Fluid Coupling Overview
  A fluid coupling includes three components, in addition to the hydraulic fluid:
  The casing, also known as the shell (which must have an oil-limited seal around the travel shafts), provides the fluid and turbines.
  Two turbines (fan like components):
  One connected to the insight shaft; referred to as the pump or impellor, primary wheel input turbine
  The other connected to the result shaft, referred to as the turbine, result turbine, secondary wheel or runner
  The driving turbine, known as the ‘pump’, (or driving torus) is usually rotated by the prime mover, which is typically an internal combustion engine or electrical electric motor. The impellor’s motion imparts both outwards linear and rotational motion to the fluid.
  The hydraulic fluid is definitely directed by the ‘pump’ whose shape forces the circulation in the direction of the ‘output turbine’ (or powered torus). Here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ lead to a net power on the ‘output turbine’ causing a torque; therefore causing it to rotate in the same direction as the pump.
  The movement of the fluid is successfully toroidal – traveling in one direction on paths that can be visualised as being on the top of a torus:
  When there is a difference between insight and result angular velocities the movement has a component which is definitely circular (i.e. across the rings formed by sections of the torus)
  If the input and output phases have similar angular velocities there is absolutely no net centripetal power – and the movement of the fluid is normally 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 its stall acceleration. The stall speed is thought as the highest speed of which the pump can change when the output turbine is usually locked and maximum insight power is used. Under stall circumstances all the engine’s power will be dissipated in the fluid coupling as heat, probably leading to damage.
  Step-circuit coupling
  A modification 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 consists of a reservoir to which some, however, not all, of the essential oil gravitates when the result shaft is stalled. This decreases the “drag” on the insight shaft, leading to reduced fuel intake when idling and a decrease in the vehicle’s inclination to “creep”.
  When the result shaft begins to rotate, the essential oil is trashed of the reservoir by centrifugal push, 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 insight and output angular velocities are similar. Hence a fluid coupling cannot achieve 100 percent power transmission effectiveness. Because of slippage which will occur in virtually any fluid coupling under load, some power will be dropped in fluid friction and turbulence, and dissipated as high temperature. Like other fluid dynamical devices, its efficiency will increase gradually with increasing scale, 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 automatic transmission fluids are used. Increasing density of the fluid escalates the amount of torque that can be transmitted at a given input speed. Nevertheless, hydraulic fluids, much like other fluids, are subject to changes in viscosity with temperatures change. This network marketing leads to a transformation in transmission functionality and so where undesired performance/efficiency change needs to be held to a minimum, a motor essential oil or automated transmission fluid, with a higher viscosity index ought to 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 utilized for braking it is also referred to as a retarder.

Fluid Coupling Applications
  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 a few Diesel locomotives within the power transmission system. Self-Changing Gears made semi-automatic transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple systems 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 automotive applications, the pump typically is linked to the flywheel of the engine-in fact, the coupling’s enclosure may be section of the flywheel proper, and thus is switched by the engine’s crankshaft. The turbine is connected to the input shaft of the transmission. While the transmission is in equipment, as engine swiftness increases torque is definitely transferred from the engine to the input shaft by the motion of the fluid, propelling the vehicle. In this respect, 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 best known for their use in Daimler cars together with a Wilson pre-selector gearbox. Daimler utilized these throughout their range of luxury vehicles, until switching to automated gearboxes with the 1958 Majestic. Daimler and Alvis had been both also known for their military vehicles and armored cars, some of which also utilized the mixture of pre-selector gearbox and fluid flywheel.
  The many prominent utilization of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 engines where it was utilized as a barometrically controlled hydraulic clutch for the centrifugal compressor and the Wright turbo-compound reciprocating engine, in which three power recovery turbines extracted approximately 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-swiftness turbine rotation to low-speed, high-torque result to drive the propeller.