What is Sliding Mesh Gearbox?
In a Sliding Mesh Gearbox, all the gear shift engagemets are done by manually sliding the gears of the main shaft and engaging them with the corresponding gears on the countershaft by the shift lever by the driver. That’s why it is called Sliding Mesh Gearbox.
All the gears on the main shaft are free to move as they are internally spined and mess with the external splines of the main shaft.
It is the simplest type of gearbox. The arrangements of gears in Sliding Mesh Gearbox are shown below in a neutral position. The gear housing and bearing are not shown. The clutch gear is strongly fixed to the clutch shaft. It remains always connected to the driver gear of countershaft. Three other gears are also rigidly fixed to the countershaft (layshaft).
They are the first speed gear, second speed gear, and reverse speed gear. Two gears are mounted on the splined main shaft which can be slide by the shifter yoke when the shift lever is operated. These gears are the second, first and reverse speed gear. A reverse idler gear is always mounted on another third shaft and it remains connected to the reverse gear of countershaft.
Gears in neutral
When the engine is running and the clutch is engaged, the clutch shaft gear drives the countershaft gear. The countershaft rotates opposite to the direction of the clutch shaft. Note that in a neutral position, only the clutch shaft gear is connected to the countershaft gear. Other gears are free and hence transmission main shaft is not running. The vehicle is stationary.
FIRST OR LOW-SPEED GEAR
By operating the shift lever, the bigger gear on the main shaft is moved along the shaft to mesh with the first gear of the countershaft. The clutch shaft turns in the same direction as the main shaft. Since the larger main shaft gear is engaged with the smaller countershaft gear, a gear reduction of approximately 3:1 is obtained. For each revolution of the main shaft, the clutch-shaft turns three times. Gear reduction in the differential at the rear wheels produces a higher gear ratio of 12:1, between the engine crankshaft and the wheels.
SECOND SPEED GEAR:
By operating gear shift lever, the larger gear on the main shaft is de-meshed from the first gear of the countershaft and then the smaller gear of the main shaft has meshed with the second gear of the countershaft. The clutch shaft turns in the same direction as the main shaft. A gear reduction of approximately 2:1 is obtained. The differential gear reduction increases the gear ratio to 8:1.
THIRD, TOP OR HIGH-SPEED GEAR:
By operating the gear shift lever, the second gear of the main shaft and countershaft are de-meshed, and then the second and top gear of the main shaft is forced axially against the clutch shaft gear. External teeth on the clutch shaft gear mesh with internal teeth in the second and top gear. The clutch shaft turns with the main shaft and a gear ratio of 1:1 is achieved. The differential reduction produces a gear ratio of 4:1 between the engine crankshaft and the wheels.
By operating the gear shift lever, the larger gear on the main shaft meshes with the reverse idler gear. The countershaft reverse gear is always in mesh with the reverse idler gear. Interposing the idler gear between the countershaft reverse gear and the main shaft bigger gear. The main shaft turns in the opposite direction of the clutch shaft. This reverses the rotation of the wheel so that the vehicle backs.