Fluid Coupling
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A fluid coupling or hydraulic coupling is a
hydrodynamic In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in moti ...
or 'hydrokinetic' device used to transmit rotating mechanical power.Fluid coupling
''encyclopedia2.thefreedictionary.com''
It has been used in
automobile A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, peopl ...
transmission Transmission or transmit may refer to: Science and technology * Power transmission ** Electric power transmission ** Transmission (mechanical device), technology that allows controlled application of power *** Automatic transmission *** Manual tra ...
s as an alternative to a mechanical
clutch A clutch is a mechanical device that allows an output shaft to be disconnected from a rotating input shaft. The clutch's input shaft is typically attached to a motor, while the clutch's output shaft is connected to the mechanism that does th ...
. 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 A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy ( hydrostatic energy i.e. flow, pressure). Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. They gen ...
and
motor An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gene ...
combinations.


History

The fluid coupling originates from the work of Hermann Föttinger, who was the chief designer at the AG Vulcan Works in
Stettin Szczecin ( , , ; ; ; or ) is the capital and largest city of the West Pomeranian Voivodeship in northwestern Poland. Located near the Baltic Sea and the German border, it is a major seaport, the largest city of northwestern Poland, and se ...
. His patents from 1905 covered both fluid couplings and
torque converter A torque converter is a device, usually implemented as a type of fluid coupling, that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the ...
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 Following Sinclair's discussions with the London General Omnibus Company begun in October 1926, and trials on an Associated Daimler bus chassis,
Percy Martin Percy Martin (1871-1958) was an American-born British engineer and automobile manufacturer. Born in Columbus, Ohio 19 June 1871 he obtained a degree in mechanical engineering, specializing in electrical engineering, from Ohio State University in 1 ...
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 and in 1934 was featured in the Singer Eleven branded as Fluidrive. These couplings are described as constructed under Vulcan-Sinclair and Daimler patents. In 1939
General Motors Corporation General Motors Company (GM) is an American multinational automotive manufacturing company headquartered in Detroit, Michigan, United States. The company is most known for owning and manufacturing four automobile brands: Chevrolet, Buick, GMC, ...
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 locomotive A diesel locomotive is a type of railway locomotive in which the prime mover (locomotive), power source is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is con ...
s using fluid couplings were also produced in the 1930s.


Overview

A fluid coupling consists of three components, plus the
hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backho ...
: * The housing, also known as the ''shell'' (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'', or ''primary wheel'' ''input turbine''. ** The other connected to the output shaft, known as the ''turbine'', ''output turbine'', ''secondary wheel'' or ''runner'' The driving turbine, known as the 'pump', (or ''driving torus'') is rotated by the
prime mover Prime mover may refer to: Philosophy *Unmoved mover, a concept in Aristotle's writings Engineering * Prime mover (engine or motor), a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc.) into ener ...
, which is typically an
internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...
or
electric motor An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...
. The impeller's motion imparts both outwards linear and rotational motion to the fluid. The
hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backho ...
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
toroid In mathematics, a toroid is a surface of revolution with a hole in the middle. The axis of revolution passes through the hole and so does not intersect the surface. For example, when a rectangle is rotated around an axis parallel to one of its ...
al - travelling in one direction on paths that can be visualised as being on the surface of a
torus In geometry, a torus (: tori or toruses) is a surface of revolution generated by revolving a circle in three-dimensional space one full revolution about an axis that is coplanarity, coplanar with the circle. The main types of toruses inclu ...
: * 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.


Slip

A fluid coupling cannot develop output torque when the input and output angular velocities are identical.Why is the output speed of a turbo coupling always lower than the input speed?
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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 In fluid dynamics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between Inertia, inertial and viscous forces. At low Reynolds numbers, flows tend to ...
.


Hydraulic fluid

As a fluid coupling operates kinetically, low-
viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
fluids are preferred. Generally speaking, multi-grade
motor oil Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, deterge ...
s or
automatic transmission fluid Automatic transmission fluid (ATF) is a hydraulic fluid that is essential for the proper functioning of vehicles equipped with Automatic transmission, automatic transmissions. Usually, it is coloured red or green to differentiate it from motor ...
s are used. Increasing density of the fluid increases the amount of
torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
that can be transmitted at a given input speed.Does the type of operating fluid influence the transmission behaviour?
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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 The viscosity index (VI) is an arbitrary, unit-less measure of a fluid's change in viscosity relative to temperature change. It is mostly used to characterize the viscosity-temperature behavior of lubricating oils. The lower the VI, the more the ...
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''.Fluid couplings glossary
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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. 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 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 drives.


