Clockwork (windup) mechanisms

Last updated: February 5, 2010.

Batteries not included—as a child, those are just about the most disappointing words you can read when you buy a new toy. In the 1970s and before, that wasn't such a big issue because the vast majority of toys worked an entirely different way. Instead of using electricity stored chemically in batteries, they relied on windup power and clockwork mechanisms. Clockwork has certainly stood the test of time: the earliest clockwork device, known as the Antikythera mechanism, dates from ancient Greece and is thought to be at least 2000 years old. Why has clockwork technology been such a firm favorite for so long? How exactly does it work? Let's take a closer look!

Photo: It may not look much, but even the simplest clockwork toy is a perfect example of miniaturized mechanical engineering! Wind it up and it jumps along on its pink plastic feet. How do all the parts work to make a funny, jumping man? See the box at the bottom of this article for an explanation.

What is clockwork?

Clockwork means, literally, "working like a clock"—that much is obvious! But most modern clocks are electronic: powered by electricity and regulated by quartz crystals, they have relatively few moving parts. If you want to understand clockwork, you need to understand how clocks used to work in the days when you wound them with a key. Like an old-fashioned clock, a clockwork device is completely mechanical and has these essential parts:

• A key (or crown) you wind to add energy.
• A spiral spring to store the energy you add with the key. (Pendulum clocks store energy with weights that rise and fall, but other clocks and windup wristwatches use springs instead.)
• A set of gears through which the spring's energy is released. The gears control how quickly (or slowly) a clockwork machine can do things, but they also control how powerful it is.
• A mechanism the gears drive that makes the device do useful or interesting things. In a clock, the mechanism is the set of hands that sweep around the dial to tell you the time. In a clockwork car, the gears would drive the wheels that power it over your floor.

Adding and storing energy

A basic law of science called the conservation of energy tells us that we can't do anything without energy. If you want a clockwork car to drive across your carpet, you have to give it enough energy to do just that before you release it; in other words, you have to wind it up.

The mainspring

What happens when you wind? If you've ever wound a clockwork toy, you'll know that the key (sometimes it's a little plastic knob called a crown) can be quite stiff and hard to turn. Why is that? When you turn the key, you're tightening a sturdy metal spring, called the mainspring, and storing up energy; the mainspring is the mechanical equivalent of a battery. Clockwork springs are usually thick twists of steel, so tightening them (forcing them to occupy a much smaller space) is actually quite hard work—in both the everyday and the scientific senses of the word. With each turn of the screw, your fingers are doing work (as we say in science): they're moving a force (pushing against the spring's tendency to expand) through a distance—in other words, compressing the spring.

Since you're doing work with your fingers, you're using energy, but that energy doesn't vanish into thin air: it's stored in the spring as potential energy. Tightening the mainspring in a windup toy is like pushing a rollercoaster car up a hill. Just as you can get the energy in a rollercoaster car back by letting it roll down the hill, so you can get the energy back from a mainspring by releasing it to drive a clockwork mechanism—the potential energy is converted into kinetic energy (as well as heat and sound energy) in the whirring gears.

If you want a clockwork device to entertain you (or do something useful) for a while, you need to give it plenty of energy. Windup clocks and watches are designed to have springs that will store enough energy to keep the mechanism working for a day or more. Clockwork toys aren't anything like as well made (or as impressive) and if you get more than a minute or two's entertainment for your thirty seconds or so of winding you're doing well. Generally, more interesting clockwork devices that run for longer have bigger and sturdier springs capable of storing much more energy. The size and tension of the spring control how much energy it will hold. The harder a spring is to turn and the longer you wind it, the more energy it will store.

Using energy

Virtually all clockwork devices have gears, which are wheels with teeth that mesh together. As you'll discover by reading our main article on gears, there are generally two reasons why you use them: to make a wheel go faster (with less force) or to make it go more slowly (with more force). Clockwork mechanisms use gears in both these ways. In a pocket watch, gears transform the speed of a rotating shaft so it drives the second hand at one speed, the minute hand at 1/60 that speed, and the hour hand at 1/3600 the speed. Clockwork toy cars often use gears to make themselves race along at surprising speed: as the mainspring uncoils, it turns a wheel around quite quickly and then gears step this speed up to drive the car's wheels even faster. Something like a clockwork tank would use gears the opposite way so it can climb over obstacles: in this case, the wheels (or tracks) would take power from the spring, step down the speed, and generate more climbing force at the same time (like the low gears you'd use on a bicycle or a car for climbing a hill).

Cams and cranks

Virtually all clockwork toys use their mainspring to generate rotational power—to turn wheels, in other words. If you want them to do something other than turn, roll, or rotate, you have to use a cam or a crank to transform their rotational (round-and-round) motion into reciprocating (back-and-forth) motion.

When you see a clockwork robot walking along, it's probably using cranks driven by wheels to power its legs. The wheels rotate on the same shaft, at the same speed, driven by gears powered from the mainspring, and each leg is connected to the leg by a separate crank. One leg will be connected to the top of one of the wheels, while the other leg will be connected to the bottom of the other wheel. As the two wheels turn, the cranks will move around out of step and the two legs will connect with the ground alternately, making the robot shuffle along.

Slowly moving cams are another way of getting clockwork toys to do interesting things—but only once in a while. Suppose you want to build a clockwork Charlie Chaplin whose bowler hat automatically lifts in the air maybe every 30 seconds or so but stays on his head the rest of the time. You could run a gear from the toy's mainspring and power a cam—an egg-shaped wheel with a lever on top. Each time the point of the cam reaches the vertical, it will push up the lever and Charlie's hat will lift in the air.

Photo: How a cam works: As the green cam turns, the blue box rises into the air. You can use a cam like this, driven by a rotating wheel, to make something happen every so often. The slower the wheel turns, the less often it'll happen.

Some clockwork toys, such as the clockwork smiley man in our top photo, produce intermittent movement using more elaborate mechanisms, such as Geneva drives (effectively, cranks that slide up and down in slots).

Resistance is useless?

If you wind up a clockwork car as much as you can, then let the key go, without putting the car on the ground, you'll hear the gears inside the mechanism screech and squeal as the spring releases its energy amazingly quickly. Since there's very little resistance except friction (the rubbing force between touching surfaces) in the gearbox, there's nothing really for the mechanism to work against and it can deliver energy very fast. Put it on a rug and the energy is delivered much more slowly (and quietly). Now the spring has to work against the resistance of the fabric, which works like a brake on the wheels and the gears that power them.

When you're designing clockwork toys and other devices, you always need to take into account what they're actually going to do (the surfaces they'll work on, for example). Then you have to choose a spring that can store enough energy to keep the mechanism working long enough, and gears that can release that energy at exactly the right rate.

Who needs batteries when clockwork mechanisms are so much fun?