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.
