Editing Gear (section)

==Materials==

[[File:Cog Wheel and stone spindle.jpg|thumb|right|Wooden cogwheel driving a lantern pinion or [[cage gear]]]]

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 mill]]s, 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 or 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.

[[File:Storckensohn cog wheels closeup.jpg|thumb|A cast gearwheel (above) meshing with a cogged mortise wheel (below). The wooden cogs are held in place by nails.]]
Still, because of cost or other considerations, some early metal gears had wooden cogs, each tooth forming a type of specialised [[mortise and tenon#Types|'through' mortise and tenon]] joint<ref>{{cite book
 |title=A Treatise on Gear Wheels
 |last=Grant
 |first=George B.
 |year=1893
 |place=Lexington, MA; Philadelphia, PA
 |edition=6th, illus.
 |publisher=George B. Grant
 |page=[https://archive.org/details/treatiseongearwh00granrich/page/21 21] |url=https://archive.org/details/treatiseongearwh00granrich}}</ref> 

More recently [[engineering plastic]]s 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]],<ref>{{citation|chapter-url=http://motionsystemdesign.com/mechanical-pt/plastic-gears-more-reliable-0798/index.html|chapter=Plastic gears are more reliable when engineers account for material properties and manufacturing processes during design.|first=Zan|last=Smith|title=Motion System Design|year=2000|postscript=.|access-date=7 January 2011|archive-date=14 July 2011|archive-url=https://web.archive.org/web/20110714120543/http://motionsystemdesign.com/mechanical-pt/plastic-gears-more-reliable-0798/index.html|url-status=dead}}</ref> and don't 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.

[[File:Cogwheel in Malbork.jpg|thumb|200px|Wooden gears of a historic [[windmill]]]]
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 mill]]s and [[grist mills]], some dating back over 100 years.<ref>{{cite book
 |title=Dudley's Handbook of Practical Gear Design and Manufacture
 |last=Radzevich
 |first=Stephen P.
 |year=2012
 |place=Boca Raton, FL.
 |edition=2nd
 |publisher=CRC Press, an imprint of Taylor & Francis Group
 |pages=691, 702
 |url=http://download.drgearbox.com/books/2012%20-%20Radzevich%20-%20Dudleys%20Handbook%20of%20Practical%20Gear%20Design%20and%20Manufacture.pdf}}</ref>