Explainer #16 · Machines & Mechanisms
How do gears work?
A gear is a wheel with teeth. When two gears touch, the teeth of one sit in the gaps of the other, so one can't turn without the other. The second gear always turns the opposite way. If a small gear turns a gear with twice as many teeth, the big gear turns only once for every two turns of the small one: slower, but with twice the turning force (100 Nm in becomes 200 Nm out). Swap them round and you get speed instead of force. It's just like the gears on a bike: in the easy gear you pedal fast and climb hills slowly, in the hard gear you pedal slowly and fly along on the flat.
The principle
A gear is a wheel with teeth. You find gears in bikes, clocks, kitchen mixers, drills and almost every machine that turns. Their job is to pass turning from one place to another, and to change how fast and how hard it turns on the way.
Step 1: The teeth lock together. Each tooth on one gear sits in a gap on the other gear. When one gear turns, its teeth push on the teeth of the other, so it has to turn as well. The teeth can’t slip past each other, which is why gears are used when the timing has to be exact, like in a clock.
Step 2: The direction flips. Turn the top gear one way and its teeth push the other gear round the opposite way. Think of two people holding hands and spinning around each other’s hands: where they touch, both move in the same direction, so the wheels themselves turn in opposite directions.
Step 3: Small turns big = slow but strong. In the video the big gear has twice as many teeth as the small one. Every tooth that passes counts the same on both gears, so the big gear only makes one turn for every two turns of the small one. In return it turns with twice the force. In the video the small gear is turned with 100 Nm (newton-metres, a way of measuring turning force), and the big gear comes out with 200 Nm. It’s like opening a jar: a jar with a wide lid is easier to twist open than a narrow one. That’s the easy gear on a bike: you pedal fast, the bike goes slowly, but climbing a hill feels light.
Step 4: Big turns small = fast but weaker. Now turn the big gear instead. One turn of the big gear spins the small gear twice, so it goes twice as fast, but with only half the force: 200 Nm in on the big gear becomes 100 Nm out on the small one. That’s the hard gear on a bike: you pedal slowly and push harder, and the bike flies along on the flat.
Step 5: Gears in a row. Add a third gear and each gear flips the direction again, so the first and the third gear turn the same way. The gear in the middle doesn’t change the speed of the last one; that only depends on the sizes of the first and the last gear. The same goes for force: turn the top gear with 100 Nm and the big middle gear gets 200 Nm, but it hands that on to a small gear again, so the bottom gear ends up with 100 Nm, just like the top. The middle gear only flips the direction. Long rows of gears like this carry turning across a machine, for example inside a clock or a printer.
Common myth
Gears create extra power
Gears can’t make energy out of nothing. They only trade one thing for another: more turning force means less speed, and more speed means less force. Put in one turn’s worth of effort and, at best, you get the same amount back out, just in a different form. Real gears even lose a little along the way to rubbing and noise.How this video was made: Rendered 3D animation, simplified and drawn flat. The gears are shown with a simple tooth shape, and a white dot on each gear makes it easy to count the turns; a ring shows where each dot started.
