Overdrive is a device interposed between the transmission and propeller shaft to permit the propeller shaft to turn faster than the transmission main shaft. It is called because it provides a speed ratio over that of the high-speed ratio.
In other words, It is the highest gear in the transmission. The overdrive permits the engine to operate only about 70% of the propeller shaft speed, when the vehicle is operating in the high-speed ranges, which in turn extends the engine life, improve the field consumption, and reduces vibration and noises.
The overdrive is essentially suited to high powered cars employing three-speed gearboxes, since in order to produce flexible top gear performance a low gear final drive may be necessary, resulting in the vehicle’s engine running faster at high speeds than is desired.
Generally, an overdrive is fitted to the top gear only, but 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 transmission between the transmission and the propeller shaft. A slightly higher rear-axle gear ratio is used with an overdrive than without one.
The overdrive has two essential devices, a freewheeling mechanism and a planetary gear set. These are also explained in the coming articles.
Overdrive Transmission Construction
It consists of the following parts
- A set of planetary gear.
- A solenoid and pawl arrangement for locking the sun gear.
- A rail and fork assembly linked to a dash control knob for disconnecting the overdrive when not in use.
- A freewheel assembly or overrunning clutch that drives the main shaft below the cut-in speed.
The planetary gears are used to increase speed by arranging to have the ring gear driven by the planet-pinion cage when the sun gear is locked. Thus, the increase in speed of the main shaft the power available to drive the wheel decreases.
The overdrive ratio can be used only when the engine is running fast enough to generate enough torque to offset this handicap. The maximum speed at which the engine can do this is called Cut in Speed. Below this speed, the drive is made direct by unlocking the sun gear.
The ring gear is splined to the outer case of the freewheel assembly, which is a part of the overdrive main shaft. When the pawl is not engaged in the gear plate, the sun gear is unlocked and the planetary gears cannot transmit power.
Then the unit is in direct drive. In this case, the power flows from the transmission main shaft to the freewheel assembly and then to the overdrive main shaft.
- If the driver wants to go into overdrive, when the car is travelling above a predetermined cut-in speed ( usually 35 to 55 km/hr), momentarily releases the accelerator pedal.
- If the driver wants to come out of the overdrive he merely pushes the accelerator pedal past the full throttle position.
- The driver wants to lock out of the overdrive, he pulls a control knob on the car dash.
Overdrive Electric Controls
The overdrive electric control serves the following purposes:
- It energizes the solenoid as the car reaches cut in speed.
- Overdrive disconnects the ignition circuit momentarily
- It opens the solenoid circuit when the Kick-down switch is close as the driver wants to come out of overdrive.
The figure shows a wiring circuit of the electric control system used with the overdrive. When the driver wants to go into overdrive, he pushes in the control knob on the dash. When the car reaches cut-in speed, the governor closes its contacts to connect the overdrive relay winding to the battery.
The overdrive relay
When the driver wants to come 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 the upper contacts causes to open the overdrive relay circuit.
The overdrive relay, therefore, opens its contacts to open the solenoid circuit. Also, closing the lower contacts in the kick-down switch causes to ground the ignition. With this interruption of ignition system action, the engine stops delivering power and begins to slow down. With this action, the thrust on the solenoid pawl is relieved, and the spring pressure pulls the pawl out of the notch in the sun gear control plate. It causes to unground the ignition coil and thereby permit the ignition system to function again.
This series of actions take place so quickly that no appreciable lag is noticeable in power delivery.
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