spring, in mechanics, any of several elastic devices used variously to store and to furnish energy, to absorb shock, to sustain the pressure between contacting surfaces, and to resist tensional or compressional stress. Springs are made of an elastic material, e.g., specially formulated steel alloys or certain types of rubber or plastic. A torsion spring that stores energy, e.g., for operating a watch, is a metal strip wound spirally around a fixed center. For reducing concussion in some heavy trucks and railroad cars, helical, or coil, springs are used. Coil springs are commonly used for the same purpose in automobiles, as are leaf springs that consist of flat bars clamped together. These have been replaced in some vehicles by torsion bars that absorb stresses by twisting. The helical-coil compression spring provides the force to keep the operating surfaces together in the friction clutch (see transmission). The extension spring is employed for the spring balance; the distance through which it is extended depends on the weight suspended from it. The disk spring, which consists of a laminated series of convex disks, is widely employed for heavy loads.