A fully synchronized transmission uses an inertia synchronizer, mainly composed of a sleeve, synchronizer ring, and other components. Its characteristic is to achieve synchronization through friction. The sleeve, synchronizer ring, and the gear ring of the gear to be engaged all have chamfers (locking angles). Friction is generated when the inner conical surface of the synchronizer ring contacts the outer conical surface of the gear ring of the gear to be engaged.
The locking angle and conical surface are appropriately selected during design. The conical friction causes the sleeve and gear ring to be engaged to synchronize rapidly, while also generating a locking action to prevent the gears from engaging before synchronization. After the inner conical surface of the synchronizer ring contacts the outer conical surface of the gear ring of the gear to be engaged, under the action of friction torque, the gear speed rapidly decreases (or increases) to be equal to the synchronizer ring speed. Both rotate synchronously, and the relative speed of the gear to the synchronizer ring becomes zero, thus the inertia torque also disappears. At this point, driven by the applied force, the sleeve engages unimpeded with the synchronizer ring's gear teeth, and further engages with the gear ring of the gear to be engaged, completing the shifting process.
The gear ratio (number of teeth) of output shaft third gear 6 to input shaft third gear 2 (z6/z2) is greater than the gear ratio of output shaft fourth gear 5 to input shaft fourth gear 4 (z5/z4). From the relationship between the speed and number of teeth of meshing gears (n2/n6=z6/z2, n4/n5=z5/z4), it can be concluded that the speed ratio of gear 2 to gear 6 (n2/n6) is greater than the speed ratio of input shaft fourth gear 4 to output shaft fourth gear 5 (n4/n5). Furthermore, the speeds of output shaft third gear 6 and gear 5 are the same (n6=n5). Therefore, during transmission, the speed of gear 2 is always higher than the speed of gear 4, i.e., n2 > n4.
When shifting the transmission from a lower gear (third gear) to a higher gear (fourth gear), first, the clutch pedal must be pressed to disengage the clutch. Then, the sleeve 3 is moved to the right via the shift lever, entering the neutral position. At the moment sleeve 3 and gear 2 just separate, their speeds are still equal, i.e., n3=n2. Since n2 > n4, it can be concluded that n3 > n4, meaning the speed of sleeve 3 is greater than the speed of gear 4. If sleeve 3 is immediately pushed towards the gear ring on gear 4 at this point, gear clash will occur.
At this time, since the transmission is in neutral, there is no connection between the sleeve and the gears. The clutch driven plate is also disengaged from the engine. Therefore, the speeds of both the sleeve and the gears are gradually decreasing separately. Because the gears are connected to other gears, the output shaft, universal joint drive, drive axle, running gear, and the entire vehicle, their inertia is very large, so n4 decreases slowly. However, the sleeve is only connected to the input shaft and the clutch driven plate, having very little inertia, so n3 decreases faster.
Since n3 was originally greater than n4, and n3 decreases faster than n4, it is inevitable that after a while, the condition n3=n4 (synchronization) will occur. It is best to move sleeve to the right and engage fourth gear at the moment n3=n4. The smaller the inertia of the series of parts connected to the sleeve, the faster n3 decreases, and the less time is required to achieve synchronization. Furthermore, with the same speed difference, the impact force between the gears is also smaller. Therefore, the rotational inertia of the clutch driven part should be as small as possible.
