Guide

Linked by
Shared Axle
Driven Gear
Driving Gear
12-Toot h
60-Toot h
Compound Gear
Reduction
Gear Reduction
= 25 / 1
Say “25 to 1 Compound
Gear Reduction”
(60 / 12) x (60 / 12)
(5 / 1) x (5 / 1)
G.3
cont.
Compound Gears and Compound Gear Reductions (Grades 6-8)
In certain situations, a design may require more mechanical advantage than a single gear ratio can
provide or is otherwise impractical. For example, if a VEX IQ robot design requires a 12:500 gear
ratio it is a problem because there is no 500-tooth gear available. In this situation, a designer can
use multiple gear reductions in the same mechanism. This is called a Compound Gear Reduction.
In a Compound Gear system, there are multiple gear pairs. Each pair has its own Gear Ratio,
and a shared axle connects the pairs to each other. The resulting Compound Gear system still
has a Driving Gear and a Driven Gear, and still has a Gear Reduction. However, it’s now called a
Compound Gear Reduction and is calculated by multiplying the gear reductions of each of the
individual gear pairs.
That means the output (Driven Gear) shaft is 25 times slower than the input (Driving Gear)
shaft, and has 25 times as much torque. Compound Gear Reductions add up quickly!
Teacher Note: The VEX IQ Gear Ratio Simulator (G.8) and Gear Ratio Exercises (G.9) can be used to
help understand this section.
For the above example with 12-tooth and 60-tooth gears, the overall Gear Reduction is
calculated this way: