Gear is a mechanical device that transfers rotating or back-and-forth motion and power from one part of a machine to another. Gears vary greatly in size and use. They range from the tiny gears that drive the hands of a watch to the huge gears that turn the propeller of a supertanker. The most common type of gear consists of a metal or plastic wheel or disk with slots called teeth around the edge. Such a gear has an axle in its center. A rack gear is a flat rack with teeth on one side.
Gears always work in pairs. The teeth of one gear mesh (fit together) with the teeth of the other gear. The axle of one gear is connected to a power source, such as an electric motor. When the power axle turns, its gear turns and causes the second gear to rotate in the opposite direction. This action powers the axle of the second gear to do useful work.
Gears may be used to increase or decrease the speed of rotation. They thus enable various parts of machines to operate at different speeds.
Most gears are made of steel, but other materials, such as bronze or nylon and other plastics, are also used. Metal gears are usually lubricated with oil or grease to keep their surfaces smooth and cool while they rotate.
Types of gears include spur gears, helical gears, bevel gears, worm gears, planetary gears, and rack-and-pinion gears. Spur gears are simple gears with straight teeth and axles that are parallel to each other. Helical gears have teeth that are set at an angle to the axle. Such gears are quieter at high speeds, but they tend to generate a sideways force, reducing efficiency. Double helical gears or herringbone gears solve this problem by using two rows of teeth that generate opposite forces.
Bevel gears usually mesh at a right angle to transmit power between intersecting axles. Worm gears have a worm, an endless screw around an axle, that meshes at a right angle with a large gear similar to a spur gear. As the worm turns, it causes the gear to rotate at a much slower speed. Automatic automobile transmissions use planetary gears. Planetary gears are also called epicyclic gears._In such gears, a number of spur gears, called _planet gears, rotate around a central gear called a sun (see Transmission (Planetary gears) ). Rack-and-pinion gears used in automobile steering mechanisms consist of a spur gear that runs back and forth along a rack gear.
