A transmission is a machine in a power transmission system, which provides controlled application of the power. Often the term transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device. In British English, the term transmission refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive), differential, and final drive shafts. In American English, however, the term refers more specifically to the gearbox alone, and detailed usage differs.[note 1] The most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are also used on pedal bicycles, fixed machines, and where different rotational speeds and torques are adapted. Often, a transmission has multiple gear ratios (or simply "gears") with the ability to switch between them as speed varies. This switching may be done manually (by the operator) or automatically. Directional (forward and reverse) control may also be provided. Single-ratio transmissions also exist, which simply change the speed and torque (and sometimes direction) of motor output. In motor vehicles, the transmission generally is connected to the engine crankshaft via a flywheel or clutch or fluid coupling, partly because internal combustion engines cannot run below a particular speed. The output of the transmission is transmitted via the driveshaft to one or more differentials, which drives the wheels. While a differential may also provide gear reduction, its primary purpose is to permit the wheels at either end of an axle to rotate at different speeds (essential to avoid wheel slippage on turns) as it changes the direction of rotation. Conventional gear/belt transmissions are not the only mechanism for speed/torque adaptation. Alternative mechanisms include torque converters and power transformation (e.g. diesel-electric transmission and hydraulic drive system). Hybrid configurations also exist. Automatic transmissions use a valve body to shift gears using fluid pressures in response to speed and throttle input.
1 Explanation 2 Uses 3 Simple 4 Multi-ratio systems
5 Uncommon types
5.1 Dual clutch transmission 5.2 Continuously variable 5.3 Infinitely variable 5.4 Electric variable
6.1 Electric 6.2 Hydrostatic 6.3 Hydrodynamic
7 See also 8 Notes 9 References 10 Further reading 11 External links
Explanation Transmission types Manual Sequential manual Non-synchronous Preselector
Automatic Manumatic Semi-automatic Electrohydraulic Dual-clutch
Uses Gearboxes have found use in a wide variety of different—often stationary—applications, such as wind turbines. Transmissions are also used in agricultural, industrial, construction, mining and automotive equipment. In addition to ordinary transmission equipped with gears, such equipment makes extensive use of the hydrostatic drive and electrical adjustable-speed drives.
The main gearbox and rotor of a
Furthermore, the engine provides its highest torque and power outputs unevenly across the rev range resulting in a torque band and a power band. Often the greatest torque is required when the vehicle is moving from rest or traveling slowly, while maximum power is needed at high speed. Therefore, a system is required that transforms the engine's output so that it can supply high torque at low speeds, but also operate at highway speeds with the motor still operating within its limits. Transmissions perform this transformation. A diagram comparing the power and torque bands of a "torquey" engine versus a "peaky" one The dynamics of a car vary with speed: at low speeds, acceleration is limited by the inertia of vehicular gross mass; while at cruising or maximum speeds wind resistance is the dominant barrier. Many transmissions and gears used in automotive and truck applications are contained in a cast iron case, though more frequently aluminium is used for lower weight especially in cars. There are usually three shafts: a mainshaft, a countershaft, and an idler shaft. The mainshaft extends outside the case in both directions: the input shaft towards the engine, and the output shaft towards the rear axle (on rear wheel drive cars. Front wheel drives generally have the engine and transmission mounted transversely, the differential being part of the transmission assembly.) The shaft is suspended by the main bearings, and is split towards the input end. At the point of the split, a pilot bearing holds the shafts together. The gears and clutches ride on the mainshaft, the gears being free to turn relative to the mainshaft except when engaged by the clutches.
Manual Main article: Manual transmission 16-speed (2x4x2) ZF 16S181 — opened transmission housing (2x4x2) 16S181 — opened planetary range housing (2x4x2) Manual transmissions come in two basic types:
A simple but rugged sliding-mesh or unsynchronized/non-synchronous
system, where straight-cut spur gear sets spin freely, and must be
synchronized by the operator matching engine revs to road speed, to
avoid noisy and damaging clashing of the gears
The now ubiquitous constant-mesh gearboxes, which can include
non-synchronised, or synchronized/synchromesh systems, where typically
diagonal cut helical (or sometimes either straight-cut, or
double-helical) gear sets are constantly "meshed" together, and a dog
clutch is used for changing gears. On synchromesh boxes, friction
cones or "synchro-rings" are used in addition to the dog clutch to
closely match the rotational speeds of the two sides of the
(declutched) transmission before making a full mechanical engagement.
