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What is Fluid Coupling or Fluid Drive? It’s Construction and Working Principle

Fluid coupling or Fluid Drive is a transmission driveline combination which replaced the flywheel with a hydraulic coupling and works on the same function as a modern torque converter, only without torque multiplication.

The fluid drive is one name for liquid coupling to transmit turning an effort from the engine to the clutch. It is located between the crankshaft and the clutch. Because the liquid coupling is a major part of the engine flywheel assembly, it is sometimes called a fluid flywheel.

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Fluid Coupling

The Figure shows the diagram of a liquid coupling. It consists of, in general, two half-dough nut-shaped shells provided with interior fins that radiate from the hubs. One shell is attached on the crankshaft and is known as driving member or an impeller. Another shell is fixed on the driven shaft and is known as a driven member or a runner.

diagram of a liquid coupling

The 2 shells are very close with their ends facing each other and attached in the housing so that they can rotate without rubbing each other. It is a complete assembly of a liquid coupling. The liquid (oil) is filled in the housing.

When the crankshaft turns, the oil trapped in the impeller fins impinges on the runner fins and causes it to move. In this way, the liquid transmits engine force to the clutch plate without any metal to metal contact.

The lag of the runner behind the impeller is called a slip. It depends upon the engine speed, and load. The slip is maximum when the vehicle is at rest and the throttle open, to causes the impeller to start circulating the oil. The oil under these conditions moves in two general directions at the same time. It rotates at right angles to the shaft (rotary flow) and also circulates between the impeller and runner (vortex flow).

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Fluid Coupling or Fluid Drive Working Principle

The working principle of a liquid coupling is shown in the figure with the help of two fans. When one fan is turned on and face toward the other the air stream makes the second fan to turn, even though it is not switched on.

The first fan is the driving member and the second fan is the driven member or the runner. Air is the fluid used as the medium of power transmission. This is simple fluid coupling, air serving as the fluid.

Vortex Flow

The Vortex flow is at right angles to the rotary flow and is produced by oil trapped in the fins of the impeller. The oil flies out against the rounded interior because of the centrifugal force. The rotary flow starts the movement of the runner when it attains sufficient force and volume.

Vortex Flow

The vortex flow is high when the slip is 100% and decreases as the runner speed approaches that of the impeller. As a result, the efficiency of the liquid coupling increases rapidly from 0 at rest to nearly 99% at higher speeds.

In a typical liquid coupling, the runner would reach the same speed as the impeller, so as to receive all the power given to the coupling by the engine. In commercial design, the runner speed becomes nearly equal to that of the impeller only at the best operating condition, when the efficiency of the coupling is highest.

The liquid coupling is not suited for use with an ordinary gearbox. It is generally used in conjunction with epicyclic gears to provide a semi or fully automatic gearbox.

The liquid coupling is always a major part of the engine flywheel assemble and hence it is also known as a fluid flywheel.

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Fluid Flywheel

Another design of the fluid flywheel is shown in the Figure. It consists of a slip housing which is rotated by the engine crankshaft. Inside the housing is a driven rotor which is connected by a shaft to the gearbox. The flywheel housing is divided into a number of cells by means of radial vanes.

design of the fluid flywheel

These cells correspond to similar openings in the driven rotor. As the driving member rotates, the fluid flows outwards due to the centrifugal force and circulates from the flywheel to the driven rotor.

Because the fluid is also carried cut round by the driving member, the fluid tends to rotate the driven rotor. Because the fluid is also carried out round by the driving member, the fluid tends to rotate the driven rotor. Thus the torque is transmitted to the crankshaft to the gearbox shaft.

The liquid flywheel requires less attention than friction clutches. Also, it needs no adjustment. The drive is taken up smoothly, torsional vibration of the crankshaft and the transmission shocks when braking and going down a hill, and the clutch pedal is eliminated.

Advantage of a Fluid Drive

  1. A liquid coupling, when used with a conventional clutch and transmission enables the driver to use the clutch and gears with less skill and fatigue than with an all-mechanical linkage.
  2. If the driver fails to de-clutch or shift to neutral before coming to a stop, the coupling will slip sufficiently to prevent stalling the engine.
  3. Unskilled clutch engagement or selection of improper gear will not produce the chattering and bucking.
  4. Any sudden load is cushioned and absorbed by the coupling so that the dynamic stresses on the gear teeth of the transmission and rear axle are greatly reduced.
  5. The vehicle may be started in high gear when equipped with a liquid coupling because the coupling shills sufficiently to enable the engine to speed up.
  6. Liquid coupling at low speeds is not as efficient a mechanical ones. It reduces engine braking when slowing down and requires high speed to start the vehicle.

That’s It. Thanks for reading. If you have any question on the “fluid coupling or fluid drive” leave a comment I’ll respond. If you find this article helpful please share with your friends.

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About Saif M

Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at www.theengineerspost.com

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