Fluid Coupling Overview and Applications

Fluid Coupling Overview
  A fluid coupling includes three components, plus the hydraulic fluid:
  The casing, also referred to as the shell (which will need to have an oil-restricted seal around the get shafts), provides the fluid and turbines.
  Two turbines (lover like components):
  One linked to the input shaft; referred to as the pump or impellor, primary wheel input turbine
  The other linked to the result shaft, referred to as the turbine, result turbine, secondary wheel or runner
  The generating turbine, known as the ‘pump’, (or driving torus) is rotated by the primary mover, which is normally an internal combustion engine or electric powered electric motor. The impellor’s motion imparts both outwards linear and rotational motion to the fluid.
  The hydraulic fluid is normally directed by the ‘pump’ whose form forces the movement 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 drive on the ‘result turbine’ leading to a torque; therefore leading to it to rotate in the same direction as the pump.
  The movement of the fluid is successfully toroidal – exploring in one direction on paths that can be visualised to be on the top of a torus:
  If there is a notable difference between input and result angular velocities the movement has a component which is definitely circular (i.e. round the bands formed by parts of the torus)
  If the insight and output stages have similar angular velocities there is absolutely no net centripetal pressure – and the movement 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 flow of fluid from one turbine to the other.
  Stall speed
  An important characteristic of a fluid coupling is normally its stall swiftness. The stall quickness is thought as the best speed of which the pump can turn when the output turbine is usually locked and maximum insight power is used. Under stall circumstances all of the engine’s power would be dissipated in the fluid coupling as heat, possibly leading to damage.
  Step-circuit coupling
  A modification to the simple fluid coupling is the step-circuit coupling that was formerly produced as the “STC coupling” by the Fluidrive Engineering Company.
  The STC coupling includes a reservoir to which some, however, not all, of the oil gravitates when the result shaft can be stalled. This reduces 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 result shaft begins to rotate, the oil is thrown out of the reservoir by centrifugal pressure, and returns to the primary body of the coupling, so that normal power transmitting is restored.
  A fluid coupling cannot develop result torque when the input and result angular velocities are identical. Hence a fluid coupling cannot achieve 100 percent power transmission performance. Due to slippage which will occur in any fluid coupling under load, some power will always be dropped in fluid friction and turbulence, and dissipated as temperature. Like other fluid dynamical devices, its efficiency will increase steadily with increasing level, as measured by the Reynolds amount.
  Hydraulic fluid
  As a fluid coupling operates kinetically, low viscosity fluids are preferred. Generally speaking, multi-grade motor natural oils or automatic transmission liquids are used. Increasing density of the fluid increases the amount of torque which can be transmitted at a given input speed. However, hydraulic fluids, much like other fluids, are at the mercy of changes in viscosity with heat range change. This leads to a change in transmission efficiency and so where undesirable performance/efficiency change needs to be kept to a minimum, a motor essential oil or automated transmission fluid, with a high viscosity index ought to be used.
  Hydrodynamic braking
  Fluid couplings can also act as hydrodynamic brakes, dissipating rotational energy as temperature through frictional forces (both viscous and fluid/container). Whenever a fluid coupling can be used for braking additionally it is referred to as a retarder.

Fluid Coupling Applications
  Fluid couplings are used in many commercial application including rotational power, specifically in machine drives that involve high-inertia starts or continuous cyclic loading.
  Rail transportation
  Fluid couplings are found in a few Diesel locomotives as part of the power transmitting system. Self-Changing Gears produced semi-automated transmissions for British Rail, and Voith produce turbo-transmissions for railcars and diesel multiple systems which contain various combinations of fluid couplings and torque converters.
  Fluid couplings were used in a number of early semi-automated transmissions and automated transmissions. Because the late 1940s, the hydrodynamic torque converter offers replaced the fluid coupling in motor vehicle applications.
  In motor vehicle applications, the pump typically is linked to the flywheel of the engine-in reality, the coupling’s enclosure may be part of the flywheel proper, and therefore is switched by the engine’s crankshaft. The turbine is linked to the insight shaft of the transmitting. While the transmission is in gear, as engine acceleration increases torque is normally transferred from the engine to the input shaft by the motion of the fluid, propelling the automobile. In this respect, the behavior of the fluid coupling highly resembles that of a mechanical clutch traveling a manual transmitting.
  Fluid flywheels, as unique from torque converters, are best known for their use in Daimler vehicles in conjunction with a Wilson pre-selector gearbox. Daimler utilized these throughout their selection of luxury cars, until switching to automated gearboxes with the 1958 Majestic. Daimler and Alvis had been both also known for his or her military automobiles and armored cars, some of which also utilized the combination of pre-selector gearbox and fluid flywheel.
  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 managed hydraulic clutch for the centrifugal compressor and the Wright turbo-substance reciprocating engine, where three power recovery turbines extracted around 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 output to operate a vehicle the propeller.