Fluid Coupling Overview
  A fluid coupling contains three components, plus the hydraulic fluid:
  The housing, also known as the shell (which must have an oil-limited seal around the get shafts), provides the fluid and turbines.
  Two turbines (lover like components):
  One connected to the input shaft; referred to as the pump or impellor, primary wheel input turbine
  The other connected to the output shaft, referred to as the turbine, output turbine, secondary wheel or runner
  The driving turbine, referred to as the ‘pump’, (or driving torus) is usually rotated by the primary mover, which is normally an internal combustion engine or electric powered 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 shape forces the flow in the 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 pressure on the ‘result turbine’ leading to a torque; therefore leading to it to rotate in the same direction as the pump.
  The motion of the fluid is successfully toroidal – venturing in one path 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 element which is definitely circular (i.e. across the bands formed by parts of the torus)
  If the insight and output stages have identical angular velocities there is no net centripetal drive – and the motion of the fluid is circular and co-axial with the axis of rotation (i.e. round the edges of a torus), there is absolutely no movement of fluid from one turbine to the various other.
  Stall speed
  An important characteristic of a fluid coupling is its stall swiftness. The stall rate is defined as the best speed at which the pump can turn when the result turbine can be locked and maximum insight power is applied. Under stall conditions all the engine’s power will be dissipated in the fluid coupling as heat, possibly resulting in 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 Organization.
  The STC coupling consists of a reservoir to which some, but not all, of the essential oil gravitates when the output shaft is normally stalled. This decreases the “drag” on the insight shaft, leading to reduced fuel intake when idling and a reduction in the vehicle’s tendency to “creep”.
  When the result shaft starts to rotate, the essential oil is thrown out of the reservoir by centrifugal push, and returns to the main body of the coupling, to ensure that normal power transmitting 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 100 percent power transmission performance. Because of slippage that may occur in virtually any fluid coupling under load, some power will be lost in fluid friction and turbulence, and dissipated as warmth. Like other fluid dynamical products, its efficiency will increase gradually with increasing scale, 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 oils or automated transmission fluids are used. Increasing density of the fluid increases the quantity of torque that can be transmitted at confirmed input speed. Nevertheless, hydraulic fluids, very much like other liquids, are at the mercy of adjustments in viscosity with temperatures change. This prospects to a switch in transmission functionality and so where unwanted performance/efficiency change needs 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 may 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 known as a retarder.

Fluid Coupling Applications
  Industrial
  Fluid couplings are used in many industrial application including 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-automatic transmissions for British Rail, and Voith produce turbo-transmissions for railcars and diesel multiple units which contain numerous combinations of fluid couplings and torque converters.
  Automotive
  Fluid couplings were found in a variety of early semi-automated transmissions and automated 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 linked to the flywheel of the engine-in fact, the coupling’s enclosure could be area of the flywheel correct, and therefore is switched by the engine’s crankshaft. The turbine is linked to the input shaft of the transmitting. While the transmission is in gear, as engine speed increases torque is certainly transferred from the engine to the input shaft by the motion of the fluid, propelling the automobile. In this regard, the behavior of the fluid coupling strongly resembles that of a mechanical clutch generating a manual transmitting.
  Fluid flywheels, as specific from torque converters, are best known for their make use of in Daimler cars together with a Wilson pre-selector gearbox. Daimler utilized these throughout their selection of luxury vehicles, until switching to automated gearboxes with the 1958 Majestic. Daimler and Alvis had been both also known for their military automobiles and armored vehicles, a few of which also used the combination of pre-selector gearbox and fluid flywheel.
  Aviation
  The many 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 managed hydraulic clutch for the centrifugal compressor and the Wright turbo-substance 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, using three fluid couplings and gearing, converted low-torque high-quickness turbine rotation to low-speed, high-torque result to drive the propeller.