A spring is defined in its most simplest form as an elastic device used to store energy. Springs are usually made from coiled spring steel wire. There are a large number of spring designs, broken down into three primary categories; compression, extension and torsion.
Compression Springs Extension Springs Torsion Springs Constant Force Springs
There is a fourth, less common category, “constant force springs” such as those found in a tape measure. Springs can be made out of round, flat or square wire. Small springs can be wound from “pre-hardened” wire stock while larger springs are made from “annealed steel wire” and hardened after fabrication. Some non-ferrous metals are also used, including phosphor bronze and titanium, for parts requiring corrosion resistance and beryllium copper for springs carrying electrical current (because of its low electrical resistance).
When a coil spring is compressed or stretched slightly from rest, the force it exerts is approximately proportional to its change in length. The “Rate” of a spring is the change in the force it exerts, divided by the change in its deflection. A compression or extension spring has “Units of Force” divided by distance. A torsion spring has “Units of Torque” divided by angle and degree. The inverse of Spring Rate is Compliance.
Depending on the design and required operating environment, any material can be used to construct a spring, so long as the material has the required combination of rigidity and elasticity.
Example: How to Measure Compression Springs: