The method of increasing the speed of
propeller shaft more than that of the engine speed is called as the over drive. A view of the overdrive can be seen in Figure
The part which performs this action
is called as the overdrive unit. If the main shaft in the gear box rotates at higher speed than the clutch shaft, then the transmission system is said to be with the over drive. Over drive unit is fixed next to the gear box. It is placed in between the gear box and the propeller shaft.
Construction of Overdrive unit
In over drive two shafts are present.
They are input shaft and output shaft.Input shaft is the main shaft of gear box.Output shaft is the propeller shaft.The parts of the overdrive can be seen in Figure. An epicyclic gear train is mounted on the input shaft. In this epicyclic gear train, a sun gear, three planet gears, a planet carrier and a ring gear are present.
The casing is stationary when the
sun gear is locked with casing. Now the overdrive unit starts functioning and the speed of the output shaft is increased.When the sun gear is locked with the planet carrier, the usual input shaft speed is given to the output shaft.
Advantages of overdrive
1. Engine life is increased
2. Achieves reduction in fuel consumption.
3. Less vibration
4. Low noise
5. Excess gear ratio can be achieved
6. Wear of engine is lower.
7. Connecting over drive unit is easy.