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{{Short description|Device used to transmit rotating mechanical power}}
{{About| hydrodynamic fluid couplings|hydroviscous fluid couplings|Viscous coupling unit}}
[[File:Fluid flywheel, part section (Autocar Handbook, 13th ed, 1935).jpg|thumb|[[Daimler Company#Daimler's semi-automatic transmissions|Daimler]] car fluid flywheel of the 1930s]]

A '''fluid coupling''' or '''hydraulic coupling''' is a [[hydrodynamics|hydrodynamic]] or 'hydrokinetic' device used to transmit rotating mechanical power.<ref name="dic">[http://encyclopedia2.thefreedictionary.com/Fluid+coupling Fluid coupling] ''encyclopedia2.thefreedictionary.com''</ref> It has been used in [[automobile]] [[Transmission (mechanics)|transmission]]s 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 (mechanics)|shock loading]] of the power transmission system is essential.

Hydrokinetic drives, such as this, should be distinguished from [[hydrostatic drive]]s, such as [[hydraulic pump]] and [[hydraulic motor|motor]] combinations.

==History==
The fluid coupling originates from the work of [[Hermann Föttinger]], who was the chief designer at the [[AG Vulcan Stettin|AG Vulcan Works]] in [[Stettin]].<ref name="mjn">{{cite book |last=Nunney |first=Malcolm James |title=Light and Heavy Vehicle Technology |url= https://books.google.com/books?id=eL6TBaSnd78C |year=2007 |publisher=Butterworth-Heinemann |isbn=978-0-7506-8037-0 |pages=317}}</ref> His patents from 1905 covered both fluid couplings and [[torque converter]]s.

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 1920s<ref name="mjn"/> 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.<ref name=DCMB>{{cite book|last1=Douglas-Scott-Montagu|first1=Edward |author-link1=Edward Douglas-Scott-Montagu, 3rd Baron Montagu of Beaulieu|last2=Burgess-Wise|first2=David |title=Daimler Century: The Full History of Britain's Oldest Car Maker|url=https://books.google.com/books?id=kuWmAAAACAAJ|year=1995|publisher=Patrick Stephens|isbn=978-1-85260-494-3}}</ref>

During 1930 [[Daimler Company|The Daimler Company of Coventry, England]] began to introduce a transmission system using a fluid coupling and [[Preselector gearbox#Wilson gearbox|Wilson self-changing gearbox]] for buses and their [[Daimler Double-Six sleeve-valve V12|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 and in 1934 was featured in the [[Singer Eleven]] branded as Fluidrive. These couplings are described as constructed under Vulcan-Sinclair and Daimler patents.<ref name=DCMB/>

In 1939 [[General Motors|General Motors Corporation]] introduced [[hydramatic|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]]s using fluid couplings were also produced in the 1930s.<ref name="locbook">{{cite book|last=Ransome-Wallis|first=Patrick |title=Illustrated Encyclopedia of World Railway Locomotives|url=https://books.google.com/books?id=rsOYinjYGCkC|year=2012|publisher=Dover Publications|isbn=978-0-486-41247-4|page=64}}</ref>

==Overview==
[[File:Fluid coupling make Transfluid Industrial Transmission model KPTO.JPG|right|thumb|250px|Fluid coupling on Transfluid's industrial transmission model KPTO]]
A fluid coupling consists of three components, plus the [[hydraulic fluid]]:
* The housing, also known as the ''shell''<ref name="gloss"/> (which must have an oil-tight seal around the drive shafts), contains the fluid and turbines.
* Two turbines (fanlike components):
** One connected to the input shaft; known as the ''pump'' or ''impeller'',<ref name="gloss"/> or ''primary wheel'' ''input turbine''.<ref name="gloss"/>
** 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''{{efn|name=GM term|A [[General Motors Corporation|General Motors]] term}}) is rotated by the [[Engine|prime mover]], which is typically an [[internal combustion engine]] or [[electric motor]]. The impeller'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''{{efn|name=GM term}}). 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 [[toroid]]al - 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 [[poloidal]] component
* 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 (i.e. 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 output turbine is locked and full input torque (at the stall speed) is applied. Under stall conditions all of the engine's power at that speed 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.<ref>{{cite book|last=Bolton|first=William F. |title=Railwayman's Diesel Manual: A Practical Introduction to the Diesel-powered Locomotive, Railcar and Multiple-unit Powered Train for Railway Staff and Railway Enthusiasts|url=https://books.google.com/books?id=i1TznQEACAAJ|year=1963|edition=4th|publisher=Ian Allan Publishing|isbn=978-0-7110-3197-5|pages=97–98}}</ref>

