Torque Converter

 

Torquee cconverter Torqu onverter A torque converter is a type of fluid coupling which 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 torque converter connects the power source   to the loa source load. d. It is usually located between the engine's flexplate and the transmission. The equivalent location in a manual transmission would be the mechanicall clu mechanica clutch. tch. The key key characteristic characteristic of a torque converter is its ability to multiply torque when the output rotational rotational speed is so low that that it allows the fluid coming off the curved vanes of the turbine turbine to be deflected off the stator while it is locked against its one-way clutch, thus thus providing the equivalent of a reduction gear. This is a feature beyond that of the  the  simple fluid co coupling, upling, which can match rotational speed but does not multiply multip ly torqu torque, e, thus reduces power. Some of these devices are are also equipped with a "lockup" mechanism which which rigidly binds the engine to the transmission when their speeds are nearly equal, equ al, to avoid

converter nverter cut-away ZF torque co

slippage and a resulting loss of efficiency. ficiency.

Contents Hydraulic systems Mechanical systems Usage Function Theory of Operation Torque Torqu e co converter nverter elements

A cut-away model of a torque converter

Operational phases Efficiency and torque multiplication Lock-up torque Lock-up torqu e converters Capacity and failure modes failure modes

Manufacturers Current Past See also References External links

Hydraulic systems By far the most common form of torque converter in automobile transmissions is the hydrokinetic device described in this article. There are also hydrostatic systems which are widely used in small machines such ascompact excavators.

Mechanical systems

 

There are also mechanical designs for continuously variable variable transmissions and these also have the ability to multiply torque. They include the pendulum-basedConstan pendulum-basedConstantinesco tinesco torque t orque con converter verter, the Lambert fr friction iction g gearing earing disk di sk drive tr transmissi ansmission onand the Varioma Variomatic tic with expanding pulleys and a belt drive.

Usage Automatic transmissionson automobiles, automobiles, such as cars, buses, and on/off highway trucks. Forwarders and other heavy hea vy duty vehicles. Marine propulsion systems. Industrial power transmission such asconveyor asconveyor drives, almost all modern forklifts, winches  winches,, drilling rigs, rigs, construction equipment, equipm ent, and railway railwa y locomo locomotives tives..

Function Theory of Operation Torque converter equations of motion are dominated byLeonhard Euler's Euler's eighteenth century turbomachine equation:

The equation expands to include the fifth power of radius; as a result, r esult, torque converter properties are very dependent on the size of the device.

Torque converter elements A fluid coupling is a two element drive that is incapable of multiplying torque, while a torque converter has at least one extra element  —the stator—which alters the drive's characteristics characteristics during periods of high slippage, producing an increas increasee in output torque. In a torque converter there are at least three rotating elements: the impeller, impeller, which is mechanically mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator . In practice, however however,, the stator is mounted on an overrunning clutch, which prevents the stator from counter coun ter-rotat -rotating ing with resp respect ect to the prime mover but allows allo ws forward rotation ro tation.. Modifications to the basic three element design have been periodically incorporated, especially in applications where higher than normal torque multiplication is required. Most commonly, commonly, these have taken the form of multiple turbines and stators, each set being designed to produce differing amounts of torque multiplication. For example, the Buick Dynaflow automatic transmission was a nonshifting design and, under normal conditions, relied solely upon the converter to multiply torque. The Dynaflow used a five element converter to produce the wide range of torque multiplication needed to propel a heavy vehicle. Although not strictly a part of classic torque converter design, many automotive converters include a lock-up clutch to improve cruising power transmission efficiency efficiency and reduce heat. The application of the clutch locks the turbine to the impeller, impeller, causing all power transmission to be mechanical, thus eliminating losses associated with fluid drive.

Operational phases A torque converter has three stages of operation:

Stall. The prime mover is applying power to the impeller but the turbine cannot rotate. For example, in an automobile, this stage of operation would occur when the driver has placed the transmission transmission in gear but is preventing the vehicle from moving by continuing to apply thebrakes. At stall, the torque converter can produce maximum

