Fluid Coupling Overview
A fluid coupling consists of three components, plus the 
hydraulic fluid:
The housing, also referred to as the shell (which will need to have an oil-tight seal around the drive shafts), contains the fluid and turbines.
Two turbines (enthusiast like components):
One connected to the input shaft; referred to as the pump or impellor, primary wheel input turbine
The other connected to the result shaft, known as the turbine, result turbine, secondary steering wheel or runner
The traveling 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 movement imparts both outwards linear and rotational movement to the fluid.
The hydraulic fluid is certainly directed by the ‘pump’ whose form forces the flow in the 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 push on the ‘result turbine’ leading to a torque; therefore leading to it to rotate in the same direction as the pump.
The motion of the fluid is successfully toroidal – venturing in one direction on paths which can be visualised as being on the surface of a torus:
When there is a difference between insight and output angular velocities the motion has a element which is definitely circular (i.e. across the bands formed by parts of the torus)
If the insight and output stages have similar angular velocities there is no net centripetal force – and the movement of the fluid is usually circular and co-axial with the axis of rotation (i.e. across the edges of a torus), there is absolutely no flow of fluid in one turbine to the other.
Stall speed
A significant characteristic of a fluid coupling can be its stall speed. The stall speed is thought as the highest speed at which the pump can turn when the output turbine is certainly locked and optimum insight power is used. Under stall conditions all of the engine’s power will be dissipated in the fluid coupling as heat, perhaps leading to damage.
Step-circuit coupling
An adjustment to the easy fluid coupling may be the step-circuit coupling that was formerly manufactured as the “STC coupling” by the Fluidrive Engineering Firm.
The STC coupling consists of a reservoir to which some, however, not all, of the oil gravitates when the output shaft is certainly stalled. This decreases the “drag” on the input shaft, leading to reduced fuel intake when idling and a reduction in the vehicle’s tendency to “creep”.
When the result shaft begins to rotate, the essential oil is trashed of the reservoir by centrifugal force, and returns to the main body of the coupling, so that normal power transmitting is restored.
Slip
A fluid coupling cannot develop output torque when the insight and output angular velocities are similar. Hence a fluid coupling cannot achieve 100 percent power transmission performance. Due to slippage which will occur in any fluid coupling under load, some power will be lost in fluid friction and turbulence, and dissipated as heat. Like other fluid dynamical devices, its efficiency tends to increase gradually with 
increasing level, as measured by the Reynolds amount.
Hydraulic fluid
As a fluid coupling operates kinetically, low viscosity fluids are preferred. Generally speaking, multi-grade motor oils or automatic transmission liquids are used. Raising density of the fluid increases the amount of torque that can be transmitted at a given input speed. Nevertheless, hydraulic fluids, very much like other liquids, are subject to changes in viscosity with temperature change. This leads to a change in transmission functionality therefore where unwanted performance/efficiency change has to be held to the very least, a motor oil or automatic transmission fluid, with a high viscosity index ought to be used.
Hydrodynamic braking
Fluid couplings can also become hydrodynamic brakes, dissipating rotational energy as 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 used in many commercial application involving rotational power, especially in machine drives that involve high-inertia starts or constant cyclic loading.
Rail transportation
Fluid couplings are located in a few Diesel locomotives as part of the power transmitting system. Self-Changing Gears produced semi-automatic transmissions for British Rail, and Voith produce turbo-transmissions for railcars and diesel multiple models which contain several combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were used in a variety of early semi-automatic transmissions and automatic transmissions. Because the late 1940s, the hydrodynamic torque converter has replaced the fluid coupling in motor vehicle applications.
In automotive applications, the pump typically is linked to the flywheel of the engine-in reality, the coupling’s enclosure could be portion of the flywheel proper, and thus is switched by the engine’s crankshaft. The turbine is linked to the insight shaft of the transmission. While the transmitting is in gear, as engine swiftness increases torque is definitely 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 strongly resembles that of a mechanical clutch traveling a manual transmission.
Fluid flywheels, as unique 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 range of luxury vehicles, until switching to automatic gearboxes with the 1958 Majestic. Daimler and Alvis were both also known for his or her military vehicles and armored vehicles, some of which also utilized the mixture of pre-selector gearbox and fluid flywheel.
Aviation
The most prominent use 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, using three fluid couplings and gearing, converted low-torque high-rate turbine rotation to low-speed, high-torque result to drive the propeller.