A coil spring, also known as a helical spring, is a mechanical device which is typically used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces. They are made of an elastic material formed into the shape of a helix which returns to its natural length when unloaded.

Under tension or compression, the material (wire) of a coil spring undergoes torsion. The spring characteristics therefore depend on the shear modulus, not Young's Modulus.

A coil spring may also be used as a torsion spring: in this case the spring as a whole is subjected to torsion about its helical axis. The material of the spring is thereby subjected to a bending moment, either reducing or increasing the helical radius. In this mode, it is the Young's Modulus of the material that determines the spring characteristics.

Metal coil springs are made by winding a wire around a shaped former - a cylinder is used to form cylindrical coil springs.

Variants

Types of coil spring are:

• Tension/extension coil springs, designed to resist stretching. They usually have a hook or eye form at each end for attachment.
• Compression coil springs, designed to resist being compressed. A typical use for compression coil springs is in car suspension systems.
  • Volute springs are used as heavy load compression springs. A strip of plate is rolled into the shape of both a helix and a spiral. When compressed, the strip is stiffer edge-on than a wire coil, but the spiral arrangement allows the turns to overlap rather than bottoming out on each other.
• Torsion springs, designed to resist twisting actions. Often associated to clothes pegs or up-and-over garage doors.

Applications

Coil springs have many applications; notable ones include:

• Buckling springs in computer keyboards
• Mattress coils in innerspring mattresses
• Upholstery coil springs in upholstery

See also

• Timmis system, an early form of coil spring used on railway axles.
• Spring (device)