 

torque multiplication if sufficient ficient input power is applied (the resulting multiplication is called thestall ratio). ratio). The stall phase actually lasts for a brief period when the load (e.g., vehicle) initially starts to move, as there will be a very larg difference between pump and turbine speed. ference between impeller and turbine Acceleration. The load is accelerating but there still is a relatively large dif speed. Under this condition, the converter will produce torque multiplication that is less than what could be achieved under stall conditions. The amount of multiplication will depend upon the actual dif ference between pump and turbine speed, as well as various other design factors. .T Torque orque multiplication m ultiplication has Coupling. The turbine has reached approximately 90 percent of the speed of the impeller essentially ceased and the torque converter is behaving in a manner similar to a simple fluid coupling. In modern automotive applications, it is usually at this stage of operation where the lock-up clutch is applied, a procedure that tends to improve fuel efficiency efficiency.. The key to the torque converter's ability to multiply torque lies in the stator. In the classic fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the impeller to oppose the direction of impeller rotation, leading to a significant loss of efficiency and the generation of considerable waste heat. Under the same condition in a torque converter, the returning fluid will be redirected by the stator so that it aids the rotation of the impeller, instead of impeding it. The result is that much of the energy in the returning fluid is recovered and added to the energy being applied to the impeller by the prime mover mover.. This action causes causes a substantial increa increase se in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is initially traveling in a direction opposite to impeller rotation, the stator will likewise attempt to counter-rotate counter-rotate as it forces the fluid to change direction, an efffect ect that is pr prevented evented by the one-way stator clutch clutch.. Unlike the radially straight blades used in a plain fluid coupling, a torque converter's turbine and stator use angled and curved blades. The blade shape of the stator is what alters the path of the fluid, forcing it to coincide with the impeller rotation. The matching curve of the turbine blades helps to correctly direct the returning fluid to the stator so the latter can do its job. The shape of the blades is important as minor variations can result in significant changes to the converter's performance. During the stall and acceleration phases, in which torque multiplication occurs, the stator remains stationary due to the action of its one-way clutch. clutch. However, However, as the torque converter approaches the coupling phase, the energy and volume of the fluid returning from the turbine will gradually decrease, causing pressure on the stator to likewise decrease. Once in the coupling phase, the returning fluid will reverse direction direction and now rotate in the direction of the impeller and turbine, an effect which will attempt to forward-rotate the stator stat or.. At thi thiss point, the stator stat or clutch clut ch will relea r elease se and the impelle impellerr, turbi turbine ne and st stator ator wil willl all (more or less) l ess) turn tur n as a unit unit.. Unavoidably, some of the fluid's kinetic energy will be lost due to friction and turbulence, causing the converter to generate waste heat (dissipated in many applications by water cooling). This effect, often referred to as pumping loss, will be most pronounced at or near stall conditions. In modern designs, the blade geometry minimizes oil velocity at low impeller speeds, which allows the turbine to be stalled for long periods with little danger of overheating (as when a vehicle with an automatic transmission is stopped at a traf fic signal or in traf tr affic fic congestion while still in gear).

Efficiency and torque multiplication A torque converter cannot achieve 100 percent coupling ef ficiency ficiency.. The classic three element torque converter has an ef ficiency curve that resembles ∩: zero efficiency at stall, generally increasing efficiency during the acceleration phase and low efficiency in the coupling phase. The loss of efficiency as the converter enters the coupling phase is a result of the turbulence and fluid flow interference generated by the stator, and as previously mentioned, is commonly overcome by mounting the stator on a one-way clutch. Even with the benefit of the one-way stator clutch, a converter cannot achieve the same level of efficiency in the coupling phase as an equivalently sized fluid coupling. Some loss is due to the presence of the stator (even though rotating as part of the assembly), as as it always generates generates some power-absorbing turbulen turbulence. ce. Most of the loss, however, however, is caused by the curved and angled turbine blades, which do not absorb kinetic energy from the fluid mass as well as radially straight blades. Since the turbine blade geometry is a crucial factor in the converter's ability to multiply torque, trade-offs between torque multiplication and coupling efficiency are inevitable. In automotive applications, where steady improvements in fuel economy have been mandated by market forces and government edict, the nearly universal use of a lock-up clutch has helped to eliminate the converter from the efficiency equation during cruising operation.

