Manual transmission
A manual transmission, also known as manual gearbox, standard transmission, or stick shift, is a multi-speed motor vehicle transmission system where gear changes require the driver to manually select the gears by operating a gear stick and clutch.
Early automobiles used sliding-mesh manual transmissions with up to three forward gear ratios. Since the 1950s, constant-mesh manual transmissions have become increasingly commonplace, and the number of forward ratios has increased to 5-speed and 6-speed manual transmissions for current vehicles.
The alternative to a manual transmission is an automatic transmission. Common types of automatic transmissions are the hydraulic automatic transmission and the continuously variable transmission. The automated manual transmission and dual-clutch transmission are internally similar to a conventional manual transmission, but are shifted automatically.
Alternatively, there are semi-automatic transmissions. These systems are based on the design of, and are technically similar to, a conventional manual transmission. They have a gear shifter which requires the driver's input to manually change gears, but the driver is not required to engage a clutch pedal before changing gear. Instead, the mechanical linkage for the clutch pedal is replaced by an actuator, servo, or solenoid and sensors, which operate the clutch system automatically when the driver touches or moves the gearshift. This removes the need for a physical clutch pedal.
Overview
A manual transmission requires the driver to operate the gear stick and clutch in order to change gears. Most manual transmissions for cars allow the driver to select any gear ratio at any time, for example shifting from second to fourth gear, or fifth to third gear. However, sequential manual transmissions, which are commonly used in motorcycles and racing cars, only allow the driver to select the next-higher or next-lower gear.In a vehicle with a manual transmission, the flywheel is attached to the engine's crankshaft, therefore rotating at engine speed. A clutch disc sits between the flywheel and the transmission pressure plate which is attached to the transmission input shaft, controlling whether the transmission is connected to the engine or not. The clutch pedal controls the pressure plate or not connected to the engine. When the engine is running and the clutch is engaged, the flywheel spins the clutch pressure plate and hence the transmission.
The design of most manual transmissions for cars is that gear ratios are selected by locking selected gear pairs to the output shaft inside the transmission. This is a fundamental difference compared with a typical hydraulic automatic transmission, which uses an epicyclic design, and a hydraulic torque converter. An automatic transmission that allows the driver to control the gear selection is called a manumatic transmission, and is not considered a manual transmission. Some automatic transmissions are based on the mechanical build and internal design of a manual transmission but have added components which automatically control the timing and speed of the gear shifts and clutch; this design is typically called an automated manual transmission.
Contemporary manual transmissions for cars typically use five or six forward gears ratios and one reverse gear; however, transmissions with between two and seven gears have been produced at times. Transmissions for trucks and other heavy equipment often have between eight and twenty-five gears, in order to keep the engine speed within the optimal power band for all typical road speeds. Operating such transmissions often uses the same pattern of shifter movement with a single or multiple switches to engage the next sequence of gears.
History
1890s to 1940s
Many of the first automobiles were rear-engined, with a simple belt-drive functioning as a single-speed transmission. The 1891 Panhard et Levassor is considered a significant advance in automotive transmissions since it used a three-speed manual transmission. This transmission, along with many similar designs that it inspired, was a non-synchronous design where gear changes involved sliding the gears along their shafts so that the desired cogs became meshed. The driver was therefore required to use careful timing and throttle manipulation when shifting, so the gears would be spinning at roughly the same speed when engaged; otherwise, the teeth would refuse to mesh. This was difficult to achieve, so gear changes were often accompanied by grinding or crunching sounds, resulting in the gearboxes being nicknamed "crash boxes". Even after passenger cars had switched to synchronous transmissions, many transmissions for heavy trucks, motorcycles and racing cars remained non-synchronous, in order to withstand the forces required or provide a faster shift time.1950s to 1980s
The first car to use a manual transmission with synchromesh was the 1929 Cadillac. Most North American marques had adopted synchronized manual transmissions, usually for second and high gears, by the mid-1930s. In 1947, Porsche patented the split ring synchromesh system. The 1952 Porsche 356 was the first car to use a transmission with synchromesh on all forward gears. In the early 1950s, most cars only had synchromesh for the shift from third gear to second gear.Up until the late 1970s, most transmissions had three or four forward gear ratios, although five-speed manual transmissions were occasionally used in sports cars such as the 1948 Ferrari 166 Inter and the 1953 Alfa Romeo 1900 Super Sprint. Five-speed transmissions became widespread during the 1980s, as did the use of synchromesh on all forward gears.
