What is a Gear?

   A gear is a wheel with teeth designed to fit into the teeth of a similar wheel. Gears are used to transmit power from one section of a machine to another or from one machine to another. Often they also have the job of changing the direction of power transmission or of varying the speed of different parts of the machine, stepping it up or slowing it down. For example, if a large wheel with 32 teeth is meshed with a small wheel having only 16 teeth, the small wheel will have to revolve twice for every rotation of the large wheel. While the large wheel moves slowly, it forces the small wheel to turn rapidly, and not only is the power transmitted from one wheel to the next but also the speed of the moving parts is varied.
   The simplest form of gear is the spur gear, just a toothed wheel, or cog-wheel. When two spur gears of dif­ferent sizes, as in the example, are meshed, the smaller of the two gears is known as a pinion gear. Some other common types of gears are the bevel gear, the helical gear, and the worm gear. All these types are used in automobiles. The worm gear is used in the steering mechanism and in the speedometer. The worm gear is also sometimes used in the driving mechanism. However, the other types find more common application here. The pinion and bevel gears are found at the differential end of the drive shaft; helical gears are found in the differential and transmission systems. When the driver shifts gears, he slides one gearwheel backward or forward to mesh with one gearwheel on the countershaft. Since the several gears on this countershaft and the sliding gears each have a different number of teeth, shifting gears changes the ratio of engine speed to wheel speed.