Helical springs are made out of thick wires coiled into a helix. They are of two types, viz., (i) closecoiled helical springs and (ii) open-coiled helical springs.
In close-coiled helical spring, the wire is coiled very closely and so it is assumed that each turn of the coil is practically a plane perpendicular to the axis of the helix. The close-coiled spring is a torsion spring and the bending stress is negligible.
In open-coiled helical spring, the wire is not coiled closely but with large gap between the two consecutive turns as a result of which there is bending stress also. Thus, open-coiled helical springs belong to the categories of torsion and bending springs.