A chain is a
serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a
rope in that it is flexible and
linear, rigid, and load-bearing in
tension. A chain may consist of two or more links. Chains can be classified by their design, which can be dictated by their use:
Those designed for lifting, such as when used with a
hoist; for pulling; or for securing, such as with a
bicycle lock, have links that are
torus shaped, which make the chain flexible in two
dimensions (the fixed third dimension being a chain's length). Small chains serving as
jewellery are a mostly decorative analogue of such types.
Those designed for transferring power in machines have links designed to mesh with the teeth of the
sprockets of the machine, and are flexible in only one dimension. They are known as
roller chains, though there are also non-roller chains such as
Lead shank (or stud chain), used on horses that are misbehaving
Pull switch, an electrical switch operated by a ball chain
Lavatory chain, the chain attached to the cistern of an old-fashioned W.C. in which the flushing power is obtained by a gravity feed from above-head height. Although most cisterns no longer work like that, the phrase "pull the chain" is still encountered to mean "flush the toilet".
Rigid chain actuator, a type of chain that only bends in one direction, allowing it to operate under compression
Silent chain, a type of chain in which the links engage the sprockets similarly to gear teeth
Stud link chain, a type of chain with metal between the sides of each link, keeping the attached links in place. This helps prevent bunching when the chain is run out from a storage bin, as for use in anchoring ships.
Short link chain, a chain where the gap between attached links is small relative to thickness.
Several methods are available to connect chain ends to each other or to other objects, and to apply a load to a chain away from the ends. These methods are usually specific to the type of chain, and must be of the correct size.
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The metal link chain has been in use since at least 225 BC.