Fluid coupling
Encyclopedia
A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. It has been used in automobile
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/or controlled start-up without shock loading of the power transmission system is essential.
, who was the chief designer at the AG Vulcan Works in Stettin. His patents from 1905 covered both fluid couplings and torque converter
s.
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.
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 locomotive
s using fluid couplings were also produced in the 1930s
The driving turbine, known as the 'pump', (or driving torusA General Motors term) 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
:
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.
The very best efficiency a fluid coupling can achieve is 94 percent, that is for every 100 revolutions input, there will be 94 revolutions output. Like other fluid dynamical devices, its efficiency tends to increase gradually with increasing scale, as measured by the Reynolds number.
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.
s as part of the power transmission system. Self-Changing Gears
made semi-automatic transmissions for British Rail, and Voith
manufacture turbo-transmissions for railcar
s and diesel multiple unit
s which contain various combinations of fluid couplings and torque converters.
s and automatic transmission
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
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 behavior of the fluid coupling strongly resembles that of a mechanical clutch
driving a manual transmission
.
Fluid flywheels, as distinct from torque converter
s, 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.
reciprocating engine, in which three power recovery turbines extracted approximately 20 percent of the energy or about 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
.
, where 
is the mass density of the fluid, 
is the impeller speed, and 
is the impeller diameter. 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
.
Manufacturers of industrial fluid couplings include Voith
, Transfluid, TwinDisc, Siemens
, PARAG, Fluidomat, and Reuland Electric.
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
transmission
Transmission (mechanics)
A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...
s as an alternative to a mechanical clutch
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
. 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.
History
The fluid coupling originates from the work of Dr. Hermann FöttingerHermann Föttinger
Hermann Föttinger was a German engineer and inventor. In the course of his life he submitted over 100 patent applications, but he is most notable for inventing fluid coupling....
, who was the chief designer at the AG Vulcan Works in Stettin. His patents from 1905 covered both fluid couplings and torque converter
Torque converter
In modern usage, a torque converter is generally a type of hydrodynamic fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load...
s.
In 1930 Harold Sinclair, working with the Daimler company
Daimler-Benz
Daimler-Benz AG was a German manufacturer of automobiles, motor vehicles, and internal combustion engines; founded in 1926. An Agreement of Mutual Interest - which was valid until year 2000 - was signed on 1 May 1924 between Karl Benz's Benz & Cie., and Daimler Motoren Gesellschaft, which had...
, 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.
In 1939 General Motors Corporation
General Motors
General Motors Company , commonly known as GM, formerly incorporated as General Motors Corporation, is an American multinational automotive corporation headquartered in Detroit, Michigan and the world's second-largest automaker in 2010...
introduced Hydramatic drive
Hydramatic
Hydramatic was an automatic transmission developed by both General Motors' Cadillac and Oldsmobile divisions. Introduced in 1939 for the 1940 model year vehicles, the Hydramatic was the first fully automatic mass-produced transmission developed for passenger automobile use.-History:During the...
, the first fully automatic automotive transmission system installed in a mass produced automobile. The Hydramatic employed a fluid coupling.
The first Diesel locomotive
Diesel locomotive
A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...
s using fluid couplings were also produced in the 1930s
Overview
A fluid coupling consists of three components, plus the hydraulic fluid:- 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 (fan like components):
- One connected to the input shaft; known as the pump or impellor, 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 torusA General Motors term) is rotated by the prime mover
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
, which is typically an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
or electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
. 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
Toroidal
Toroidal describes something which resembles or relates to a torus or toroid:*Torus*Toroid, a doughnut-shaped object which resembles a torus*Toroidal inductors and transformers, a type of electrical device...
- travelling in one direction on paths that can be visualised as being on the surface of a torus
Torus
In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...
:
- If there is a difference between input and output angular velocities the motion has a component which is circular (i.e. 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 (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 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.
Slip
A fluid coupling cannot develop output torque when the input and output angular velocities are identical. 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.The very best efficiency a fluid coupling can achieve is 94 percent, that is for every 100 revolutions input, there will be 94 revolutions output. 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. Generally speaking, multi-grade motor oilMotor oil
Motor oil or engine oil is an oil used for lubrication of various internal combustion engines. The main function is to lubricate moving parts; it also cleans, inhibits corrosion, improves sealing, and cools the engine by carrying heat away from moving parts.Motor oils are derived from...
s or automatic transmission fluid
Automatic Transmission Fluid
Automatic transmission fluid is the fluid used in vehicles with self shifting or automatic transmissions. It is typically colored red or green to distinguish it from motor oil and other fluids in the vehicle...
s are used. Increasing density of the fluid increases the amount of torque that can be transmitted at a given input speed.
