Engines use pistons and cylinders, so the power they produce is a continual back-and-forth, push-and-pull, or reciprocating motion. But many machines (and virtually all vehicles) rely on wheels that turn round and round—in other words, rotational motion. There are various different ways of turning reciprocating motion into rotational motion (or vice-versa). The wheels are driven by a crank and connecting rod: a simple lever-linkage that connects one side of a wheel to a piston so the wheel turns around as the piston pumps back and forth.

An alternative way to convert reciprocating into rotational motion is to use gears. This is what brilliant Scottish engineer James Watt (1736–1819) decided to do in 1781 when he discovered the crank mechanism he needed to use in his improved design of steam engine was, in fact, already protected by a patent. Watts design is known as a sun and planet gear) and consists of two or more gear wheels, one of which (the planet) is pushed up and down by the piston rod, moving around the other gear (the Sun), and causing it to rotate.