You see
gears in
just about everything that has spinning parts. Car engines
and transmissions
contain lots of gears. If you ever open up a VCR and look
inside, you will see it is full of gears. Wind-up, grandfather
and pendulum
clocks contain plenty of gears, especially if they have
bells or chimes. You probably have a power meter on the side
of your house, and if it has a see-through cover you can see
that it contains 10 or 15 gears. Gears are everywhere where
there are engines
and motors
producing rotational motion.

In this edition of HowStuffWorks,
you will learn about gear ratios and gear trains so you'll
understand what all of these different gears are doing. You
might also want to read How Gears
Work to find out more about different kinds of gears and
their uses.

Putting Gears to Work Gears are generally
used for one of four different reasons:

To reverse the direction of rotation

To increase or decrease the speed of rotation

To move rotational motion to a different axis

To keep the rotation of two axes synchronized

You can see effects 1, 2 and 3 in the figure above. In this
figure, you can see that the two gears are rotating in
opposite directions, that the smaller gear is spinning twice
as fast as the larger gear, and that the axis of
rotation of the smaller gear is to the right of the axis
of rotation of the larger gear.

The fact that one gear is spinning twice as fast as the
other is because of the ratio between the gears -- the gear
ratio. In this figure, the diameter of the gear on
the left is twice that of the gear on the right. The gear
ratio is therefore 2:1 (pronounced "two to one"). If you watch
the figure, you can see the ratio: Every time the larger gear
goes around once, the smaller gear goes around twice. If both
gears had the same diameter, they would rotate at the same
speed but in opposite directions.

Understanding the Concept of Gear
Ratio Understanding the concept of the gear ratio is
easy if you understand the concept of the circumference
of a circle. Keep in mind that the circumference of a circle
is equal to the diameter of the circle multiplied by Pi (Pi
is equal to 3.14159...). Therefore, if you have a circle or a
gear with a diameter of 1 inch, the circumference of that
circle is 3.14159 inches.

The following figure shows how the circumference of a
circle with a diameter of 1.27 inches is equal to a linear
distance of 4 inches:

Let's say that you have another circle whose diameter is
0.635 inches (1.27 inches / 2), and you roll it in the same
way as in this figure. You'll find that, because its diameter
is half of the circle's in the figure, it has to complete two
full rotations to cover the same 4-inch line. This explains
why two gears, one half as big as the other, have a gear ratio
of 2:1. The smaller gear has to spin twice to cover the same
distance covered when the larger gear spins once.

Most gears that
you see in real life have teeth. The teeth have three
advantages:

They prevent slippage between the gears. Therefore,
axles connected by gears are always synchronized exactly
with one another.

They make it possible to determine exact gear ratios.
You just count the number of teeth in the two gears and
divide. So if one gear has 60 teeth and another has 20, the
gear ratio when these two gears are connected together is
3:1.

They make it so that slight imperfections in the actual
diameter and circumference of two gears don't matter. The
gear ratio is controlled by the number of teeth even if the
diameters are a bit off.

Gear Trains To create large gear ratios,
gears are often connected together in gear trains, as
shown here:

The right-hand (purple) gear in the train is actually made
in two parts, as shown above. A small gear and a larger gear
are connected together, one on top of the other. Gear trains
often consist of multiple gears in the train, as shown in the
next two figures.

In the case above, the purple gear turns at a rate twice
that of the blue gear. The green gear turns at twice the rate
of the purple gear. The red gear turns at twice the rate as
the green gear. The gear train shown below has a higher gear
ratio:

In this train, the smaller gears are one-fifth the size of
the larger gears. That means that if you connect the purple
gear to a motor spinning at 100 revolutions per minute (rpm),
the green gear will turn at a rate of 500 rpm and the red gear
will turn at a rate of 2,500 rpm. In the same way, you could
attach a 2,500-rpm motor to the red gear to get 100 rpm on the
purple gear. If you can see inside your power meter and it's
of the older style with five mechanical dials, you will see
that the five dials are connected to one another through a
gear train like this, with the gears having a ratio of 10:1.
Because the dials are directly connected to one another, they
spin in opposite directions (you will see that the numbers are
reversed on dials next to one another).

Worm Gears If you want to create a high gear
ratio, nothing beats the worm gear. In a worm gear, a
threaded shaft engages the teeth on a gear. Each time the
shaft spins one revolution, the gear moves one tooth forward.
If the gear has 40 teeth, you have a 40:1 gear ratio in a very
small package. Here's one example from a windshield
wiper.

A mechanical odometer
is another place that uses a lot of worm gears:

There are three worm gears visible in this
odometer. See How
Odometers Work for more
information.

Planetary Gears There are
many other ways to use gears. One specialized gear train is
called a planetary gear train. Planetary gears solve
the following problem. Let's say you want a gear ratio of 6:1
with the input turning in the same direction as the output.
One way to create that ratio is with the following three-gear
train:

In this train, the blue gear has six times the diameter of
the yellow gear (giving a 6:1 ratio). The size of the red gear
is not important because it is just there to reverse the
direction of rotation so that the blue and yellow gears turn
the same way. However, imagine that you want the axis of the
output gear to be the same as that of the input gear. A common
place where this same-axis capability is needed is in an electric
screwdriver. In that case, you can use a planetary gear
system, as shown here:

In this gear system, the yellow gear (the sun)
engages all three red gears (the planets)
simultaneously. All three are attached to a plate (the
planet carrier), and they engage the inside of
the blue gear (the ring) instead of the outside.
Because there are three red gears instead of one, this gear
train is extremely rugged. The output shaft is attached to the
blue ring gear, and the planet carrier is held stationary --
this gives the same 6:1 gear ratio. You can see a picture of a
two-stage planetary gear system on the electric
screwdriver page, and a three-stage plenetary gear system
of the sprinkler
page. You also find planetary gear systems inside automatic
transmissions.

Another interesting thing about planetary gearsets is that
they can produce different gear ratios depending on which gear
you use as the input, which gear you use as the output, and
which one you hold still. For instance, if the input is the
sun gear, and we hold the ring gear stationary and attach the
output shaft to the planet carrier, we get a different gear
ratio. In this case, the planet carrier and planets orbit the
sun gear, so instead of the sun gear having to spin six times
for the planet carrier to make it around once, it has to spin
seven times. This is because the planet carrier circled the
sun gear once in the same direction as it was spinning,
subtracting one revolution from the sun gear. So in this case,
we get a 7:1 reduction.

You could rearrange things again, and this time hold the
sun gear stationary, take the output from the planet carrier
and hook the input up to the ring gear. This would give you a
1.17:1 gear reduction. An automatic
transmission uses planetary gearsets to create the
different gear ratios, using clutches
and brake bands to hold different parts of the gearset
stationary and change the inputs and outputs.

An Example Imagine the following situation:
You have two red gears that you want to keep synchronized, but
they are some distance apart. You can place a big gear between
them if you want them to have the same direction of rotation:

Or you can use two equal-sized gears if you want them to
have opposite directions of rotation:

However, in both of these cases the extra gears are likely
to be heavy and you need to create axles for them. In these
cases, the common solution is to use either a chain or
a toothed belt, as shown here:

The advantages of chains and belts are light weight, the
ability to separate the two gears by some distance, and the
ability to connect many gears together on the same chain or
belt. For example, in a car engine,
the same toothed belt might engage the crankshaft, two camshafts
and the alternator. If you had to use gears in place of the
belt, it would be a lot harder.

For more information on gears and their applications, check
out the links on the next page!