Gears change turning — Engineering, 4–6 years
Gears are toothed wheels that pass turning from one part to another. They can make a turn faster, slower or go in the opposite direction.
What gears do
A gear is a wheel with little teeth around its edge. When one gear turns, its teeth push the teeth of the next gear, so that one turns too. Two touching gears turn in opposite directions, and their sizes can change how fast they turn.
Why use gears?
People needed one turning part to make another part turn, even when the useful part was somewhere else. A smooth wheel could slip, so teeth were added to keep the turning together. Different-sized gears also help when a machine needs more speed or more pushing strength.
A ten-tooth gear
Imagine a small gear with 10 teeth touching a larger gear with 20 teeth. First, turn the small gear through one full turn: its 10 teeth push along all the teeth of the large gear. The large gear therefore turns half a turn, and it turns the opposite way. One turn has become a slower half-turn.
The bigger gear is not faster
It is easy to think that the larger gear must turn faster because its edge travels around a bigger circle. In a pair of touching gears, the teeth meet at the same rate, so the larger gear needs more time to move all the way round. It turns more slowly, although its edge travels farther in one turn.
Gears in everyday things
A bicycle uses gears between the pedals and the back wheel. A low gear helps when going uphill because the wheel turns less for each pedal turn, while another gear helps on flatter ground. Clocks, toy cars and some kitchen machines also use gears to pass turning from one part to another.
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