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Spring and Coils
A spring is a flexible elastic object used to store mechanical energy. Springs are usually made out of hardened steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel 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).

The rate of a spring is the change in the force it exerts, divided by the change in deflection of the spring. That is, it is the gradient of the force versus deflection curve. For an extension or compression spring it has the units of lbf/in, N/mm, or similar. For a torsional spring it has the units of Nm/radian or ft lb/degree, for example. 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 adds, and the compliance of springs in series, adds.

Simple non-coiled springs were used throughout human history. In the bronze age more sophisticated spring devices were used, this can be known from the spread of the tweezers in many cultures. The Greek engineer Ctesibius of Alexandria developed a method for making bronze with springlike characteristics by producing an alloy of bronze with an increased proportion of tin, and then hardening it with hammering after it is cast. Coiled springs were intoduced in the 15th centutry.

Types
The most common types of spring are:

  • Coil Spring or Helical Spring - a spring (made by winding a wire around a cylinder) and the conical spring - these are types of torsion spring, because the wire itself is twisted when the spring is compressed or stretched. These are in turn of two types:
  • Tension Springs are designed to become longer under load. Their turns are normally touching in the unloaded position, and they have a hook, eye or some other means of attachment at each end.
  • Compression Springs are designed to become shorter when loaded. Their turns are not touching in the unloaded position, and they need no attachment points. A volute spring is a compression spring in the form of a cone so that under compaction the coils are not forced against each other, thus permitting longer travel.
  • Leaf Spring - a flat springy sheet, used in vehicle suspensions, electrical switches, bows.
  • V-Spring - used in antique firearm mechanisms such as the wheellock, flintlock and percussion cap locks.
  • Spiral Spring or 'Clock Spring' - a spring of the type as used in clocks, galvanometers, and places where electricity must be carried to partially-rotating devices such as steering wheels.
  • Cantilever Spring - a spring which is fixed only at one end.

Other types include:

  • Belleville Washer or Belleville Spring - a disc shaped spring commonly used to apply tension to a bolt (and also in the initiation mechanism of pressure-activated landmines).
  • 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.
  • Gas Spring - a volume of gas which is compressed.
  • Rubber Band - a tension spring where energy is stored by stretching the material.

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