 

The maximum amount of torque multiplication produced by a converter is highly dependent on the size and geometry of the turbine and stator blades, and is generated only when the converter is at or near the stall phase of operation. Typical stall torque multiplication ratios range from 1.8:1 to 2.5:1 for most automotive applications (although multi-elem multi-element ent des designs igns as used in the Buick Dynaflow and Chevrolet Turboglide could produce more). Specialized converters designed for industrial, rail, or heavy marine power transmission systems are capable of as much as 5.0:1 multiplication. Generally speaking, there is a trade-off between maximum torque multiplication and efficiency—high stall ratio converters tend to be relatively inef inefficient ficient below the coupling speed, whereas low stall ratio converters tend to provide less possible torque multiplication. The characteristics of the torque converter must be carefully matched to the torque curve of the power source and the intended application. Changing the blade geometry of the stator and/or turbine will change the torque-stall characteristics, as well as the overall efficiency of the unit. For example, drag racing automatic automatic transmissions often use converters modified to produce high stall speeds to improve off-the-line torque, and to get into the power band of the engine more quickly. Highway vehicles generally use lower stall torque converters to limit heat production, and provide a more firm feeling to the vehicle's characteristics. A design feature once found in someGeneral someGeneral Motors automatic transmissions was the variable-pitch stator, in which the blades' angle of attack could be varied in response to changes in engine speed and load. The effect of this was to vary the amount of torque multiplication produced produced by the converter. converter. At the normal angle of attack, the stator caused the converter converter to produce a moderate amount of multiplication but with a higher level of efficiency. If the driver abruptly opened the throttle, a valve would switch the stator pitch to a different angle of o f attack, iincreasi ncreasing ng torque torqu e multipl ultiplication ication at the t he expense of ef e fficiency. iciency. Some torque converters use multiple stators and/or multiple turbines to provide a wider range of torque multiplication. Such multipl element converters are more common in industrial industrial environments than in automotive transmissions, but automotive applications such as Buick's Triple Turbine Dynaflow and Chevrolet's Chevrolet's Turboglide also existed. The T he Buick Dynaflow utilized the torque-multiplying characteristics of its planetary gear set in conjunction with the torque converter for low gear and bypassed the first turbine, using only the second turbine as vehicle speed increased. The unavoidable trade-off with this arrangement was low efficiency and eventually these transmissions were discontinued in favor of the more efficient three speed units with a conventional three element torque converter.. It is also found that efficiency of torque converter is maximum at very low speeds. converter

Lock-up torque converters As described above, impelling losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that that physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and virtually no power loss. The first automotive application application of the lock-up principle was Packard's Packard's Ultramatic transmission, introduced in 1949, which locked up the converter at cruising speeds, unlocking when the throttle was floored for quick acceleration or as the vehicle slowed down. This feature was also present in someBorg-W someBorg-Warner arner transmissions produced during the 1950s. It fell out of favor in subsequent years due to its extra complexity and cost. In the late 1970s lock-up clutches started to reappear in response to demands for improved fuel economy,, and are now nearly universal in automotive applications. economy

Capacity and failure modes As with a basic fluid coupling the theoretical torque capacity of a converter is proportional to of the fluid (kg/m³),

is the impeller speed (rpm), and

, where

is the mass density

is the diameter(m).[1] In practice, the maximum torque capacity is limited

by the mechanical characteristics of the materials used in the converter's components, as well as the ability of the converter to dissipate heat heat (often through water cooling). As an aid to strength, reliability and economy of production, most automotive converter housings are of welded construction. Industrial units are usually assembled with bolted housings, a design feature that eases the process of inspection and repair, but adds to the cost of producing the converter .

 

In high performance, racing and heavy duty commercial converters, the pump and turbine may be further strengthened by a process called furnace brazing, brazing, in which molten brass is drawn into seams and joints to produce a stronger bond between the blades, hubs and annular ring(s). Because the furnace brazing process creates a small radius at the point where a blade meets with a hub or annular ring, a theoretical decrease in turbulence will occur , resulting in a corresponding increase in effficiency. iciency. Overloading a converter can result in several failure modes, some of them potentially dangerous in nature:

