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.
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.
