Gear ratio: What is it and how does it work?

When you hear somebody talking about three to one ratios and torque, it may sound like Greek, but what you’re hearing is a very old and still very current technology. What we are discussing is gear ratios, or the science of movement. So how does that work? Simply put, a set of matching teeth, meshing with a different sized wheel, defines the ratio.

This simple diagram breaks the formula down to its simplest terms.

In other words, a large gear with 30 teeth, meshed with a small gear with ten teeth of the same size is a 3 to 1 ratio. If the large gear is the drive gear and the small the driven, the ratio is 3 to 1, if reversed, it is 1 to 3. The ratio controls how fast or slow the motor or device is to work through the rotation.

This x-ray shows the gears driving the precise ratio to keep accurate time in a wrist watch. But gear ratio has evolved to control everything from automobiles to locomotives and direct drive turntables.

The speed ratio for a pair of meshing gears can be computed from the ratio of the radii of the pitch circles and the ratio of the number of teeth on each gear. That sounds complicated, but the idea is very simple. And with the advent of a sliding gear transmission, controlled by a clutch intervention system, transportation moved into the modern age with cars.

This vintage movie released by General Motors is the best example of a complex gear ratio I found. Very cool and very informative. Check it out!