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When a conventional spring, without stiffness variability features, is compressed or stretched from its resting position, it exerts an opposing [[force]] approximately proportional to its change in length (this approximation breaks down for larger deflections). The ''rate'' or ''spring constant'' of a spring is the change in the force it exerts, divided by the change in [[deflection (engineering)|deflection]] of the spring. That is, it is the [[gradient]] of the force versus deflection [[curve]]. An [[tension (physics)|extension]] or [[compression (physical)|compression]] spring's rate is expressed in units of force divided by distance, for example or N/m or lbf/in. A [[torsion spring]] is a spring that works by twisting; when it is twisted about its axis by an angle, it produces a [[torque]] proportional to the angle. A torsion spring's rate is in units of torque divided by angle, such as [[newton metre|N·m]]/[[radian|rad]] or [[ft·lbf]]/degree. The inverse of spring rate is compliance, that is: if a spring has a rate of 10 N/mm, it has a compliance of 0.1 mm/N. The stiffness (or rate) of springs in parallel is [[additive map|additive]], as is the compliance of springs in series.
Springs are made from a variety of elastic materials, the most common being spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from [[annealing (metallurgy)|annealed]] steel and hardened after manufacture. Some [[non-ferrous metal]]s are also used, including [[phosphor bronze]] and [[titanium]] for parts requiring corrosion resistance, and low-[[Electrical resistance and conductance|resistance]] [[beryllium copper]] for springs carrying [[
==History==
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