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.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 locomotiveDiesel locomotive
A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...
s as part of the power transmission system. Self-Changing Gears
Self-Changing Gears
Self-Changing Gears was a British company, set up and owned equally by Walter Gordon Wilson and John Davenport Siddeley to develop and exploit the Wilson or pre-selector gearbox...
made semi-automatic transmissions for British Rail, and Voith
Voith
The Voith GmbH, which is headquartered in Germany, is a family-run corporation in the mechanical engineering sector with worldwide operations....
manufacture turbo-transmissions for railcar
Railcar
A railcar, in British English and Australian English, is a self-propelled railway vehicle designed to transport passengers. The term "railcar" is usually used in reference to a train consisting of a single coach , with a driver's cab at one or both ends. Some railways, e.g., the Great Western...
s and diesel multiple unit
Diesel multiple unit
A diesel multiple unit or DMU is a multiple unit train consisting of multiple carriages powered by one or more on-board diesel engines. They may also be referred to as a railcar or railmotor, depending on country.-Design:...
s which contain various combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were used in a variety of early semi-automatic transmissionSemi-automatic transmission
A semi-automatic transmission is an automobile transmission that does not change gears automatically, but rather facilitates manual gear changes by dispensing with the need to press...
s and automatic transmission
Automatic transmission
An automatic transmission is one type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually...
s. Since the late 1940s, the hydrodynamic torque converter
Torque converter
In modern usage, a torque converter is generally a type of hydrodynamic fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load...
has replaced the fluid coupling in automotive applications.
In automotive applications, the pump typically is connected to the flywheel
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
of the engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
—in fact, the coupling's enclosure may be part of the flywheel
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
proper, and thus is turned by the engine's crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
. The turbine is connected to the input shaft of the transmission
Transmission (mechanics)
A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...
. While the transmission is in gear, as engine speed increases torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
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
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
driving a manual transmission
Manual transmission
A manual transmission, also known as a manual gearbox or standard transmission is a type of transmission used in motor vehicle applications...
.
Fluid flywheels, as distinct from torque converter
Torque converter
In modern usage, a torque converter is generally a type of hydrodynamic fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load...
s, are best known for their use in Daimler
Daimler Motor Company
The Daimler Motor Company Limited was an independent British motor vehicle manufacturer founded in London by H J Lawson in 1896, which set up its manufacturing base in Coventry. The right to the use of the name Daimler had been purchased simultaneously from Gottlieb Daimler and Daimler Motoren...
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 Majestic
The Daimler Majestic 101 was launched by the Daimler Motor Company of Coventry in July 1958 and was in production until 1962. The six cylinder, four door saloon, with new three speed Borg Warner automatic transmission, power steering and four-wheel disc brakes, vacuum-servo assisted, was very...
. Daimler and Alvis
Alvis Cars
Alvis Car and Engineering Company Ltd was a British manufacturing company that existed in Coventry, England from 19191967. In addition to automobiles designed for the civilian market, the company also produced racing cars, aircraft engines, armoured cars and other armoured fighting vehicles, the...
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.
Aviation
The most prominent use of fluid couplings in aeronautical applications was in the Wright turbo-compoundWright R-3350
The Wright R-3350 Duplex-Cyclone was one of the most powerful radial aircraft engines produced in the United States. It was a twin row, supercharged, air-cooled, radial engine with 18 cylinders. Power ranged from 2,200 to over 3,700 hp , depending on the model...
reciprocating engine, in which three power recovery turbines extracted approximately 20 percent of the energy or about 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
Propeller
A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...
.
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 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. 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 economyFuel economy in automobiles
Fuel usage in automobiles refers to the fuel efficiency relationship between distance traveled by an automobile and the amount of fuel consumed....
.
Manufacture
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
Voith
The Voith GmbH, which is headquartered in Germany, is a family-run corporation in the mechanical engineering sector with worldwide operations....
, Transfluid, TwinDisc, Siemens
Siemens
Siemens may refer toSiemens, a German family name carried by generations of telecommunications industrialists, including:* Werner von Siemens , inventor, founder of Siemens AG...
, PARAG, Fluidomat, and Reuland Electric.