Cold Wound Coil Spring uses materials with 0.1mm~10mm diameter. In general, the shape of the materials is round section, but for special purposes, there are nonlinear sectional spring that is used for the engine valves of autos recently, and square sectional spring.
Coiling on the cylinder-shape is most common, but for special purposes, there are ping tail end spring in which the diameter of the left part coil is minimized, cylinder-shaped spring, cone-shaped spring, and double-headed drum type spring.
In case of tension coil spring, the loop of the spring and the end of the loop are various in terms of the shape.
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