The helical springs are made up of a wire coiled in the form of a helix and are primarily intended for compressive or tensile loads. The cross-section of the wire from which the spring is made may be circular, square or rectangular. Helical compression springs have applications to resist applied compression forces or in the push mode, store energy to provide the "push". Different forms of compression springs are produced.
The helical springs are said to be closely coiled when the spring wire is coiled so close that the plane containing each turn is nearly at right angles to the axis of the helix and the wire is subjected to torsion. in other words, in a closely coiled helical spring, the helix angle is very small, it is usually less than 10 degree. The major stresses produced in helical springs are shear stresses due to twisting. The load applied is parallel to or along the axis of the spring. In open coiled helical springs, the spring wire is coiled in such a way that there is a gap between the two consecutive turns, as a result of which the helix angle is large.
Terms used ¡n Compression Springs
The following terms used in connection with compression springs
Solid length (Ls).
When the compression spring is compressed until the coils come in contact with each other, then the spring is said to be solid. The solid length of a spring is the product of total number of coils and the diameter of the wire.
Free length (Lo).
The free length of a compression spring is the length of the spring in the free or unloaded condition.
The force applied to a spring that causes a deflection.
Wire Diameter (d) - The diameter of the wire that is wound into a helix.
Spring Index (C) - The ratio of mean coil diameter to wire diameter. A low index indicates a tightly wound spring (a relatively large wire size wound around a relatively small diameter mandrel giving a high rate).
Coil Diameter (D) - The mean diameter of the helix, i.e., (D outer + Dinner)/2.
Active Coils (Na) - The number of coils which actually deform when the spring is loaded, as opposed to the inactive turns at each end which are in contact with the spring seat or base.
Total Coils (Nt)- The number of coils or turns in the spring.
Pitch (p) - The distance from center to center of the wire in adjacent active coils
Pitch Angle (a) - The angle between the coils and the base of the spring. The pitch angle is calculated from the equation
Compression Spring Rate: