Trading speed for strength
How Do Gears Work?
Two toothed wheels of different sizes swap turning speed for turning force.
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Step 1 of 5
Teeth make wheels talk to each other.
Everything a gear does follows from the teeth not being allowed to slip. Two smooth wheels touching would just slip. Teeth lock them together, so one wheel cannot turn without turning the other exactly.
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Step 2 of 5
Different sizes turn at different rates.
A gear with 10 teeth driving one with 30 must go round three times to turn the big one once. Count teeth and you know the ratio exactly.
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Step 3 of 5
Slower means stronger.
The trade is the same as a lever: going three times slower gives three times the turning force. That is how a small motor moves a heavy load.
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Step 4 of 5
A bicycle lets you pick the trade.
Low gear: the pedals spin fast, the wheel turns slowly, and you can climb a hill. High gear: the opposite, and you fly along the flat.
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Step 5 of 5
Gears also reverse and redirect.
Meshed gears turn opposite ways, so a gear train can reverse direction. Angled teeth can even turn the motion through 90 degrees — as in a hand drill.
A clock is a gear train: the same rotation, geared down again and again, drives the second, minute and hour hands.
The short version
Count the teeth and you have the ratio: 10 driving 30 means three turns for one, and three times slower is three times the turning force. Meshed gears also reverse direction, and angled ones turn motion through 90 degrees.
Try it yourself
Change gear on a bicycle while it is upside down and turn the pedals. Watch how many wheel turns each pedal turn buys.