What is a Manual Gearbox?
A manual gearbox (also known as a ‘manual’ or ‘stick shift’) is a type of transmission used in automotive applications. It generally utilizes a driver-operated clutch operated by a pedal or lever, for regulating torque transfer from the engine to the transmission, and a gear-shift either operated by hand (as in a car) or by foot (as on a motorcycle). Other types of transmission in mainstream automotive use are the automatic transmission, semi-automatic transmission, and the continuously variable transmission (CVT).Overview
Manual transmissions often feature a driver-operated clutch and a movable gear selector. Most automobile manual transmissions allow the driver to select any forward gear 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 ratio. This second type of transmission is sometimes called a sequential manual transmission. Sequential transmissions are commonly used in auto racing for their ability to make quick shifts. 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 epicyclical (planetary) gearing controlled by brake bands and/or clutch packs to select gear ratio. Automatic transmissions that allow the driver to manually select the current gear are called semi-automatic transmissions.
Contemporary automotive manual transmissions are generally available with four to six forward gears and one reverse gear, although manual transmissions have been built with as few as two and as many as eight gears. Tractor units have at least 9 gears and as many as 24. Some manuals are referred to by the number of forward gears they offer (e.g., 5-speed) 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.
Unsynchronized transmission
Even though automotive transmissions are now almost universally synchronised, heavy trucks and machinery as well as dedicated racing transmissions are usually not; such transmissions are colloquially referred to as “crash boxes.” Non-synchronized designs are used for several reasons. 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. For racing of production-based transmissions, sometimes half the teeth (or “dogs”) on the synchros are removed to speed the shifting process, at the expense of greater wear. Heavy duty trucks utilize unsynchronized transmissions in the interest of saving weight. Military edition trucks, which do not have to obey weight laws, usually have a synchronized transmission. Highway use heavy-duty trucks in the United States are limited to 80,000 pounds GVWR, and the lighter the curb weight for the truck, the more cargo can be carried, and with a synchronizer adding weight to a truck that could otherwise be used to carry cargo, most drivers are simply taught how to double clutch, initially, and then most eventually gravitate to shifting without the clutch. Similarly, most modern motorcycles still utilize unsynchronized transmissions as synchronizers are generally not necessary or desirable. Their low gear inertias and higher strengths mean that forcing the gears to alter speed is not damaging, and the selector method on modern motorcycles (pedal operated) is not conducive to having the long shift time of a synchronized gearbox. Because of this, it is still necessary to synchronize gear speeds by blipping the throttle when shifting into a lower gear on a motorcycle.Synchronized transmission
Modern gearboxes are constant mesh, i.e., all input and drive gears are always in mesh. Only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, while the others are allowed to rotate freely. This greatly reduces the skill required to shift gears. Most modern cars are fitted with a synchronized gear box, although it is entirely possible to construct a constant mesh gearbox without synchromesh, as found in a motorcycle, for example. In a constant mesh gearbox, the transmission gears are always in mesh and rotating, but the gears are not rigidly connected to the shafts on which they rotate. Instead, the gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism for any individual gear consists of a collar (or “dog collar”) on the shaft which is able to slide sideways so that teeth (or “dogs”) on its inner surface bridge two circular rings with teeth on their outer circumference: one attached to the gear, one to the shaft (one collar typically serves for two gears; sliding in one direction selects one transmission speed, in the other direction selects the other). When the rings are bridged by the collar to that particular gear is rotationally locked to the shaft and determines the output speed of the transmission. 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. The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings (also called blocker rings or balk rings, the latter being spelled “baulk” in the UK). 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 and that of another engaged. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed. The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356; cone synchronizers were called “Porsche-type” for many years after this. In the early 1950s only the second-third shift was synchromesh in most cars, requiring only a single synchro and a simple linkage; drivers’ manuals in cars suggested that if the driver needed to shift from second to first, it was best to come to a complete stop then shift into first and start up again. With continuing sophistication of mechanical development, however, fully synchromesh transmissions with three speeds, then four speeds, and then five speeds, became universal by the 1980s. Many modern manual transmission cars, especially sports cars, now offer six speeds. Reverse gear, however, is usually not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears while moving into reverse is not required. Among the cars that have synchromesh in reverse are the 1995-2000 Ford Contour and Mercury Mystique, ’00-’05 Chevrolet Cavalier, Mercedes 190 2.3-16, the V6 equipped Alfa Romeo GTV/Spider (916), certain Chrysler, Jeep, and GM products which use the New Venture NV3500 and NV3550 units, and almost all Lamborghinis.Internals: Shafts
Like other transmissions, a manual transmission has several shafts with various gears and other components attached to them. Typically, a rear-wheel-drive transmission has three shafts: an input shaft, a countershaft and an output shaft. The countershaft is sometimes called a lay shaft. 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 main shaft. The input and output ends of this combined shaft rotate independently, at different speeds, which is possible because one piece slides into a hollow bore in the other piece, where it is supported by a bearing. Sometimes the term main shaft refers to just the input shaft or just the output shaft, rather than the entire assembly. In some transmissions, it’s possible for the input and output components of the main shaft to be locked together to create a 1:1 gear ratio, causing the power flow to bypass the countershaft. The main shaft then behaves like a single, solid shaft, a situation referred to as direct drive. Even in transmissions that do not feature direct drive, it’s an advantage for the input and output to lie along the same line, because this reduces the amount of torsion that the transmission case has to bear. Less than one possible design, the transmission’s input shaft has just one pinion gear, which drives the countershaft. Along the countershaft are mounted gears of various sizes, which rotate when the input shaft rotates. These gears correspond to the forward speeds and reverse. 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. Reverse is typically implemented differently, see the section on Reverse. Most front-wheel-drive transmissions for transverse engine mounting are designed differently. For one thing, they have an integral final drive and differential. For another, they usually have only two shafts; input and countershaft, sometimes called input and output. The input shaft runs the whole length of the gearbox, and there is no separate input pinion. At the end of the second (counter/output) shaft is a pinion gear that mates with the ring gear on the differential. Front-wheel and rear-wheel-drive transmissions operate similarly. When the transmission is in neutral, and the clutch is disengaged, the input shaft, clutch disk and countershaft can continue to rotate under their own inertia. In this state, the engine, the input shaft and clutch, and the output shaft all rotate independently.Dog clutch
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. The shaft then spins together with that gear. 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.
Synchromesh
If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at different speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter together. For this reason, a modern dog clutch in an automobile has a synchronizer mechanism or synchromesh, which consists of a cone clutch and blocking ring. Before the teeth can engage, the cone clutch engages first which brings the selector and gear to the same speed using friction. Moreover, until synchronization occurs, the teeth are prevented from making contact, because further motion of the selector is prevented by a blocker (or “baulk”) ring. When synchronization occurs, friction on the blocker ring is relieved and it twists slightly, bringing into alignment certain grooves and notches that allow further passage of the selector which brings the teeth together. Of course, the exact design of the synchronizer varies from manufacturer to manufacturer.
Sequential manual
Some transmissions do not allow the driver to arbitrarily select any gear. Instead, the driver may only ever select the next-lowest or next-highest gear ratio. These transmissions often provide clutch control, but the clutch is only necessary when selecting first or reverse gear from neutral. Most gear changes can be performed without the clutch. Sequential transmissions are generally controlled by a forward-backward lever, foot pedal, or set of paddles mounted behind the steering wheel. In some cases, these are connected mechanically to the transmission. In many modern examples, these controls are attached to sensors which instruct a transmission computer to perform a shift—many of these systems can be switched into an automatic mode, where the computer controls the timing of shifts, much like an automatic transmission.
