mechanical · 32 minutes · OETT Sample Confirmed
Gears: Direction & Speed
Which way gears turn and how fast they move
What to know
- touching outer teeth reverse direction
- driver: the gear supplying motion
- driven: the gear being turned
- RPM: turns per minute
- idler: a gear between two others
Gear questions usually ask which way a gear turns, how fast it turns, or both.
The core rule
Two gears with touching outer teeth turn in opposite directions. Driven RPM = driver RPM × driver teeth ÷ driven teeth.
Start with the gear supplying motion, called the driver. Reverse direction each time the outer teeth meet another gear. A larger driven gear turns more slowly because it has more teeth to move past. RPM means revolutions, or full turns, per minute. A simple idler gear between the first and last gears changes direction, but not their speed ratio.
Worked examples
Example 1: easy
Two external gears turn opposite directions.
- Find the direction arrow on the first gear.
- The touching outer teeth make the second gear turn the opposite way.
Each contact between outer gear teeth reverses the turning direction.
Example 2: medium
A 12-tooth gear driving 36 teeth makes the driven gear turn one-third as fast.
- Use driven speed = driver speed × 12/36.
- The 36-tooth gear turns one-third as fast as the 12-tooth gear, in the opposite direction.
It has three times as many teeth, so each full turn takes three times as long.
Example 3: hard
In A–idler–C, two reversals make C match A while the A:C tooth ratio sets RPM.
- A turns the middle gear backward; the middle gear turns C back again. A and C turn the same way.
- For C speed, multiply A speed by A teeth ÷ C teeth.
The simple middle gear changes direction but cancels out of the first-to-last speed ratio.
Common mistakes
A larger driven gear spins faster because it has more teeth.
More teeth take longer to pass the contact point. The larger driven gear turns more slowly and can provide more turning force.
Put it into practice
Use guided explanations, check your answers, and try a new variation.