Fluid coupling

Daimler car fluid flywheel of the 1930s
This article is about hydrodynamic fluid couplings, for "hydroviscous fluid couplings" see Viscous coupling unit.

A fluid coupling or hydraulic coupling is a hydrodynamic device used to transmit rotating mechanical power.[1] 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.


The fluid coupling originates from the work of Dr. Hermann Föttinger, who was the chief designer at the AG Vulcan Works in Stettin.[2] His patents from 1905 covered both fluid couplings and torque converters.

Dr 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 1920s[2] 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.[3]

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.[3]

In 1939 General Motors Corporation introduced Hydramatic drive, the first fully automatic automotive transmission system installed in a mass-produced automobile.[2] The Hydramatic employed a fluid coupling.

The first Diesel locomotives using fluid couplings were also produced in the 1930s[4]


Fluid coupling on Transfluid's industrial transmission model KPTO.

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

The driving turbine, known as the 'pump', (or driving torus[lower-alpha 1]) 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[lower-alpha 2]). 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:

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 output 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.

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 contains a reservoir to which some, but not all, of the oil gravitates when the output shaft is stalled. This reduces the "drag" on the input shaft, resulting in reduced fuel consumption when idling and a reduction in the vehicle's tendency to "creep".

When the output shaft begins to rotate, the oil is thrown out of the reservoir by centrifugal force, and returns to the main body of the coupling, so that normal power transmission is restored.[6]


A fluid coupling cannot develop output torque when the input and output angular velocities are identical.[7] Hence a fluid coupling cannot achieve 100 percent power transmission efficiency. Due to slippage that will occur in any fluid coupling under load, some power will always be lost in fluid friction and turbulence, and dissipated as heat. Like other fluid dynamical devices, its efficiency tends to increase gradually with increasing scale, as measured by the Reynolds number.

Hydraulic fluid

As a fluid coupling operates kinetically, low viscosity fluids are preferred.[7] Generally speaking, multi-grade motor oils or automatic transmission fluids are used. Increasing density of the fluid increases the amount of torque that can be transmitted at a given input speed.[8] However, hydraulic fluids, much like other fluids, are subject to changes in viscosity with temperature change. This leads to a change in transmission performance and so where unwanted performance/efficiency change has to be kept to a minimum a motor oil, or automatic transmission fluid, with a high viscosity index should be used.

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.[5]



Fluid couplings are used in many industrial application involving rotational power,[9][10] 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 railcars and diesel multiple units which contain various combinations of fluid couplings and torque converters.


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 enginein 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 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 armored cars, some of which also used the combination 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 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.


Generally speaking, the power transmitting capability of a given fluid coupling is strongly related to pump speed, a characteristic that generally works well with applications where the applied load doesn't fluctuate to a great degree. The torque transmitting capacity of any hydrodynamic coupling can be described by the expression , where is the mass density of the fluid, is the impeller speed, and is the impeller diameter.[11] In the case of automotive applications, where loading can vary to considerable extremes, is only an approximation. Stop-and-go driving will tend to operate the coupling in its least efficient range, causing an adverse effect on fuel economy.


Fluid couplings are relatively simple components to produce. For example, the turbines can be aluminum castings or steel stampings, and the housing can also be a casting or made from stamped or forged steel.

Manufacturers of industrial fluid couplings include Voith,[12] Transfluid,[13] TwinDisc,[14] Siemens,[15] Parag,[16] Fluidomat,[17] Reuland Electric,[18] and Howden North America "Gýrol Fluid Drive".[19]


List of fluid coupling patents.

This is not an exhaustive list but is intended to give an idea of the development of fluid couplings in the 20th century.

Patent number Publication date Inventor Link
GB190906861 02 Dec 1909 Hermann Föttinger
US1127758 09 Feb 1915 Jacob Christian Hansen-Ellehammer
US1199359 26 Sep 1916 Hermann Föttinger
US1472930 06 Nov 1923 Fritz Mayer
GB359501 23 Oct 1931 Voith
US1937364 28 Nov 1933 Harold Sinclair
US1987985 15 Jan 1935 Schmieske and Bauer
US2004279 11 Jun 1935 Hermann Föttinger
US2127738 23 Aug 1938 Fritz Kugel
US2202243 28 May 1940 Noah L Alison
US2264341 02 Dec 1941 Arthur and Sinclair
US2491483 20 Dec 1949 Gaubatz and Dolza
US2505842 02 May 1950 Harold Sinclair
US2882683 21 Apr 1959 Harold Sinclair

See also



  1. Fluid coupling encyclopedia2.thefreedictionary.com
  2. 1 2 3 Nunney, Malcolm James (2007). Light and Heavy Vehicle Technology. Butterworth-Heinemann. p. 317. ISBN 978-0-7506-8037-0.
  3. 1 2 Douglas-Scott-Montagu, Edward; Burgess-Wise, David (1995). Daimler Century: The Full History of Britain's Oldest Car Maker. Patrick Stephens. ISBN 978-1-85260-494-3.
  4. Ransome-Wallis, Patrick (2012). Illustrated Encyclopedia of World Railway Locomotives. Dover Publications. p. 64. ISBN 978-0-486-41247-4.
  5. 1 2 3 4 5 Fluid couplings glossary voithturbo.com
  6. Bolton, William F. (1963). Railwayman's Diesel Manual: A Practical Introduction to the Diesel-powered Locomotive, Railcar and Multiple-unit Powered Train for Railway Staff and Railway Enthusiasts (4th ed.). Ian Allan Publishing. pp. 97–98. ISBN 978-0-7110-3197-5.
  7. 1 2 Why is the output speed of a turbo coupling always lower than the input speed? voithturbo.com from Voith - Fluid couplings FAQ
  8. Does the type of operating fluid influence the transmission behaviour? voithturbo.com from Voith - Fluid couplings FAQ
  9. Industry/Sector Industrial and other uses of fluid couplings voithturbo.com
  10. Process Uses of fluid coupling by process voithturbo.com
  11. Hydrodynamic couplings and converters. Automotive Handbook (3rd ed.). Robert Bosch. p. 539. ISBN 0-8376-0330-7.
  12. Voith: Fluid Coulings, voith.com
  13. Transfluid: Fluid couplings, transfluid.eu
  14. TwinDisc: Fluid couplings, twindisc.com
  15. Siemens: Hydrodynamic couplings, automation.siemens.com
  16. PARAG Engineering: Fluid Couplings
  17. Fluidomat fluidomat.com
  18. Reuland Electric Co., Inc.
  19. http://www.howden.com/en/products/other/gyrolfluiddrives/default.htm
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