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 locomotive A diesel locomotive is a type of railway locomotive in which the prime mover (locomotive), power source is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is con ...
s as part of the power transmission system. Self-Changing Gears made semi-automatic transmissions for British Rail, and
Voith The Voith Group is a global technology company. With its broad portfolio of systems, products, services and digital applications, Voith trades in the markets of energy, paper, raw materials and transport. Founded in 1867, Voith today has aroun ...
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 transmission A semi-automatic transmission is a multiple-speed Transmission (mechanics), transmission where part of its operation is Automation, automated (typically the actuation of the clutch), but the driver's input is still required to launch the vehicle f ...
s and
automatic transmission An automatic transmission (AT) or automatic gearbox is a multi-speed transmission (mechanics), transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions. The 1904 ...
s. 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 A flywheel is a mechanical device that uses the conservation of angular momentum to store rotational energy, a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, a ...
of the
engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ge ...
—in fact, the coupling's enclosure may be part of the
flywheel A flywheel is a mechanical device that uses the conservation of angular momentum to store rotational energy, a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, a ...
proper, and thus is turned by the engine's
crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...
. The turbine is connected to the input shaft of the
transmission Transmission or transmit may refer to: Science and technology * Power transmission ** Electric power transmission ** Transmission (mechanical device), technology that allows controlled application of power *** Automatic transmission *** Manual tra ...
. While the transmission is in gear, as engine speed increases,
torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
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 A clutch is a mechanical device that allows an output shaft to be disconnected from a rotating input shaft. The clutch's input shaft is typically attached to a motor, while the clutch's output shaft is connected to the mechanism that does th ...
driving a
manual transmission A manual transmission (MT), also known as manual gearbox, standard transmission (in Canadian English, Canada, British English, the United Kingdom and American English, the United States), or stick shift (in the United States), is a multi-speed ...
. 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 Alvis is a given name and a surname (close to the uncommon Scottish surname Alves). Alvis may also refer to: *Alvi, a Muslim community in South Asia, who claims descent from the fourth Rashidun caliph, Ali ibn Abi Talib *Alvis Car and Engineering ...
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 Centrifugal compressors, sometimes called impeller compressors or radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery. They achieve pressure rise by adding energy to the continuous flow of fluid through th ...
and the Wright turbo-compound reciprocating engine, in which three power recovery turbines extracted approximately 20 percent of the energy or about 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 A propeller (often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon a working flu ...
.


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 r\,N^2D^5, where r is the mass density of the fluid (kg/m3), N is the impeller speed (
rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to hertz. Standards ISO 80000-3:2019 def ...
), and D is the impeller diameter ( m). In the case of automotive applications, where loading can vary to considerable extremes, r\,N^2D^5 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.


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 The Voith Group is a global technology company. With its broad portfolio of systems, products, services and digital applications, Voith trades in the markets of energy, paper, raw materials and transport. Founded in 1867, Voith today has aroun ...
, Transfluid, TwinDisc,
Siemens Siemens AG ( ) is a German multinational technology conglomerate. It is focused on industrial automation, building automation, rail transport and health technology. Siemens is the largest engineering company in Europe, and holds the positi ...
, Parag, Fluidomat, Reuland Electric and TRI Transmission and Bearing Corp.TRI Transmission and Bearing Corp
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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.


See also

*
Torque amplifier A torque amplifier is a mechanical device that amplifies the torque of a rotating shaft without affecting its rotational speed. It is mechanically related to the capstan (nautical), capstan seen on ships. Its most widely known use is in power steer ...
*
Torque converter A torque converter is a device, usually implemented as a type of fluid coupling, that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the ...
* Water brake


Notes


References


External links

* Fluid Coupling, The Principles of Operation, fil

{{DEFAULTSORT:Fluid Coupling Rotating shaft couplings Mechanical power transmission Automotive transmission technologies