Fluid Coupling Overview
A fluid coupling contains 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 get shafts), provides the fluid and turbines.
Two turbines (enthusiast like components):
One connected to the insight shaft; known as the pump or impellor, primary wheel input turbine
The other connected to the result shaft, referred to as the turbine, output turbine, secondary wheel or runner
The driving turbine, referred to as the ‘pump’, (or driving torus) can be rotated by the primary mover, which is normally an interior combustion engine or electrical engine. 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 stream 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 push on the ‘output turbine’ causing a torque; hence leading to it to rotate in the same path as the pump.
The movement of the fluid is efficiently toroidal – going in one direction on paths which can be visualised to be on the surface of a torus:
If there is a difference between input and output angular velocities the movement has a component which is normally circular (i.e. round the bands formed by parts of the torus)
If the insight and output levels have similar angular velocities there is absolutely no net centripetal drive – and the motion 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 from one turbine to the various other.
Stall speed
A significant characteristic of a fluid coupling is usually its stall velocity. The stall rate is thought as the best speed at which the pump can turn when the output 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 easy fluid coupling may be the step-circuit coupling which was formerly produced as the “STC coupling” by the Fluidrive Engineering Business.
The STC coupling includes a reservoir to which some, however, not all, of the oil gravitates when the result shaft is definitely stalled. This decreases the “drag” on the input shaft, resulting in reduced fuel consumption when idling and a decrease in the vehicle’s tendency to “creep”.
When the output shaft starts to rotate, the oil is thrown out of the reservoir by centrifugal force, 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 input and result angular velocities are similar. Hence a fluid coupling cannot achieve completely power transmission performance. Because of slippage which will occur in any fluid coupling under load, some power will always be lost in fluid friction and turbulence, and dissipated as high temperature. Like other fluid dynamical devices, its efficiency tends to increase steadily 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 natural oils or automated transmission fluids are used. Raising density of the fluid escalates the quantity of torque which can be transmitted at confirmed input speed. However, hydraulic fluids, very much like other fluids, are subject to adjustments in viscosity with temperatures change. This qualified prospects to a change in transmission overall performance and so where unwanted performance/efficiency change has to be kept to the very least, a motor oil or automated transmission fluid, with a higher viscosity index ought to be used.
Hydrodynamic braking
Fluid couplings can also act as hydrodynamic brakes, dissipating rotational energy as warmth through frictional forces (both viscous and fluid/container). When a fluid coupling is utilized for braking additionally it is referred to as a retarder.
Fluid Coupling Applications
Industrial
Fluid couplings are used in many industrial application involving rotational power, specifically in machine drives that involve high-inertia starts or constant cyclic loading.
Rail transportation
Fluid couplings are found in a few Diesel locomotives as part of 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 several combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were used in a number of early semi-automatic transmissions and automatic transmissions. Since 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 section of the flywheel correct, and therefore is switched by the engine’s crankshaft. The turbine is linked to the input shaft of the transmission. While the transmitting is in equipment, as engine swiftness increases torque is usually transferred from the engine to the input shaft by the movement of the fluid, propelling the automobile. In this regard, the behavior of the fluid coupling highly resembles that of a mechanical clutch generating a manual transmission.
Fluid flywheels, as distinctive from torque converters, are best known for their make use of in Daimler vehicles together with a Wilson pre-selector gearbox. Daimler utilized these throughout their range of luxury cars, until switching to automatic gearboxes with the 1958 Majestic. Daimler and Alvis had been both also known for their military vehicles and armored vehicles, a few of which also utilized the combination of pre-selector gearbox and fluid flywheel.
Aviation
The many prominent use 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 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-rate turbine rotation to low-speed, high-torque result to drive the propeller.