Compound gear train

A single gear pair can only reduce speed so far before the small gear becomes impractically tiny. Compound trains solve this by mounting two gears on the same shaft: a large gear is driven by the previous stage, and a small gear on the same shaft drives the next one. The overall ratio is simply the product of every individual pair's ratio, which is how a small motor can end up driving a clock hand once per hour.

1. 1Add stages one at a time and watch the output speed drop — each new stage multiplies the previous ratio.
2. 2Notice the direction: every external mesh flips rotation, so with an even number of stages the final output spins the same way as the input.
3. 3Compare the overall ratio to what a single pair of the same overall size would need — compounding gets there with far smaller gears.