What Is a Reduction Gear?

An automobile drivetrain is an example of a multi-stage gear reduction system.
A reduction gear is a system of gears set up to reduce the speed but increase the strength of an engine or motor.
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  • Written By: James Back
  • Edited By: Jenn Walker
  • Last Modified Date: 25 January 2015
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A reduction gear is part of a mechanical system of gears and shafts used to reduce the rotational speed of the input shaft to a slower rotational speed on the output shaft. This reduction in output speed helps increase increase torque. Reduction gears are commonly used in automobile and truck drivetrains to reduce the high rotational speeds of the engine to a usable, slower speed at the tires.

A simple reduction gear consists of two gears that have the same size teeth but are of different diameters. The number of teeth is proportional to the circumference of the gear; the smaller circumference gear will have fewer teeth than the larger one. For example, a gear with a circumference of 16 inches (40.64 cm) will have twice as many teeth as one with a circumference of 8 inches (20.32 cm). When these gears mesh in a reduction gearbox, the smaller gear makes two revolutions for every one revolution of the larger gear — it spins twice as fast. Conversely, the amount of torque available on the larger shaft would be twice as much as that on the smaller. A reduction gear is a closed system in which the amount of input and output power remains constant. As the speed of the output decreases, the torque is increased proportionally.


The number of teeth on the gears determines the gear ratio of the reduction gear. This is expressed in the ratio of the number of teeth on the larger gear to the number of teeth on the smaller gear. A single-stage gear reduction system consisting of two gears, one with 30 teeth and the other with 10 teeth, would have a gear ratio of 30:10, or 3:1. In this system, the larger gear would turn at one-third the speed of the smaller, while having three times the available torque.

Multi-stage gear reduction gearboxes allow for much higher gear ratios than are practical with single-stage systems. Additional shafts and gears are used in these systems. For example, a small gear is added to the output shaft of the first set of gears to serve as the input driver for a second set of gears. Additional sets of gears can be added as needed. The final gear ratio is determined by multiplying the ratio of each set of gears. For example, a reduction gearbox consisting of three sets of gears with gear ratios of 3:1, 4:1, and 5:1 would yield a final gear ratio of 60:1.

An automobile drivetrain is an example of a multi-stage gear reduction system. A typical engine turns at 1500 to 3000 revolutions per minute (RPM) — this is much greater than is needed at the tires. A transmission reduces the RPMs to the driveshaft, sufficiently increasing the torque to move the vehicle. The rear-end further reduces the speed to a usable level, while multiplying the amount of torque available to the drive wheels. Most electrically-powered hand tools use some form of a reduction gear to control output speed while increasing available torque. This allows small, relatively low-powered electric motors to perform work that would require much larger, more powerful motors.



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Post 1

This sounds like the idea behind putting my car in low gear before heading up a mountain. I get more torque, but I don't move as fast.

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