SPRING, in mechanics, an elastic body, structure or contrivance, that yields under force or stress, but returns to its former posi tion or shape when the stress is withdrawn. A strip of soft india-rubber exhibits great elastic qualities, but is not durable enough for use in machinery, and, therefore, springs are made mostly of steel, though many are of iron and brass. Springs are used for various purposes — diminishing concussion, as in car riages; for motive power, acting through the tendency of a metallic coil to unwind itself, as in clocks and watches; or to communicate motion by sudden release from a state of ten sion, as the spring of a gun-lock, etc.; others are employed to measure weight and other force, as in the spring-balance, as regulators to control the movement of wheel-works, etc.
A spiral spring is one shaped like the main spring or hair spring of a watch. Such a spring lies wholly between two parallel planes whose distance apart is equal to the width of the wire or tape of which the spring is made. They resist distortion, when used, for example, to actuate a watch or chronometer balance, with a force which is very closely proportional to the amount of the distortion. It is this prop erty which insures the isochronism of the motion of a watch or chronometer balance.
Helical springs, also commonly called spiral springs, are formed by winding wire into a helix about a right circular cylinder. If wound tight, they resist elongation, or if wound loose, with considerable spaces between the coils, they may be used to resist com pression. They are also made in various forms to resist tortional strains. Where a stiff strong spring is desired, flat steel plates, often in groups tied together, are found efficient. These are called laminated springs. The cer tainty with which a metal spring yields to a given force has led to its employment in the spring-balance, a weighing device con sisting essentially of a helical extension spring with a handle above and a hook or support for the thing to be weighed. Frequently a circular pointer is attached which indicates the weight on a dial. Air-cylinders with pistons or plung ers are often used as springs, being found es pecially efficient in cylinder printing presses. Consult Routh, (Rigid Dynamics", Thomson and Tait, (Natural Philosophy' ; Kent, (Me chanical Engineers' Pocket-book) (1916).