Gears, pulleys and cams
Design & Technology · Key Stage 2 · The School
Gears
Two meshed gears turn in OPPOSITE directions. A small gear driving a big one turns it slower but with more turning force; a big gear driving a small one does the reverse. Gears trade speed against force — you never get both, and a bicycle's gears are exactly this trade.
Pulleys
A belt over two wheels transmits movement, and both turn the SAME way — unless you cross the belt, which reverses one. Different wheel sizes change speed just as gears do. Pulleys can drive things a distance apart, which gears cannot.
Cams
A CAM is a shaped wheel. As it rotates, a follower resting on it rises and falls — turning rotation into up-and-down movement. The cam's shape decides the movement: a round-but-off-centre cam gives a smooth rise and fall, a pear shape gives a pause then a lift, a snail shape gives a slow rise and sudden drop.
A driver gear with 10 teeth turns a driven gear with 30. Ratio 30 to 10, which is three to one — the big gear turns once for every three turns of the small one, so it is three times SLOWER and carries three times the turning force. And they turn in opposite directions. Add an idler gear between them and the ratio is unchanged but they now turn the same way, which is what an idler is for.