Fluid Coupling Overview
　　A fluid coupling consists of three components, in addition to the hydraulic fluid:
　　The casing, also known as the shell (which will need to have an oil-restricted seal around the get shafts), contains the fluid and turbines.
　　Two turbines (enthusiast like components):
　　One connected to the input shaft; known as the pump or impellor, primary steering wheel input turbine
　　The other connected to the output shaft, referred to as the turbine, output turbine, secondary wheel or runner
　　The generating turbine, referred to as the ‘pump’, (or driving torus) is certainly rotated by the primary mover, which is typically an internal combustion engine or electrical motor. The impellor’s movement 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 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 force on the ‘result turbine’ causing a torque; therefore leading to it to rotate in the same direction as the pump.
　　The movement of the fluid is effectively toroidal – travelling in one direction on paths that can be visualised to be on the surface of a torus:
　　When there is a difference between insight and output angular velocities the motion has a element which is normally circular (i.e. across the rings formed by sections of the torus)
　　If the input and output stages have identical angular velocities there is no net centripetal drive – and the motion of the fluid can be circular and co-axial with the axis of rotation (i.e. round the edges of a torus), there is absolutely no flow of fluid from one turbine to the other.
　　Stall speed
　　An important characteristic of a fluid coupling can be its stall speed. The stall swiftness is defined as the highest speed at which the pump can change when the output turbine is locked and maximum insight power is applied. Under stall conditions all of the engine’s power will 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 which was formerly produced as the “STC coupling” by the Fluidrive Engineering Company.
　　The STC coupling consists of a reservoir to which some, but not all, of the oil gravitates when the output shaft is definitely stalled. This reduces the “drag” on the insight shaft, resulting in reduced fuel intake when idling and a decrease in the vehicle’s inclination to “creep”.
　　When the output shaft begins to rotate, the oil is trashed of the reservoir by centrifugal drive, and returns to the primary 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 similar. Hence a fluid coupling cannot achieve 100 percent power transmission performance. Because of slippage that may occur in virtually 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 level, as measured by the Reynolds number.
　　Hydraulic fluid
　　As a fluid coupling operates kinetically, low viscosity fluids are preferred. Generally speaking, multi-grade motor natural oils or automated transmission fluids are used. Raising density of the fluid increases the amount of torque which can be transmitted at a given input speed. However, hydraulic fluids, very much like other fluids, are subject to adjustments in viscosity with heat range change. This prospects to a modification in transmission overall performance and so 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 act as hydrodynamic brakes, dissipating rotational energy as temperature through frictional forces (both viscous and fluid/container). When 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 industrial application regarding rotational power, specifically in machine drives that involve high-inertia starts or constant cyclic loading.
　　Rail transportation
　　Fluid couplings are located in some Diesel locomotives within the power transmission system. Self-Changing Gears made semi-automatic transmissions for British Rail, and Voith produce turbo-transmissions for railcars and diesel multiple systems which contain different 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 past due 1940s, the hydrodynamic torque converter provides replaced the fluid coupling in motor vehicle applications.
　　In motor vehicle applications, the pump typically is connected to the flywheel of the engine-in truth, the coupling’s enclosure could be part of the flywheel appropriate, and thus is switched by the engine’s crankshaft. The turbine is linked to the insight shaft of the transmitting. While the transmission is in equipment, as engine rate increases torque is 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 highly resembles that of a mechanical clutch driving a manual transmitting.
　　Fluid flywheels, as specific from torque converters, are most widely known for their use in Daimler cars 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 had been both also known for their military automobiles and armored cars, a few of which also utilized the mixture of pre-selector gearbox and fluid flywheel.
　　Aviation
　　The most prominent usage of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 engines where it had been 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 around 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 output to operate a vehicle the propeller.