The former type was standard in many vintage cars (alongside e.g.
epicyclic and multi-clutch systems) before the development of
constant-mesh manuals and hydraulic-epicyclic automatics, older
heavy-duty trucks, and can still be found in use in some agricultural
equipment. The latter is the modern standard for on- and off-road
transport manual and semi-automatic transmission, although it may be
found in many forms; e.g., non-synchronised straight-cut in racetrack
or super-heavy-duty applications, non-synchro helical in the majority
of heavy trucks and motorcycles and in certain classic cars (e.g. the
Fiat 500), and partly or fully synchronised helical in almost all
modern manual-shift passenger cars and light trucks.
Manual transmissions are the most common type outside North America
and Australia. They are cheaper, lighter, usually give better
performance, but the newest automatic transmissions and CVTs give
better fuel economy. It is customary for new
drivers to learn, and be tested, on a car with a manual gear change.
Non-synchronous Main article: Non-synchronous transmission Some commercial applications use non-synchronized manual transmissions that require a skilled operator. Depending on the country, many local, regional, and national laws govern operation of these types of vehicles (see Commercial Driver's License). This class may include commercial, military, agricultural, or engineering vehicles. Some of these may use combinations of types for multi-purpose functions. An example is a power take-off (PTO) gear. The non-synchronous transmission type requires an understanding of gear range, torque, engine power, and multi-functional clutch and shifter functions. Also see Double-clutching, and Clutch-brake sections of the main article. Float shifting is the process of shifting gears without using the clutch.
Main article: Automatic transmission
Main article: Semi-automatic transmission
A hybrid form of transmission where an integrated control system
handles manipulation of the clutch automatically, but the driver can
still—and may be required to—take manual control of gear
selection. This is sometimes called a "clutchless manual", or
"automated manual" transmission. Many of these transmissions allow the
driver to fully delegate gear shifting choice to the control system,
which then effectively acts as if it was a regular automatic
transmission. They are generally designed using manual transmission
"internals", and when used in passenger cars, have synchromesh
operated helical constant mesh gear sets.
Early semi-automatic systems used a variety of mechanical and
hydraulic systems—including centrifugal clutches, torque converters,
electro-mechanical (and even electrostatic) and servo/solenoid
controlled clutches—and control schemes—automatic declutching when
moving the gearstick, pre-selector controls, centrifugal clutches with
drum-sequential shift requiring the driver to lift the throttle for a
successful shift, etc.—and some were little more than regular
lock-up torque converter automatics with manual gear selection.
Most modern implementations, however, are standard or slightly
modified manual transmissions (and very occasionally modified
automatics—even including a few cases of CVTs with "fake" fixed gear
ratios), with servo-controlled clutching and shifting under command of
the central engine computer. These are intended as a combined
replacement option both for more expensive and less efficient "normal"
automatic systems, and for drivers who prefer manual shift but are no
longer able to operate a clutch, and users are encouraged to leave the
shift lever in fully automatic "drive" most of the time, only engaging
manual-sequential mode for sporty driving or when otherwise strictly
Specific types of this transmission include: Easytronic,
Geartronic, as well as the systems used as standard in all ICE-powered
Smart-MCC vehicles, and on geared step-through scooters such as the
Honda Super Cub
Uncommon types Dual clutch transmission Main article: Dual clutch transmission This arrangement is also sometimes known as a direct shift gearbox or powershift gearbox. It seeks to combine the advantages of a conventional manual shift with the qualities of a modern automatic transmission by providing different clutches for odd and even speed selector gears. When changing gear, the engine torque is transferred from one gear to the other continuously, so providing gentle, smooth gear changes without either losing power or jerking the vehicle. Gear selection may be manual, automatic (depending on throttle/speed sensors), or a 'sports' version combining both options.
