A boy winds up his clockwork radio with a hand crank.

Cranks and cams

by Chris Woodford. Last updated: March 13, 2011.

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.

Animation showing how a crankshaft turns rotary motion into reciprocal motion.

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.

Animation showing how a cam turns rotary motion into reciprocal motion.

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!

Cranks in action!

It's all very well to talk about things in theory with little animated graphics, but it's much more interesting to see how these things work in actual machines. The following selection of photos shows you some examples of cranks working hard in real life. I've not found any good pictures of cams, but I'll keep looking...

Engine crankshaft Car and boat engines have multiple cylinders that turn a single drive shaft, called the crankshaft. Each cylinder fires at a slightly different time so, at any given moment, there's always at least one cylinder adding power and driving the vehicle along. The cylinders are attached to the crankshaft by rods that connect to the piston rods inside the cylinders. Photo: The shiny crankshaft from a brand new marine engine. Picture by Marco Bernardini of Funadium published on Flickr in 2009 under a Creative Commons Licence.
Steam engine crankshaft Early steam engines were so big that they were permanently fixed in place, often occupying entire huge buildings. An engine like this typically makes power with a single piston moving in and out of a single cylinder. If you need it drive a wheel, you can attach the piston to a beam. As the piston moves up and down, the beam rocks back and forth, pulling on a crankshaft and connecting rod that turn the wheel. Photo: A fairly small beam engine in the engine hall at Think Tank, the science and engineering museum in Birmingham, England. You can just about see the cylinder and piston on the left, the rocking beam on top, and the crankshaft and connecting rod driving the wheel on the right.
Hand-cranked pump Hand cranks are great ways to drive machines with the help of muscle power! Our top photo shows a hand crank being used to generate electricity. Here we have a hand crank turning a rotary pump that's being used to transport fuel down a pipeline. The crank is turning an impeller (a kind of turbine-like device) sealed inside the red casing, which powers the fluid along. Photo: Hand-cranked rotary pump. Picture by Ian Schell courtesy of of US Army.
Bicycle pedals Here's one of the most familiar cranks of them all! We always talk about the "pedals" on a bicycle when we really mean the cranks—two levers that help to multiply the force supplied by our leg muscles as they pump up and down. In theory, the longer the cranks, the better—because that gives more leverage. In practice, though, the cranks have to be short enough for your legs to operate the pedals comfortably and not so long that they bang on the ground. The cranks combine with the gears to make the bicycle just about the most efficient form of transportation so far invented.