What is Overdrive Gearbox? How does it work?


Under the heading overdrive gearbox, its two essential devices-freewheeling mechanism and planetary gear set have also been described. Liquid coupling, hydraulic drive, and torque converter have been described briefly.


Overdrive gearbox is a device interposed between the transmission and propeller shaft to permit the propeller shaft to turn faster than, or overdrive, the transmission main shaft. It is called overdrive because it provides a gear ratio over that of the high-speed ratio. The overdrive reduces the effort of the engine and operates on about 70% of the propeller shaft speed.

When the vehicle is operating in the high-speed ranges, which in turn extends the engine life, reduces fuel consumption and reduces noises and vibration. The overdrive is normally made for high powered cars using three-speed transmission since in order to produce flexible high-speed gear performance a low-speed gear final drive may be necessary, resulting in the engine runs more faster at high speeds than is desired.

Overdrive gearbox
Overdrive gearbox

Normally, an overdrive is fitted to the top gear only, but in some sports cars have overdrive on second, third and top gear, giving seven forward speeds. Overdrive is usually employed to supplement conventional transmission. It is bolted to the rear of the gearbox between the gearbox and the propeller shaft. A higher rear-axle gear ratio is obtained with an overdrive than without one. The overdrive includes two important devices, a freewheel mechanism, and a planetary gear set. These are also explained in the below articles.

Overdrive gearbox
Overdrive gearbox


The figure shows the design of the overdrive.
It consists of the following parts:

1. A set of planetary gears.
2. A solenoid and pawl arrangement for locking the sun gear.
3. A rail and fork assembly linked to a dash control knob for disconnecting the overdrive when not in use.
4. A freewheel assembly or over-running clutch that drives the main shaft below the cut-in speed.

The planetary gears are employed to increase speed by arranging the ring gear driven by the planet-pinion cage when the sun gear is locked. Because when the speed of the main shaft increases that decreases the power available to drive the wheels, the overdrive ratio can be used only when the engine is running fast enough to develop enough torque to offset this handicap.

The maximum speed at which the engine can do gear is splined to the outer case of the freewheel assembly, which is a part of the overdrive I main shaft. When the pawl not engaged in the gear plate, the sun gear is unlocked and planetary gears cannot able to transmit power. Then the unit is in direct drive. At this condition, the power flows from the gearbox main shaft to the freewheel assembly and next to the overdrive main shaft.


If the driver wants to go into overdrive when the car is traveling above a pre-determined cut-in speed(usually 35 to 55 km/h), he momentarily releases the accelerator pedal. If the driver wants to disengage from the overdrive, he pushes the accelerator pedal past the full throttle position. If the driver wants to the lockout of the overdrive, he pulls a control knob on the car dash.

Watch the video below to know how the overdrive transmission works:


The overdrive electric control serves the following purposes:

  1. It energizes the solenoid when the car reaches the cut-in speed.
  2. It disconnects the ignition circuit momentarily.
  3. It opens the solenoid circuit when the Kick-down switch is closed as the driver wants to come out of overdrive.

The below image shows the wiring circuit of the electric control system used with the overdrive in vehicles. When the driver wants to go into the overdrive, he pushes in the control knob on the dash. When the car reaches out-in speed, the governor closes its contacts to connect the overdrive relay winding to the battery. The overdrive relay, in turn, closes its contacts to connect the solenoid to the battery. Now, the overdrive is ready to go into action. When the driver momentarily releases the accelerator pedal, the solenoid sends the pawl into a notch in the sun gear control plate. This puts the transmission into overdrive.

Overdrive Gearbox electric control system
Overdrive Gearbox electric control system

 When the driver wants to driver wants me out of overdrive, he pushes the accelerator pedal past the full throttle position. It causes the upper contacts of the kick-down switch to open and the lower contacts to close. The opening of upper contact results to open the overdrive relay circuit. The overdrive relay, therefore, opens the contact points to open the solenoid circuit. Also when we close the lower contacts in the kick-down switch it causes to ground the ignition.

By this interruption of ignition system action, the engine stops its delivering
power and begins to slow down. By this action, the thrust power on solenoid pawl is relieved and the Spring pressure automatically pulls the pawl out of the notch in the sun gear control plate. t causes to underground the ignition coil and thereby permit the ignition system to function again. This series of the action takes place so quickly that no appropriate lag is noticeable in power delivery.

Overdrive Parts
Overdrive Parts

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Creative Blogger And Automobile Engineer.

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