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Compression Springs
A compression spring is an open-coil helical spring that offers resistance to a compressive force applied axially. Compression springs are usually coiled as a constant diameter cylinder. Other common forms of compression springs--such as conical, tapered, concave, convex or various combinations of these--are used as required by the application. While square, rectangular, or special-section wire may have to be specified, round wire is predominant in compression springs because it is readily available and adaptable to standard coiler tooling.  The illustration shown here is recommended as a guide in specifying compression springs. The functional design characteristics of the spring should be given as mandatory specifications. Secondary characteristics, which may well be useful for reference, should be identified as advisory data. This practice controls the essential requirements, while providing as much design flexibility as possible to the spring manufacturer in meeting these requirements. Compression springs should be stress-relieved to remove residual bending stresses produced by the coiling operation. Depending on design and space limitations, compression springs may be categorized according to stress level as follows:
1. Springs which can be compressed solid without permanent set, so that an extra operation for removing set is not needed. These springs are designed with torsional stress levels when compressed solid that do not exceed about 40 percent of the minimum tensile strength of the material.
2. Springs which can be compressed solid without further permanent set after set has initially be removed. These may be pre-set by the spring manufacturer as an added operation, or they may be pre-set later by the user prior to or during the assembly operation. These are springs designed with torsional stress levels when compressed solid that do not exceed 60 percent of the minimum tensile strength of the material.
3. Springs which cannot be compressed solid without some further permanent set taking place because set cannot be completely removed in advanced. These springs involve torsional stress levels which exceed 60 percent of the minimum tensile strength of the material. The spring manufacturer will usually advise the user of the maximum allowable spring deflection without set whenever springs are specified in this category. In designing compression springs the space allotted governs the dimensional limits of a spring with regard to allowable solid height and outside and inside diameters. These dimensional limits, together with the load and deflection requirements, determine the stress level. It is extremely important to consider carefully the space allotted to insure that the spring will function properly to begin with, thereby avoiding costly design changes.
Solid Height of a Compression Spring
The solid height of a compression spring is defined as the length of the spring when under sufficient load to bring all coils into contact with the adjacent coils and additional load causes not further deflection. Solid height should be specified by the user as a maximum, with the actual number of coils in the spring to be determined by the spring manufacturer.
As square or rectangular wire is coiled, the wire cross-section deforms slightly into a keystone or trapezoidal shape, which increased the solid height considerably. The dimensional change is a function of the spring index and the thickness of the material. When calculating maximum solid height, allowance must be made for all the factors which apply, such as material, finish, and manufacturing tolerances.
How to Determine Rate Rate which is the change in load per unit deflection, may be determined by the following procedure:
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