Fluid Coupling Overview
A fluid coupling contains three components, in addition to the hydraulic fluid:
The housing, also known as the shell (which must have an oil-tight seal around the drive shafts), provides the fluid and turbines.
Two turbines (enthusiast like components):
One linked to the insight shaft; known as the pump or impellor, primary steering wheel input turbine
The other connected to the output shaft, known as the turbine, output turbine, secondary steering wheel or runner
The driving turbine, referred to as the ‘pump’, (or driving torus) is 
definitely rotated by the prime mover, which is typically an internal combustion engine or electric motor. The impellor’s motion imparts both outwards linear and rotational movement to the fluid.
The hydraulic fluid is usually directed by the ‘pump’ whose shape forces the flow in direction of the ‘output turbine’ (or driven torus). Right here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ lead to a net pressure on the ‘result turbine’ causing a torque; hence causing it to rotate in the same path as the pump.
The motion of the fluid is effectively toroidal – travelling in one direction on paths that can be visualised to be on the surface of a torus:
When there is a notable difference between insight and output angular velocities the motion has a component which is certainly circular (i.e. across the bands formed by sections of the torus)
If the input and output phases have similar angular velocities there is no net centripetal power – and the motion of the fluid is normally 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
A significant characteristic of a fluid coupling is usually its stall velocity. The stall velocity is thought as the highest speed at which the pump can turn when the result turbine can be locked and maximum insight power is applied. Under stall conditions all the engine’s power would be dissipated in the fluid coupling as heat, probably resulting in damage.
Step-circuit coupling
A modification to the simple fluid coupling may be the step-circuit coupling that was formerly manufactured as the “STC coupling” by the Fluidrive Engineering Business.
The STC coupling consists of a reservoir to which some, but not all, of the essential oil gravitates when the output shaft is stalled. This decreases the “drag” on the input shaft, leading to reduced fuel consumption when idling and a reduction in the vehicle’s inclination to “creep”.
When the output shaft starts to rotate, the oil is trashed of the reservoir by centrifugal power, and returns to the main body of the coupling, to ensure that normal power transmission is restored.
Slip
A fluid coupling cannot develop result torque when the insight and output angular velocities are identical. Hence a fluid coupling cannot achieve completely power transmission efficiency. Because of slippage that will occur in any fluid coupling under load, some power will always be dropped in fluid friction and turbulence, and dissipated as high temperature. Like other fluid dynamical products, its efficiency will increase gradually with increasing 
scale, as measured by the Reynolds number.
Hydraulic fluid
As a fluid coupling operates kinetically, low viscosity liquids are preferred. In most cases, multi-grade motor natural 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 at the mercy of adjustments in viscosity with temperature change. This prospects to a switch in transmission performance therefore where undesired performance/efficiency change has to be held to the very least, a motor oil or automated transmission fluid, with a higher viscosity index ought to be used.
Hydrodynamic braking
Fluid couplings can also act as hydrodynamic brakes, dissipating rotational energy as high temperature through frictional forces (both viscous and fluid/container). When a fluid coupling is used for braking additionally it is known as a retarder.
Fluid Coupling Applications
Industrial
Fluid couplings are found in many industrial application concerning rotational power, especially in machine drives that involve high-inertia begins or continuous cyclic loading.
Rail transportation
Fluid couplings are located in some Diesel locomotives within the power transmission system. Self-Changing Gears produced semi-automatic transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple units which contain numerous combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were found in a number of early semi-automatic transmissions and automatic transmissions. Since the past due 1940s, the hydrodynamic torque converter provides replaced the fluid coupling in automotive applications.
In automotive applications, the pump typically is linked to the flywheel of the engine-in fact, the coupling’s enclosure may be area of the flywheel appropriate, and therefore is switched by the engine’s crankshaft. The turbine is linked to the insight shaft of the transmitting. While the transmitting is in equipment, as engine quickness increases torque is usually transferred from the engine to the insight shaft by the movement of the fluid, propelling the vehicle. In this respect, the behavior of the fluid coupling highly resembles that of a mechanical clutch generating a manual transmission.
Fluid flywheels, as distinctive from torque converters, are best known for their use in Daimler cars together with a Wilson pre-selector gearbox. Daimler used these throughout their selection of luxury vehicles, until switching to automatic gearboxes with the 1958 Majestic. Daimler and Alvis were both also known because of their military automobiles and armored vehicles, a few of which also used the combination of pre-selector gearbox and fluid flywheel.
Aviation
The many prominent use of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 motors where it was utilized as a barometrically managed hydraulic clutch for the centrifugal compressor and the Wright turbo-compound reciprocating engine, where three power recovery turbines extracted around 20 percent of the energy or around 500 horsepower (370 kW) from the engine’s exhaust gases and then, using three fluid couplings and gearing, converted low-torque high-speed turbine rotation to low-speed, high-torque result to drive the propeller.