Gears

Rotating gears, with their intermeshed teeth, have played a crucial role in the history of technology. Not only are gear mechanisms important for increasing the applied twisting force, or torque, but gears are also useful for changing the speed and direction of force.

One of the oldest machines is a potter’s wheel, and primitive gears associated with these kinds of wheels probably existed for thousands of years. In the fourth century B.C., Aristotle wrote about wheels using friction between smooth surfaces to convey motions. Built around 125 B.C., the Antikythera Mechanism employed toothed gears for calculating astronomical positions. One of the earliest written references to toothed gears was made by Hero of Alexandria, c. 50 A.D.

Through time, gears have played a crucial role in mills, clocks, bicycles, cars, washing machines, and drills. Because they are so useful in amplifying forces, early engineers used them for lifting heavy construction loads. The speed-changing properties of gear assemblies were put to use when ancient textile machines were powered by the movement of horses or water. The rotational speed of these power supplies was often insufficient, so a set of wooden gears was used to increase the speed for textile production.

When two gears are intermeshed, the rotational speed ratio is the reciprocal ratio of the number n of teeth on the two gears. Thus, a small gear turns faster than its larger partner. The torque ratio has an opposite relationship. The larger gear experiences greater torque, and the higher torque implies lower velocity. This is useful, for example, for electric screwdrivers, in which the motor can produce a small amount of torque at high speed, but we wish to have a slow output speed with increased torque.

Among the simplest gears are spur gears, with their straight-cut teeth.

Helical gears in which the teeth are set at an angle have the advantage of running more smoothly and quietly and usually being able to handle greater torques.