What is a coupling?
A coupling is a mechanical device which is used to hold two rotating shafts together. It is a permanent joint, unlike clutch which can be disengaged on the will of operator.
With the help of couplings, we can join two shafts which are intersecting, colinear or parallel (with a small distance).
Oldham coupling is used to connect two parallel shafts which are a small distance apart.
Difference between clutch and coupling:
We all know that a clutch (which is generally used in automobiles) is also used for connecting two rotating shafts but there is a basic difference between a clutch and coupling. In clutch we can disengage the rotating shaft at our will whenever we want but in coupling it is not possible.
Practical example of coupling:
In our domestic electricity generator, we see that a shaft is rotated by an engine by converting chemical energy of fuel into mechanical energy. This rotating shaft of engine is then connected with the shaft of an electric alternator with the help of a coupling. And finally, the electric alternator converts mechanical energy of shaft into electric energy.
Here coupling is an important part which transmits rotating motion of engine shaft to electric alternator shaft.
Requirements of a good coupling
- It should be capable of transmitting torque from driving shaft to driven shaft
- It should align the shafts properly
- It should be able to dismantle for the purpose of maintenance
- It should provide safety to man and machine in case of coupling failure
Types of couplings
There are two types of couplings
A flexible coupling can accommodate a slight misalignment between shafts (up to 0.5-degree angular misalignment and 5 mm axial misalignment) but a rigid coupling cannot tolerate any such misalignment.
Examples of rigid coupling muff coupling, clamp coupling, rigid flange coupling etc.
Example of flexible coupling is Bushed pin flexible coupling.
Difference between rigid and flexible couplings
- Flexible coupling can accommodate a slight misalignment between shafts but a rigid coupling cannot accommodate any misalignment
- Flexible coupling can absorb shocks and vibrations but a rigid coupling cannot absorb shocks and vibrations
- Rigid coupling is simple and inexpensive but flexible couplings are expensive
Some common types of couplings are discussed below.
A muff coupling is also known as a sleeve or box coupling. It is a type of rigid coupling. It consists a sleeve and a key which is used to join input and output shafts.
Image source: Coal handling plants
Advantages of muff coupling
- It is simple, it has only two parts a sleeve and a key
- Since it has no projecting parts hence it is safe to use
- It has compact construction
- It is cheaper compared to other types of couplings
Disadvantages of muff coupling
- It is difficult to assemble or dismantle
- Since it is a rigid coupling so it cannot accommodate any misalignment
- Due to absence of flexible elements it can not absorbs shocks and vibrations
- It requires more axial space compared with other types of couplings
It is also known as compression or split muff coupling. It is a type of rigid coupling. It is same as muff coupling the only difference is in clamp coupling sleeve is divided into two axially symmetric parts. Which are assembled together to form a coupling.
Image source: Huco
Advantages of clamp coupling
- It is Easy to assemble and dismantle
- It is easy to remove without axial displacement of any shaft
- Its diametrical dimensions are small compared to muff coupling
Disadvantages of clamp coupling
- Dynamic balancing is difficult hence it is not suitable for high speed applications
- It is Not suitable for shock loads
- Necessary to provide a guard for safety
Rigid flange coupling
As the name suggests it is a rigid coupling. It has two flanges one on driving shaft and one on driven shaft. These two flanges are joined together with the help of nut and bolt.
Advantages of rigid flange coupling
- It has High torque transmitting capacity
- It is easy to assemble and dismantle
- It has simple construction and it is easy to design and manufacture
Disadvantages of rigid flange coupling
- It can not tolerate misalignment between shafts
- It can not used where shocks and vibrations are present
- It requires some radial space
Bushed pin flexible coupling
As the name suggests it is a type of flexible coupling. It consists two flanges mounted each on input and output shaft. Both the flanges are connected with rubber bush in between, which provides resistance to shocks and slight misalignment.
Image source: HMK direct
Advantages of Bushed pin flexible coupling
- 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 of Bushed pin flexible coupling
- It has high cost due to additional parts
- It requires more radial space compared with other types of couplings