Gear

Jai Ganesh · 2024-07-22 16:57:16

Gear

Gist

A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another (compatible) toothed part to transmit rotational power.

Summary

Gear, machine component consisting of a toothed wheel attached to a rotating shaft. Gears operate in pairs to transmit and modify rotary motion and torque (turning force) without slip, the teeth of one gear engaging the teeth on a mating gear. If the teeth on a pair of mating gears are arranged on circles, i.e., if the gears are toothed wheels, the ratios of the rotary speeds and torques of the shafts are constant. If the teeth are arranged on noncircular bodies the speed and torque ratios vary.

Most gears are circular. To transmit motion smoothly and with a nonvarying speed ratio at every instant, the contacting surfaces of gear teeth must be carefully shaped to a specific profile. If the smaller of a gear pair (the pinion) is on the driving shaft, the pair acts to reduce speed and to amplify torque; if the pinion is on the driven shaft the pair acts as a speed increaser and a torque reducer. If the driven gear has twice as many teeth as the pinion, for example, the torque of the driven gear is twice the pinion torque, whereas the pinion speed is twice the speed of the driven gear.

The shafts that gears connect must be relatively close, but they may have practically any spatial relationship with respect to one another; they may be parallel or nonparallel and intersecting or nonintersecting. For each of these arrangements of the shafts, gears having appropriate capabilities can be made. Parallel shafts can be connected by gears with teeth that are straight lengthwise and parallel to the shaft axes (spur gears) or by gears with twisted, screwlike teeth (helical gears). Intersecting shafts are connected by gears with tapered teeth arranged on truncated cones (bevel gears). Nonparallel, nonintersecting shafts are usually connected by a worm and gear. The worm resembles a screw, and the gear resembles a quarter section of a long nut that has been bent around a cylinder. The commonest angle between nonparallel shafts, either intersecting or nonintersecting, is a right angle (90°).

Because it is basically a screw, a worm gear may have only one thread (tooth), whereas to maintain continuous contact with parallel shaft gears (spur and helical), the pinion must have at least five teeth. For this reason, to obtain a large speed ratio in a single gear pair, a worm and gear are well suited. If the shafts must be parallel, it may be necessary to use several gear pairs in series (a train) to obtain a large ratio.

Details

A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another (compatible) toothed part to transmit rotational power. While doing so, they can change the torque and rotational speed being transmitted (in inverse proportion) and also change the rotational axis of the power being transmitted. The teeth on the two meshing gears all have the same shape.

The basic principle behind the operation of gears is analogous to the basic principle of levers. Meshing gears of different diameters produce three changes — (i) a change in torque, creating a mechanical advantage, (ii) an inverse change in rotational speed and (iii) a change in the sense of the rotation, a clockwise rotation becoming an anti-clockwise one and vice-versa. The diameters of the gears are measured at a point between the root and tips of the gear teeth called the pitch circle.

A gear may also be known informally as a cog.

Two or more meshing gears, working in a sequence, are called a gear train or a transmission. The gears in a transmission are analogous to the wheels in a crossed, belt pulley system. An advantage of gears is that the teeth of a gear prevent slippage. In transmissions with multiple gear ratios—such as bicycles, motorcycles, and cars—the term "gear" (e.g., "first gear") refers to a gear ratio rather than an actual physical gear. The term describes similar devices, even when the gear ratio is continuous rather than discrete, or when the device does not actually contain gears, as in a continuously variable transmission (CVT). Sometimes a CVT is referred to as an "infinitely variable transmission".

Furthermore, a gear can mesh with a linear toothed part, called a rack, producing movement in a straight line instead of rotation (movement in a circle).

History

The earliest examples of gears date from the 4th century BC in China (Zhan Guo times – Late East Zhou dynasty), which have been preserved at the Luoyang Museum of Henan Province, China. The earliest preserved gears in Europe were found in the Antikythera mechanism an example of a very early and intricate geared device, designed to calculate astronomical positions. Its time of construction is now estimated between 150 and 100 BC. Aristotle mentions gears around 330 BC, (wheel drives in windlasses). He said that the direction of rotation is reversed when one gear wheel drives another gear wheel. Philon of Byzantium was one of the first who used gears in water raising devices. Gears appear in works connected to Hero of Alexandria, in Roman Egypt circa AD 50, but can be traced back to the mechanics of the Library of Alexandria in 3rd-century BC Ptolemaic Egypt, and were greatly developed by the Greek polymath Archimedes (287–212 BC).

