Fluid coupling

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This article is about hydrodyamic fluid complings, for "hydroviscous fluid couplings" see Viscous coupling unit.

A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power.<ref name="dic">Fluid coupling encyclopedia2.thefreedictionary.com</ref> 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/or controlled start-up without shock loading of the power transmission system is essential.


The fluid coupling orginates from the work of Dr. Herman Fottinger, who was the chief designer at the Vulcan Works in Stettin.<ref name="mjn">Light and heavy vehicle technology, Malcolm James Nunney, page 317. (Google books link: [1])</ref>. His patents from 1905 covered both fluid couplings and torque converters.

In 1930 Harold Sinclair, working with the Daimler company, devised a transmission system using a fluid coupling and planetary gearing for buses in an attempt to mitigate the lurching he had experienced while riding on London buses during the 1920s.<ref name="mjn"/>

In 1939 General Motors Corporation introduced Hydramatic drive, the first fully automatic automotive transmission system installed in a mass produced automobile<ref name="mjn"/>. The Hydramatic employed a fluid coupling.

The first Diesel locomotive using fluid couplings were also produced in the 1930s<ref name="locbook">Illustrated Encyclopedia of World Railway Locomotives, Patrick Ransome-Wallis, page 64, ISBN 0486412474, 9780486412474. Google books link: [2]</ref>


A fluid coupling consists of three components, plus the hydraulic fluid:

  • The housing, (which must have an oil tight seal around the drive shafts) - contains the fluid and turbines. (also known as the shell<ref name="gloss"/>)
  • Two turbines (fan like components):
    • One connected to the input shaft; known as the pump or impellor<ref name="gloss"/>, primary wheel<ref name="gloss"/> input turbine
    • The other connected to the output shaft, known as the turbine, output turbine, secondary wheel<ref name="gloss"/> or runner

The driving turbine, known as the 'pump', (or driving torus) is rotated by the prime mover, which is typically an internal combustion engine or electric motor. The impellor's motion imparts both outwards linear and rotational motion to the fluid.

The hydraulic fluid is directed by the 'pump' whose shape forces the flow in the direction of the 'output turbine' (or driven torus). Here, any difference in the angular velocities of 'input stage' and 'output stage' result in a net force on the 'output turbine' causing a torque; thus causing it to rotate in the same direction as the pump.

The motion of the fluid is effectively toroidal - travelling in one direction on paths that can be visualised as being on the surface of a torus:

  • If there is a difference between input and output angular velocities the motion has a component which is circular (ie round the rings formed by sections of the torus)
  • If the input and output stages have identical angular velocities there is no net centripetal force - and the motion of the fluid is circular and co-axial with the axis of rotation (ie 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 its stall speed. The stall speed is defined as the highest speed at which the pump can turn when the ouput turbine is locked and maximum input power is applied. Under stall conditions all of the engine's power would be dissipated in the fluid coupling as heat, possibly leading to damage.


The fluid coupling develops no torque when the input and output angular velocities are the same.<ref name="slip">Why is the output speed of a turbo coupling always lower than the input speed? voithturbo.com from Voith - fluid couplings FAQ</ref> Thus a fluid coupling cannot achieve 100% efficiency in power transmission, additionally some of the energy transferred to the fluid by the pump will be lost to friction (transformed to heat).

This speed difference is called slip or slippage.

Hydraulic fluid

Because the coupling operates kinetically, with change of direction of motion of the fluid, low viscosity fluids are preferred.<ref name="flu">Does the type of operating fluid influence the transmission behaviour? voithturbo.com from Voith - fluid couplings FAQ</ref> Increasing density of the fluid increases the amount of torque that can be transmitted at a given input speed.<ref name="flu"/> Temperature stability is also important as frictional losses cause heating.

Hydrodynamic braking

Fluid couplings can also act as hydrodynamic brakes, dissipating rotational energy as heat through frictional forces (both viscous and fluid/container). When a fluid coupling is used for braking it is also known as a retarder.<ref name="gloss">Fluid couplings glossary voithturbo.com</ref>


A number of modifications to the basics design have been made, including delay chambers<ref name="gloss"/> which are designed to reduce the load on the input device on start up (ie a low angular input velocity).

Main article: torque converter

If the turbine blades are angled (ie not directly radial to the axis of rotation) the turbines can be used to alter torque - dependant of the pitch of the input and output stages being different. Such an arrangement is called a torque converter. Some modern torque converters include an additional component called a stator - an auxiliary turbine attatched to the output stage via a ratchet that provides additional torque multiplication at low speeds.

Additionally, torque converters can function as reversing gears and are produced in packages containing multiple stages that provide different torque multiplication factors.

Automotive Applications

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. As engine speed increases while the transmission is in gear, torque is transferred from the engine to the input shaft by the motion of the fluid, propelling the vehicle. In this regard, the behavior 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.


Fluid couplings are relatively simple components - the 'turbines' can be cast - aluminium is one suitable material, the shell can also be cast or made from pressed or forged metal.

Voith is a German major manufacturer of fluid coupling, especially in the industrial sector, Other manufacturers include TwinDisc, based in Australia<ref>TwinDisc company website twindisc.com : fluid couplings-[3]</ref>, Siemens <ref>Siemens - Hydrodynamic couplings automation.siemens.com</ref>, Parag<ref>Parag Parag Fluid Couplings</ref> & Fluidomat in India<ref>Fluidomat fluidomat.com</ref> and many others through the world.

See also