How Gear Ratios Work

Other Uses for Gears

There are many other ways to use gears. For example, you can use conical gears to bend the axis of rotation in a gear train by 90 degrees. The most common place to find conical gears like this is in the differential of a rear-wheel-drive car. A differential bends the rotation of the engine 90 degrees to drive the rear wheels:

Another 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. One way to create that ratio is with the following three-gear train:

In this train, the red gear has three times the diameter of the yellow gear, and the blue gear has two times the diameter of the red gear (giving a 6:1 ratio). However, imagine that you want the axis of the output gear to be the same as that of the input gear. A common place to need this same-axis capability 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 engages all three red gears simultaneously. They are all three attached to a plate, and they engage the inside of the blue gear instead of the outside. Because there are three red gears instead of one, this gear train is extremely rugged. The ouput shaft is taken from the plate, and the blue gear is held stationary. You can see a picture of an two-stage planetary gear system on the electric screwdriver page.