A gearDefinition of "gear" in the Oxford Learner's Dictionary online. Accessed on 2024-07-27. or gearwheel,Definition of "gearwheel" in the Collins English Dictionary online. Accessed on 2024-07-27.Definition of "gearwheel" in the Merriam-Webster dictionary online. Accessed on 2018-09-20.Definition of "gearwheel" in the Oxford Learner's Dictionary online. Accessed on 2024-07-27. also called a toothed wheel, or cog is a rotating machine part typically used to transmit rotational motion or torque by means of a series of "teeth" that engage with compatible teeth of another gear or other part. The teeth can be integral saliences or cavities machined on the part, or separate pegs inserted into it. In the latter case, the gear is usually called a cogwheel. A cog may be one of those pegsDefinition of "cog" in the Oxford Learner's Dictionary online. Accessed on 2024-07-29.Definition of "cog" in the Merriam-Webster Dictionary online. Accessed on 2024-07-29.Definition of "cog" in the Cambridge Dictionary Plus online. Accessed on 2024-07-29. or the whole gear.Definition of "cog" in the Collins English Dictionary online. Accessed on 2024-07-29. Two or more meshing gears are called a gear train.
The smaller member of a pair of meshing gears is often called pinion. Most commonly, gears and gear trains can be used to trade torque for rotational speed between two axles or other rotating parts or to change the axis of rotation or to invert the sense of rotation. A gear may also be used to transmit linear force or linear motion to a rack, a straight bar with a row of compatible teeth.
Gears are among the most common mechanical parts. They come in a great variety of shapes and materials, and are used for many different functions and applications. Diameters may range from a few μm in micromachines,Shuailong Zhang, Mohamed Elsayed, Ran Peng, Yujie Chen (2021): "Reconfigurable multi-component micromachines driven by optoelectronic tweezers". Nature Communications, volume 12, issue 1. to a few mm in watches and toys to over 10 metres in some mining equipment.Matthew Jaster (2013): "MY GEAR is Bigger than YOUR GEAR: Industry Battles it Out for World's Largest Gear Title" . Online article from Gear Technology magazine. Archived on 2024-07-30 . Other types of parts that are somewhat similar in shape and function to gears include the sprocket, which is meant to engage with a link chain instead of another gear, and the timing pulley, meant to engage a timing belt. Most gears are round and have equal teeth, designed to operate as smoothly as possible; but there are several applications for non-circular gears, and the Geneva drive has an extremely uneven operation, by design.
Gears can be seen as instances of the basic lever "machine". When a small gear drives a larger one, the mechanical advantage of this ideal lever causes the torque T to increase but the rotational speed ω to decrease. The opposite effect is obtained when a large gear drives a small one. The changes are proportional to the gear ratio r, the ratio of the tooth counts: namely, , and . Depending on the geometry of the pair, the sense of rotation may also be inverted (from clockwise to anti-clockwise, or vice versa).
Most vehicles have a transmission or "gearbox" containing a set of gears that can be meshed in multiple configurations. The gearbox lets the operator vary the torque that is applied to the wheels without changing the engine's speed. Gearboxes are also used in many other machines, such as lathes and conveyor belts. In all those cases, terms like "first gear", "high gear", and "reverse gear" refer to the overall torque ratios of different meshing configurations, rather than to specific physical gears. These terms may be applied even when the vehicle does not actually contain gears, as in a continuously variable transmission.
History
The earliest surviving gears date to 4th century BC China,Derek J. de Solla Price, On the Origin of Clockwork, Perpetual Motion Devices, and the Compass, p.84 during the Warring States period, which have been preserved at the Luoyang Museum in Henan Province, China.
In Europe, Aristotle mentions gears around 330 BC, as wheel drives in windlasses. He observed 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). The earliest surviving gears in Europe were found in the Antikythera mechanism, an example of a very early and intricate geared device, designed to calculate astronomical positions of the sun, moon, and planets, and predict eclipses. Its time of construction is now estimated between 150 and 100 BC.
The Chinese engineer Ma Jun (–265) described a south-pointing chariot. A set of differential gears connected to the wheels and to a pointer on top of the chariot kept the direction of the latter unchanged as the chariot turned.Joseph Needham (1986). Science and Civilization in China: Volume 4, Part 2, page 298. Taipei: Caves Books, Ltd.
Another early surviving example of geared mechanism is a complex 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.
Geared mechanical water clocks were built in China by 725.
Around 1221, a geared astrolabe was built in Isfahan showing the position of the moon in the zodiac and its phase, and the number of days since the new moon.
The worm gear was invented in the Indian subcontinent, for use in roller cotton gins, some time during the 13th–14th centuries.Irfan Habib, Economic History of Medieval India, 1200-1500, page 53, Pearson Education
A complex astronomical clock, called the Astrarium, was built between 1348 and 1364 by Giovanni Dondi dell'Orologio. It 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.
