You see gears in just about everything that has spinning parts. For example, 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
**How Stuff Works**
, you will learn about gears, gear ratios and gear trains so that you can understand what all the different gears you see are doing.

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 axis 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 for the larger gear. The fact that one gear is spinning twice as fast as the other results from the **ratio** between the gears, or the **gear ratio **(Check out our gear ratio chart for more info). 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. You can see that if both gears had the same diameter, they would rotate at the same speed but in opposite directions.

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