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).
Types of spring
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 attachement 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.
- 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).
- 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.
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, and only when the deformation (extension or contraction) is small compared to the rod's overall length. For deformations beyond the elastic limit, atomic bonds get broken or rearranged, and a spring may snap, buckle, or permanently deform. Many materials have no clearly defined elastic limit, and Hooke's law can not be meaningfully applied to these materials.
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.
Contrary to popular belief, springs do not appreciably "creep" or get "tired" with age. Spring steel has a very high resistance to creep under normal loads. The sag observed in older automobiles is really due to the springs being occasionally compressed beyond their yield point, causing plastic deformation. This can happen when the vehicle hits a large bump or pothole, especially when heavily loaded. Most vehicles will accumulate a number of such impacts over their working life, leading to a lower ride height and eventual bottoming-out of the suspension.