Cranks and cams
by Chris Woodford. Last updated: September 6, 2016.
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.
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 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. But most of the time the box just sits there, motionless, with the cam slowly spinning beneath it. If you look closely at the way I've drawn the cam, you can see the secret of how it works. Three "quarters" of it is like a circle, so anything resting on it for three quarters of the time will neither rise nor fall, but stay motionless. The other quarter stretches out into an ellipse shape and that's the part that lifts and lowers every time it reaches the top.
The object that moves up and down, or otherwise tracks the cam's movement, is called (surprise, surprise) the follower. It could be a simple rod or lever (as I've shown here), a pointer, a rotating roller, or something more intricate and complex. The cam, too, can vary in design. A classic cam is oval (pear-shaped) like the one I've drawn here, but others are heart-shaped, hexagonal, round but eccentric (a circle that rotates about an off-centered point), elliptical (a more symmetrical oval than our pear-shaped cam), snail-shaped (a circle with a tail), or pretty much anything else you can think of. Where the shape of the cam controls the path that the follower traces, both its size and shape control the timing of its motion. So, for example, if I stretched my green oval taller, the blue box would not only move higher but take longer to return to position. That can be handy if cams are being used for things like opening and closing valves on engines.
Artwork: Cam shapes: Here are eight typical designs, but you can make cams any shape and size you need to (even a completely irregular shape). The cam turns around the blue circle in each case.
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...
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.
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 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.
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.
Photo: Bicycle pedals would be better referred to as "bicycle cranks," because it's the cranks attached to the pedals that do the work!
Force or speed?
Cranks are closely related to levers and gears: all three of these can be used to magnify force or speed, although not both at the same time. Suppose you're turning the handle of the clockwork radio in our top picture. Your hand goes round in a larger circle than the mechanism in the center, so it's traveling faster. A crank used in this way reduces speed and increases force. Most cranks work like this, providing leverage to give extra force. A coffee grinder is another good example: it takes quite a lot of force to power the crushers (burrs) that smash the beans to pieces and the handle of the grinder multiplies the force you apply with your hands. If you use a crank the opposite way, using the power from a cylinder and piston to drive a rotating wheel, you can use it to increase the speed of a machine, although the force you get will be reduced at the same time.
Why can't you increase the speed and the force simultaneously? Because of a basic law of physics called the conservation of energy. Suppose you're cranking the clockwork radio. You apply a certain force to the handle to move it a certain distance in one second. That takes a certain amount of energy, which your body supplies. Now the law of conservation of energy tells us nothing can magically create energy out of thin air: the energy you supply to the outside (the crank handle) must be the same (or a bit less) than the energy received at the inside (the center of the crank where the clockwork mechanism is located). We know we're getting more force at the center of the crank than at the outside; we also know that the inside of the crank turns more slowly (less distance each second) than the outside. So the greater force is compensated by that force moving a shorter distance each second. Every second, the energy supplied to the inner, clockwork mechanism at the center (the larger force times the shorter distance) is virtually the same as the energy we put in at the outer part of the crank (the smaller force times the longer distance).
Artwork: How a crank works: If you turn the outside of the crank through a quarter circle, it moves a distance from point A to point B. The inside of the crank moves a much shorter distance from point C to point D. Since the distance AB is much bigger than the distance CD, and both these distances are covered in the same time, it's clear the outside of the crank is moving faster than the inside, so the crank reduces speed at the center. What's less obvious is that the crank increases force in exactly the same proportion. So if it gives a ten-fold reduction in speed, it also gives a ten-fold increase in force.