Follow one contact
These gears mesh on the outside, and you view both from the same side. Gear A has 18 teeth and turns clockwise at 60 RPM, or revolutions per minute. Gear B has 36 teeth.
B turns in the opposite direction. It has twice as many teeth, so it turns half as fast: 60 × 18 ÷ 36 = 30 RPM, counterclockwise.
Twice as many teeth, half the speed.
Add an intermediate gear
Add gear C with 12 teeth. B turns at 30 RPM, counterclockwise. At the next contact the direction reverses again, and C turns at 30 × 36 ÷ 12 = 90 RPM, clockwise.
A and C turn in the same direction because there are two contacts. The intermediate tooth count cancels in the overall speed calculation: 60 × 18 ÷ 12 = 90 RPM.
Two contacts reverse the direction twice.
Try a new pair
In this new pair, A has 24 teeth and turns clockwise at 30 RPM. B has 12 teeth.
Follow the contact to find B’s direction. Then compare the tooth counts to find its speed. Choose both parts together.
Follow the single contact from A to B.
Try it
How fast, and in which direction, does B turn in this new pair?
- 30 RPM · Clockwise
- 60 RPM · Counterclockwise
Show the answer
60 RPM · Counterclockwise
Correct. One contact makes B turn counterclockwise. Its speed is 30 × 24 ÷ 12 = 60 RPM because it has half as many teeth as A.
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