A spur gear is the most basic type of gear available. It consists of nothing more than a cylinder or disk with radially projecting teeth aligned parallel to the rotational axis. The simplicity of spur gears means that they are commonly used in number of machines, from cars to household appliances. Because they are used often, spur gears must be built of materials, like plastics and metals, that are easily fabricated and molded, but also strong and durable.
Acetal is a plastic polymer that is used either in its pure state or slightly altered state—e.g. Derlin—for a number of spur gears. The acetal polymer is much stronger than common plastic, though it can be easily molded to any shape, including a spur gear. Once acetal has hardened in the shape of a spur gear, it is stif, strong and resistant to abrasion. The malleability, strength and resilience make it an ideal material for spur gears.
Cast iron is, like acetal, an easily molded material. It is also highly resistant to rust. Cast iron is not pure iron, and because of this, any given batch of cast iron will have different ingredients. These different ingredients cohere for different degrees of strength and durability. Cast iron is used in machine parts because it is relatively inexpensive, rust resistant and easy to mold, though it may be either incredibly strong or incredibly weak, depending upon the admixture.
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Stainless steel is a metal alloy commonly used in the casting of spur gears. A metal alloy is a metal composed of two or more distinct elements that are melted together. Like cast iron, it is highly resistant to oxidation, and like acetal, it is resistant to abrasions and other weakening blemishes. Stainless steel’s resistance to rust and scarring is due to the infusion of chromium. The strength, durability and corrosion resistance make stainless steel a popular material for spur gears.