Manual Transmission Introduction
As the vehicle’s only source of motive force, the engine must be able to cope with a vast range of operating conditions from start-up to high-speed driving. The transmission plays a major role in ensuring that this will be possible.
Suppose that your vehicle was provided with a 1st gear only. It couldn’t be driven at high speeds, and even in the low to mid-speed range, its operation would be exceptionally loud. If on the other hand, only a 5th gear was provided, you would not be able to start your vehicle at all. It is the role of the transmission to ensure that the engine can deliver a sufficient amount of torque and speed to deal with every possible situation that may be encountered during driving.
The functions of the transmission are basically as follows:
• To transfer the engine output power to the final driven gear.
• To provide sufficient torque at starting, ascending, accelerating, engine braking, etc.
• To provide low to high speed driving capability.
• To change the direction of wheel rotation.
2. Flow of Power in a Drivetrain
Engine rotation is transmitted to the transmission mainshaft through the action of the clutch. This mainshaft then drives a countershaft using a set of meshing gears which have been selected by the driver’s gearshift lever. The countershaft’s output gear relays the motion to the differential assembly’s final driven gear, and the differential transmits it to the left and right drive shafts. These shafts ultimately turn the left and right wheels respectively.
4 Final driven gear
5 Drive shaft
3. Gear Ratio
The actual operation performed by the gearshift lever is to change the gear size — thenumber of teeth — of the drive gear (the mainshaft gear) and the driven gear (the countershaft gear).
This loss in speed is balanced by a corresponding increase in output torque, making this type of gearing relationship most suitable for standing starts or uphill driving.
Alternatively when a small gear is driven by a larger gear, the result will be a decrease in torque and an increase in speed. If a 40-tooth drive gear drives a 10-tooth driven gear — the opposite setup to that mentioned above — one revolution will cause the wheels to rotate a total for four times. Consequently, this type of gearing relationship is most suitable for driving at high-speeds.
The term “gear ratio” refers to the ratio of the number of teeth on the drive gear to the number of teeth on the driven gear. Thus, the gear ratio for the first example above is 4 and the second example is 0.25.
4. Reverse Gear
The transmission also enables the vehicle to be driven in reverse. Since it is impossible to reverse the engine’s rotation in order to back up the vehicle, the direction of rotation is reversed inside the transmission.
As shown in the following diagram, when in the forward-driving condition, the rotation of the engine is transmitted from the mainshaft gear to the countershaft gear, and then to the differential/final gear and the wheels.
1 From Engine
2 Mainshaft gear
3 Countershaft gear
4 Final driven gear
5 To Driveshafts (wheels)
* The arrows show the direction of transmission of torque (i.e., the torque path).
When the driver moves the gearshift lever into the reverse position, another gear — the reverse idler gear — moves into mesh with the mainshaft and the countershaft, causing the counter gear to rotate in the opposite direction to normal. As a result, the vehicle will now move in the reverse direction.
5. Major Components
A manual transmission consists of the following major components:
• Transmission housing
• Clutch housing
• Reverse idler gear
• Shift fork assembly
• Shift arm assembly
1 Clutch housing
2 Transmission housing
3 Countershaft assembly
4 Mainshaft assembly
5 Reverse idler gear
6 Shift fork assembly
7 Shift arm assembly
3 5th gear shift fork
4 3rd and 4th gear shift fork
5 Shift fork assembly
6 Shift arm assembly
7 Shift rod
8 Shift lever
9 1st and 2nd gearshift fork
Transmission Housing and Clutch Housing
The transmission housing, which is closed off at one end by the clutch housing, holds almost all of the transmission components. In addition, many individual configurations also position the differential inside the transmission casing.
Mainshaft and Countershaft
The mainshaft and countershaft are fitted with 4 or 5 pairs of forwarding gears having mutually differing gear rations, with a pair of reversing gears, and with synchronizers which ensure that engagement of the gears can be performed smoothly. When the gearshift lever is at the neutral position, all pairs of gears with the exception of the reversing gears are rotating in a state of constant engagement; however, one of the gears in each pair will be rotating freely with respect to its shaft so that there will be no net transfer of torque. When the driver moves the gearshift lever to select a gear, the motion of the lever is transferred to the relevant synchronizer via the shift fork mentioned later and the corresponding free gear is fixed to its shaft. In this way, one pair of gears will transfer torque from the mainshaft to the countershaft.
Reverse Idler Gear
The reverse idler gear engages with the main- and countershafts to cause the latter to rotate in the opposite direction to normal. This action allows the vehicle to be driven in backward.
Shift Arm and Shift Fork Assemblies
The function of the shift arm and shift fork assemblies is to select and shift gears in accordance with the motion of the gearshift lever. This gearshift-lever motion is transmitted to the shift arm assembly through the action of either a shift rod or sift cables. Following this, the shift arm assembly engages with the appropriate shift fork and this fork slides a synchronizer sleeve so that the required gear can be fixed to its shaft.