Hydraulic Coupling

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A fluid coupling or hydraulic coupling is a hydrodynamic or ‘hydrokinetic’ device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch. It also has widespread application in marine and industrial machine drives, where variable speed operation and controlled start-up without shock loading of the power transmission system is essential.

Hydrokinetic drives, such as this, should be distinguished from hydrostatic drives, such as hydraulic pump and motor combinations.

History

The fluid coupling originates from the work of Hermann Föttinger, who was the chief designer at the AG Vulcan Works in Stettin.His patents from 1905 covered both fluid couplings and torque converters.

Dr Gustav Bauer of the Vulcan-Werke collaborated with English engineer Harold Sinclair of Hydraulic Coupling Patents Limited to adapt the Föttinger coupling to vehicle transmission in an attempt to mitigate the lurching Sinclair had experienced while riding on London buses during the 1920 Following Sinclair’s discussions with the London General Omnibus Company begun in October 1926, and trials on an Associated Daimler bus chassis, Percy Martin of Daimler decided to apply the principle to the Daimler group’s private cars.

During 1930 The Daimler Company of Coventry, England began to introduce a transmission system using a fluid coupling and Wilson self-changing gearbox for buses and their flagship cars. By 1933 the system was used in all new Daimler, Lanchester and BSA vehicles produced by the group from heavy commercial vehicles to small cars. It was soon extended to Daimler’s military vehicles. These couplings are described as constructed under Vulcan-Sinclair and Daimler patents.

In 1939 General Motors Corporation introduced Hydramatic drive, the first fully automatic automotive transmission system installed in a mass-produced automobile.The Hydramatic employed a fluid coupling.

The first diesel locomotives using fluid couplings were also produced in the 1930s

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Applications

Industrial

Fluid couplings are used in many industrial application involving rotational power, especially in machine drives that involve high-inertia starts or constant cyclic loading.

Rail transportation

Fluid couplings are found in some Diesel locomotives as part of the power transmission system. Self-Changing Gears made semi-automatic transmissions for British Rail, and Voith manufacture turbo-transmissions for diesel multiple units which contain various combinations of fluid couplings and torque converters.

Automotive

Fluid couplings were used in a variety of early semi-automatic transmissions and automatic transmissions. Since the late 1940s, the hydrodynamic torque converter has replaced the fluid coupling in automotive applications.

In automotive applications, the pump typically is connected to the flywheel of the engine—in fact, the coupling’s enclosure may be part of the flywheel proper, and thus is turned by the engine’s crankshaft. The turbine is connected to the input shaft of the transmission. While the transmission is in gear, as engine speed increases, torque is transferred from the engine to the input shaft by the motion of the fluid, propelling the vehicle. In this regard, the behaviour of the fluid coupling strongly resembles that of a mechanical clutch driving a manual transmission.

Fluid flywheels, as distinct from torque converters, are best known for their use in Daimler cars in conjunction with a Wilson pre-selector gearbox. Daimler used these throughout their range of luxury cars, until switching to automatic gearboxes with the 1958 Majestic. Daimler and Alvis were both also known for their military vehicles and armoured cars, some of which also used the combination of pre-selector gearbox and fluid flywheel.

Aviation

The most 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 controlled 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 then, using three fluid couplings and gearing, converted low-torque high-speed turbine rotation to low-speed, high-torque output to drive the propeller.