After the clutch is the transmission or gearbox in the transmission system. The torque and speed of the vehicle have to change depending upon the various requirements of the vehicle. Sometimes we have to move the vehicle in the reverse direction.
The gearbox serves all these functions. Various types of gearboxes, construction, and working, gear ratio, etc. are vividly elaborated in this unit. Dismantling and assembling, diagnosis of gearbox troubles their causes and remedies, etc. make the students experts in this area.
Spur gears or straight-cut gears are the simplest type of gears. It consists of a disk with teeth projecting radially. The edge of each tooth is straight and aligned parallel to the axis of rotation. These gears mesh together correctly only if fitted to parallel shafts.
Helical or “dry fixed” gears offer a refinement over spur gears. The leading edges of the teeth are not parallel to the axis of rotation but are set at an angle. Since the gear is curved, this angling makes the tooth shape a segment of a helix. Helical gears can be meshed in parallel or in crossed orientations. The angled teeth engage more gradually than the spur gear teeth, causing them to run more smoothly and quietly
Spiral bevel gear
When it is necessary to transmit quietly and smoothly a large torque through a right angle, spiral bevel gear can be used. Spiral bevel gears have teeth cut in helix spiral form.
A hypoid gear is the same as a spiral bevel gear whose main difference is that the mating gears axes do not intersect. The hypoid gear is offset from the gear center. The teeth on a hypoid gear are helical.
In mechanical engineering, a gear ratio is a direct measure of the ratio of the rotational speeds of two or more interlocking gears. As a general rule, when dealing with two gears, if the driving gear (the one directly receiving the rotational force from the engine) is bigger than the driven gear, the latter will turn more quickly, and vice versa.
We can express this basic concept with the formula Gear ratio = T2/T1=N1/N2, where T1 is the number of teeth on the driver gear, T2 is the number of teeth on the driven gear, N1 is the speed of driver gear and N2 is the speed of the driven gear.
Synchromesh gear box
This type of gearbox is similar to the constant mesh type in that all the gears on the main shaft are in constant mesh with the corresponding gears on the lay shaft. The gears on the lay shaft are fixed to it while those on the main shaft are free to rotate on the same.
Its working is also similar to the constant mesh type, but in the former, there is one definite improvement over the latter. This is the provision of a synchromesh device which avoids the necessity of double-declutching.
In the above figure the engine shaft, gears B, C, D, E are free on the main shaft and are always in mesh with corresponding gears in the lay shaft. Thus all the gears on the main shaft as well as on the lay shaft continue to rotate so long as shaft A is rotating. Menders F1 and F2 are free to slide on splines on the main shaft. G1 and G2 are ring shaped members having internal teeth fit onto the external teeth members F1 and F2 respectively.
K1 and K2 are dog teeth on B and D respectively and these also fit onto the teeth of G1 and G2. S1and S2 are the forks. T1and T2 are the ball supported by springs. These tend to prevent the sliding of members G1 (G2) on F1(F2). However, when force is applied in G1 (G2) through fork S1 (S2) exceeds a certain value, the balls are overcome and member G1 (G2) slides over F1 (F2). There are usually six of these balls symmetrically placed circumferentially in one synchromesh device. M1, M2, N1, N2, P1, P2, R1, R2 are the frictional surfaces.
There are many mechanisms used for selecting the desired gear. Broadly speaking these can be divided into two categories, the mechanism where the gear shift lever is mounted on the top of the transmission case and the other where the gear shifting lever is mounted on the steering column. However, in these two types, most of the mechanism is similar and only the external linkage is different.
When a particular gear is to be engaged the corresponding selector rod is moved in the desired direction. To avoid unwanted engagement of gears, slots are made on the selector rods and the sleeves are provided with spring-loaded balls. These balls resist the movements of the forks until some force is applied to the gear lever to overcome their resistance.