Coil springs are used in virtually every man-made device due to their easily determined operational characteristics, reliability, and low cost of manufacture in quantity. Springs store mechanical energy when either stretched or compressed from their tempered relaxed state and release the energy when the force is removed from them. Springs are used to retain parts and mechanisms, return an equal and opposite force when compressed or extended, absorb shock, and protect mechanical components. Compression coil springs are particularly durable because their very design limits the amount of motion possible well within their modulus of elasticity.
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Coil Spring Theory
Coil springs work on the principle of continuous cross sectional torsion of the round or oval wire over its entire length simultaneously. As the ends of the spring are displaced, the spirally wound wire acts as a moment arm to the sections 90 degrees from it in either direction. Since the spiral is most often uniform, the compression is uniform over the spring's entire length.
Coil springs are used in four basic applications--compression, tension, torsion and extension. Compression springs are formed over cylindrical formers with a specified spacing between rows to resolve the compressive displacement into the gaps between the coils. Tension springs start from a relaxed state of many coils with no gaps between them. Torsion springs provide the holding force of a clothes pin, or a machine. Extension springs are similar to tension springs except that they can never achieve a relaxed state, since their gaps are held closed by the spring rate.
Aside from the basic compression and tension types, coil springs can be further loosely grouped based on linear or variable rate per inch of displacement, oval or round in addition to cylindrical shape, how the ends of the spring are finished, and other characteristics such as metallurgy that allows springs to operate isothermally, or independent of temperature changes. For example, compression springs may have flat and ground ends, or open ends which may be cut to length. Tension springs may have either closed loops or open hooks on each end. Springs may have bolt loops formed in each end, or straight sections without coils such as is often used with automotive brakes.
Spring makers have many different formulas for making springs including metallurgy, dimensional proportions, elasticity, and mounting specifics, but in general, springs are formed from soft ductile metal material, whether wire, rod, or bar, by winding them on cylindrical formers. They are stretched to their exact relaxed dimension, and then heat tempered to a precise temperature and quenched. The resultant metal is elastic, tough, hard, and extremely resistant to being stretched outside its elastic range; however, once it is, it will stay deformed, and must either be discarded or remade.
Because springs have virtually no internal friction, and very low mass compared to their capacity to store energy, they can snap to their relaxed state with explosive velocity, which can injure, maim or kill if they launch themselves as projectiles. Extreme caution should always be exercised when working with springs of any type.
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