Fluid Coupling Overview
A fluid coupling consists of three components, in addition to the hydraulic fluid:
The casing, also known as the shell (which must have an oil-restricted seal around the drive shafts), provides the fluid and turbines.
Two turbines (lover like components):
One connected to the input shaft; referred to as the pump or impellor, primary wheel input turbine
The other connected to the output shaft, known as the turbine, result turbine, secondary wheel or runner
The generating turbine, known as the ‘pump’, (or driving torus) is normally rotated by the prime mover, which is normally an internal combustion engine or 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 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 drive on the ‘output turbine’ causing a torque; thus causing it to rotate in the same path as the pump.
The movement of the fluid is efficiently toroidal – travelling in one direction on paths that can be visualised as being on the surface of a torus:
When there is a notable difference between input and output angular velocities the movement has a element which is usually circular (i.e. round the rings formed by parts 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 is definitely circular and co-axial with the axis of rotation (i.e. round the edges of a torus), there is no circulation of fluid from one turbine to the other.
Stall speed
An important characteristic of a fluid coupling is normally its stall quickness. The stall rate 
is thought as the best speed at which the pump can change when the output turbine is certainly locked and maximum input power is applied. Under stall conditions all the engine’s power would be dissipated in the fluid coupling as heat, possibly resulting in damage.
Step-circuit coupling
A modification to the easy fluid coupling may be the step-circuit coupling that was formerly produced as the “STC coupling” by the Fluidrive Engineering Company.
The STC coupling contains a reservoir to which some, however, not all, of the essential oil gravitates when the result shaft is usually stalled. This decreases the “drag” on the input shaft, leading to reduced fuel usage when idling and a decrease in the vehicle’s inclination to “creep”.
When the output shaft starts to rotate, the oil is thrown out of the reservoir by centrifugal pressure, and returns to the main body of the coupling, so 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 completely power transmission efficiency. Due to slippage that may occur in any fluid coupling under load, some power will be lost in fluid friction and turbulence, and dissipated as high temperature. Like other fluid dynamical products, its efficiency will increase gradually with increasing level, as measured by the 
Reynolds quantity.
Hydraulic fluid
As a fluid coupling operates kinetically, low viscosity fluids are preferred. In most cases, multi-grade motor natural oils or automatic transmission fluids are used. Increasing density of the fluid escalates the amount of torque which can be transmitted at a given input speed. However, hydraulic fluids, much like other fluids, are subject to changes in viscosity with temperatures change. This prospects to a switch in transmission overall performance therefore where undesirable performance/efficiency change has to be kept to the very least, a motor oil or automated transmission fluid, with a high viscosity index should be used.
Hydrodynamic braking
Fluid couplings may also become hydrodynamic brakes, dissipating rotational energy as high temperature through frictional forces (both viscous and fluid/container). When a fluid coupling is utilized for braking it is also referred to as a retarder.
Fluid Coupling Applications
Industrial
Fluid couplings are used in many industrial application regarding rotational power, especially in machine drives that involve high-inertia starts or continuous cyclic loading.
Rail transportation
Fluid couplings are located in a few Diesel locomotives within the power transmitting system. Self-Changing Gears produced semi-automatic transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple devices which contain various combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were used in a variety of early semi-automated transmissions and automated transmissions. Because the past due 1940s, the hydrodynamic torque converter has replaced the fluid coupling in automotive applications.
In automotive applications, the pump typically is linked to the flywheel of the engine-in truth, the coupling’s enclosure could be portion of the flywheel appropriate, and therefore is switched by the engine’s crankshaft. The turbine is connected to the insight shaft of the transmitting. While the transmitting is in gear, as engine swiftness increases torque is usually transferred from the engine to the input 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 specific from torque converters, are most widely known for their make use of in Daimler vehicles together with a Wilson pre-selector gearbox. Daimler used these throughout their selection of luxury cars, until switching to automatic gearboxes with the 1958 Majestic. Daimler and Alvis were both also known for their military automobiles and armored vehicles, some of which also used the combination of pre-selector gearbox and fluid flywheel.
Aviation
The many prominent utilization of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 engines where it was utilized as a barometrically controlled hydraulic clutch for the centrifugal compressor and the Wright turbo-compound reciprocating engine, in which three power recovery turbines extracted approximately 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.