gear

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 .

Bibliography: See D. W. Dudley, ed., Gear Handbook (1962); H. J. Watson, Modern Gear Production (1970); R. J. Drago, Fundamentals of Gear Design (1988).

gear / gi(ə)r/ • n. 1. (often gears) one of a set of toothed wheels that work together to alter the relation between the speed of a driving mechanism (such as the engine of a vehicle or the crank of a bicycle) and the speed of the driven parts (the wheels). ∎  a particular function or state of adjustment of engaged gears: he was tooling along in fifth gear. 2. inf. equipment that is used for a particular purpose. ∎  a person's personal possessions and clothes. ∎  clothing, esp. of a specified kind: designer gear. ∎ Naut. a ship’s rigging. • v. [tr.] design or adjust the gears in (a machine) to give a specified speed or power output: it's geared too high for serious off-road use. ∎  (gear something to) adjust or adapt something to suit a special purpose or need. PHRASES: in gear with a gear engaged: the captain revved the engines and put them in gear. ∎ fig. done with more energy or effort: I've got to get my act in gear. PHRASAL VERBS: gear down (or up) change to a lower (or higher) gear. gear for make ready or prepared: a nation geared for war. gear up equip or prepare oneself: the region started to gear up for the tourist season.

gear Wheel, usually toothed, attached to a rotating shaft. The teeth of one gear engage those of another in order to transmit and modify speed of rotation and torque.