Fluid Coupling Overview and Applications

Fluid Coupling Overview
  A fluid coupling contains three components, in addition to the hydraulic fluid:
  The housing, also known as the shell (which will need to have an oil-restricted seal around the travel shafts), contains the fluid and turbines.
  Two turbines (enthusiast like components):
  One connected to the input shaft; referred to as the pump or impellor, primary wheel input turbine
  The other linked to the output shaft, referred to as the turbine, output turbine, secondary steering wheel or runner
  The generating turbine, referred to as the ‘pump’, (or driving torus) is rotated by the primary mover, which is typically an interior combustion engine or electric motor. The impellor’s motion imparts both outwards linear and rotational motion to the fluid.
  The hydraulic fluid is certainly directed by the ‘pump’ whose shape forces the flow in the direction of the ‘output turbine’ (or driven torus). Right here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ lead to a net pressure on the ‘output turbine’ leading to a torque; thus causing it to rotate in the same path as the pump.
  The motion of the fluid is effectively toroidal – going in one path on paths which can be visualised as being on the top of a torus:
  When there is a difference between input and result angular velocities the movement has a component which is normally circular (i.e. across the bands formed by sections of the torus)
  If the insight and output phases 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. across the edges of a torus), there is no circulation of fluid from one turbine to the other.
  Stall speed
  A significant characteristic of a fluid coupling is certainly its stall velocity. The stall rate is thought as the highest speed of which the pump can turn when the result turbine is locked and maximum input power is used. Under stall circumstances all of the engine’s power would 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 that was formerly manufactured as the “STC coupling” by the Fluidrive Engineering Company.
  The STC coupling consists of a reservoir to which some, however, not all, of the essential oil gravitates when the output shaft is certainly stalled. This reduces the “drag” on the insight shaft, resulting in reduced fuel usage when idling and a decrease in the vehicle’s inclination to “creep”.
  When the result shaft begins to rotate, the oil is thrown out of the reservoir by centrifugal pressure, and returns to the main body of the coupling, so that normal power transmitting 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 efficiency. Due to slippage that will occur in virtually any fluid coupling under load, some power will always be dropped in fluid friction and turbulence, and dissipated as warmth. Like other fluid dynamical products, its efficiency tends to increase steadily with increasing scale, as measured by the Reynolds number.
  Hydraulic fluid
  As a fluid coupling operates kinetically, low viscosity fluids are preferred. In most cases, multi-grade motor oils or automatic transmission fluids are used. Raising density of the fluid increases the quantity of torque that can be transmitted at a given input speed. Nevertheless, hydraulic fluids, much like other liquids, are subject to adjustments in viscosity with heat change. This prospects to a change in transmission efficiency and so where undesired performance/efficiency change has to be held to the very least, a motor essential oil or automatic transmission fluid, with a high viscosity index should be used.
  Hydrodynamic braking
  Fluid couplings may also become hydrodynamic brakes, dissipating rotational energy as heat through frictional forces (both viscous and fluid/container). When a fluid coupling is used for braking additionally it is referred to as a retarder.

Fluid Coupling Applications
  Fluid couplings are used in many commercial application involving rotational power, especially in machine drives that involve high-inertia begins or constant cyclic loading.
  Rail transportation
  Fluid couplings are located in a few Diesel locomotives within the power transmitting system. Self-Changing Gears made semi-automatic transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple systems which contain various combinations of fluid couplings and torque converters.
  Fluid couplings were used in a variety of early semi-automated transmissions and automated transmissions. Since the past due 1940s, the hydrodynamic torque converter has replaced the fluid coupling in automotive applications.
  In motor vehicle applications, the pump typically is connected to the flywheel of the engine-in truth, the coupling’s enclosure could be section 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 speed increases torque is transferred from the engine to the insight 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 transmission.
  Fluid flywheels, as specific from torque converters, are best known for their make use of in Daimler vehicles in conjunction 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 were both also known for his or her military automobiles and armored cars, a few of which also used the mixture of pre-selector gearbox and fluid flywheel.
  The most 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 controlled hydraulic clutch for the centrifugal compressor and the Wright turbo-compound 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-quickness turbine rotation to low-speed, high-torque result to drive the propeller.


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