How Gears Work

by Marshall Brain
Source: HowStuffWorks

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:

1. To reverse the direction of rotation
2. To increase or decrease the speed of rotation
3. To move rotational motion to a different axis
4. 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.

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.