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. For compatible external gears, equal tangential motion at the pitch contact links angular displacement to pitch radius and tooth count. The driven rotation is negative relative to the driver and scaled by driver teeth divided by driven teeth. The simplified teeth make this kinematic relationship visible without representing an engineered tooth profile or load rating.
Engineering and scientific investigation · Discover & try
How do gear teeth change turns and direction?
Reading level 5 of 5 · AdvancedChange
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Driver
The gear receiving the input turn.
Driven gear
The gear moved by the driver.
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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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