Overheating: Continuous high levels of slippage may overwhelm the converter's ability to dissipate heat, resulting in damage to the elastomer seals that retain fluid inside the converter. This will cause the unit to leak and eventually stop functioning due to lack of fluid. theone-way stator clutchbecome permanently locked Stator clutch seizure: The inner and outer elements of theone-way together,, thus preventing the stator from rotating during the coupling phase. Most together M ost often, seizure is precipitated by severe loading and subsequent distortion of the clutch components. Eventually , galling of the mating parts occurs,  which triggers seizure. A converter with a seizedstator clutch will will exhibit very poor efficiency ficien cy during tthe he coupling phase, and in a motor vehicle, fuel consumption will drastically increase. Converter overheating under such conditions will usually occur if continued operation is attempted. clutch, resulting in Stator clutch breakage: A very abrupt application of power can cause shock loading of thestator clutch, breakage. If this occurs, the stator will freely counter-rotate in the direction opposite to that of the pump and almost no power transmission will take place. In an automobile, the ef fect is similar to a severe case of transmission slippage and the vehicle is all but incapable of moving under its own power . , pump Blade deformation and fragmentation: If subjected to abrupt loading or excessive heating of the converter and/or turbine blades may be deformed, separated from their hubs and/or annular rings, or may break up into fragments. At the least, such a failure will result in a significant loss of ef ficiency, ficiency, producing symptoms similar (although less pronounced) to those accompanying stator clutch failure. In extreme cases, catastrophic destruction of the converter will occur. Ballooning: Prolonged operation under excessive loading, very abrupt application of load, or operating a torque converter at very high RPM may cause the shape of the converter's conver ter's housing to be physically distorted due to inter internal nal pressure and/or the stress imposed by inertia. Under extreme conditions, ballooning will cause the converter housin to rupture, resulting in the violent dispersal of hot oil and metal fragments over a wide area.

Manufacturers Current Valeo, produces pr oduces Torque converter conver ter ffor or For Ford, d, GM, Mazda, Subaru Subar u Allison Transmission Transmission,, used in bus, refuse, fire, construction, distribution, military and specialty applications BorgWarner, BorgW arner, used in automobiles Subaru,, used in automobiles Subaru Isuzu, used in automobiles Twin Disc, Disc, used in vehicle, marine and oilfield applications Voith TurboTurbo-T Transmissions, ransm issions, used in many diesel locomot l ocomotives ives and diesel mu multiple ltiple units u nits ZF Friedrichshafen, automobiles, forestry for estry machines, popular in citybus applications Jatco, used in automobiles Aisin AW, AW, used in automobiles LuK USA LLC L LC,, produces prod uces Torque Converters Conver ters fo forr Ford, GM, Allison llison,, and Hyundai Hy undai Exedy,, used in automobiles Exedy

Past Lysholm-Smith, named after its inventor, Alf Lysholm,[2] produced by Leyland Motors and used in buses from 1933-9 1 933-9 and also some British Rail Derby Lightweightand Ulster Transport Authority diesel multiple units Mekydro,[3] used in British Rail Class 35 Hymek  locomotives.  locomotives. Packard,, used in the Ultramatic automobile transmission Packard tra nsmission system Rolls-Royce RollsRoyce (Twin (Twin Disc) Disc ), used in s some ome British U United nited Tractio Traction n diesel m multip ultiple le units Vickers-Coates[4]

 

See also Clutch

Torque amplifier

Fluid coupling

Water brake

Servomechanism

References ed.).Robert Bosch. Bosch. p. 539. ISBN ISBN  0-8376-03301. Hydrodynamic couplings and converters. Automotive Handbook (3rd ed.).Robert 7. 2. "Espacenet - Original document"(http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR =1900118A&KC=A&FT=D&ND=3&date=19330307&DB=worldwide.espacenet.com&locale=en_EP) . Worldwide.espac Worldwide .espacenet. enet.com. com. 1933-03-0 1933 -03-07 7. Retrieved Retriev ed 20142014-07-21. 07-21. 3. "Archiv "Archived ed copy" (https://web (https ://web.archi .archive.org ve.org/web/2 /web/201003 0100302142 02142045/ht 045/http:// tp://www www .intertrain .inter trains.co.u s.co.uk/glos k/glossary/m sary/m/meky /mekydro-tr dro-tran an origi nal (http://www (http: //www.inter .intertrain trains.co.u s.co.uk/glos k/glossary/m sary/m/mek /mekydro-trans ydro- transmission mission.html) .html)on on smission.html).. Archived from the original smission.html) 2010-03-02. Retrieved 2009-10-31. 4. [1] (https://news.google.com/newspapers?nid=1301&dat=19300220&id=8_RjAAAAIBAJ&sjid=0JYDAAAAIBA (https://news.google.com/newspapers?nid=1301&dat=19 300220&id=8_RjAAAAIBAJ&sjid=0JYDAAAAIBAJ&pg= J&pg= 7237,1332667)

External links HowStuffWorks HowStuf fWorks article on torque converters YouTube ouTube video about torque converters Retrieved Retrie ved from "https: "https://en. //en.wikiped wikipedia.org ia.org/w/ind /w/index.ph ex.php?titl p?title=T e=T o orque_conver rque_ converter&ol ter&oldid=8 did=837525 37525755 755"

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