Fluid Coupling Overview
A fluid coupling consists of three components, plus the hydraulic fluid:
The housing, also called the shell (which will need to have an oil-restricted seal around the travel shafts), contains the fluid and turbines.
Two turbines (fan like components):
One connected to the insight shaft; referred to as the pump or impellor, primary wheel input turbine
The other connected to the result shaft, referred to as the turbine, result turbine, secondary wheel or runner
The generating turbine, referred to as the ‘pump’, (or driving torus) is rotated by the prime mover, which is normally an interior combustion engine or electric electric motor. The impellor’s motion imparts both outwards linear and rotational movement to the fluid.
The hydraulic fluid is definitely directed by the ‘pump’ whose form forces the circulation in the direction of the ‘output turbine’ (or driven torus). Right here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ result in a net force on the ‘result turbine’ causing a torque; therefore causing it to rotate in the same direction as the pump.
The movement of the fluid is successfully toroidal – traveling in one direction on paths which can be visualised as being on the top of a torus:
If there is a notable difference between 
input and output angular velocities the motion has a component which is normally circular (i.e. round the rings formed by parts of the torus)
If the input and output phases have similar angular velocities there is absolutely no net centripetal power – and the motion of the fluid is circular and co-axial with the axis of rotation (i.e. across the edges of a torus), there is absolutely no stream of fluid from one turbine to the various other.
Stall speed
An important characteristic of a fluid coupling is certainly its stall rate. The stall rate is thought as the best speed of which the pump can turn when the output turbine is usually locked and maximum input power is applied. Under stall circumstances all of the engine’s power will be dissipated in the fluid coupling as heat, possibly resulting in damage.
Step-circuit coupling
A modification to the simple fluid coupling is the step-circuit coupling which was formerly produced as the “STC coupling” by the Fluidrive Engineering Company.
The STC coupling includes a reservoir to which some, however, not all, of the oil gravitates when the output shaft can be stalled. This reduces the “drag” on the input shaft, resulting in reduced fuel intake when idling and a reduction in the vehicle’s tendency to “creep”.
When the output shaft starts to rotate, the oil is trashed of the reservoir by centrifugal force, and returns to the main body of the coupling, to ensure that normal power transmission is restored.
Slip
A fluid coupling cannot develop output torque when the input and result angular velocities are identical. 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 always be dropped in fluid friction and turbulence, and dissipated as high temperature. Like other fluid dynamical gadgets, its efficiency tends to increase gradually with increasing scale, as measured by the Reynolds amount.
Hydraulic fluid
As a fluid coupling operates kinetically, low viscosity liquids are preferred. Generally speaking, multi-grade motor oils or automatic transmission liquids are used. Increasing density of the fluid increases the quantity of torque that can be transmitted at a given input speed. Nevertheless, hydraulic fluids, much like other liquids, are at the mercy of adjustments in viscosity with temperatures change. This prospects to a switch in transmission overall performance therefore where unwanted performance/efficiency change has to be held to the very least, a motor essential oil or automatic transmission fluid, with a higher viscosity index should be used.
Hydrodynamic braking
Fluid couplings can also become hydrodynamic brakes, dissipating rotational energy as warmth through frictional forces (both viscous and fluid/container). Whenever a fluid coupling can be used for braking it is also referred to as a retarder.
Fluid Coupling Applications
Industrial
Fluid couplings are found in many industrial application including rotational power, specifically in machine drives that involve high-inertia starts or constant cyclic loading.
Rail transportation
Fluid couplings are located in some Diesel locomotives as part of the power transmitting system. Self-Changing Gears made semi-automated transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple devices which contain several combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were used in a number of early semi-automated transmissions and automated transmissions. Because the late 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 portion of the flywheel proper, and therefore is turned by the engine’s crankshaft. The turbine is linked to the input shaft of the transmission. While the transmission is in equipment, as engine speed increases torque is usually transferred from the engine to the input shaft by the motion of the fluid, propelling the automobile. In this respect, the behavior of the fluid coupling highly resembles that of a mechanical clutch generating a manual transmission.
Fluid flywheels, as unique from torque converters, are most widely known for their make use of in Daimler cars together 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 had been both also known for their military automobiles and armored cars, some of which also utilized the mixture of pre-selector gearbox and fluid flywheel.
Aviation
The most prominent usage of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 motors where it was used as a barometrically managed 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 result to drive the propeller.