Main article: Continuously variable transmission
The continuously variable transmission (CVT) is a transmission in
which the ratio of the rotational speeds of two shafts, as the input
shaft and output shaft of a vehicle or other machine, can be varied
continuously within a given range, providing an infinite number of
possible ratios. The CVT allows the driver or a computer to select the
relationship between the speed of the engine and the speed of the
wheels within a continuous range. This can provide even better fuel
economy if the engine constantly runs at a single speed. The
transmission is, in theory, capable of a better user experience,
without the rise and fall in speed of an engine, and the jerk felt
when changing gears poorly.
CVTs are increasingly found on small cars, and especially
high-gas-mileage or hybrid vehicles. On these platforms, the torque is
limited because the electric motor can provide torque without changing
the speed of the engine. By leaving the engine running at the rate
that generates the best gas mileage for the given operating
conditions, overall mileage can be improved over a system with a
smaller number of fixed gears, where the system may be operating at
peak efficiency only for a small range of speeds.
CVTs are also found in agricultural equipment; due to the high-torque
nature of these vehicles, mechanical gears are integrated to provide
tractive force at high speeds. The system is similar to that of a
hydrostatic gearbox, and at 'inching speeds' relies entirely on
hydrostatic drive. German tractor manufacturer
Infinitely variable The IVT is a specific type of CVT that includes not only an infinite number of gear ratios, but an "infinite" range as well. This is a turn of phrase, it actually refers to CVTs that are able to include a "zero ratio", where the input shaft can turn without any motion of the output shaft while remaining in gear. The gear ratio in that case is not "infinite" but is instead "undefined". Most (if not all) IVTs result from the combination of a CVT with an epicyclic gear system with a fixed ratio. The combination of the fixed ratio of the epicyclic gear with a specific matching ratio in the CVT side results in zero output. For instance, consider a transmission with an epicyclic gear set to 1:−1 gear ratio; a 1:1 reverse gear. When the CVT side is set to 1:1 the two ratios add up to zero output. The IVT is always engaged, even during its zero output. When the CVT is set to higher values it operates conventionally, with increasing forward ratios. In practice, the epicyclic gear may be set to the lowest possible ratio of the CVT, if reversing is not needed or is handled through other means. Reversing can be incorporated by setting the epicyclic gear ratio somewhat higher than the lowest ratio of the CVT, providing a range of reverse ratios.
Electric variable The Electric Variable Transmission (EVT) combines a transmission with an electric motor to provide the illusion of a single CVT. In the common implementation, a gasoline engine is connected to a traditional transmission, which is in turn connected to an epicyclic gear system's planet carrier. An electric motor/generator is connected to the central "sun" gear, which is normally un-driven in typical epicyclic systems. Both sources of power can be fed into the transmission's output at the same time, splitting power between them. In common examples, between one-quarter and half of the engine's power can be fed into the sun gear. Depending on the implementation, the transmission in front of the epicyclic system may be greatly simplified, or eliminated completely. EVTs are capable of continuously modulating output/input speed ratios like mechanical CVTs, but offer the distinct benefit of being able to also apply power from two different sources to one output, as well as potentially reducing overall complexity dramatically. In typical implementations, the gear ratio of the transmission and epicyclic system are set to the ratio of the common driving conditions, say highway speed for a car, or city speeds for a bus. When the driver presses on the gas, the associated electronics interpret the pedal position and immediately set the gasoline engine to the RPM that provides the best gas mileage for that setting. As the gear ratio is normally set far from the maximum torque point, this set-up would normally result in very poor acceleration. Unlike gasoline engines, electric motors offer efficient torque across a wide selection of RPM, and are especially effective at low settings where the gasoline engine is inefficient. By varying the electrical load or supply on the motor attached to the sun gear, additional torque can be provided to make up for the low torque output from the engine. As the vehicle accelerates, the power to the motor is reduced and eventually ended, providing the illusion of a CVT. The canonical example of the EVT is Toyota's Hybrid Synergy Drive. This implementation has no conventional transmission, and the sun gear always receives 28% of the torque from the engine. This power can be used to operate any electrical loads in the vehicle, recharging the batteries, powering the entertainment system, or running the air conditioning system. Any residual power is then fed back into a second motor that powers the output of the drivetrain directly. At highway speeds this additional generator/motor pathway is less efficient than simply powering the wheels directly. However, during acceleration, the electrical path is much more efficient than an engine operating so far from its torque point. GM uses a similar system in the Allison Bus hybrid powertrains and the Tahoe and Yukon pick-up trucks, but these use a two-speed transmission in front of the epicyclic system, and the sun gear receives close to half the total power.
