Fluid Coupling Overview and Applications

Fluid Coupling Overview
  A fluid coupling includes three components, plus the hydraulic fluid:
  The housing, also called the shell (which will need to have an oil-tight seal around the travel 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 linked to the output shaft, referred to as the turbine, result turbine, secondary steering wheel or runner
  The traveling turbine, referred to as the 'pump', (or driving torus) is normally rotated by the primary mover, which is typically an internal combustion engine or electric powered electric 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 stream in the direction of the 'output turbine' (or powered torus). Right here, any difference in the angular velocities of 'input stage' and 'output stage' result in a net pressure on the 'output turbine' leading to a torque; therefore leading to it to rotate in the same path as the pump.
  The movement of the fluid is efficiently toroidal - venturing in one path on paths which can be visualised to be on the surface of a torus:
  When there is a notable difference between insight and result angular velocities the movement has a element which is circular (i.e. round the bands formed by parts of the torus)
  If the insight and output levels have similar angular velocities there is absolutely no net centripetal force - and the movement of the fluid is certainly circular and co-axial with the axis of rotation (i.e. round the edges of a torus), there is no stream of fluid from one turbine to the additional.
  Stall speed
  A significant characteristic of a fluid coupling is usually its stall quickness. The stall rate is thought as the best speed of which the pump can change when the result turbine is normally locked and optimum input power is used. Under stall conditions all of the engine's power would be dissipated in the fluid coupling as heat, probably resulting in damage.
  Step-circuit coupling
  An adjustment to the simple fluid coupling is the step-circuit coupling which was formerly produced as the "STC coupling" by the Fluidrive Engineering Organization.
  The STC coupling contains a reservoir to which some, but not all, of the essential oil gravitates when the output shaft is definitely stalled. This decreases the "drag" on the input shaft, resulting in reduced fuel intake when idling and a decrease in the vehicle's inclination to "creep".
  When the output shaft starts to rotate, the essential oil is trashed of the reservoir by centrifugal pressure, and returns to the primary body of the coupling, to ensure that normal power transmitting is restored.
  A fluid coupling cannot develop result torque when the insight and result angular velocities are identical. Hence a fluid coupling cannot achieve completely power transmission effectiveness. Due to slippage that will occur in virtually any fluid coupling under load, some power will always be lost in fluid friction and turbulence, and dissipated as high temperature. Like other fluid dynamical devices, its efficiency tends to increase gradually with increasing scale, as measured by the Reynolds amount.
  Hydraulic fluid
  As a fluid coupling operates kinetically, low viscosity fluids are preferred. Generally speaking, multi-grade motor oils or automated transmission liquids are used. Raising density of the fluid escalates the quantity of torque which can be transmitted at a given input speed. However, hydraulic fluids, very much like other liquids, are subject to changes in viscosity with temp change. This leads to a switch in transmission efficiency therefore where undesired performance/efficiency change needs to be kept to the very least, a motor essential oil or automated transmission fluid, with a higher viscosity index ought to be used.
  Hydrodynamic braking
  Fluid couplings can also become hydrodynamic brakes, dissipating rotational energy as heat through frictional forces (both viscous and fluid/container). When a fluid coupling can be used for braking additionally it is known as a retarder.

Fluid Coupling Applications
  Fluid couplings are used in many industrial application including rotational power, especially in machine drives that involve high-inertia starts or constant cyclic loading.
  Rail transportation
  Fluid couplings are located in some Diesel locomotives within the power transmitting system. Self-Changing Gears produced semi-automatic transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple models which contain various combinations of fluid couplings and torque converters.
  Fluid couplings were found in a number of early semi-automated transmissions and automatic transmissions. Since the late 1940s, the hydrodynamic torque converter offers replaced the fluid coupling in automotive applications.
  In automotive applications, the pump typically is connected to the flywheel of the engine-in truth, the coupling's enclosure may be section of the flywheel proper, and therefore is turned by the engine's crankshaft. The turbine is linked to the insight shaft of the transmission. While the transmitting is in gear, as engine quickness increases torque is transferred from the engine to the insight shaft by the motion of the fluid, propelling the vehicle. In this regard, 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 most widely known for their make use of in Daimler vehicles together 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 had been both also known for their military vehicles and armored vehicles, a few of which also used the mixture of pre-selector gearbox and fluid flywheel.
  The many prominent usage of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 motors where it was utilized as a barometrically managed hydraulic clutch for the centrifugal compressor and the Wright turbo-substance reciprocating engine, where 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-quickness turbine rotation to low-speed, high-torque output to drive the propeller.

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