Have you ever looked inside a grandfather clock or a small
alarm clock, seen all the gears and springs and thought,
"Wow -- that's complicated!"? While clocks normally are fairly
complicated, they do not have to be confusing or mysterious.
In fact, as you learn how a clock works, you can see how clock
designers faced and solved a number of interesting problems to
create accurate timekeeping devices. In this edition of HowStuffWorks,
we'll help you understand what makes clocks tick, so the next
time you look inside one you can make sense of what's
Pendulum clocks have been used to keep time since 1656, and
they have not changed dramatically since then. Pendulum clocks
were the first clocks made to have any sort of accuracy. When
you look at a pendulum clock from the outside, you notice
several different parts that are important to the mechanism of
all pendulum clocks:
There is the face of the clock, with its hour and minute
hand (and sometimes even a "moon phase" dial!).
There are one or more weights (or, if the clock is more
modern, a keyhole used to wind a spring inside the clock --
we will stick with weight-driven clocks in this article).
And of course there is the pendulum itself. In most wall
clocks that use a pendulum, the pendulum swings once per
second. In small cuckoo clocks the pendulum might swing
twice a second. In large grandfather clocks, the pendulum
swings once every two seconds.
Let's Start with the Weight! So let's start
with the weight and see what it is doing. The idea behind the
weight is to act as an energy storage device so that the clock
can run for relatively long periods of time unattended. When
you "wind" a weight-driven clock, you pull on a cord that
lifts the weight. That gives the weight "potential
energy" in the Earth's gravitational field. As we will see
in a moment, the clock uses that potential energy as the
weight falls to drive the clock's mechanism.
So let's say that we wanted to use a falling weight to
create the simplest possible clock -- a clock that has just a
second hand on it. We want the second hand on this simple
clock to work like a normal second hand on any clock, making
one complete revolution every 60 seconds. We might try to do
that, as shown in the figure on the right, simply by attaching
the weight's cord to a drum and then attaching a second
hand to the drum as well. This, of course, would not work. In
this simple mechanism, releasing the weight would cause it to
fall as fast as it could, spinning the drum at about 1,000 rpm
until the weight clattered on the floor.
Still, it's headed in the right direction. Let's say we put
some kind of friction device on the drum -- some sort
of brake pad or something that would slow the drum down. This
might work. We would certainly be able to devise some scheme
based on friction to get the second hand to make approximately
one revolution per minute. But it would only be approximate.
As the temperature and the humidity in the air changed, the
friction in the device would change. Thus our second hand
would not keep very good time.
So, back in the 1600s, people who wanted to create accurate
clocks were trying to solve the problem of how to cause the
second hand to make exactly one revolution per minute. The
Dutch astronomer Christiaan Huygens is credited with
first suggesting the use of a pendulum. Pendulums are useful
because they have an extremely interesting property: The
period (the amount of time it takes for a pendulum to go back
and forth once) of a pendulum's swing is related only to the
length of the pendulum and the force of
gravity. Since gravity is constant at any given spot on
the planet, the only thing that affects the period of a
pendulum is the length of the pendulum. The amount of
weight does not matter. Nor does the length of the arc that
the pendulum swings through. Only the length of the pendulum
Experiment You can prove this fact to
yourself -- that the only thing affecting the period of a
pendulum is the length of the pendulum -- by performing the
following experiment. For this experiment you will need:
A watch with a second hand (or a numeric seconds display
on a digital watch)
For the weight you can use
anything. In a pinch, a coffee mug or a book will do -- it
doesn't really matter. Tie the string to the weight. Then
suspend your pendulum over the edge of the table so that the
length of the pendulum is about 2 feet, as shown here:
Now pull the weight back about a foot and let your pendulum
start swinging. Time it for 30 or 60 seconds and count how
many times it swings back and forth. Remember that number. Now
stop the pendulum and restart it, but this time pull it back
only 6 inches initially so it is swinging through a much
smaller arc. Count the number of swings again through the same
30- or 60-second time period. What you will find is that the
number you get is the same as the first number you counted. In
other words, the angle of the arc through which the pendulum
swings does not affect the pendulum's period. Only the length
of the pendulum's string matters. If you play around with the
length of your pendulum you will find that you can adjust it
so that it swings back and forth exactly 60 times in one
Once someone noticed this fact about pendulums, it was
realized that you could use the phenomenon to create an
accurate clock. The figure below shows how you can create a
clock's escapement using a pendulum.
