Over drive notes PPT

 This post gives the basic idea about an over drive. The  over drive is one of the main transmission system used in automobiles. Now discuss about an Over drive below.

OVER DRIVE:

1. ž Over drive is a device which is used to set up the gear ratio in the vehicle. It is mounted in between transmission and propeller shaft.
2. ž  It provides a high cruising speed with comparatively low engine speed (up to about 20-25% less) on long journeys.
3. ž  It is generally fixed at the top gear only.

OPERATION

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 1.  The shift to overdrive can be obtained when the car is running above a predetermined cut in speed from 32-51 km/hr by lifting the foot momentarily from the accelerator.

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 2.  The action of the overdrive is controlled by a centrifugally operated switch.

ž  3.  As the car slows down below another predetermined speed (slightly lower than cut in speed), it is returned to third speed automatically in most designs.

ž 4. Greater power and acceleration are needed through the overdrive gear ratio when a car runs on the road.

ž  5.  So the car is shifted back to direct gear when an overdrive is depressed by the accelerator to the full throttle position.

HOW AN OVER DRIVE UNIT WORKS:

ž  The overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmission unit.

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    It can either couple the input drive shaft directly to the output shaft (propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft (1:1 + n).

ž    Thus the output shaft may be "overdriven" relative to the input shaft.

ž    In newer transmissions, the overdrive speed(s) are typically as a result of combinations of planetary/epicyclic gear sets which are integrated in the transmission.

ADVANTAGES:

ž  1. Saves fuel

ž  2. It allows a lower engine speed to maintain the car at highway speed.

ž  3. Wear on the engine and accessories are reduced.

ž  4. It does not require as much as power to keep it moving.

  5.Therefore, the engine can turn more slowly, produce less power and still maintain vehicle speed.