1990s to present
Six-speed manual transmissions started to emerge in high-performance vehicles in the early 1990s, such as the 1990 BMW 850i and the 1992 Ferrari 456. The first 6-speed manual transmission was introduced in the 1967 Alfa Romeo 33 Stradale. The first 7-speed manual transmission was introduced in the 2012 Porsche 911.In 2008, 75.2% of vehicles produced in Western Europe were equipped with manual transmission, versus 16.1% with automatic and 8.7% with other.
Internals
Shafts
A manual transmission has several shafts with various gears and other components attached to them. Most modern passenger cars use 'constant-mesh' transmissions consisting of three shafts: an input shaft, a countershaft and an output shaft.The input shaft is connected to the engine and spins at engine speed whenever the clutch is engaged. The countershaft has gears of various sizes, which are permanently meshed with the corresponding gear on the input shaft. The gears on the output shaft are also permanently meshed with a corresponding gear on the countershaft; however, the output shaft gears are able to rotate independently of the output shaft itself. Through the use of collars, the speed of the output shaft becomes temporarily locked to the speed of the selected gear. Some transmission designs—such as in the Volvo 850 and S70—have two countershafts, both driving an output pinion meshing with the front-wheel-drive transaxle's ring gear. This allows for a narrower transmission since the length of each countershaft is halved compared with one that contains four gears and two shifters.
The fixed and free gears can be mounted on either the input or output shaft or both. For example, a five-speed transmission might have the first-to-second selectors on the countershaft, but the third-to-fourth selector and the fifth selector on the main shaft. This means that when the vehicle is stopped and idling in neutral with the clutch engaged and the input shaft spinning, the third-, fourth-, and fifth-gear pairs do not rotate.
When neutral is selected, none of the gears on the output shaft are locked to the shaft, allowing the input and output shafts to rotate independently. For reverse gear, an idler gear is used to reverse the direction in which the output shaft rotates. In many transmissions, the input and output shafts can be directly locked together to create a 1:1 gear ratio which is referred to as direct-drive.
In a transmission for longitudinal engined vehicles, it is common for the input shaft and output shaft to be located on the same axis, since this reduces the torsional forces to which the transmission casing must withstand. The assembly consisting of both the input and output shafts is referred to as the main shaft. Independent rotation of the input and output shafts is made possible by one shaft being located inside the hollow bore of the other shaft, with a bearing located between the two shafts.
In a transmission for transverse engined vehicles, also called a transaxle, there are usually only two shafts: input and countershaft. The input shaft runs the whole length of the gearbox, and there is no separate input pinion. These transmissions also have an integral differential unit, which is connected via a pinion gear at the end of the counter/output shaft.
Dog clutch
In a modern constant-mesh manual transmission, the gear teeth are permanently in contact with each other, and dog clutches are used to select the gear ratio for the transmission. When the dog clutches for all gears are disengaged, all of the gears are able to spin freely around the output shaft. When the driver selects a gear, the dog clutch for that gear is engaged, locking the transmission's output shaft to a particular gear set. This means the output shaft rotates at the same speed as the selected gear, thus determining the gear ratio of the transmission.The dog clutch is a sliding selector mechanism that sits around the output shaft. It has teeth to fit into the splines on the shaft, forcing that shaft to rotate at the same speed as the gear hub. However, the clutch can move back and forth on the shaft, to either engage or disengage the splines. This movement is controlled by a selector fork that is linked to the gear lever. The fork does not rotate, so it is attached to a collar bearing on the selector. The selector is typically symmetric: it slides between two gears and has a synchromesh and teeth on each side in order to lock either gear to the shaft. Unlike some other types of clutches, a dog clutch provides non-slip coupling and is not suited to intentional slipping.