Cranks and cams

Last updated: January 29, 2010.

What sort of problems keep you awake at night? If you'd been an engineer during the Industrial Revolution, tinkering with steam engines and such, cams and cranks were the kind of thing you'd have worried about. They're cunning inventions that convert the push-pull motion of engines and machines into the spinning and whirring motion of wheels—or vice-versa. Cranks and cams are just as useful today and you can find them in everything from car engines and bicycle pedals to windup radios and electric toothbrushes. Here's a quick look at how they work!

Photo: This radio doesn't need batteries because it's powered by a simple hand-crank: when you crank the handle, you spin a small electricity generator (dynamo) inside the case that stores energy in rechargeable batteries. Although you're turning the crank, if you do it for a while you soon get a distinct feeling that you're moving your hand back and forth instead of round-and-round. So the crank is really converting a back-and-forth motion of your hand into rotary motion in the generator. Photo by Robert J. Fluegel courtesy of US Navy.

Cranks

Engines that make their power with pistons usually need a way of converting back-and-forth (reciprocating) motion into round-and-round (rotational) motion—a way of driving a wheel, in other words. Most engines use cranks to do this. A crank is simply an off-center connection that provides energy to (or takes energy from) a rotating wheel. As the crank pushes back and forth, the wheel rotates (or vice-versa). In this example, as the red wheel rotates, the green crank pushes the black and blue connecting rods back and forth, converting the wheel's rotary motion into reciprocal motion. So the red wheel moves round, but the blue rod moves back and forth.

The same mechanism could be used the opposite way to drive the wheel from a piston. You'd just hitch the blue rod up to the piston so that as it moved in and out of its cylinder, the red wheel would go round and round. Steam engine wheels are driven exactly like this.

Cams

Cams generally do the opposite job to cranks: they turn rotary motion into reciprocating motion. Whatever you need to move up and down (or back and forth) rests on top of an oval wheel, sometimes mounted off-center (the cam). As the cam rotates, the object it supports rises up and down. In this example, you can see the blue box rises and falls as the green cam turns round and round. In reality, you'd design the green wheel so the movement of the box was a little smoother—but it's not so easy to draw with a mouse!