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
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
Photo: "Clockwork" is literally how clocks work. This is the clockwork mechanism
inside the Union Station Tower Clock in Portland, Oregon, which dates from 1896. Credit: Photographs in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
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 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 much force it can produce (for climbing inclines, perhaps).
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.
Photo: It may not look much (left), but even the simplest clockwork toy is a perfect example of miniaturized mechanical engineering (right)! 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.
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.
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.
Artwork: A typical watch mainspring. The tightly wound spring (red) is entirely contained inside a cylindrical box called the barrel (gray) that has gear teeth around its edge. The longer and thinner the spring, the more energy it can hold inside a barrel of a certain size. Artwork from US Patent 525,265: Mainspring-barrel for watches by Agile N. Gauthier, patented August 28, 1894, courtesy of US Patent and Trademark Office.
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
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.
How much energy, exactly? Clearly, the size and tension of
the spring are crucial. The harder a spring
is to turn and the longer you wind it, the more energy it will store.
But you can be much more exact than that if you want to be:
there are mathematical equations that tell you how much torque
(turning force) and stored energy you can achieve with a spring
of a certain length, width, thickness, and stiffness (measured
by the Young's modulus
of the material it's made from). I'm not going to go into the math
in any more detail (you can find a brief outline of it here if you're interested). Cutting to the chase, it's no surprise to find that a longer or thinner spring (one that you can wind
up with more turns) stores more energy, while a shorter or thicker spring gives more torque.
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 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.
Animation: How two opposing cranks (blue) can make a robot walk. The cranks are powered by
two wheels driven from the same axle. Whether this robot would actually walk or just wobble from side to side
is a matter for debate (and experiment).
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.
Animation: 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).
How will it work in practice?
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, and how much force they need to produce through their gears to make their own parts
move smoothly). Then you have to choose a spring that can store enough energy
to keep the mechanism working for a while, and gears that can produce the right amount of
torque (turning force) to do something useful. Real cars have gearboxes so they can
produce more force or speed to suit the driving conditions (starting from standstill
or racing down the highway), and large fuel tanks so they can do that for a decent
amount of time; exactly the same principle applies to toy cars (and other clockwork
So that's clockwork for you, in a nutshell. Who needs batteries when clockwork mechanisms are so much fun?
How does a clockwork toy work?
Now we've looked at the basic idea of clockwork, let's peek inside an actual clockwork machine:
the clockwork smiley man in our top photo. If you're going to try this, be careful of the mainspring:
it's a tightly compressed bit of metal with a sharp edge that could whip out and hit you in the face. Eye
protection is a good idea... and take care!
First, we take off the yellow outer case and expose the crux of the mechanism. What we've got here is a
Geneva drive that makes the legs
hop intermittently. Here's how it works:
You wind the white plastic crown.
The mainspring inside the white case stores up the energy. You can just see the dark, sinister shadow of the
spring looming inside the case like a shark moving about underwater!
The gears inside the case take power from the spring and drive the single crank on the outside of the case at
fairly low speed.
The crank has a small plastic knob protruding from it. As it turns, it moves up and down the pink slot, rocking
the top part of the legs back and forth.
Thanks to the pivot, as the top part of the legs rocks, the feet jump up and down.
Now if we get rid of the legs and break into the white case, we can see first the gearbox (left)
and then, taking off the gears and another plastic layer, the mainspring underneath (right). This is what the spring looks like when it's totally wound down. When it's wound up tightly, it fits entirely inside the white box. It's quite hard to squeeze sturdy metal into such a small space—and that's why it can store energy so effectively: the harder you have to work to compress a
spring, the more energy it can hold:
Clockwork toys from yesteryear
Ages before PlayStations, long before the first battery-powered toys, children still needed entertaining. Back in the 19th century, it was clockwork that pulled off the tricky job of keeping kids amused. I've dug back through the archives of the US Patent and Trademark Office to find a few examples of clockwork toys that illustrate the principles I've been explaining in this article.
Artwork: A simple clockwork boat from US Patent 301,846: Toy
Boat by William A. Wright, patented July 8, 1884, courtesy of US Patent and Trademark Office.
The first one is a basic toy boat with a clockwork propeller. You wind the crown (blue) at the top to tighten the mainspring (red). As the spring slowly unwinds, it drives a series of gears (green) and a central driveshaft (orange) that spins the propeller (purple). This is just about the most basic clockwork mechanism you could imagine. The only technical challenge for the inventor would have been figuring out how many gears to use to make the propeller turn at the right speed for exactly the right amount of time: not so fast that the spring wound down immediately; not so slow that the boat didn't really go anywhere.
Artwork: A clockwork gymnast powered by an oscillating cam mechanism from US Patent 140,883: Automatic Toys by Henry Brower, patented July 15, 1873, courtesy of US Patent and Trademark Office.
The second example is much more ingenious and interesting, because it uses a cam to produce a toy with more irregular and unpredictable movement. In Figure 1, on the left, we can see a toy gymnast with his hands soldered to a central green axle that disappears into the mysterious box on the right, where the clockwork mechanism is concealed. Thanks to his pivoted arms, the gymnast flips up and down, back and forth, doing a variety of athletic moves that are quite hard to predict. But the only thing that powers his movements is the green axle to which his hands are connected. How does it work—and how does it produce unpredictable movement when all it can do is rotate?
We can see the mechanism on the right, which I've colored and numbered to make it easier to understand. At the top (1), there's a fairly conventional mainspring. As it unwinds, it powers a pair of red gears (2) that are meshed together, so they rotate in opposite directions. The gear on the left has a cam firmly attached to it (3). As the cam rotates, it pushes the entire vertical blue bar (4) first to the right and then back to the left—so it converts the gear's rotary motion into reciprocating motion. The blue bar pivots about a point at the bottom (5) and is held fairly firm by a spring (purple) so it moves smoothly. In the middle of the blue bar, there's a yellow gear with the axle (6) running through it to which the gymnast's hands are attached. As the blue bar moves left-right, left-right, the yellow gear alternately meshes with the red gear on the left, then the red gear on the right, and back again. The two red gears rotate in opposite directions, so the yellow gear must turn first one way, then the other way, with a brief pause as it changes direction. Since the yellow gear powers the gymnast, its constant reversals give the man his apparently unpredictable movement. Simple, but actually quite ingenious!
And here are two more: a doll that uses a mainspring (blue) to turn a wheel (red) and crank (green) that moves its arms back and forth in a crawling motion; and a bear that rocks up and down powered by a mainspring (blue) and gear wheels (red circles) that operate the various body parts using long cranks inside the body (red lines).
New and Complete Clock and Watchmakers' Manual by Mary Booth. John Wiley, 1869. Forget the date! This remains a very useful introduction to mechanical clocks and (since it's out of copyright) you can read the whole thing online for free.
Wind-up mobile targets developing regions by Spencer Kelly, BBC News, 2 October 2009. A wind-up cellphone destined for people who lack reliable electricity supplies. The secret here is an ultra-thin clockwork dynamo.
Engineering the 10,000-Year Clock by David Kushner, IEEE Spectrum, October 27, 2011. How do you design a mechanical clock that will keep time for thousands of years into the future? This article explores the long-range engineering challenges behind The Clock of the Long Now.
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