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# Spring (device)

A spring is a flexible elastic object used to store mechanical energy. Springs are commonly made out of steel or brass.

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## Types of spring

The most common types of spring are:

Other types include:

• the Belleville spring, a disc shaped spring commonly used to apply tension and also in the initiation mechanism of pressure-activated landmines.
• the spring washer, used to apply a constant tensile force along the axis of a fastener)
• the torsion spring (any spring designed to be twisted rather than compressed or extended)

## Theory

In classical physics, a spring can be seen as a device 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).

This law actually holds only approximately, when the deformation (extension or contraction) is small compared to the rod's overall length. For large enough deformations, the atom bonds get broken or rearranged, and the rod may snap, buckle, or permanently deform. Even if that limit is not reached, the force may deviate noticeably from Hooke's law.

Hooke's law is actually a mathematical consequence of the fact that the potential energy of the rod is a minimum when it has its relaxed length. Any smooth function of one variable approximates a quadratic function when examined near enough to its minimum point; and therefore the force — which is the derivative of energy with respect to displacement — will approximate a linear function.

## Custom fabrication of springs

It is easy to hand-make a coil spring using easily available steel piano wire. To make the spring, clamp one end of the wire to a stud bolt, and fix the bolt in the chuck of a drill rotating counter-clockwise. Hold the other end of the wire in a vice while slowly paying out the wire, allowing it to wrap around the bolt.