A transmission (also called a gearbox) is a mechanical device invented by Louis Renault (who founded

Renault Renault S.A., commonly referred to as Groupe Renault ( , , , also known as the Renault Group in English), is a French Multinational corporation, multinational Automotive industry, automobile manufacturer established in 1899. The company curr ...

) which uses a gear set—two or more

gears A gear or gearwheel is a rotating machine element, machine part typically used to transmit rotation, rotational motion and/or torque by means of a series of teeth that engage with compatible teeth of another gear or other part. The teeth can ...

working together—to change the speed, direction of rotation, or torque multiplication/reduction in a

machine A machine is a physical system that uses power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromol ...

. Transmissions can have a single fixed-gear ratio, multiple distinct

gear ratio A gear train or gear set is a machine element of a mechanical system formed by mounting two or more gears on a frame such that the teeth of the gears engage. Gear teeth are designed to ensure the Pitch circle diameter (gears), pitch circles of e ...

s, or continuously variable ratios. Variable-ratio transmissions are used in all sorts of machinery, especially vehicles.

Applications

Early uses

Early transmissions included the right-angle drives and other gearing in

windmill A windmill is a machine operated by the force of wind acting on vanes or sails to mill grain (gristmills), pump water, generate electricity, or drive other machinery. Windmills were used throughout the high medieval and early modern period ...

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 mi ...

-powered devices, and

steam Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...

-powered devices. Applications of these devices included

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

s, mills and hoists.

Bicycles

Bicycle A bicycle, also called a pedal cycle, bike, push-bike or cycle, is a human-powered transport, human-powered or motorized bicycle, motor-assisted, bicycle pedal, pedal-driven, single-track vehicle, with two bicycle wheel, wheels attached to a ...

s traditionally have used

hub gear A hub gear, internal-gear hub, internally geared hub or just gear hub is a gear ratio changing system commonly used on bicycles that is implemented with planetary or epicyclic gears. The gears and lubricants are sealed within the shell of the h ...

or Derailleur gear transmissions, but there are other more recent design innovations.

Automobiles

Since the

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 ...

and

power Power may refer to: Common meanings * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power, a type of energy * Power (social and political), the ability to influence people or events Math ...

output of 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 ...

(ICE) varies with its

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 ...

automobiles 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 ...

power band The power band of an internal combustion engine or electric motor is the range of operating speeds under which the engine or motor is able to output the most power, that is, the maximum energy per unit of time. This usually means that maximum ...

to produce optimal power,

fuel efficiency Fuel efficiency (or fuel economy) is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical energy, chemical potential energy contained in a carrier (fuel) into kinetic energy or Mechanical work, w ...

, and smooth operation. Multiple gear ratios are also needed to provide sufficient acceleration and velocity for safe and reliable operation at modern highway speeds. ICEs typically operate over a range of approximately 600–7000 rpm, while the vehicle's speeds requires the wheels to

rotate Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis interse ...

in the range of 0–1800 rpm. In the early mass-produced automobiles, the standard transmission design was manual: the combination of gears was selected by the driver through a lever (the

gear stick A gear stick (rarely spelled ''gearstick''), gear lever (both UK English), gearshift or shifter (both U.S. English), more formally known as a transmission lever, is a metal lever attached to the transmission of an automobile. The term ''gear s ...

) that displaced gears and gear groups along their axes. Starting in 1939, cars using various types of

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 ...

became available in the US market. These vehicles used the engine's own power to change the effective gear ratio depending on the load so as to keep the engine running close to its optimal rotation speed. Automatic transmissions now are used in more than two thirds of cars globally, and on almost all new cars in the US. Most currently-produced passenger cars with gasoline or diesel engines use transmissions with 4–10 forward gear ratios (also called speeds) and one reverse gear ratio.

Electric vehicle An electric vehicle (EV) is a motor vehicle whose propulsion is powered fully or mostly by electricity. EVs encompass a wide range of transportation modes, including road vehicle, road and rail vehicles, electric boats and Submersible, submer ...

s typically use a fixed-gear or two-speed transmission with no reverse gear ratio, as electric motors can operate at a wider range of RPM, and provide their full torque even when close to 0 RPM. Reversing the motor is achieved electrically.

Motorcycles

Fixed-ratio

The simplest transmissions used a fixed ratio to provide either a gear reduction or increase in speed, sometimes in conjunction with a change in the orientation of the output shaft. Examples of such transmissions are used in

helicopter A helicopter is a type of rotorcraft in which Lift (force), lift and thrust are supplied by horizontally spinning Helicopter rotor, rotors. This allows the helicopter to VTOL, take off and land vertically, to hover (helicopter), hover, and ...

s and

wind turbine A wind turbine is a device that wind power, converts the kinetic energy of wind into electrical energy. , hundreds of thousands of list of most powerful wind turbines, large turbines, in installations known as wind farms, were generating over ...

s. In the case of a wind turbine, the first stage of the gearbox is usually a planetary gear, to minimize the size while withstanding the high torque inputs from the turbine.

Multi-ratio

Many transmissions – especially for transportation applications – have multiple gears that are used to change the ratio of input speed (e.g. engine rpm) to the output speed (e.g. the speed of a car) as required for a given situation. Gear (ratio) selection can be manual, semi-automatic, or automatic.

Manual

A manual transmission requires the driver to manually select the gears by operating a

and

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 ...

