Fluid Coupling Overview
A fluid coupling includes three components, in addition to the hydraulic fluid:
The casing, also called the shell (which must have an oil-tight seal around the get shafts), contains the fluid and turbines.
Two turbines (enthusiast like components):
One connected to the input shaft; known as the pump or impellor, primary wheel input turbine
The other connected to the output shaft, referred to as the turbine, output turbine, secondary wheel or runner
The driving turbine, referred to as the ‘pump’, (or driving torus) can be rotated by the prime mover, which is typically an interior combustion engine or electric electric motor. The impellor’s motion imparts both outwards linear and rotational motion to the fluid.
The hydraulic fluid can be directed by the ‘pump’ whose form forces the stream in the direction of the ‘output turbine’ (or driven torus). Here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ lead to a net force on the ‘output turbine’ leading to a torque; thus causing it to rotate in the same path as the pump.
The movement of the fluid is successfully toroidal – exploring in one path on paths which can be visualised to be on the surface of a torus:
When there is a difference between input and output angular velocities the movement has a element which is certainly circular (i.e. across the rings formed by sections of the torus)
If the input and output levels have similar angular velocities there is absolutely no net centripetal force – and the movement of the fluid can be circular and co-axial with the axis of rotation (i.e. across the edges of a torus), there is no movement of fluid in one turbine to the additional.
Stall speed
An important characteristic of a fluid coupling is certainly its stall velocity. The stall velocity is thought as the highest speed of which the pump can turn when the result turbine is locked and optimum insight power is used. Under stall circumstances all the engine’s power would be dissipated in the fluid coupling as heat, perhaps leading to damage.
Step-circuit coupling
A modification to the simple fluid coupling is the step-circuit coupling that was formerly produced as the “STC coupling” by the Fluidrive Engineering Organization.
The STC coupling contains a reservoir to which some, but not all, of the essential oil gravitates when the result shaft is stalled. This reduces the “drag” on the insight shaft, resulting in reduced fuel consumption when idling and a reduction in the vehicle’s tendency to “creep”.
When the result shaft begins to rotate, the oil is thrown out of the reservoir by centrifugal force, and returns to the primary body of the coupling, to ensure that normal power transmitting is restored.
Slip
A fluid coupling cannot develop output 
torque when the insight and result angular velocities are similar. Hence a fluid coupling cannot achieve 100 percent power transmission efficiency. Because of slippage that may occur in virtually any fluid coupling under load, some power will be lost in fluid friction and turbulence, and dissipated as high temperature. Like other fluid dynamical gadgets, its efficiency tends to increase steadily with increasing level, as measured by the Reynolds number.
Hydraulic fluid
As a fluid coupling operates kinetically, low viscosity fluids are preferred. In most cases, multi-grade motor oils or automated transmission liquids are used. Increasing density of the fluid increases the quantity of torque that can be transmitted at a given input speed. However, hydraulic fluids, very much like other liquids, are subject to adjustments in viscosity with temperature change. This network marketing leads to a switch in transmission functionality and so where unwanted performance/efficiency change has to be kept to a minimum, a motor essential oil or automated transmission fluid, with a high viscosity index should be used.
Hydrodynamic braking
Fluid couplings can also become hydrodynamic brakes, dissipating rotational energy as high temperature through frictional forces (both viscous and fluid/container). Whenever a fluid coupling is utilized for braking additionally it is known as a retarder.
Fluid Coupling Applications
Industrial
Fluid couplings are used in many commercial application regarding rotational power, specifically in machine drives that involve high-inertia starts or constant cyclic loading.
Rail transportation
Fluid couplings are found in some Diesel locomotives as part of the power transmitting system. Self-Changing Gears made semi-automatic transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple systems which contain several combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were 
used in a variety of early semi-automatic transmissions and automated transmissions. Since the late 1940s, the hydrodynamic torque converter offers replaced the fluid coupling in motor vehicle applications.
In automotive applications, the pump typically is linked to the flywheel of the engine-in truth, the coupling’s enclosure may be part of the flywheel proper, and thus is turned by the engine’s crankshaft. The turbine is connected to the insight shaft of the transmitting. While the transmission is in equipment, as engine quickness increases torque is definitely transferred from the engine to the insight shaft by the motion of the fluid, propelling the automobile. In this regard, the behavior of the fluid coupling strongly resembles that of a mechanical clutch driving a manual transmission.
Fluid flywheels, as distinct from torque converters, are best known for their use in Daimler vehicles in conjunction with a Wilson pre-selector gearbox. Daimler utilized these throughout their selection of luxury vehicles, until switching to automated gearboxes with the 1958 Majestic. Daimler and Alvis were both also known for their military automobiles and armored vehicles, a few of which also used the mixture of pre-selector gearbox and fluid flywheel.
Aviation
The many prominent usage of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 engines where it had been utilized as a barometrically controlled hydraulic clutch for the centrifugal compressor and the Wright turbo-substance reciprocating engine, in which three power recovery turbines extracted approximately 20 percent of the energy or about 500 horsepower (370 kW) from the engine’s exhaust gases and then, using three fluid couplings and gearing, converted low-torque high-rate turbine rotation to low-speed, high-torque output to drive the propeller.