When two external gears mesh, their teeth push each other. They turn in opposite directions. A gear with more teeth turns less far for the same number of teeth passing the contact. Follow the red markers to count turns. Divide input-gear teeth by second-gear teeth to find the turns per input turn. For example, 12 teeth driving 36 gives one third of a turn.
Engineering and scientific investigation · Discover & try
How do gear teeth change turns and direction?
Reading level 1 of 5 · EarlyChange
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Driver
The gear receiving the input turn.
Driven gear
The gear moved by the driver.
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See how it works
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Ideal external-gear kinematics with compatible tooth spacing. Tooth profiles are illustrative; friction and backlash are omitted.
What to look for
- One turn of the driver gives one opposite turn of the driven gear.
- One input turn makes the second gear turn only half as far.
- One input turn gives one-third of a turn at the driven gear.
Explain what you noticed
A 15-tooth driver makes one turn beside a 45-tooth external gear. What does the driven gear do?
Keep this idea: For external gears, tooth counts set the turn ratio and adjacent gears reverse direction.
Follow the sources
- University of Colorado Boulder / TeachEngineering — What Are Gears and What Do They Do?
- University of Colorado Boulder / TeachEngineering — Get in Gear
Source material checked 2026-10-29. Explanations and activities by Nepedia. Source links do not imply endorsement.
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