Fluid Coupling Overview
A fluid coupling includes three components, in addition to the hydraulic fluid:
The casing, also referred to as the shell (which must have an oil-limited seal around the travel 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 linked to the output shaft, known as the turbine, result turbine, secondary wheel or runner
The driving turbine, known as the ‘pump', (or driving torus) is rotated by the primary mover, which is normally an interior combustion engine or electrical electric motor. The impellor's movement imparts both outwards linear and rotational movement to the fluid.
The hydraulic fluid is directed by the ‘pump' whose shape forces the stream in the direction of the ‘output turbine' (or powered torus). Right here, any difference in the angular velocities of ‘input stage' and ‘output stage' result in a net drive on the ‘result turbine' causing a torque; hence causing it to rotate in the same direction as the pump.
The motion of the fluid is successfully toroidal – travelling in one path on paths which can be visualised as being on the top of a torus:
When there is a difference between insight and output angular velocities the movement has a element which is definitely circular (i.e. across the rings formed by parts of the torus)
If the input and output stages have similar angular velocities there is no net centripetal power – and the motion of the fluid is definitely 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 additional.
Stall speed
A significant characteristic of a fluid coupling is its stall quickness. The stall acceleration is thought as the highest speed of which the pump can change when the result turbine can be locked and maximum input power is applied. Under stall conditions all of the engine's power will be dissipated in the fluid coupling as heat, possibly leading to damage.
Step-circuit coupling
An adjustment to the simple fluid coupling is the step-circuit coupling which was formerly manufactured as the “STC coupling” by the Fluidrive Engineering Organization.
The STC coupling contains a reservoir to which some, however, 
not all, of the oil gravitates when the result shaft can be stalled. This reduces 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 result shaft begins to rotate, the oil is trashed of the reservoir by centrifugal drive, and returns to the main body of the coupling, to ensure 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 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 steadily with increasing level, as measured by the Reynolds amount.
Hydraulic fluid
As a fluid coupling operates kinetically, low viscosity fluids 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 which can be transmitted at a given input speed. Nevertheless, hydraulic fluids, very much like other fluids, 
are at the mercy of adjustments in viscosity with temp change. This prospects to a switch in transmission efficiency and so where undesirable performance/efficiency change needs to be held to a minimum, a motor 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 warmth through frictional forces (both viscous and fluid/container). Whenever a fluid coupling can be used for braking additionally it is known as a retarder.
Fluid Coupling Applications
Industrial
Fluid couplings are found in many commercial application including rotational power, specifically in machine drives that involve high-inertia begins or continuous cyclic loading.
Rail transportation
Fluid couplings are located in some 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 units which contain different combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were used in a number of early semi-automated transmissions and automated transmissions. Since the late 1940s, the hydrodynamic torque converter provides 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 may be part of the flywheel proper, and therefore is switched by the engine's crankshaft. The turbine is connected to the input shaft of the transmission. While the transmitting is in gear, 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 regard, the behavior of the fluid coupling strongly resembles that of a mechanical clutch generating a manual transmitting.
Fluid flywheels, as distinctive from torque converters, are most widely known for their make use of 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 had been both also known because of their military automobiles and armored vehicles, some of which also utilized the combination 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 motors where it was utilized as a barometrically managed hydraulic clutch for the centrifugal compressor and the Wright turbo-substance reciprocating engine, in which three power recovery turbines extracted around 20 percent of the energy or around 500 horsepower (370 kW) from the engine's exhaust gases and, using three fluid couplings and gearing, converted low-torque high-velocity turbine rotation to low-speed, high-torque result to drive the propeller.