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 get shafts), provides the fluid and turbines.
　　Two turbines (enthusiast like components):
　　One linked to the insight 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 certainly rotated by the prime mover, which is normally an internal combustion engine or electric electric motor. The impellor’s movement imparts both outwards linear and rotational motion to the fluid.
　　The hydraulic fluid is normally directed by the ‘pump’ whose form forces the flow in the direction of the ‘output turbine’ (or powered torus). Right here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ lead to a net push on the ‘result turbine’ causing a torque; therefore leading to it to rotate in the same path as the pump.
　　The movement of the fluid is efficiently toroidal – exploring in one direction on paths which can be visualised to be on the top of a torus:
　　If there is a difference between input and result angular velocities the motion has a element which is definitely circular (i.e. across the bands formed by parts of the torus)
　　If the input and output phases have similar angular velocities there is absolutely no net centripetal power – and the motion of the fluid can be circular and co-axial with the axis of rotation (i.e. across the edges of a torus), there is no movement of fluid in one turbine to the additional.
　　Stall speed
　　A significant characteristic of a fluid coupling is its stall velocity. The stall speed is defined as the best speed of which the pump can change when the result turbine is certainly locked and optimum input power is applied. Under stall conditions all of the engine’s power will be dissipated in the fluid coupling as heat, perhaps resulting in damage.
　　Step-circuit coupling
　　A modification to the simple fluid coupling is the step-circuit coupling which 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 normally stalled. This decreases the “drag” on the input shaft, leading to reduced fuel usage when idling and a reduction in the vehicle’s inclination to “creep”.
　　When the output shaft starts to rotate, the essential oil is thrown out of the reservoir by centrifugal drive, and returns to the main body of the coupling, so that normal power transmission is restored.
　　Slip
　　A fluid coupling cannot develop output torque when the insight and output angular velocities are similar. Hence a fluid coupling cannot achieve completely power transmission effectiveness. Because of slippage that will occur in any fluid coupling under load, some power will be lost in fluid friction and turbulence, and dissipated as temperature. Like other fluid dynamical devices, its efficiency tends to increase steadily with increasing level, as measured by the Reynolds number.
　　Hydraulic fluid
　　As a fluid coupling operates kinetically, low viscosity liquids are preferred. In most cases, multi-grade motor natural oils or automated transmission fluids are used. Increasing density of the fluid increases the amount of torque which can be transmitted at confirmed input speed. However, hydraulic fluids, much like other liquids, are subject to changes in viscosity with heat change. This prospects to a modification in transmission overall performance therefore where unwanted performance/efficiency change needs to be held to a minimum, a motor oil or automated transmission fluid, with a high viscosity index should be used.
　　Hydrodynamic braking
　　Fluid couplings may also become hydrodynamic brakes, dissipating rotational energy as warmth through frictional forces (both viscous and fluid/container). Whenever a fluid coupling is used for braking it is also referred to as a retarder.

Fluid Coupling Applications
　　Industrial
　　Fluid couplings are found in many commercial application concerning 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 made semi-automated transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple systems which contain different combinations of fluid couplings and torque converters.
　　Automotive
　　Fluid couplings were found in a number of early semi-automated transmissions and automatic transmissions. Since 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 connected to the flywheel of the engine-in truth, the coupling’s enclosure could be portion of the flywheel appropriate, and therefore is turned by the engine’s crankshaft. The turbine is connected to the input shaft of the transmitting. While the transmitting is in equipment, as engine acceleration increases torque can be 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 strongly resembles that of a mechanical clutch traveling a manual transmitting.
　　Fluid flywheels, as specific from torque converters, are most widely known for their use in Daimler vehicles in conjunction with a Wilson pre-selector gearbox. Daimler used these throughout their range of luxury vehicles, until switching to automatic gearboxes with the 1958 Majestic. Daimler and Alvis had been both also known for his or her military vehicles and armored vehicles, a few of which also utilized the mixture of pre-selector gearbox and fluid flywheel.
　　Aviation
　　The most prominent utilization of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 motors where it was used as a barometrically managed hydraulic clutch for the centrifugal compressor and the Wright turbo-compound reciprocating engine, where three power recovery turbines extracted approximately 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-acceleration turbine rotation to low-speed, high-torque result to drive the propeller.