Fluid Coupling Overview
  A fluid coupling consists of three components, in addition to the hydraulic fluid:
  The housing, also called the shell (which will need to have an oil-tight seal around the drive shafts), provides the fluid and turbines.
  Two turbines (fan like components):
  One connected to the input shaft; referred to as the pump or impellor, primary steering wheel input turbine
  The other connected to the result shaft, known as the turbine, result turbine, secondary wheel or runner
  The driving turbine, known as the ‘pump’, (or driving torus) is 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 form forces the flow in direction of the ‘output turbine’ (or driven torus). Right here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ result in a net pressure on the ‘result turbine’ leading to a torque; therefore causing it to rotate in the same direction as the pump.
  The motion of the fluid is effectively toroidal – travelling in one path on paths that can be visualised as being on the surface of a torus:
  If there is a notable difference between input and output angular velocities the motion has a component which is definitely circular (i.e. round the bands formed by parts of the torus)
  If the input and output stages have identical angular velocities there is no net centripetal force – 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 no stream of fluid in one turbine to the additional.
  Stall speed
  A significant characteristic of a fluid coupling is normally its stall rate. The stall swiftness is defined as the best speed at which the pump can turn when the output turbine is locked and maximum input power is applied. Under stall circumstances all of the engine’s power will be dissipated in the fluid coupling as heat, perhaps resulting in damage.
  Step-circuit coupling
  An adjustment to the simple fluid coupling is the step-circuit coupling that was formerly produced as the “STC coupling” by the Fluidrive Engineering Firm.
  The STC coupling consists of a reservoir to which some, however, not all, of the essential oil gravitates when the result shaft is definitely stalled. This decreases the “drag” on the insight shaft, resulting in reduced fuel intake when idling and a decrease in the vehicle’s tendency to “creep”.
  When the output shaft begins to rotate, the essential oil is thrown out of the reservoir by centrifugal force, 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 100 percent power transmission performance. Due to slippage which will occur in virtually any fluid coupling under load, some power will be lost in fluid friction and turbulence, and dissipated as temperature. Like other fluid dynamical products, its efficiency tends to increase steadily with increasing level, as measured by the Reynolds quantity.
  Hydraulic fluid
  As a fluid coupling operates kinetically, low viscosity liquids are preferred. Generally speaking, multi-grade motor natural oils or automated transmission liquids are used. Increasing density of the fluid increases the amount of torque that can be transmitted at a given input speed. However, hydraulic fluids, very much like other liquids, are subject to changes in viscosity with temperatures change. This qualified prospects to a change in transmission efficiency therefore where unwanted performance/efficiency change has to be kept to a minimum, a motor essential oil or automated 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). When a fluid coupling is utilized for braking additionally it is known as a retarder.

Fluid Coupling Applications
  Fluid couplings are used in many commercial application concerning rotational power, specifically in machine drives that involve high-inertia begins or continuous cyclic loading.
  Rail transportation
  Fluid couplings are located in a few Diesel locomotives as part of the power transmitting system. Self-Changing Gears made semi-automatic 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-automatic transmissions and automatic transmissions. Since the past due 1940s, the hydrodynamic torque converter offers replaced the fluid coupling in motor vehicle applications.
  In automotive applications, the pump typically is linked to the flywheel of the engine-in reality, the coupling’s enclosure could be part of the flywheel appropriate, and therefore is switched by the engine’s crankshaft. The turbine is connected to the insight shaft of the transmission. While the transmission is in equipment, as engine rate increases torque is certainly 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 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 cars in conjunction with a Wilson pre-selector gearbox. Daimler used these throughout their range of luxury vehicles, until switching to automated gearboxes with the 1958 Majestic. Daimler and Alvis were both also known for their military vehicles and armored vehicles, a few of which also used the combination of pre-selector gearbox and fluid flywheel.
  The many prominent use of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 engines where it was used as a barometrically managed 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, using three fluid couplings and gearing, converted low-torque high-swiftness turbine rotation to low-speed, high-torque result to drive the propeller.