Fig. 1. Tension spring

Without springs, modern civilization would not exist. How and who discovered the first spring will never be known, but what we do know is that a spring, which is a piece of steel with memory, when flexed returns to its original shape. More formally, a spring is an object that stores potential energy by straining the bonds between the atoms of an elastic material. Hooke’s law of elasticity states that the extension of an elastic rod (its distended length minus its relaxed length) is linearly proportional to its tension, the force used to stretch it. Similarly, the contraction (negative extension) is proportional to the compression (negative tension).

Springs are usually made from hardened steel. Small springs can be wound from pre-hardened stock, while larger ones are typically made from material in the annealed condition and hardened after fabrication. Some non-ferrous metals are also used, including phosphor bronze for parts requiring corrosion resistance and beryllium copper for springs carrying electrical current (because of its low electrical resistance).

Fig. 2. Compression spring

Common Types of Springs

Torsion springs, such as the helical or coil spring (made by winding a wire around a cylinder) and the conical spring, are so named because the wire itself is twisted when the spring is compressed or stretched. There are two types:
  • Tension springs (Fig. 1) are designed to become longer under load. Their turns are normally touching in an unloaded position, and they have a hook, eye, or some other means of attachment at each end.
  • Compression springs (Fig. 2) are designed to become shorter when loaded. Their turns are not touching in the unloading position, and they need no attachment points.
“Spring steel” is a common name for a low-alloy, medium- to high-carbon (0.7 to 1.0%) steel, which can be heat treated to produce good spring characteristics (Table 1). Heat treatment typically involves hardening in the 1475-1500°F (800 to 815°C) range followed by tempering. The desired spring properties are imparted largely by the time at temperature and by proper quenching. Austempering is often used to produce a bainitic structure.

Other Types of Springs

A rubber band is an example of a tension spring, and one in which all the energy is stored by stretching the material. Some other types of springs include:
  • Leaf springs, used in vehicle suspensions, electrical switches, and bows
  • Spiral springs, used in clocks and galvanometers
  • Belleville springs (or washers), a disc-shaped spring commonly used to apply tension to a bolt. It is the initiation mechanism of pressure-activated landmines.
  • Spring washers, used to apply a constant tensile force along the axis of a fastener
  • Torsion spring, which is twisted rather than compressed or extended
  • Cantilever springs, which are fixed only at one end
  • Gas spring in which a volume of gas is compressed