Wednesday, 20 February 2013

Manual transmission system in automobile

A manual transmission, also known as a manual gearbox or standard transmission is a type of transmission used in motor vehicle applications.

It uses a driver-operated clutch engaged and disengaged by a foot pedal or hand lever , for regulating torque transfer from the engine to the transmission; and a gear stick operated by foot or by hand .
A conventional, 5-speed manual transmission is often the standard equipment in a base-model car; other options include automated transmissions such as an automatic transmission , a semi-automatic transmission , or a continuously variable transmission .

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Most automobile manual transmissions allow the driver to select any forward gear ratio at any time, but some, such as those commonly mounted on motorcycles and some types of racing cars, only allow the driver to select the next-higher or next-lower gear. This type of transmission is sometimes called a sequential manual transmission .

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Manual transmissions are characterized by gear ratios that are selectable by locking selected gear pairs to the output shaft inside the transmission. Conversely, most automatic transmissions feature epicyclic gearing controlled by brake bands and/or clutch packs to select gear ratio.
Contemporary automobile manual transmissions typically use four to six forward gears and one reverse gear, although automobile manual transmissions have been built with as few as two and as many as eight gears.
Transmission for heavy trucks and other heavy equipment usually have at least 9 gears so the transmission can offer both a wide range of gears and close gear ratios to keep the engine running in the power band .
Some heavy vehicle transmissions have dozens of gears, but many are duplicates, introduced as an accident of combining gear sets, or introduced to simplify shifting.
Some manuals are referred to by the number of forward gears they offer as a way of distinguishing between automatic or other available manual transmissions. Similarly, a 5-speed automatic transmission is referred to as a "5-speed automatic."
The earliest form of a manual transmission is thought to have been invented by Louis-René Panhard and Emile Levassor in the late 19th century. This type of transmission offered multiple gear ratios and, in most cases, reverse.

Newer manual transmissions on cars have all gears mesh at all times and are referred to as constant-mesh transmissions, with "synchro-mesh" being a further refinement of the constant mesh principle.
To shift to a higher gear, the transmission is put in neutral and the engine allowed to slow down until the transmission parts for the next gear are at a proper speed to engage.
To shift to a lower gear, the transmission is put in neutral and the throttle is used to speed up the engine and thus the relevant transmission parts, to match speeds for engaging the next lower gear.
Other drivers will depress the clutch, shift to neutral, then engage the clutch momentarily to force transmission parts to match the engine speed, then depress the clutch again to shift to the next gear, a process called double clutching .

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Double clutching is easier to get smooth, as speeds that are close but not quite matched need to speed up or slow down only transmission parts, whereas with the clutch engaged to the engine, mismatched speeds are fighting the rotational inertia and power of the engine.

The friction material, such as brass , in synchronizers is more prone to wear and breakage than gears, which are forged steel, and the simplicity of the mechanism improves reliability and reduces cost. In addition, the process of shifting a synchromesh transmission is slower than that of shifting a non-synchromesh transmission.
In Europe, heavy duty trucks use synchronized gearboxes as standard. Similarly, most modern motorcycles use unsynchronized transmissions: their low gear inertias and higher strengths mean that forcing the gears to alter speed is not damaging, and the pedal operated selector on modern motorcycles, with no neutral position between gears , is not conducive to having the long shift time of a synchronized gearbox.

