Gear
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 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 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.

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 don't need lubrication. Plastic gears may 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- 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.
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