A fluid coupling is a coupling that uses fluid to transmit power from the driving side to the driving side.
Fluid coupling is also known as hydraulic coupling.
It is named fluid or hydraulic coupling because fluid is used in this coupling. The fluid used in this type of coupling can be oil or any hydraulic fluid.
Fluid coupling is used in automobile transmission as an alternative to a mechanical clutch.
In this coupling, fluid gets its kinetic energy from the driven side and transfers this kinetic energy to the driven side.

Let’s understand fluid coupling with an example:-

Let’s take the example of a bowl filled with water. When the bowl is rotated, the water starts rotating and the water starts coming out of the bowl when the speed of rotation increases because centrifugal force will act on it.
Then we pace another bowl on top of the first bowl of the same shape and size. The two bowls are placed in such a way that the bowling bottom is not in contact with a bowl on top and the bowl on top is free to rotate.
When the bottom bowl is rotated again and as the speed of rotation increases the water starts moving upward and starts hitting the bowl on top.
When the speed of rotation of the bottom bowl reaches a very high value, the water moves upward in a circular motion with such pressure that the bowl on the top also starts rotating due to the pressure of the water.

Let’s take another example:-
Two table fans are placed in front of each other. One of them is connected to the power supply and it starts rotating.
When the blades of one fan rotate, the air starts flowing towards the other fan and due to the pressure of air created by the first fan, the blades of the second fan also start rotating.

From these two examples, we can see that one driver member transfers its power to the driven member through fluid without any mechanical connection.

### Parts Of Fluid Coupling:

1) Housing:-
It is also known as the shell. It is the outer covering of the fluid coupling. It has oil tight seal around the drive shaft. It also protects the impeller and turbine from outside damage.

2) Impeller Or Pump:-
It is the rotating part of the driving member. It is known as an impeller or pump because it is used for the fluid present inside the housing.

3) Turbine:
The turbine is the driven member which rotates due to fluid pressure created by the driving member or housing.

4) Hydraulic Fluid:-

### Working Of Fluid Coupling:

The impeller is connected to the input shaft. The input shaft is connected either to an electric motor or engine.
The turbine is connected to the output shaft and the output shaft is connected to the object where circulatory motion is to be transmitted.
The impeller and the turbine are not in mechanical contact with each other and fluid is present between these two as the housing is completely filled with fluid.

When the driving member starts rotating, the impeller connected to it starts rotating at high speed.
Due to the high-speed rotation of the impeller, kinetic energy will be imparted to the fluid near the impeller. Due to the imparted kinetic energy, the fluid starts moving toward the turbine and will collide with the blades of the turbine.
When the fluid in the fluid coupling collides with the blade of the turbine, the turbine also starts rotating and transmits its rotation to the output shaft.

So, this is the working of fluid coupling. There are some terms related to fluid coupling that you should know:-

1) Stall Speed:-
Stall speed is the highest speed at which the impeller or pump can rotate when the output turbine is locked and full input torque is applied.
During the stall condition, all engine’s power at that speed is dissipated as heat which can cause damage.
2) Slip:-
Slip is a condition when the input and output angular velocities are identical. In this condition, a fluid coupling cannot develop torque, and hence fluid coupling cannot achieve 100 percent transmission efficiency.

1) No wear and tear as there is no mechanical connection between the driving and driven members.
2) Multiple motors can start sequentially for reducing the aggregate value of the current drawn.
3) When the fluid coupling is used, the motor is selected based on operating duty or load rather than starting load which results in low capital cost.
4) Torque can be easily controlled by changing the fill level i.e. the amount of fluid in the coupling.
5) In fluid couplings there is very effective dampening of shocks, load fluctuations, and torsional vibrations.

1) There is always a slight difference speed of the impeller or pump and the turbine.
2) Fluid couplings are not suitable for application that require a greater speed range.
3) In fluid coupling, torque cannot be developed when the driving and the driven shaft are rotating at the same angular velocity.
4) Maintenance cost is more.
5) Leakages have to be checked regularly.

### Applications Of Fluid Coupling:-

1) Used in Automatic Transmission:
Fluid coupling was used in lots of semi-automatic and automatic transmission systems. In motor vehicle applications.
For use in an automatic transmission, the input shaft of the fluid coupling is connected to the flywheel of the engine, and its output shaft is connected to the insight shaft of the transmission.
2) Fluid couplings also have industrial applications like providing rotational power, particularly in machine drives that have high inertia begin or continuous cyclic loading.
3) Fluid couplings are used in the semi-automated transmission of many country’s Railways.