Gears! What are they good for?

We talk a lot on this blog about gears, and we take for granted that our readers know all about the subject matter. But since we’re getting some new readers, we thought it would be a good idea to cover some gear basics. To start out, a gear is just a part of a transmission device; it helps transmit rotational torque by applying force to another gear or toothed piece of equipment. Gears are found, for instance, in the transmission of your car and, of course, on your bicycle. They allow you to multiply your force or to change the rotational speed or direction of a force.

Gears are extremely useful because of this force multiplication. Since the rotational speed of a gear is in proportion to its circumference and speed of rotation, a larger gear will turn more slowly than a smaller gear with which it’s meshed. Think of bicycle tire spinning: The outside edge of the tire is spinning slower than the inside edge, but both edges are making the same number of rotations. A larger gear has more teeth than a smaller gear, so when the two meshed gears are turning, the smaller gear will have made more revolutions than the larger gear. This relationship is called the “gear ratio,” and it generates the mechanical advantage of gears.

Gear ratio is the relation between the number of teeth on two gears that are meshed.
You can express this relation mathematically. For example, if one gear with 26 teeth is driven by a gear with 14 teeth, the gear ratio is 1/1.86, or 1:1.86. (That is, the mathematical expression of gear ratio is the number of teeth on one gear divided by the number of teeth on the other gear, basically.) A piece of equipment’s gear ratio is used to determine its performance and capability.

(An automobile transmission: Look a those gears!)

Gear ratios are specifically determined by manufacturers of engines and other equipment to achieve a certain result. In some contexts--such as in automobile transmissions--gear ratios can be varied to adapt to the car’s needs at different rates of speed. The lowest gear in a car’s transmission will have a high gear ratio, such as 2:1 or 3:1, which allows the car to achieve a smooth start from a full stop. However, since the engine has to make two or three revolutions for every revolution of the transmission the engine cannot make the car in low gear go very fast in first gear. In second gear, the gear ratio is lower, meaning that the engine does not have to work as hard to give the car additional acceleration. In the higher gears, the gear ratios get progressively lower. These are good for moderate acceleration and reducing the number of engine revolutions needed to keep the car at a comfortable cruising speed. The gears in a transmission, therefore, serve to either magnify or reduce the output of an engine, based on the car’s speed at the time.

We hope that this little post helped to clear up some questions about what exactly gears are and how they function in a practical, day-to-day capacity!