Transmission gears are always in mesh and rotating, but gears on one shaft can freely rotate or be locked to the shaft.
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The gearshift lever manipulates the collars using a set of linkages , so arranged so that one collar may be permitted to lock only one gear at any one time; when "shifting gears", the locking collar from one gear is disengaged before that of another is engaged.
In a synchromesh gearbox, to correctly match the speed of the gear to that of the shaft as the gear is engaged the collar initially applies a force to a cone-shaped brass clutch attached to the gear, which brings the speeds to match prior to the collar locking into place.
Reverse gear is usually not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears into reverse while moving is not required - and often highly undesirable, particularly at high forward speed.
Shafts Like other transmissions, a manual transmission has several shafts with various gears and other components attached to them.
In a rear-wheel-drive transmission, the input and output shaft lie along the same line, and may in fact be combined into a single shaft within the transmission. This single shaft is called a mainshaft.
Each of the forward gears on the countershaft is permanently meshed with a corresponding gear on the output shaft. However, these driven gears are not rigidly attached to the output shaft: although the shaft runs through them, they spin independently of it, which is made possible by bearings in their hubs.
The gear selector does not engage or disengage the actual gear teeth which are permanently meshed. Rather, the action of the gear selector is to lock one of the freely spinning gears to the shaft that runs through its hub.
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The output shaft's speed relative to the countershaft is determined by the ratio of the two gears: the one permanently attached to the countershaft, and that gear's mate which is now locked to the output shaft.
The dog clutch is a sliding selector mechanism which is splined to the output shaft, meaning that its hub has teeth that fit into slots on the shaft, forcing that shaft to rotate with it. However, the splines allow the selector to move back and forth on the shaft, which happens when it is pushed by a selector fork that is linked to the gear lever.
The success in engaging the gear without clutching can deceive the driver into thinking that the RPM of the layshaft and transmission were actually exactly matched. Nevertheless, approximate rev. matching with clutching can decrease the general change between layshaft and transmission and decrease synchro wear.
Reverse is also a pair of gears: one gear on the countershaft and one on the output shaft. However, whereas all the forward gears are always meshed together, there is a gap between the reverse gears. Moreover, they are both attached to their shafts: neither one rotates freely about the shaft.
The idler has teeth which mesh with both gears, and thus it couples these gears together and reverses the direction of rotation without changing the gear ratio.
By contrast, most reverse gears are spur gears , meaning that they have straight teeth, in order to allow for the sliding engagement of the idler, which is difficult with helical gears.
Attempting to select reverse while the vehicle is moving forward causes severe gear wear . However, most manual transmissions have a gate that locks out reverse directly from 5th gear to help prevent this.
In the 1970s, as fuel prices rose and fuel economy became an important selling feature, 4-speed transmissions with an overdrive 4th gear or 5-speeds were offered in mass market automobiles and even compact pickup trucks, pioneered by Toyota . 6-speed transmissions started to emerge in high-performance vehicles in the early 1990s.

Recently Porsche announced the next-generation 911 will be available with a 7-speed manual transmission, the first of its kind for a normal automobile with the first six gear ratios the same as the 6-speed gearbox and the 7th gear being of a higher ratio.
The slowest gears in most automotive applications allow for three to four engine rotations for each output revolution . "High" gear in a three or four speed manual transmission allows the output shaft to spin at the same speed as the engine .
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In all vehicles using a transmission , a coupling device is used to separate the engine and transmission when necessary. This is because an internal-combustion engine must continue to run when in use, although a few modern cars shut off the engine at a stoplight.
Without the clutch, changing gears would be very difficult, even with the vehicle moving already: deselecting a gear while the transmission is under load requires considerable force .
As well, selecting a gear requires the revolution speed of the engine to be held at a very precise value which depends on the vehicle speed and desired gear – the speeds inside the transmission have to match.
In most vehicles with manual transmission, gears are selected by manipulating a lever called a gear stick, shift stick, gearshift, gear lever, gear selector, or shifter connected to the transmission via linkage or cables and mounted on the floor, dashboard, or steering column.
Four-speed transmissions with floor-mounted shifters were sometimes referred to as "four on the floor" during the period when the steering column was the more common shifter location. The latter, often being the standard non-performance transmission, usually had only three forward speeds and was referred to as "three on the tree."
Newer small cars and MPVs , like the Suzuki MR Wagon , the Fiat Multipla , the Toyota Matrix , the Pontiac Vibe , the Chrysler RT platform cars, the Honda Element and the Honda Civic may feature a manual or automatic transmission gear shifter located on the vehicle's instrument panel, similar to the mid-1950s Chryslers and Powerglide Corvairs .

Many vehicles offer a 5-speed or 6-speed manual, whereas the automatic option would typically be a 4-speed. This is generally due to the increased space available inside a manual transmission compared with an automatic, since the latter requires extra components for self-shifting, such as torque converters and pumps. However, automatic transmissions are now adding more speeds as the technology matures.
The increased number gears allows for better use of the engine's power band , allowing increased fuel economy, by staying in the most fuel-efficient part of the power band, or higher performance, by staying closer to the engine's peak power. However, a manual transmission has more space to put in more speeds, as the 991 Generation of the Porsche 911 and the 2014 Chevy Corvette has a 7- speed manual transmission.
To reduce wear in these applications, some manual transmissions will have a very low, "granny" gear which provides the leverage to move the vehicle easily at very low speeds. This reduces wear at the clutch because the transmission requires less input torque. However the issue of handling stops on hills is easy to learn, and due to the driving of common cars of low importance.
Off-road vehicles and trucks often feature manual transmissions because they allow direct gear selection and are often more rugged than their automatic counterparts. Conversely, manual transmissions are no longer popular in many classes of cars sold in North America, Australia and some parts of Asia, although they remain dominant in Europe, Asia, Africa and Latin America.
Nearly all cars are available with an automatic transmission option, and family cars and large trucks sold in the US are predominantly fitted with automatics, however in some cases if a buyer wishes he/she can have the car fitted with a manual transmission at the factory.

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