Manual Transmissions (MT) and Automated Manual Transmissions (AMT)
Since they are very common from the basic principle point of view, you find basic information on both transmission types on this page.
Gearbox layouts for MT and AMT
MT / AMT gearboxes might be designed in one of the three principle layouts: two-shaft layout, three-shaft layout or the counterhaft layout, all described on this page.
Two-shaft gearbox layout
The two-shaft gearbox design is the most simple gearbox layout. The gearbox has an input shaft driven by the clutch, and an output shaft driving the vehicle wheels. The input and output shafts are connected by multiple gearwheel pairs, one pair for each forward gear (the figure shows only two gearwheel pairs). The shifting mechanism can be mounted on both the input or the output shaft. Typically, the synchronizers are located at the larger diameter gearwheel, so on the output shaft for lower gears and on the input shaft for higher gears.
Depending on the driveline layout, the output of the gearbox might be at the opposite end as the input (typically, but not exclusively for rear-wheel drive) or at the same end (typically for front-wheel drive).
The additional gear-wheel pair shown in front-wheel drive layout is the final drive, a fixed ratio. A final drive is needed for each vehicle, but is not necessarily integrated into the gearbox. It can also be included in the driven axis, in this case it is not shown in the gearbox schematics.
In order to achieve the opposite rotation direction on the output shaft compared to the forward gears, the reverse gear requires an additional intermediate gear between the input and the output shafts. The intermediate gear is located on the reversing shaft and is active only in reverse gear. Considering the short reverse shaft as well, a two-shaft transmission has indeed three shafts.
The two-shaft gearbox layout is simple, but the installation length is relative high, as all forward gears and also the reverse gears require their own gearwheel pairs, all mounted side-by-side. Therefore this layout is feasible only up to 5 forward gears in case of transversal engine and up to 7 gears in case of longitudinal engine. If more gears are needed, a more compact layout with three shafts is applied.
Three-shaft gearbox layout
The three-shaft gearbox design comprises an input shaft driven by the clutch, and two output shafts united in the same final drive. With this layout, the final drive needs to be integrated into the gearbox, so the gearbox has only one output towards the driven wheel despite the two output shafts.
The forward gears are achieved through gearwheel pairs connecting the input shaft and one of the output shafts, similarly to the two-shaft layout. Approximately one half of the gearwheel pairs is using the first output shaft, and the other half is achieved via the second output shaft.
The benefit of the three-shaft layout is the so-called dependent gear, when a dedicated gearwheel on the input shaft is connected to not only one but two gearwheels, this way capable of driving the one or the other output shaft, but only one at a time. With this design, the length of one gearwheel pair is saved, so the gearbox has an overall length of a two-shaft gearbox with one forward gear less.
As long as there is only one dependent gear, all gear ratios can be defined independently. If the length of the gearbox needs to be further reduced, double dependency is applied. This means that there are two gears on the input shaft which are connected to two other gearwheels each, saving the length of two gearwheel pairs.
A three-shaft gearbox with double dependency has an overall length of a two-shaft gearbox with two forward gears less, but there is a restriction: the gear ratios are not fully independent any more. There are four gears involved in the double dependency out of which only three gear ratios are independent, the fourth ratios is determined by the others.
There are three basic variants to achieve a reverse gear in a three-shaft transmission.
As the gearbox has inherently three shafts, the most compact solution is to use a gearwheel on one output shaft as intermediate gear to reverse the rotation direction, and use the other output shaft to drive the final drive. In this case, the input shaft drives an unlocked gearwheel on the one output shaft, which in turn is in contact with the locked gearwheel of the reverse gear on the other output shaft.
The second variant is to place the reverse gear on a dedicated third output shaft. In this case, the input shaft drives an unlocked gearwheel on the one output shaft, which in turn is in contact with the locked gearwheel of the reverse gear on the dedicated output shaft.
The third variant is a dedicated reverse shaft with two gearwheels. One gearwheel on the input shaft is driving the first gearwheel on the reverse shaft, while the other gearwheel of the reverse shaft is driving a locked gearwheel on one of the output shafts.
The above variants might be divided into further sub-variants (not disclosed further here) depending on the exact location of the shifting mechanism, and whether the intermediate gear is used for a forward gear as well or not.
In case of the second and third variants, the gearbox has indeed four shafts and some additional gearwheels, but in return, the center lines of the gearbox shafts can be placed with higher flexibility, as there is no constraint on the center distance between the output shafts.
Countershaft gearbox layout
Countershaft gearboxes are different from two-shaft or three-shaft gearboxes in the sense that the forward gears are achieved through two pairs of gearwheels instead of one pair. The input shaft drives the countershaft (sometimes referred as the layshaft) through one constant gearwheel pair. The countershaft and the output shaft are connected by multiple pairs of gearwheels (the figure shows only two such gearwheel pairs), achieving the forwards gear similarly to the input and output shaft of a two-shaft gearbox. In a countershaft gearbox however, the gear ratios are multiplied by the ratio of the constant gearwheel pair.
The benefit of a countershaft gearbox compared to the two-shaft or three-shaft gearbox is that the gear ratios between the countershaft and the input shaft are reduced, therefore the smaller gears of the gearwheel pairs are bigger in diameter. Since bigger diameter gearwheels can be designed to withstand higher loads, a countershaft gearbox has higher torque capacity than a two-shaft or three-shaft gearbox of comparable dimensions, or it has a more compact housing in case of the same torque capacity.
The countershaft gearbox has three shafts with only two geometrical axis, because the input and output shafts are in line. This gives the countershaft gearbox a further benefit: if the input and output shafts are locked together, a real direct drive is achieved, i.e. a „bonus” forward gear without additional gearwheels.
The reverse gear is achieved the same way as in a two-shaft transmission. There is a reverse shaft with an intermediate gearwheel connecting the countershaft and the output shaft.
Thanks to the higher load capacity, countershaft gearboxes are preferred for commercial vehicles. As the required number of forward gears is higher than for passenger cars, the layout is somewhat extended and divided into two stages: the splitter and the main gearbox.
The two-speed splitter means that the ratio between the input shaft and the countershaft is not constant, but can be shifted between two gearwheel pairs. The main gearbox is made up of the gearwheel pairs between the countershaft and the output shaft and usually has three or four forward gears.
The name of the splitter comes from the fact that it splits each gear of the main gearbox into two („low split”= odd gears and „high split”= even gears), i.e. doubles the gears of the main gearbox. The result is a 6 or 8 speed gearbox (with two reverse gears).
Note that the second gearwheel of the input shaft can not only be locked to the input shaft, but also to the output shaft, which enables on one hand a direct drive, on the other hand, the second geawheel pair can be used either as part of the splitter (from the input shaft to the countershaft) or as part of the main gearbox (from the countershaft to the output shaft).
For heavy duty gearboxes, this is still not enough. The gearbox is extended with a third stage, the range gear, which again doubles the number of the gears. Together with the splitter, the gears of the main gearbox are this way quadrupled (also the reverse gear)!
The range gear separates between the lower half of the gears and the upper half of the gears. In case of a four-speed main gearbox with a two-speed splitter, the low range means gears 1 to 8, the high range means gears 9 to 16. The range gear is shifted only when changing from the low range to the high range or vica versa.
As the requested ratio step is huge, the range gear is typically not realized as a single pair of gearwheels. Instead, a planetary gearset is used, achieving a large ratio in the low range, and direct drive in the high range.
As the output is now moved after the range gear, the second shaft of the main gearbox is renamed to main shaft.
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