Atransmission (also called agearbox) is a mechanical device which uses agear set—two or moregears working together—to change the speed, direction of rotation, or torque multiplication/reduction in amachine.[1][2]
Transmissions can have a single fixed-gear ratio, multiple distinctgear ratios, or continuously variable ratios. Variable-ratio transmissions are used in all sorts of machinery, especially vehicles.
Early transmissions included the right-angle drives and other gearing inwindmills,horse-powered devices, andsteam-powered devices. Applications of these devices includedpumps,mills andhoists.[citation needed]
Bicycles traditionally have usedhub gear orDerailleur gear transmissions, but there are other more recent design innovations.
Since thetorque andpower output of aninternal combustion engine varies with itsrpm,automobiles powered by ICEs require multiple gear ratios to keep the engine within itspower band to produce optimal power,fuel efficiency, 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 torotate in the range of 0–1800 rpm.[3]
In the early mass-produced automobiles, the standard transmission design wasmanual: the combination of gears was selected by the driver through a lever (thegear stick) that displaced gears and gear groups along their axes. Starting in 1939, cars using various types ofautomatic transmission 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 vehicles typically use a fixed-gear or two-speed transmission with no reverse gear 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 inhelicopters andwind turbines. 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.[4][5]
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.
A manual transmission requires the driver to manually select the gears[6] by operating agear stick andclutch (which is usually a foot pedal for cars or a hand lever for motorcycles).
Most transmissions in modern cars usesynchromesh to synchronise the speeds of the input and output shafts. However, prior to the 1950s, most cars usednon-synchronous transmissions.
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 ofdog clutches rather than synchromesh.[7] Sequential manual transmissions also restrict the driver to selecting either the next or previous gear, in a successive order.
Asemi-automatic transmission 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.
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 asAutostick, which automatically control only theclutch, but still require the driver's input to initiate gear changes. Some of these systems are also referred to asclutchless manual systems.[8] Modern versions of these systems that are fully automatic in operation, such asSelespeed andEasytronic, can control both the clutch operation and the gear shifts automatically, without any input from the driver.[9][10]
Anautomatic transmission does not require any input from the driver to change forward gears under normal driving conditions.
The most common design of automatic transmissions is the hydraulic automatic, which typically usesplanetary gearsets that are operated usinghydraulics.[11][12] The transmission is connected to the engine via atorque converter (or afluid coupling prior to the 1960s), instead of thefriction clutch used by most manual transmissions and dual-clutch transmissions.[13]
A dual-clutch transmission (DCT) uses two separateclutches for odd and evengear sets.[14] The design is often similar to two separatemanual transmissions with their respective clutches contained within one housing, and working as one unit.[15][16] In car and truck applications, the DCT functions as an automatic transmission, requiring no driver input to change gears.
A continuously variable transmission (CVT) can change seamlessly through a continuous range ofgear ratios. 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 constantRPM while the vehicle moves at varying speeds.
CVTs are used in cars, tractors,side-by-sides, motor scooters,snowmobiles, bicycles, andearthmoving equipment.
The most common type of CVT uses twopulleys connected by abelt orchain; however, several other designs have also been used at times.
Gearboxes are often a major source ofnoise and vibration in vehicles and stationary machinery. Highersound 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 urbannoise barriers along roadways.[17]
