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 conjunction with anecm.[clarification needed]
Single stage gear reducer.
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
Sequential manual Non-synchronous Preselector
Manumatic Semi-automatic Electrohydraulic Saxomat Dual-clutch
Derailleur gears Hub gears
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Interior view of Pantigo Windmill, looking up into cap from
floor—cap rack, brake wheel, brake and wallower. Pantigo
Early transmissions included the right-angle drives and other gearing in windmills, horse-powered devices, and steam engines, in support of pumping, milling, and hoisting. Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox rotates at a slower rate than the input shaft, and this reduction in speed produces a mechanical advantage, increasing torque. A gearbox can be set up to do the opposite and provide an increase in shaft speed with a reduction of torque. Some of the simplest gearboxes merely change the physical rotational direction of power transmission. Many typical automobile transmissions include the ability to select one of several gear ratios. In this case, most of the gear ratios (often simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gears may be "overdrive" types that increase the output speed. 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. Simple
The main gearbox and rotor of a
The simplest transmissions, often called gearboxes to reflect their simplicity (although complex systems are also called gearboxes in the vernacular), provide gear reduction (or, more rarely, an increase in speed), sometimes in conjunction with a right-angle change in direction of the shaft (typically in helicopters, see picture). These are often used on PTO-powered agricultural equipment, since the axial PTO shaft is at odds with the usual need for the driven shaft, which is either vertical (as with rotary mowers), or horizontally extending from one side of the implement to another (as with manure spreaders, flail mowers, and forage wagons). More complex equipment, such as silage choppers and snowblowers, have drives with outputs in more than one direction.
Gears from a five-speed + reverse gearbox from the 1600 Volkswagen Golf (2009).
The gearbox in a wind turbine converts the slow, high-torque rotation of the turbine into much faster rotation of the electrical generator. These are much larger and more complicated than the PTO gearboxes in farm equipment. They weigh several tons and typically contain three stages to achieve an overall gear ratio from 40:1 to over 100:1, depending on the size of the turbine. (For aerodynamic and structural reasons, larger turbines have to turn more slowly, but the generators all have to rotate at similar speeds of several thousand rpm.) The first stage of the gearbox is usually a planetary gear, for compactness, and to distribute the enormous torque of the turbine over more teeth of the low-speed shaft. Durability of these gearboxes has been a serious problem for a long time. Regardless of where they are used, these simple transmissions all share an important feature: the gear ratio cannot be changed during use. It is fixed at the time the transmission is constructed. For transmission types that overcome this issue, see Continuously variable transmission, also known as CVT. Multi-ratio systems
Amphicar gearbox cutaway w/optional shift for water going propellers
Many applications require the availability of multiple gear ratios.
Often, this is to ease the starting and stopping of a mechanical
system, though another important need is that of maintaining good fuel
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. In
Most modern North American, some European and Japanese cars have an
automatic transmission that selects an appropriate gear ratio without
any operator intervention. They primarily use hydraulics to select
gears, depending on pressure exerted by fluid within the transmission
assembly. Rather than using a clutch to engage the transmission, a
fluid flywheel, or torque converter is placed in between the engine
and transmission. It is possible for the driver to control the number
of gears in use or select reverse, though precise control of which
gear is in use may or may not be possible.
Automatic transmissions are easy to use. However, in the past, some
automatic transmissions of this type have had a number of problems;
they were complex and expensive, sometimes had reliability problems
(which sometimes caused more expenses in repair), have often been less
fuel-efficient than their manual counterparts (due to "slippage" in
the torque converter), and their shift time was slower than a manual
making them uncompetitive for racing. With the advancement of modern
automatic transmissions this has changed.
Attempts to improve fuel efficiency of automatic transmissions include
the use of torque converters that lock up beyond a certain speed or in
higher gear ratios, eliminating power loss, and overdrive gears that
automatically actuate above certain speeds. In older transmissions,
both technologies could be intrusive, when conditions are such that
they repeatedly cut in and out as speed and such load factors as grade
or wind vary slightly. Current computerized transmissions possess
complex programming that both maximizes fuel efficiency and eliminates
intrusiveness. This is due mainly to electronic rather than mechanical
advances, though improvements in CVT technology and the use of
automatic clutches have also helped. A few cars, including the 2013
Subaru Impreza and the 2012 model of the Honda Jazz sold in the
UK, actually claim marginally better fuel consumption for the CVT
version than the manual version.