===Slip===
A fluid coupling cannot develop output torque when the input and output angular velocities are identical.<ref name="slip">[http://www.voithturbo.com/anfahrkupplungen_faq_sat.php?language=e&id=1 Why is the output speed of a turbo coupling always lower than the input speed?] ''voithturbo.com'' from [http://www.voithturbo.com/fluid-couplings_faq.php Voith - Fluid couplings FAQ]</ref> 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.<ref name="slip" /> Generally speaking, multi-grade [[motor oil]]s or [[automatic transmission fluid]]s are used. Increasing density of the fluid increases the amount of [[torque]] that can be transmitted at a given input speed.<ref name="flu">[http://www.voithturbo.com/anfahrkupplungen_faq_sat.php?language=e&id=8 Does the type of operating fluid influence the transmission behaviour?] ''voithturbo.com'' from [http://www.voithturbo.com/fluid-couplings_faq.php Voith - Fluid couplings FAQ]</ref> 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 brake]]s, 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">[http://www.voithturbo.com/fluid-couplings_glossary.php Fluid couplings glossary] ''voithturbo.com''</ref>

===Scoop control===
Correct operation of a fluid coupling depends on it being correctly filled with fluid. An under-filled coupling will be unable to transmit the full torque, and the limited fluid volume is also likely to overheat, often with damage to the seals.

If a coupling is deliberately designed to operate safely when under-filled, usually by providing an ample fluid reservoir which is not engaged with the impeller, then controlling its fill level may be used to control the torque which it can transmit, and in some cases to also control the speed of a load.{{efn|Where the torque needed to drive a load is proportionate to its speed.}}

Controlling the fill level is done with a 'scoop', a non-rotating pipe which enters the rotating coupling through a central, fixed hub. By moving this scoop, either rotating it or extending it, it scoops up fluid from the coupling and returns it to a holding tank outside the coupling. The oil may be pumped back into the coupling when needed, or some designs use a gravity feed - the scoop's action is enough to lift fluid into this holding tank, powered by the coupling's rotation.

Scoop control can be used for easily managed and stepless control of the transmission of very large torques. The [[Fell locomotive|Fell diesel locomotive]], a British experimental diesel railway locomotive of the 1950s, used four engines and four couplings, each with independent scoop control, to engage each engine in turn. It is commonly used to provide [[variable speed drive#Hydraulic adjustable speed drives|variable speed drive]]s.<ref>{{Cite web
|title=Variable Speed Coupling: Type SC
|website=Fluidomat
|url=http://fluidomat.com/products/type-sc.html
|access-date=2018-07-02
|archive-date=2019-04-07
|archive-url=https://web.archive.org/web/20190407204836/http://fluidomat.com/products/type-sc.html
|url-status=dead
}}</ref><ref>[https://turboresearch.wordpress.com/2010/02/04/variable-speed-fluid-drives-for-pumps/ Variable Speed Fluid Drives for Pumps]</ref>

==Applications==
===Industrial===
Fluid couplings are used in many industrial application involving rotational power,<ref>[http://www.voithturbo.com/start-up_drive-solutions_industries.htm Industry/Sector] Industrial and other uses of fluid couplings ''voithturbo.com''</ref><ref>[http://www.voithturbo.com/start-up_drive-solutions_process.htm Process] Uses of fluid coupling by process ''voithturbo.com''</ref> especially in machine drives that involve high-inertia starts or constant cyclic loading.

===Rail transportation===
Fluid couplings are found in some [[Diesel locomotive]]s 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 unit]]s which contain various combinations of fluid couplings and torque converters.

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

In [[automotive]] applications, the pump typically is connected to the [[flywheel]] of the [[internal combustion engine|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 (mechanics)|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 Company|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 [[Daimler Majestic|Majestic]]. Daimler and [[Alvis Cars|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 [[Daimler-Benz DB 601|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 R-3350|Wright turbo-compound]] reciprocating engine, in which three power recovery turbines extracted approximately 20 percent of the energy or about {{convert|500|hp}} 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]].

==Calculations==
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 does not fluctuate to a great degree. The torque transmitting capacity of any hydrodynamic coupling can be described by the expression <math>r\,N^2D^5</math>, where <math>r</math> is the mass density of the fluid (kg/m<sup>3</sup>), <math>N</math> is the impeller speed ([[Revolutions per minute|rpm]]), and <math>D</math> is the impeller diameter ([[Meter|m]]).<ref name="Bosch, Torque converter" >{{cite book
|title=Hydrodynamic couplings and converters
|series=Automotive Handbook
|year=1993
|publisher=[[Robert Bosch]]
|edition=3rd
|pages=539
|isbn=0-8376-0330-7
|ref=Bosch Automotive Handbook, 3rd ed
}}</ref> In the case of automotive applications, where loading can vary to considerable extremes, <math>r\,N^2D^5</math> 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 in automobiles|fuel economy]].

