A transmission (also called a gearbox) is a mechanical device which uses a gear set—two or more gears working together—to change the speed, direction of rotation, or torque multiplication or reduction, in a machine. It was invented by Louis Renault (the founder of Renault). It had been anticipated by Carl Benz, who in 1886 used sprockets and chains to and from an auxiliary shaft and a clutch to provide a second, low gear in the first practical car, his belt-driven Patent-Motorwagen Nr. 2. A transmission can have a single, or fixed, gear ratio or it can have variable ratios; a variable-ratio transmission can have multiple discrete gear ratios or be continuously variable. Variable-ratio transmissions are used in many kinds of machinery, especially vehicles.
Applications
Early uses Early transmissions included the right-angle drives and other gearing in windmills, horse-powered devices, and steam-powered devices. Applications of these devices included pumps, mills and hoists.
Bicycles
Bicycles conventionally have used hub gear or derailleur gear transmissions, but there are other more recent design innovations.
Automobiles
Since the torque and power output of an internal combustion engine (ICE) varies with its (rotational) speed, automobiles powered by ICEs require multiple gear ratios to keep the engine within its power 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 to rotate 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) that displaced gears and gear groups along their axes. Starting in 1939, cars using various types of automatic 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 fixed-ratio or two-speed transmissions with no reverse, as electric motors can operate efficiently across wider ranges of speed, all the way down to stopped, and change directions electrically. Electrical vehicles with more than one driven axle often have separate front and rear motors and drive trains rather than longitudinal drive shafts.
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 helicopters and wind 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.
Multi-ratio Many transmissions – especially for transportation applications – have multiple gears that are used to change the ratio of input speed (e.g. engine speed) 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 gear stick and clutch (which is usually a foot pedal for cars or a hand lever for motorcycles). Early manual transmissions were non-synchronized and used straight cut gears, the latter means that while the gears move they make a distinct and loud whining sound, nowadays making them applicable only to heavy machinery like trucks, buses and tractors since they can withstand higher forces, or motorcycles and racing cars for the simple design and reductions in weight compared to common manual transmissions. Most transmissions in modern cars use the synchromesh design to synchronise the speeds of the input and output shafts. However, prior to the 1950s, most cars used non-synchronous transmissions.
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 clutches rather than synchromesh. Sequential manual transmissions also restrict the driver to selecting either the next or previous gear, in a successive order. Due to sequential transmissions being designed for reduction in complexity, weight and size for their use cases, unlike common manual transmissions, which are designed for simplicity of use and quiet operation, they use straight cut gears (spur gears), which produce a very characteristic whining sound while spinning. Sequential transmissions can produce more than one distinct whining sound depending on the specific design, but usually the most audible whine changes in pitch with (corresponds to) the speed of the wheels (output shaft), and gets more audible the faster the gears spin.
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