Fluid Coupling Overview
  A fluid coupling contains three components, in addition to the hydraulic fluid:
  The housing, also referred to as the shell (which will need to have an oil-restricted seal around the get shafts), provides the fluid and turbines.
  Two turbines (enthusiast like components):
  One connected to the input shaft; known as the pump or impellor, primary wheel input turbine
  The other connected to the output shaft, referred to as the turbine, result turbine, secondary steering wheel or runner
  The traveling turbine, known as the ‘pump’, (or driving torus) is definitely rotated by the prime 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 normally directed by the ‘pump’ whose form forces the movement in direction of the ‘output turbine’ (or driven torus). Here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ lead to a net drive on the ‘output turbine’ leading to a torque; thus leading to it to rotate in the same direction as the pump.
  The movement of the fluid is effectively toroidal – venturing in one path on paths which can be visualised to be on the top of a torus:
  If there is a notable difference between insight and output angular velocities the motion has a component which can be circular (i.e. across the bands formed by sections of the torus)
  If the input and output phases have similar angular velocities there is no net centripetal force – and the movement of the fluid is normally circular and co-axial with the axis of rotation (i.e. round the edges of a torus), there is no flow of fluid in one turbine to the various other.
  Stall speed
  A significant characteristic of a fluid coupling is certainly its stall swiftness. The stall acceleration is thought as the best speed at which the pump can change when the result turbine is normally locked and optimum insight power is used. Under stall conditions all the engine’s power would be dissipated in the fluid coupling as heat, possibly resulting in damage.
  Step-circuit coupling
  An adjustment to the simple fluid coupling may be the step-circuit coupling that 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 essential oil gravitates when the result shaft is stalled. This reduces the “drag” on the insight shaft, resulting in reduced fuel usage 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 power, and returns to the main body of the coupling, to ensure that normal power transmission is restored.
  A fluid coupling cannot develop output torque when the input and output angular velocities are identical. Hence a fluid coupling cannot achieve completely power transmission performance. Because of slippage that will occur in any fluid coupling under load, some power will be dropped in fluid friction and turbulence, and dissipated as temperature. Like other fluid dynamical devices, its efficiency tends to increase gradually with increasing scale, as measured by the Reynolds quantity.
  Hydraulic fluid
  As a fluid coupling operates kinetically, low viscosity liquids are preferred. In most cases, multi-grade motor 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, very much like other liquids, are at the mercy of changes in viscosity with temperature change. This network marketing leads to a switch in transmission functionality therefore where unwanted performance/efficiency change has to be held to a minimum, a motor oil or automatic transmission fluid, with a high viscosity index should be used.
  Hydrodynamic braking
  Fluid couplings can also act as hydrodynamic brakes, dissipating rotational energy as high temperature through frictional forces (both viscous and fluid/container). Whenever a fluid coupling is used for braking additionally it is referred to as a retarder.

Fluid Coupling Applications
  Fluid couplings are used in many commercial application involving rotational power, specifically in machine drives that involve high-inertia begins or constant cyclic loading.
  Rail transportation
  Fluid couplings are found in some Diesel locomotives within the power transmission system. Self-Changing Gears made semi-automated transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple products which contain different combinations of fluid couplings and torque converters.
  Fluid couplings were found in a number of early semi-automated transmissions and automatic transmissions. Because the past due 1940s, the hydrodynamic torque converter provides replaced the fluid coupling in automotive applications.
  In motor vehicle applications, the pump typically is connected to the flywheel of the engine-in reality, the coupling’s enclosure may be area of the flywheel correct, and therefore is switched by the engine’s crankshaft. The turbine is connected to the input shaft of the transmission. While the transmission is in gear, as engine velocity increases torque is 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 unique from torque converters, are most widely known for their use in Daimler vehicles together with a Wilson pre-selector gearbox. Daimler used 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 automobiles and armored cars, some of which also used the mixture of pre-selector gearbox and fluid flywheel.
  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 controlled hydraulic clutch for the centrifugal compressor and the Wright turbo-compound reciprocating engine, in which 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-velocity turbine rotation to low-speed, high-torque result to operate a vehicle the propeller.