Trade speed for torque.
Mesh two gears and change their tooth counts. Watch why a larger driven gear turns more slowly—and multiplies turning force.
Reduction · 2:1
The output gear has twice as many teeth. It rotates at half the input speed, with twice the ideal torque.
Try thisSet input to 20 teeth and output to 40. Then swap them. Notice how speed and torque exchange roles.
How do gear ratios change speed and torque?
The tooth counts set the speed ratio. A larger output gear turns more slowly and provides more ideal torque. For two external gears, the output rotates opposite the input.
The mechanism,
in plain words.
Count the teeth
For a pair of external gears, the speed ratio depends on tooth count. Output speed equals input speed multiplied by input teeth divided by output teeth.
Watch the direction
Two external gears rotate in opposite directions. An additional idler can change the final direction without changing the ratio between the first and last gears.
Follow the torque
In an ideal lossless pair, reducing output speed increases output torque by the same factor. Real gearboxes lose some power through friction and other effects.
Try a taller ratio
Make the output gear smaller than the input. Output speed increases, but ideal output torque falls. Equal tooth counts give a 1:1 ratio.
A tooth-by-tooth connection Model & sources
Ratio = output teeth ÷ input teeth. Output rpm = input rpm ÷ ratio. Ideal output torque = input torque × ratio.
KHK: spur gear dimensions ↗What this model simplifies. Ideal external spur-gear pair with a common module. The flanks are sampled 20° involutes with standard full-depth proportions; root joins are simplified, not manufacturing-ready fillets. The minimum tooth count is 18 to avoid the usual low-tooth-count undercut case. The model ignores friction, inertia, backlash and shifting. It is not a complete vehicle transmission.