(which is usually a foot pedal for cars or a hand lever for motorcycles). Most transmissions in modern cars use

synchromesh A manual transmission (MT), also known as manual gearbox, standard transmission (in Canada, the United Kingdom and the United States), or stick shift (in the United States), is a multi-speed motor vehicle transmission system where gear changes ...

to synchronise the speeds of the input and output shafts. However, prior to the 1950s, most cars used

non-synchronous transmission A non-synchronous transmission, also called a crash gearbox, is a form of manual transmission based on gears that do not use synchronizing mechanisms. They require the driver to manually synchronize the transmission's input speed (engine RPM) ...

s. File:John Deere 3350 tractor cut transmission.JPG , 16-speed tractor transmission (plus 8 reverse gears) File:M5OD transmission.jpg, Mazda M5OD manual transmission (viewed from the engine side) File:Gearbox 4gears sequential.gif, Animation of a 4-speed

sequential manual transmission A sequential manual transmission, also known as a sequential gearbox or sequential transmission, is a type of Non-synchronous transmission, non-synchronous manual transmission used mostly in motorcycles and Auto racing, racing cars. It produces ...

Sequential manual

A sequential manual transmission is a type of non-synchronous transmission used mostly for motorcycles and racing cars. It produces faster shift times than synchronized manual transmissions, through the use of

dog clutch A dog clutch (also known as a dog box, dog gears, dog ring, clutch dog, or positive clutch) is a type of clutch that couples two rotating shafts or other rotating components by engagement of interlocking teeth or dogs rather than by friction. T ...

es rather than synchromesh. Sequential manual transmissions also restrict the driver to selecting either the next or previous gear, in a successive order.

Semi-automatic

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 ...

is where some of the operation is automated (often the actuation of the clutch), but the driver's input is required to move off from a standstill or to change gears.

Automated manual / clutchless manual

An automated manual transmission (AMT) is essentially a conventional manual transmission that uses automatic actuation to operate the clutch and/or shift between gears. Many early versions of these transmissions were semi-automatic in operation, such as '' Autostick'', which automatically control only the

, but still require the driver's input to initiate gear changes. Some of these systems are also referred to as ''clutchless manual'' systems. Modern versions of these systems that are fully automatic in operation, such as ''

Selespeed Selespeed is the name of an electrohydraulic manual transmission, automated manual transmission used in Alfa Romeo cars, developed by Italian company Magneti Marelli and made by Graziano Trasmissioni. The Selespeed is an automated manual gearbox w ...

'' and ''

Easytronic Easytronic is the Opel tradename for a type of transaxle-based automated manual transmission, as used in some Opel/Vauxhall cars. Easytronic is not a tiptronic gearbox design; it does not have a torque converter. It is fundamentally a convention ...

'', can control both the clutch operation and the gear shifts automatically, without any input from the driver.

Automatic

does not require any input from the driver to change forward gears under normal driving conditions.

Hydraulic automatic

The most common design of automatic transmissions is the hydraulic automatic, which typically uses planetary gearsets that are operated using

hydraulics Hydraulics () is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counterpart of pneumatics, which concer ...

. The transmission is connected to the engine via a

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 ...

(or a

fluid coupling A fluid coupling or hydraulic coupling is a hydrodynamics, hydrodynamic or 'hydrokinetic' device used to transmit rotating mechanical power.

prior to the 1960s), instead of the friction clutch used by most manual transmissions and dual-clutch transmissions. File:Automatic transmission cut.jpg, Hydraulic automatic transmission (cutaway view) File:Epicyclic gear ratios.png,

Epicyclic gearing An epicyclic gear train (also known as a planetary gearset) is a gear reduction assembly consisting of two gears mounted so that the center of one gear (the "planet") revolves around the center of the other (the "sun"). A carrier connects the ...

diagram, as used in hydraulic automatic transmissions File:ZF Automatik 6HP26.JPG, Cutaway view of a ZF 6HP hydraulic automatic transmission

Dual-clutch (DCT)

A dual-clutch transmission (DCT) uses two separate

es for odd and even gear sets. The design is often similar to two separate

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 ...

s with their respective clutches contained within one housing, and working as one unit. In car and truck applications, the DCT functions as an automatic transmission, requiring no driver input to change gears.

Continuously-variable Ratio

A continuously variable transmission (CVT) can change seamlessly through a continuous range of

s. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. The flexibility of a CVT with suitable control may allow the engine to operate at a constant

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 ...

while the vehicle moves at varying speeds. CVTs are used in cars, tractors, side-by-sides, motor scooters,

snowmobile A snowmobile, also known as a snowmachine (chiefly Alaskan), motor sled (chiefly Canadian), motor sledge, skimobile, snow scooter, or simply a sled is a motorized vehicle designed for winter travel and recreation on snow. Their engines normally ...

s, bicycles, and earthmoving equipment. The most common type of CVT uses two

pulley Sheave without a rope A pulley is a wheel on an axle or shaft enabling a taut cable or belt passing over the wheel to move and change direction, or transfer power between itself and a shaft. A pulley may have a groove or grooves between flan ...

s connected by a belt or

chain A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A ...

; however, several other designs have also been used at times.

Noise and vibration

Gearboxes are often a major source of

noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...

and vibration in vehicles and stationary machinery. Higher sound levels are generally emitted when the vehicle is engaged in lower gears. The design life of the lower ratio gears is shorter, so cheaper gears may be used, which tend to generate more noise due to smaller overlap ratio and a lower mesh stiffness etc. than the helical gears used for the high ratios. This fact has been used to analyze vehicle-generated sound since the late 1960s, and has been incorporated into the simulation of urban roadway noise and corresponding design of urban

noise barrier A noise barrier (also called a soundwall, noise wall, sound berm, sound barrier, or acoustical barrier) is an exterior structure designed to protect inhabitants of sensitive land use areas from noise pollution. Noise barriers are the most effecti ...

s along roadways.

References

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