance performed on each coupling. However, even with proper maintenance, the clutch can fail. In addition to maintenance, the root reasons for failure are:  
  • Improper installation. 
  • Inappropriate coupling choice. 
  • Operation beyond the possibilities of design. 
  1. The only way to extend the life of a clutch is to understand the cause of the failure and fix it before installing a new clutch. Some external signs of possible clutch failure are:  
  • Abnormal noise, such as screeching rattling noises. 
  • Excessive vibration or wobbling.  
  • Seal failure, indicated by a leak or contamination. 

GOOD COUPLING ASSEMBLY REQUIREMENTS   

  • Easy to connect and disconnect couplings.
  • This can cause some misalignment between the axis of rotation of two adjacent shafts. 
  • There are no projecting parts.
  • The goal is to minimize the remaining misalignment during operation in order to maximize power transmission and maximize machine uptime (the useful life of couplings, bearings, and seals).
  • We recommend that you use the manufacturer’s alignment settings to set the machine train to a defined non-zero alignment.
  • Later, when the machine is at operating temperature, the alignment conditions will be perfect.  

 APPLICATIONS  

Shaft couplings are used in machines for a variety of purposes. Its main function is to transfer power from one end to the other (e.g., motor transfer of power to the pump via a coupling).  

 Other common uses:  

  •  Change the vibration behaviour of the rotating unit.  
  •  To connect the drive and output components.  
  •  Introduce protection.  
  •  To reduce the transmission of impact loads from one shaft to another.  
  •  Slip when under overload. 

ADVANTAGES  

  • It can tolerate slight misalignment. 
  • It can work with shocks and vibrations. 
  • It can be used for transmitting high torques. 
  • It is simple in construction. 
  • It can easily be assembled or dismantled. 

DISADVANTAGES 

  • Misalignment. One of the disadvantages of using a shaft coupling is the possibility and effects of misaligning the shafts, then attaching a coupling.  
  • Vibration. Shafts can vibrate during rotation.  
  • Velocity.  
  • Backlash.  
  • Loosening. 

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