For certain applications, the slippage inherent in automatic
transmissions can be advantageous. For instance, in drag racing, the
automatic transmission allows the car to stop with the engine at a
high rpm (the "stall speed") to allow for a very quick launch when the
brakes are released. In fact, a common modification is to increase the
stall speed of the transmission. This is even more advantageous for
turbocharged engines, where the turbocharger must be kept spinning at
high rpm by a large flow of exhaust to maintain the boost pressure and
eliminate the turbo lag that occurs when the throttle suddenly opens
on an idling engine.
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
Continuously variable transmission
Hydrostatic transmissions transmit all power hydraulically, using the
components of hydraulic machinery. They are similar to electrical
transmissions, but use hydraulic fluid as the power distribution
system rather than electricity.
The transmission input drive is a central hydraulic pump and final
drive unit(s) is/are a hydraulic motor, or hydraulic cylinder (see:
swashplate). Both components can be placed physically far apart on the
machine, being connected only by flexible hoses. Hydrostatic drive
systems are used on excavators, lawn tractors, forklifts, winch drive
systems, heavy lift equipment, agricultural machinery, earth-moving
equipment, etc. An arrangement for motor-vehicle transmission was
probably used on the Ferguson F-1 P99 racing car in about 1961.
Human Friendly Transmission
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. ^ B. Paul (1979). Kinematics and Dynamics of Planar Machinery. Prentice Hall. ^ Stiesdal, Henrik (August 1999), The wind turbine: Components and operation (PDF), retrieved 2009-10-06 ^ Musial, W.; Butterfield, S.; McNiff, B. (May 2007), Improving Wind TurbineGearbox Reliability (PDF), National Renewable Energy Laboratory, retrieved July 2, 2013 ^ "Experts predict nine-, 10-speed transmissions to dominate in North America". Autoweek. May 13, 2013. ^ "Ford, GM work together on new nine-, 10-speed transmissions". Autoweek. April 15, 2013. ^ "Practical Driving Test FAQs". Dvtani.gov.uk. 2009-10-04. Retrieved 2014-04-29. ^ "Victorian restrictions on probationary drivers". Vicroads.vic.gov.au. 2014-01-14. Archived from the original on 2014-08-01. Retrieved 2014-04-29. ^ "Transportlīdzekļu vadītāja tiesību iegūšanas un atjaunošanas kārtība un vadītāja apliecības izsniegšanas, apmaiņas, atjaunošanas un iznīcināšanas kārtība" (in Latvian). Likumi.lv. Retrieved 2014-04-29. ^ "2013 Subaru Impreza Wagon AWD". fueleconomy.gov. Retrieved November 29, 2013. ^ Marshall Brain. "How Sequential Gearboxes Work". Howstuffworks.com. Retrieved July 2, 2013. ^ "Rohloff 14-speed hub". Rohloff.de. Retrieved 2014-04-29. ^ Ben Coxworth (13 March 2013). "Pinion sealed gearbox offers an alternative to those darn derailleurs". Gizmag. ^ Matt Wragg (6 June 2012). "Pinion P1.18 Gearbox: First Ride". Pinkbike. ^ "The Gearmakers". Pinion. Retrieved 2014-04-29. ^ "The Prius 'Continuously Variable Transmission'". Prius.ecrostech.com. 2001-10-10. Retrieved 2014-04-29.
Harald Naunheimer; Peter Fietkau; G Lechner (2011). Automotive transmissions : fundamentals, selection, design and application (2nd ed.). Springer. ISBN 9783642162138. Retrieved 2014-04-29.
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Manual Transmission Operation, (YouTube), Weber State University, Odgen 2012
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Spur Bevel Crown Spiral bevel Helical Herringbone
Cogset Derailleur gears Hub gear Shaft-driven bicycle Sprocket
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Part of the
Diesel engine Electric Fuel cell Hybrid (Plug-in hybrid) Internal combustion engine Petrol engine Steam engine
Continuously variable transmission
Wheels and Tires
Rim Alloy wheel Hubcap
Tubeless Radial Rain Snow Racing slick Off-road Run-flat Spare