A complex geared calendrical device showing the phase of the Moon, the day of the month and the places of the Sun and the Moon in the Zodiac was invented in the Byzantine empire in the early 6th century AD. The worm gear was invented in the Indian subcontinent, for use in roller cotton gins, some time during the 13th–14th centuries. Differential gears may have been used in some of the Chinese south-pointing chariots, but the first verifiable use of differential gears was by the British clock maker Joseph Williamson in 1720.

Examples of early gear applications include:

* 1386 AD: The Salisbury Cathedral clock: it is the world's oldest still working geared mechanical clock.
* The Astrarium of Giovanni Dondi dell'Orologio was a complex astronomical clock built between 1348 and 1364 by Giovanni Dondi dell'Orologio. The Astrarium had seven faces and 107 moving parts; it showed the positions of the sun, the moon and the five planets then known, as well as religious feast days.
* c. 13th–14th centuries: The worm gear was invented as part of a roller cotton gin in the Indian subcontinent.
* c. 1221 AD The geared astrolabe was built in Isfahan showing the position of the moon in the zodiac and its phase, and the number of days since new moon.
* c. 6th century AD: A geared calendrical device showing the phase of the Moon, the day of the month and the Zodiac was invented in the Byzantine empire.
* 725 AD: The first geared mechanical water clocks were built in China.
* 2nd century BC: The Antikythera mechanism, the world's oldest analog computer is built. It could predict the movement and position of the sun, moon and planets decades in advance and could solve different astronomical problems.
* c. 200–265 AD: Ma Jun used gears as part of a south-pointing chariot.
* In nature: in the hind legs of the nymphs of the planthopper insect Issus coleoptratus.

Etymology

Historically, cogs were teeth made of wood rather than metal, and a cogwheel technically consisted of a series of wooden gear teeth located around a mortise wheel, each tooth forming a type of specialised 'through' mortise and tenon joint. The wheel can be made of wood, cast iron, or other material. Wooden cogs were formerly used when large metal gears could not be cut, when the cast tooth was not even approximately of the proper shape, or the size of the wheel made manufacture impractical.

The cogs were often made of maple wood. In 1967 the Thompson Manufacturing Company of Lancaster, New Hampshire still had a very active business in supplying tens of thousands of maple gear teeth per year, mostly for use in paper mills and grist mills, some dating back over 100 years. Since a wooden cog performs exactly the same function as a cast or machined metal tooth, the word was applied by extension to both, and the distinction has been generally lost.

Comparison with drive mechanisms

The definite ratio that teeth give gears provides an advantage over other drives (such as traction drives and V-belts) in precision machines such as watches that depend upon an exact velocity ratio. In cases where driver and follower are proximal, gears also have an advantage over other drives in the reduced number of parts required. The downside is that gears are more expensive to manufacture and their lubrication requirements may impose a higher operating cost per hour.

Additional Information

Gears are used in tons of mechanical devices. Most importantly, they provide a gear reduction in motorized equipment. This is key because often a small motor spinning very fast can provide enough power for a device, but not enough torque, the force that causes an object to rotate on an axis or twist. For instance, an electric screwdriver has a very large gear reduction (reduction in the speed of a rotary machine such as an electric motor) because it needs lots of torque to turn screws. But the motor only produces a small amount of torque at a high speed. With a gear reduction, the output speed can be reduced while the torque is increased.

Gears also change the direction of rotation. For instance, in the differential between the rear wheels of your car, the power is transmitted by a shaft that runs down the center of the car, and the differential has to turn that power 90 degrees to apply it to the wheels.

There are a lot of intricacies in the different types of gears.

You've probably heard of gear ratios, especially when it comes to cars. The gear ratio is the number of turns the output shaft makes while the input shaft turns one time. If the gear ratio is 2:1, then the smaller gear is turning two times while the larger gear turns just once. It also means that the larger gear has twice as many teeth as the smaller gear. The larger gear is just called a "gear" while the smaller gear is also called a pinion.

One of the most primitive types of gears we could look at would be a wheel with wooden pegs sticking out of it. The problem with this type of gear is that the distance from the center of each gear to the point of contact changes as the gears rotate. This means that the gear ratio changes as the gear turns, meaning that the output speed also changes. If you used a gear like this in your car, it would be impossible to maintain a constant speed — you would be accelerating and decelerating constantly.

Many modern gears use a special tooth profile called an involute. This profile has the very important property of maintaining a constant speed ratio between the two gears.

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#1 2024-07-22 16:57:16

Gear

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