==Manufacture==
Fluid couplings are relatively simple components to produce. For example, the turbines can be aluminium 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]],<ref>[http://voith.com/en/products-services/power-transmission/fluid-couplings-10048.html Voith: Fluid Coulings], ''voith.com''</ref> Transfluid,<ref>[http://www.transfluid.eu Transfluid: Fluid couplings], ''transfluid.eu''</ref> TwinDisc,<ref>[http://www.twindisc.com/IndustrialProducts/IndFC.aspx?pid=154 TwinDisc: Fluid couplings] {{webarchive|url=https://archive.today/20130205143337/http://www.twindisc.com/IndustrialProducts/IndFC.aspx?pid=154 |date=2013-02-05 }}, ''twindisc.com''</ref> [[Siemens]],<ref>[http://www.automation.siemens.com/mcms/mechanical-drives/en/couplings/hydrodynamic-couplings/Pages/Default.aspx Siemens: Hydrodynamic couplings] {{Webarchive|url=https://web.archive.org/web/20090302044926/http://www.automation.siemens.com/mcms/mechanical-drives/en/couplings/hydrodynamic-couplings/Pages/Default.aspx |date=2009-03-02 }}, ''automation.siemens.com''</ref> Parag,<ref>{{cite web|url=http://www.fluid-coupling.com/|title=fluid-coupling -|website=fluid-coupling|access-date=16 April 2018}}</ref> Fluidomat,<ref>[http://www.fluidomat.com/ Fluidomat] ''fluidomat.com''</ref> Reuland Electric<ref>{{cite web|url=http://www.reuland.com/|title=Welcome to Reuland|website=www.reuland.com|access-date=16 April 2018}}</ref> and TRI Transmission and Bearing Corp.<ref>[http://www.turboresearch.com/products/fluid-drives-couplings TRI Transmission and Bearing Corp] ''turboresearch.com''</ref>

==Patents==
; 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.


{| class="wikitable"
|-
! Patent number
! Publication date
! Inventor
! Link
|-
| GB190906861
| 02 Dec 1909
| Hermann Föttinger
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=GB&NR=190906861A&KC=A&FT=D&ND=3&date=19091202&DB=worldwide.espacenet.com&locale=en_EP]
|-
| US1127758
| 09 Feb 1915
| Jacob Christian Hansen-Ellehammer
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=1127758A&KC=A&FT=D&ND=3&date=19150209&DB=worldwide.espacenet.com&locale=en_EP]
|-
| US1199359
| 26 Sep 1916
| Hermann Föttinger
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=1199359A&KC=A&FT=D&ND=3&date=19160926&DB=worldwide.espacenet.com&locale=en_EP]
|-
| US1472930
| 06 Nov 1923
| Fritz Mayer
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=1472930A&KC=A&FT=D&ND=3&date=19231106&DB=worldwide.espacenet.com&locale=en_EP]
|-
| GB359501
| 23 Oct 1931
| Voith
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=GB&NR=359501A&KC=A&FT=D&ND=3&date=19311023&DB=worldwide.espacenet.com&locale=en_EP]
|-
| US1937364
| 28 Nov 1933
| Harold Sinclair
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=1937364A&KC=A&FT=D&ND=3&date=19331128&DB=EPODOC&locale=en_EP]
|-
| US1987985
| 15 Jan 1935
| Schmieske and Bauer
| [http://worldwide.espacenet.com/searchResults?compact=false&ST=singleline&query=US1987985&locale=en_EP&DB=worldwide.espacenet.com]
|-
| US2004279
| 11 Jun 1935
| Hermann Föttinger
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2004279A&KC=A&FT=D&ND=3&date=19350611&DB=worldwide.espacenet.com&locale=en_EP]
|-
| US2127738
| 23 Aug 1938
| Fritz Kugel
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2127738A&KC=A&FT=D&ND=3&date=19380823&DB=worldwide.espacenet.com&locale=en_EP]
|-
| US2202243
| 28 May 1940
| Noah L Alison
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2202243A&KC=A&FT=D&ND=3&date=19400528&DB=worldwide.espacenet.com&locale=en_EP]
|-
| US2264341
| 02 Dec 1941
| Arthur and Sinclair
| [http://worldwide.espacenet.com/publicationDetails/biblio?DB=worldwide.espacenet.com&II=0&ND=3&adjacent=true&locale=en_EP&FT=D&date=19411202&CC=US&NR=2264341A&KC=A]
|-
| US2491483
| 20 Dec 1949
| Gaubatz and Dolza
| [http://worldwide.espacenet.com/searchResults?compact=false&ST=singleline&query=US2491483&locale=en_EP&DB=worldwide.espacenet.com]
|-
| US2505842
| 02 May 1950
| Harold Sinclair
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2505842A&KC=A&FT=D&ND=3&date=19500502&DB=worldwide.espacenet.com&locale=en_EP]
|-
| US2882683
| 21 Apr 1959
| Harold Sinclair
| [http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2882683A&KC=A&FT=D&ND=3&date=19590421&DB=worldwide.espacenet.com&locale=en_EP]
|}

==See also==
* [[Torque amplifier]]
* [[Torque converter]]
* [[Water brake]]

==Notes==
{{notelist}}

==References==
{{reflist}}

==External links==
* Fluid Coupling, The Principles of Operation, film [https://www.youtube.com/watch?v=leCEmJA0WsI]

{{Powertrain}}

{{DEFAULTSORT:Fluid Coupling}}
[[Category:Rotating shaft couplings]]
[[Category:Mechanical power transmission]]
[[Category:Automotive transmission technologies]]

Fluid coupling - Revision history

imported>InternetArchiveBot: Rescuing 1 sources and tagging 0 as dead.) #IABot (v2.0.9.5