spring with a variable rate, usually achieved by having unequal distance between turns so that as the spring is compressed one or more coils rests against its neighbour. Rubber band A tension spring where energy is stored by stretching the material. Spring washer Used to apply a constant tensile force along the axis of a fastener. Torsion spring Any spring designed to be twisted rather than compressed or extended. Used in torsion bar vehicle suspension systems. Wave spring Any of many wave shaped springs, washers, and expanders, including linear springs, all of which are generally made with flat wire or discs that are marcelled according to industrial terms, usually by die-stamping, into a wavy regular pattern resulting in curvilinear lobes. Round wire wave springs exist as well. Types include wave washer, single turn wave spring, multi-turn wave spring, linear wave spring, marcel expander, interlaced wave spring, and nested wave spring. Physics Hooke's law As long as not stretched or compressed beyond their elastic limit, most springs obey Hooke's law, which states that the force with which the spring pushes back is linearly proportional to the distance from its equilibrium length: where x is the displacement vector – the distance and direction the spring is deformed from its equilibrium length. F is the resulting force vector – the magnitude and direction of the restoring force the spring exerts k is the rate, spring constant or force constant of the spring, a constant that depends on the spring's material and construction. The negative sign indicates that the force the spring exerts is in the opposite direction from its displacement Coil springs and other common springs typically obey Hooke's law. There are useful springs that don't: springs based on beam bending can for example produce forces that vary nonlinearly with displacement. If made with constant pitch (wire thickness), conical springs have a variable rate. However, a conical spring can be made to have a constant rate by creating the spring with a variable pitch. A larger pitch in the larger-diameter coils and a smaller pitch in the smaller-diameter coils forces the spring to collapse or extend all the coils at the same rate when deformed. Simple harmonic motion Since force is equal to mass, m, times acceleration, a, the force equation for a spring obeying Hooke's law looks like: The mass of the spring is small in comparison to the mass of the