Stresses are determined by spring dimensions and the application's load and deflection requirements. Compression springs are stressed in torsion. Maximum stress occurs at the inner surface of the wire; the load varies as the spring is deflected, producing a range of operating stresses that influence the life of the spring.
Compression Spring Static Conditions:
Static applications require a spring to operate for a low number of cycles. Load-carrying abilities in static applications are limited by the tensile strength of the spring material, with a maximum stress that should fall between 35 and 50 percent of the minimum tensile strength. Exceeding the tensile-strength percentage can cause the spring to take a permanent set when deflected to solid. Spring Engineers Tip: Presetting, or set removal, increases the load-carrying ability of springs in static applications by increasing the spring's elastic limit. To preset, the spring is coiled longer than its required free length and then compressed to solid, causing the spring to set to its final desired length. This process increases the spring's energy storage capacity to up to 75 percent of the material's tensile strength, and is common for critical springs made from premium materials.
Compression Spring Cyclic Conditions:
Cyclic applications require a spring to operate repeatedly between specified loads. Load-carrying abilities in cyclic applications are limited by material fatigue strength. The optimum stress level is a balance between cost and reliability - reducing operating stresses will increase both reliability and cost.
Spring Engineers Tip:
Maximum stress occurs at the wire surface, and any surface defects therefore reduce fatigue life. Shot peening can improve fatigue life by creating a residually-stressed surface area which resists fatigue cracks.