The Salisbury Cathedral clock, built in 1386, is the world's oldest still working geared mechanical clock.
Differential gears were used by the British clock maker Joseph Williamson in 1720.
Etymology
The word gear is probably from Old Norse gørvi (plural gørvar) 'apparel, gear,' related to gøra, gørva 'to make, construct, build; set in order, prepare,' a common verb in Old Norse, "used in a wide range of situations from writing a book to dressing meat". In this context, the meaning of 'toothed wheel in machinery' is first attested to the 1520s; the specific mechanical sense of 'parts by which a motor communicates motion' is from 1814; specifically of a vehicle (bicycle, automobile, etc.) by 1888.
A cog is a tooth on a wheel. From Middle English cogge, from Old Norse (compare Norwegian kugg ('cog'), Swedish kugg, kugge ('cog, tooth')), from Proto-Germanic *kuggō (compare Dutch kogge ('cog boat'), German Kock), from Proto-Indo-European *gugā ('hump, ball') (compare Lithuanian gugà ('pommel, hump, hill'), from PIE *gēw- ('to bend, arch'). First used c. 1300 in the sense of 'a wheel having teeth or cogs; late 14c., 'tooth on a wheel'; cog-wheel, early 15c.
Materials
The gears of the Antikythera mechanism are made of bronze, and the earliest surviving Chinese gears are made of iron. These metals, as well as tin, have been generally used for clocks and similar mechanisms to this day.
Historically, large gears, such as those used in flour mills, were commonly made of wood rather than metal. They were cogwheels, made by inserting a series of wooden pegs or cogs around the rim of a wheel. The cogs were often made of maple wood.
Wooden gears have been gradually replaced by ones made of metal, such as cast iron at first, then steel and aluminum. Steel is most commonly used because of its high strength-to-weight ratio and low cost. Aluminum is not as strong as steel for the same geometry, but is lighter and easier to machine. Powder metallurgy may be used with alloys that cannot be easily cast or machined.
Still, because of cost or other considerations, some early metal gears had wooden cogs, each tooth forming a type of specialised 'through' mortise and tenon joint
More recently engineering plastics and composite materials have been replacing metals in many applications, especially those with moderate speed and torque. They are not as strong as steel, but are cheaper, can be mass-manufactured by injection molding, and do not need lubrication. Plastic gears can even be intentionally designed to be the weakest part in a mechanism, so that in case of jamming they will fail first and thus avoid damage to more expensive parts. Such "sacrificial" gears may be a simpler alternative to other overload-protection devices such as clutches and torque-limited or current-limited motors.
In spite of the advantages of metal and plastic, wood continued to be used for large gears until a couple of centuries ago because of cost, weight, tradition, or other considerations. 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.
Manufacture The most common techniques for gear manufacturing are dies, sand, and investment casting; injection molding; powder metallurgy; blanking; and gear cutting.
As of 2014, an estimated 80% of all gearing produced worldwide is produced by net shape molding. Molded gearing is usually powder metallurgy, plastic injection, or metal die casting. Gears produced by powder metallurgy require a sintering step after they are removed from the mold. Cast gears require gear cutting or other machining to shape the teeth to the necessary precision. The most common form of gear cutting is hobbing, but gear shaping, milling, and broaching may be used instead.
Metal gears intended for heavy duty operation, such as in the transmissions of cars and trucks, the teeth are heat treated to make them hard and more wear resistant while leaving the core soft but tough. For large gears that are prone to warp, a quench press is used.
Gears can be made by 3D printing; however, this alternative is typically used only for prototypes or very limited production quantities, because of its high cost, low accuracy, and relatively low strength of the resulting part.
Comparison with other drive mechanisms Besides gear trains, other alternative methods of transmitting torque between non-coaxial parts include link chains driven by sprockets, friction drives, belts and pulleys, hydraulic couplings, and timing belts.
One major advantage of gears is that their rigid body and the snug interlocking of the teeth ensure precise tracking of the rotation across the gear train, limited only by backlash and other mechanical defects. For this reason they are favored in precision applications such as watches. Gear trains also can have fewer separate parts (only two) and have minimal power loss, minimal wear, and long life. Gears are also often the most efficient and compact way of transmitting torque between two non-parallel axes.
On the other hand, gears are more expensive to manufacture, may require periodic lubrication, and may have greater mass and rotational inertia than the equivalent pulleys. More importantly, the distance between the axes of matched gears is limited and cannot be changed once they are manufactured. There are also applications where slippage under overload or transients (as occurs with belts, hydraulics, and friction wheels) is not only acceptable but desirable.
Ideal gear model