In an escapement there is a gear with
teeth of some special shape. There is also a pendulum, and
attached to the pendulum is some sort of device to engage the
teeth of the gear. The basic idea that is being demonstrated
in the figure is that, for each swing of the pendulum back and
forth, one tooth of the gear is allowed to "escape."
example, if the pendulum is swinging toward the left and
passes through the center position as shown in the figure on
the right, then as the pendulum continues toward the left the
left-hand stop attached to the pendulum will release its
tooth. The gear will then advance one-half tooth's-width
forward and hit the right-hand stop. In advancing forward and
running into the stop, the gear will make a sound... "tick" or
"tock" being the most common. That is where the ticking sound
of a clock or watch comes from!
One thing to keep in mind is that pendulums will not swing
forever. Therefore, one additional job of the escapement gear
is to impart just enough energy into the pendulum to overcome
friction and allow it to keep swinging. To accomplish this
task, the anchor (the name given to the gizmo attached
to the pendulum to release the escapement gear one tooth at a
time) and the teeth on the escapement gear are specially
shaped. The gear's teeth escape properly, and the pendulum is
given a nudge in the right direction by the anchor each time
through a swing. The nudge is the boost of energy that the
pendulum needs to overcome friction, so it keeps swinging.
So, let's say that you create an escapement. If you gave
the escapement gear 60 teeth and attached this gear directly
to the weight drum we discussed above, and if you then used a
pendulum with a period of one second, you would have
successfully created a clock in which the second hand turns at
the rate of one revolution per minute. By adjusting the
pendulum's length very carefully we could create a clock with
very high accuracy.
However, while accurate, this clock would have two problems
that would make it less-than-useful:
Most people want a clock to have hour and minute hands
You would have to wind the clock about every 20 minutes.
Because the drum makes one revolution every minute, the
weight would unwind to the floor very quickly. Most people
would not like a clock that had to be rewound every 20
Gear Ratio The problem of having to rewind
every 20 minutes is easy to solve. As discussed in How Gear Ratios
Work, you can create a high-ratio gear train that causes
the drum to make perhaps one turn every six to 12 hours. This
would give you a clock that you only had to rewind once a week
or so. The gear ratio between the weight drum and the
escapement gear might be something like 500:1, as shown in the
In this diagram the escapement gear has 120 teeth, the
pendulum has a period of half a second and the second hand is
connected directly to the escapement gear. Each gear in the
weight's gear train has an 8:1 ratio, so the full train's
ratio is 492:1.
You can see that if you let the escapement gear itself
drive another gear train with a ratio of 60:1, then you can
attach the minute hand to the last gear in that train. A final
train with a ratio of 12:1 would handle the hour hand. Presto!
You have a clock!
Now this clock is nice, but it has two problems:
The hour, minute and second hands are on different
axes. That problem is generally solved by using tubular
shafts on the gears and then arranging the gear trains so
that the gears driving the hour, minute and second hands
share the same axis. The tubular gear shafts are aligned one
inside the other. Look closely at any clock face and you can
see this arrangement.
Because all of these gears are connected directly
together, there is no easy way to rewind or set the
clock. That is often handled by having a gear that can
be slipped out of the train. When you pull on the stem of a
wristwatch to set the watch, that is essentially what you
are doing. In the figure above, you might imagine
temporarily removing the small black gear to either wind or
set the clock.
You can see that, even though all the
a clock make it look complicated, what a pendulum clock is
doing is really pretty simple. There are five basic parts:
Weight or spring - This provides the energy to
turn the hands of the clock.
Weight gear train - A high-ratio gear train gears
the weight drum way up so that you don't have to rewind the
clock very often.
Escapement - Made up of the pendulum, the anchor
and the escapement gear, the escapement precisely regulates
the speed at which the weight's energy is released.
Hand gear train - The train gears things down so
the minute and hour hands turn at the right rates.
Setting mechanism - This somehow disengages,
slips or ratchets the gear train so the clock can be rewound
Once you understand these pieces, clocks
are a piece of cake!
Q & A Here's a set of questions from
Watches obviously do not use pendulums, so how do
they keep time? A pendulum is one periodic mechanical
system with a precise period. There are other mechanical
systems that have the same feature. For example, a weight
bouncing on a spring has a precise period. Another example
is a wheel with a spring on its axle. In this case, the
spring causes the wheel to rotate back and forth on its
axis. Most mechanical watches use the wheel/spring
What is the difference between a weight-driven and a
spring-driven clock? Nothing, really. Both a weight
and a spring store energy. In a spring-driven clock you wind
the spring and it unwinds into the same sort of gear train
found on a weight-driven clock.
How does the moon phase dial on a grandfather clock
work? The moon phase dial works just like the hands
of the clock do. The minute hand on a clock moves at the
rate of one revolution every hour. The hour hand moves at
one revolution every 12 hours. The moon phase dial moves at
a rate of one revolution every 56 days or so. The moon's
cycle is 28 days, and the moon phase dial generally has two
moons painted on it.
For more information on pendulums, timekeeping and related
topics, check out the links on the next page!