FLUID COUPLING

Fluid coupling is the simplest form of the hydrodynamic drive consisting of two similar members with straight radial vanes referred as impeller (pump) and turbine. It is used to transmit the power from the engine to the remaining parts of the transmission. Since the fluid coupling is always a major part of the engine flywheel assembly, it is also called fluid flywheel.

The working principle of a fluid coupling can be understood easily with the help of two fans facing each other. When one fan is turned on and the air stream causes the second fan to turn even though it is not switched on. The first fan is the driving member or the impeller and the second fan is the driven member or the runner. This is the simple fluid coupling with air serving the function of fluid.  The figureshows the simple construction of a fluid flywheel. It consists of a two half dough nut shaped shells equipped with interior fins that radiates from the hubs. One shell is mounted on the crankshaft and is called impeller or driving member. The other shell is mounted on the driven shaft and is called runner or driven member. The two shells are very close with their ends facing each other and enclosed in housing, so that they can be turned without touching each other. The housing is filled with liquid / fluid. When the engine drives the impeller it sets up the fluid mass into motion, creating a fluid force. The path of the fluid force strikes on a solid object, the turbine. 

The impact of the fluid jet stream against the turbine blades sets the turbine in motion. With this energy cycle has been completed: mechanical to fluid and back to mechanical. When the impeller spins up, two separate forces are generated in the fluid. One is rotary flow, which is the rotational effort or the inertia of the impeller rotation. The other is vortex flow which circulates the fluid members (it is at right angles to the rotary flow) and is caused by the centrifugal pumping action of the rotating impeller. The lag of the runner behind the impeller is known as slip, and depends upon the engine speed and load. The slip is maximum with the vehicle at rest (turbine stationary) and the throttle open to cause the impeller to start circulating oil. As explained earlier the oil is having both rotary and vertex flow at this time. The oil flies out against the curved interior due to the centrifugal force. The rotary flow starts the movement of the runner. As the turbine begins to rotate and catch up impeller speed, flow gradually decreases because of the counter pumping action of the turbine. This permits the rotary action to become greater influence on the fluid and the resultant thrust becomes more effective in propelling the turbine. Finally, at greater speed ratios over 90% the rotary inertia or momentum of the fluid and coupling members forms a hydraulic lock or bond, and the coupling members turns as a single unit. This is referred as coupling point. In an ideal liquid coupling, the runner would attain the same speed as the impeller, so as to receive all the power imparted by the engine. In commercial designs the runner speed becomes almost equal to that of the impeller only under the best operating conditions, when the efficiency of the coupling is highest.