Non-direct Electric Main article: Diesel-electric transmission Electric transmissions convert the mechanical power of the engine(s) to electricity with electric generators and convert it back to mechanical power with electric motors. Electrical or electronic adjustable-speed drive control systems are used to control the speed and torque of the motors. If the generators are driven by turbines, such arrangements are called turbo-electric transmission. Likewise installations powered by diesel-engines are called diesel-electric. Diesel-electric arrangements are used on many railway locomotives, ships, large mining trucks, and some bulldozers. In these cases, each driven wheel is equipped with its own electric motor, which can be fed varying electrical power to provide any required torque or power output for each wheel independently. This produces a much simpler solution for multiple driven wheels in very large vehicles, where drive shafts would be much larger or heavier than the electrical cable that can provide the same amount of power. It also improves the ability to allow different wheels to run at different speeds, which is useful for steered wheels in large construction vehicles.
Continuously variable transmission
Hydrodynamic If the hydraulic pump or hydraulic motor make use of the hydrodynamic effects of the fluid flow, i.e. pressure due to a change in the fluid's momentum as it flows through vanes in a turbine. The pump and motor usually consist of rotating vanes without seals and are typically placed in proximity. The transmission ratio can be made to vary by means of additional rotating vanes, an effect similar to varying the pitch of an airplane propeller. The torque converter in most automotive automatic transmissions is, in itself, a hydrodynamic transmission. Hydrodynamic transmissions are used in many passenger rail vehicles, those that are not using electrical transmissions. In this application the advantage of smooth power delivery may outweigh the reduced efficiency caused by turbulence energy losses in the fluid.
Bearing reducer Chain drive Clutch Epicyclic gearing Hydraulic transmission Manual transmission Motorcycle transmission Transfer case
^ In American English, a gearbox can be any housing containing a gear train, even just one pair of bevel gears; a transmission is a type of gearbox that is used to dynamically change the speed-torque ratio such as in a vehicle; and automatic transmissions are usually called by that name only, although manual transmissions are often called gearboxes.
^ J. J. Uicker; G. R. Pennock; J. E. Shigley (2003). Theory of Machines and Mechanisms (3rd ed.). New York: Oxford University Press. ISBN 9780195155983..mw-parser-output cite.citation font-style:inherit .mw-parser-output .citation q quotes:"""""""'""'" .mw-parser-output .citation .cs1-lock-free a background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center .mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center .mw-parser-output .citation .cs1-lock-subscription a background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center .mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration color:#555 .mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span border-bottom:1px dotted;cursor:help .mw-parser-output .cs1-ws-icon a background:url("//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/12px-Wikisource-logo.svg.png")no-repeat;background-position:right .1em center .mw-parser-output code.cs1-code color:inherit;background:inherit;border:inherit;padding:inherit .mw-parser-output .cs1-hidden-error display:none;font-size:100% .mw-parser-output .cs1-visible-error font-size:100% .mw-parser-output .cs1-maint display:none;color:#33aa33;margin-left:0.3em .mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format font-size:95% .mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left padding-left:0.2em .mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right padding-right:0.2em
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Further reading Harald Naunheimer; Peter Fietkau; G Lechner (2011). Automotive transmissions : fundamentals, selection, design and application (2nd ed.). Springer. doi:10.1007/978-3-642-16214-5. ISBN 9783642162138. External links
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