What is Overdrive Unit Transmission

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, improves 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.

An Overdrive is a component of a transmission system which is attached at the end of the gearbox in order to provide highest gear output with minimum engine input which in turn makes the drive smooth and more fuel efficient. It is an arrangement of sun and planetary gear which are arranged in such a fashion that when overdrive is enabled, it provides high rotation per minute to the output shaft with reduced engine rotation per minute which in turn provides smooth, noise free and high fuel economy to the vehicle.

Need of an Overdrive

When we go on a long drive we shift our drive to the top gear which is said to be the direct drive, which means that the final output shaft is rotating at the engine’s rpm which in turn somehow increase the load on the engine’s shaft, so an overdrive can be used with the transmission box which can reduce engine’s load and provides high speed (higher than engine’s rpm) during a long run.

Fuel economy is the first priority of the middle class people in countries like India, as overdrive reduces the engine’s load (as discussed above) so it is obvious that with decreased engine’s load we get better fuel economy.

In high end cars like Audi A6, Mercedes E-class , luxury and smooth riding experience is the priority which cannot be compromised , so these cars having automatic transmission is equipped with overdrive which make them more luxurious.

As the engine’s load is reduced with the use of overdrive which in turn reduces the wear and tear of the engine as well as transmission box, so we can say that using overdrive also reduces the maintenance of the automobile vehicle.

If we talk about the performance, overdrive is said to provide 30% more rpm than engine’s rpm.

Working of an Overdrive

Overdrive in cars can be enabled or disabled using any of the electrical, magnetic, pneumatic actuation method through button or knob, in first case let’s just consider both the enable and disabled cases to understand its working-

Overdrive Disabled – When overdrive is disabled the input shaft passing through the sun gear rotates the sun gear which in turn rotates the constantly meshed planetary gears and then these planetary gears rotate the annulus and direct drive (same as input shaft rpm) is obtained.

Overdrive enabled- When driver enabled the overdrive, the sun gear becomes fixed which mean the annulus is now rotated by the planetary gears or in other words now the input from the input shaft is now transferred through the planetary gear to the annulus due to which overdrive is obtained, which means now the output shaft rotates with the higher rpm than input shaft due to the higher reduction ratio of planetary gears and annulus.