Fluid Coupling Overview
A fluid coupling consists of three components, plus the hydraulic fluid:
The casing, also called the shell (which must have an oil-limited seal around the travel shafts), provides the fluid and turbines.
Two turbines (lover like components):
One linked to the input shaft; known as the pump or impellor, primary steering wheel input turbine
The other linked to the output shaft, referred to as the turbine, result turbine, secondary wheel or runner
The generating turbine, referred to as the ‘pump’, (or driving torus) can be rotated by the primary mover, which is typically an internal combustion engine or electrical 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 form forces the circulation in the direction of the ‘output turbine’ (or powered 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; thus leading to it to rotate in the same direction as the pump.
The motion of the fluid is effectively toroidal – traveling in one direction on paths which can be visualised as being on the surface of a torus:
If there is a notable difference between input and result angular velocities the motion has a element which is circular (i.e. round the rings formed by sections of the torus)
If the insight and output levels have similar angular velocities there is absolutely no net centripetal drive – and the movement of the fluid can be circular and co-axial with the axis of rotation (i.e. across the edges of a torus), there is no circulation of fluid from one turbine to the various other.
Stall speed
A significant characteristic of a fluid coupling is normally its stall swiftness. The stall velocity is thought as the highest speed at which the pump can change when the result turbine is locked and optimum insight power is applied. Under stall conditions all the engine’s power will be dissipated in the fluid coupling as heat, perhaps leading to damage.
Step-circuit coupling
An adjustment to the simple fluid coupling is the step-circuit coupling that was formerly produced 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 result shaft is normally stalled. This decreases the “drag” on the input shaft, resulting in reduced fuel usage when idling and a decrease in the vehicle’s tendency to “creep”.
When the output shaft begins to rotate, the oil is trashed of the reservoir by centrifugal force, and returns to the main body of the coupling, so 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 effectiveness. Due to slippage that may occur in any fluid coupling under load, some power will always be lost in fluid friction and turbulence, and dissipated as heat. Like other fluid dynamical devices, its efficiency tends to increase steadily with increasing level, as measured by the Reynolds amount.
Hydraulic fluid
As a fluid coupling operates kinetically, low viscosity liquids are preferred. In most cases, multi-grade motor natural oils or automatic transmission fluids are used. Increasing density of the fluid increases the quantity of torque which can be transmitted at confirmed input speed. However, hydraulic fluids, much like other fluids, are 
subject to changes in viscosity with temperature change. This prospects to a switch in transmission functionality and so where undesired performance/efficiency change needs to be kept to a minimum, a motor essential oil or automatic transmission fluid, with a higher viscosity index should be used.
Hydrodynamic braking
Fluid couplings may also become hydrodynamic brakes, dissipating rotational energy as warmth through frictional forces (both viscous and fluid/container). Whenever a fluid coupling is used for braking it is also 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 begins or constant cyclic loading.
Rail transportation
Fluid couplings are located in some Diesel locomotives as part of the power transmission system. Self-Changing Gears produced semi-automated transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple units which contain different combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were found in a variety of early semi-automatic 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 connected to the flywheel of the engine-in fact, the coupling’s enclosure may be part of the flywheel proper, and therefore is turned by the engine’s crankshaft. The turbine is connected to the input shaft of the transmission. While the transmission is in gear, as engine acceleration increases torque is 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 highly resembles that of a mechanical clutch traveling a manual transmission.
Fluid flywheels, as unique from torque converters, are most widely known for their use in Daimler cars in conjunction with a Wilson pre-selector gearbox. Daimler used these throughout their selection of luxury vehicles, until switching to automated gearboxes with the 1958 Majestic. Daimler and Alvis had been both also known because of their military vehicles and armored vehicles, some of which also utilized the combination 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 engines where it was 
utilized as a barometrically managed hydraulic clutch for the centrifugal compressor and the Wright turbo-substance reciprocating engine, where three power recovery turbines extracted around 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 operate a vehicle the propeller.