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.
Shift mechanisms for MT and AMT
Gear ratios are achieved with gearwheel pairs, connecting two shafts of the gearbox. For each pair of gearwheels, the one gearwheel is permanently fixed to its shaft, while the other can be locked or unlocked to the shaft via a shifting mechanism.
If the gearwheel is not locked to the shaft, it can freely rotate without transmitting any torque. If the gearwheel is locked to the shaft, torque is transmitted between the gearbox shaft via the engaged gearwheel pair.
The most typical shifting mechanism is the synchronizer, consisting of 4 main components. The shift hub (2) is fixed the shaft. The grooves on its outer diameter are connected to the inner grooves of the shift sleeve (1). The connection between the shift hub and the shift sleeve ensures that those parts are rotating together but allows the shift sleeve to slide axially. The gearwheel (4) is not connected to the shaft, it might rotate independently. There are shift claws on the gearwheel, which fit exactly into the grooves of the shift sleeve.
The shift sleeve has two typical positions. In the disengaged position, the shift sleeve is only connected to the shift hub, consequently, the gearwheel is not locked to the shaft. However, if the shift sleeve is moved towards the gearwheel, it comes to a position, when its inner grooves get in contact with the shift claws of the gearwheel. This is the engaged position, when the shift sleeve is connected to both the shift hub and the gearwheel, establishing a form-fit connection between the shaft and the gearwheel, i.e. locking the gearwheel to the shaft.
Finally, there is a synchronizer ring (3) between the shift hub and the gearwheel. This ring comes to operation when the shift sleeve is sliding from the disengaged position to the engaged position. The shift claws of the synchronizer ring block the further movement of the shift sleeve until the speed difference between the shaft and the gearwheel is synchronized, i.e. reduced to zero. The speed difference between the shaft and the gearwheel is reduced by friction torque, generated between the ring and the conical synchronizer surface of the gearwheel.
In some applications, the synchronzer ring and the synchronizer cone is skipped. In this case the shifting mechanism is called dog clutch. Typical applications with dog clutches are motorcycles, where the gearshifts are performed without synchronization, and the heavy duty AMTs, where the synchronization is performed through external devices, which are not part of the shifting mechanism.
Whenever possible, the shifting mechanisms are applied in pairs, i.e. one shift sleeve can lock two gearwheels, but only one at a time.
If the shift sleeve is in the middle position, none of the two lockable gearwheels is locked to the shaft. If the shift sleeve is moved to the left or to the right, it engages the gearwheel pair #1 or #2.
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