Construction :-
The fluid flywheel or the hydraulic coupling as it is frequently called, has been used in cars employing automatic transmissions.

It consists of two members, the driving and the driven as shown in figure. 

The driving member is attached to the engine flywheel and the driven member, to the transmission shaft. 

The two members do not have any direct contact with each other. 

The driven member is free to slide on splines on the transmission shaft. 

The two rotors are always filled with fluid of suitable viscosity. 

These are provided with radial ribs to form a number of passages, which avoid formation of eddies and also guide the fluid to flow in the desired direction.

Torque transmission :-

A simplified diagram representing the fluid flywheel makes it easier to understand the process of transmission of torque. 

At the start, tube, X is rotating, say, at N1 RPM and tube Y is stationary. 

With the movement of fluid in X and Y, Y also starts rotating though at lower speed. 

This speed goes on increasing till it becomes equal to the speed of X. 

Then the coupling is fully engaged.

To understand how all this happens, consider a particle A, which after small intervals of time takes successively the position B, C and D. 

Thus we see that particle A gains kinetic energy as it moves from A to B in tube X, and then when it passes to tube Y, it gives the same to it, thereby increasing its speed.

Characteristics :-

Figure shows the variation of percentage slip with speed. 

The percentage sleep is defined as       N1-N2 / N1 * 100 where N1 and N2 are the speeds of driving and driven members respectively. 

It is seen that for engine speed below about 500 RPM (fixed by the designer), percentage slip is 100 which means clutch is fully disengaged. 

As the engine speed increases further to about 1000 RPM, the percentage sleep falls rapidly to about 10, beyond which the slip decreases gradually to a small value of about two percent at about 3000 RPM. 

As percentage slip represents definite loss of energy and consequently increased fuel consumption, the engine should not be allowed to run at a speed between approximately 500 and 1000 RPM. 

This condition is similar to a slipping clutch in case of ordinary friction clutches.

Advantages :-

1. No wear on moving parts.
2. No adjustment to be made.
3. No maintenance necessary except oil level.
4. Simple design.
5. No jerk on transmission when the gear engages. It damps all shocks and strains incident with connecting a revolving engine to transmission.
6. No skill required for operating it.
7. Car can stop in gear and move off also by pressing accelerator pedal only.

Disadvantages :-

The only disadvantage of the fluid flywheel is that there is a drag on the gearbox shaft even when the percentage slip is hundred. 

This makes the gear changing difficult with the ordinary crash type gearbox. 

Hence the fluid flywheel is generally used with epicyclic gearbox which avoids this difficulty.

Fluid flywheel troubleshooting :-

The faults experienced in the case of fluid flywheel are not many. In the absence of many mechanical components, the maintenance job for fluid flywheel is much easier as compared with ordinary friction clutches. The major faults that occur in flywheel are :-

1. Large slip :-

As is clear from the characteristics of a fluid flywheel, some slip always exists. But sometimes it may become excessive due to either the shortage of fluid or the fluid in the flywheel not being of proper viscosity.

2. Drag :-

If appreciable drag is experienced in the flywheel when the engine is idling it may be only due to wrong grade of fluid.

3. Vibration :-

The vibration in the fluid flywheel may be caused due to upsetting of the balance of the rotors. The unbalance may be due to reasons such as nut being changed on the bolts, oil filter plug being changed over etc.