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For other uses of this term, see Gear (disambiguation). Gears, toothed wheels or cogs are positive type drives which are used to transmit motion between two shafts or a shaft and a component having linear motion, by meshing of two or more gears. They have advantage over other drives like chains, belts etc. in case of precision machines where a definite velocity ratio is of importance and also in case where the driver and the follower are in close proximity; the downside is that gears are more expensive to manufacture and their operating cost is also relatively high. Gears of differing size are often used in pairs for a mechanical advantage, allowing the torque of the driving gear to produce a larger torque in the driven gear at lower speed, or a smaller torque at higher speed. The larger gear is known as a wheel and the smaller as a pinion. This is the principle of the automobile transmission, allowing selection between various mechanical advantages. The ratio of the rotational speeds of two meshed gears is called the Gear ratio. A gearbox is not an amplifier or a servomechanism. Conservation of energy requires that the amount of power delivered by the output gear or shaft will never exceed the power applied to the input gear, regardless of the gear ratio. Work equals the product of force and distance, therefore the small gear is required to run a longer distance and in the process is able to exert a larger twisting force or torque, than would have been the case if the gears were the same size. There is actually some loss of output power due to friction.
Spur gears The most common type of gear wheel, spur gears, are flat and have teeth projecting radially and in the plane of the wheel. The teeth of these "straight-cut gears" are cut so that the leading edges are parallel to the line of the axis of rotation. These gears can only Double helical gears Double helical gears, invented by André Citroën and also known as herringbone gears, overcome the problem of axial thrust presented by Single helical gears by having teeth that are 'V' shaped. Each gear in a double helical gear can be thought of as two standard, but mirror image, helical gears stacked. This cancels out the thrust since each half of the gear thrusts in the opposite direction. They can be directly interchanged with spur gears without any need for different bearings. Bevel gears Where two axles cross at point and engage by means of a pair of conical gears, the gears themselves are referred to as bevel gears *. These gears enable a change in the axes of rotation of the respective shafts, commonly 90°. A set of four bevel gears in a square make a differential gear, which can transmit power to two axles spinning at different speeds, such as those on a cornering automobile. Helical gears can also be designed to allow a ninety degree rotation of the axis of rotation. Worm gear If the axles are skewed, that is, non-intersecting, then a worm gear can be used. This is a gear that resembles a screw, with parallel helical teeth, and mates with a normal spur gear. The worm is always the driving gear. The worm gear can achieve a higher gear ratio than spur gears of a comparable size. Designed properly, a built in safety feature can be obtained. This gear style will self-lock if power is lost to the drive (worm). It doesn't work if the pinion is powered. Sun gear The central gear of a Planetary gear Sector gear A sector gear is merely a segment of a spur gear, such as one half or one quarter of the circumference, but still attached to the axle in the normal fashion. Such a gear will operate normally as long as the gear with which it meshes does not drive off the edge of the sector, for instance in a worm and sector automotive steering gear or its descendant the recirculating ball. It is useful for saving space and weight when only limited movement is necessary rather than the full 360 degrees of rotation. Rack and pinion Torque can be converted to linear force by a rack and pinion. The pinion is a spur gear, and meshes with a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s). A crown gear See also Crown gear A crown gear or contrate gear is a special form of bevel gear which has teeth at right angles to the plane of the wheel; it meshes with a straight cut spur gear or pinion on a right-angled axis to its own, or with an escapement such as found in mechanical clocks. Simple gears suffer from backlash, which is the error in motion that occurs when gears change direction, resulting from hard to eliminate manufacturing errors. When moving forwards, the front face of the drive gear tooth pushes on the rear face of the driven gear. When the drive gear changes direction, its rear face is now pushing on the front face of the driven gear. Unless deliberately designed to eliminate it, there is slight 'slop' in any gearing where briefly neither face of the driving gear is pushing the driven gear. This means that input motion briefly causes no output motion. Assorted schemes exist to minimize or avoid problems this creates. Epicyclic gearing See epicyclic gearing, also known as a planetary gear. Shifting of gears In some machines it is necessary to change the gear ratio to suit the task. There are several ways of doing this. For example: Friction and wear between two gears is highly dependent on the profile of the teeth. The tooth form used for most applications is involute but there are other tooth forms such as cycloidal (used in mechanical clocks) or rack (used in automobile steering). Gear materials Numerous nonferrous alloys, cast irons, powder-metallurgy and even plastics are used in the manufacture of gears. However steels are most commonly used because of their high strength to weight ratio and low cost. See also | ||||||||
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