gear, toothed wheel, cylinder, or cone that transmits motion from one part of a machine to another; it is one of the oldest means of transmitting motion. When the teeth of two gears are meshed, turning one gear will cause the other to rotate. In most cases both gears are mounted on shafts so that when one shaft turns, the other also rotates. By meshing two gears of different diameters, a variation in both speed and torque between the two shafts is obtained; the smaller gear in this case is called the pinion. A spur gear consists of a wheel with straight teeth mounted radially either on the inner circumference (internal spur gear) or outer circumference (external spur gear) of the wheel. Two meshed spur gears are used to transmit motion between parallel shafts. A rack and pinion consists of a pinion engaging and transferring motion to or from a special kind of spur gear, called a rack, consisting of a series of teeth in a straight line on a flat surface. The rack and pinion changes linear motion into rotary motion, or vice versa. A helical gear is similar to a spur gear, but its teeth are twisted instead of straight. Helical gears can be used to transmit motion between shafts that do not intersect and are at any angle with respect to each other. A bevel gear has straight or curved teeth on a conical surface near its rim. Bevel gears are used to transmit rotary motion between shafts that are not parallel and that would intersect at an angle if extended. Hypoid gears are special bevel gears used in the differential of an automobile to connect the drive shaft to the rear axle. A worm gear, meshed with a threaded cylinder, or worm, that resembles a screw, is used to transmit motion between perpendicular, nonintersecting shafts. See transmission.
See D. W. Dudley, ed., Gear Handbook (1962); H. J. Watson, Modern Gear Production (1970); R. J. Drago, Fundamentals of Gear Design (1988).
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