Gears: toothed wheels that trade speed for force
Every mechanical clock is a gear machine. Here is how a pair of gears works, and how a row of them makes a train.
How this site works. Every fact is in our own words and linked to its source, beside it. Where good sources disagree, we show each version in a “Sources differ” box.
A gear is a toothed wheel on a turning shaft. Britannica
Gears work in pairs: the teeth of one mesh with the teeth of the other, passing on turning motion and turning force without slipping. Britannica
Small drives big: slower but stronger
The smaller gear of a pair is called the pinion. Britannica
If the small pinion drives a bigger gear, the pair slows the motion down and boosts the turning force; drive it the other way and it speeds up but turns more weakly. Britannica
A driven gear with twice the pinion’s teeth turns with twice the force at half the speed. Britannica
Our own rule of thumb: count the teeth. The ratio of the teeth is the ratio of the speeds, the other way up. Try it with any numbers in the gear playground.
Round, smooth and in mesh
Most gears are round, and round gears keep the speed ratio between their shafts constant. Britannica
Gears with teeth on non-round shapes make the speed ratio change as they turn. Britannica
To run smoothly, gear teeth must be cut to a carefully chosen shape. Britannica
A spur or helical pinion needs at least five teeth to stay in mesh all the time. Britannica
Four kinds of gear
Gears on parallel shafts can have straight teeth (spur gears) or twisted, screw-like teeth (helical gears). Britannica
Shafts that meet at an angle use bevel gears, with teeth on cut-off cones. Britannica
A worm looks like a screw, and its gear like part of a long nut bent round a cylinder. Britannica
The most common angle between shafts that are not parallel is a right angle. Britannica
Worms and trains: big ratios
Because a worm is basically a screw it can have just one tooth, which makes a worm and gear good for a big speed change in one step. Britannica
For a big ratio between parallel shafts you may need several gear pairs in a row, called a train. Britannica
Gears before clocks
In a medieval-style watermill, a series of gears carries the stream’s power from the waterwheel to the millstone. Britannica
X-ray images showed 30 gear wheels inside the fragments. Britannica Nothing geared that complicated is known again until medieval cathedral clocks, a thousand years later. Britannica
Gears count as a kind of the wheel and axle, one of the six simple machines.
Next: inside a clock →