What is a safe design stress for a compression spring?

This question does not have a single, simple answer. The answer depends heavily on the type of material used (e.g. music wire, stainless steel, chrome-silicon, etc.), material grade (e.g. commercial vs. valve spring quality, standard or high strength, etc.) and the service environment (e.g., static vs. cyclic, corrosive atmosphere, extremely high or low temperatures, etc.).

A spring that has infinite fatigue life under low deflection conditions may take a set if compressed to solid height. Another spring optimized for static life in sea water may have very poor fatigue life when cycled in air.

The design process typically begins with selecting a material type appropriate for the application environment. For static conditions, the spring designer will generally select a stress level appropriate for the selected material that will assure stable spring force output over time. For cyclic conditions, not only does the force output over time have to be stable, but the spring must be able to survive the intended life without breaking. Finally, manufacturability limitations can also restrict design stress levels.

The best recommendation here is to understand what is desired from the spring in service and work with a Century Spring design engineer to develop the optimum design for the operating conditions.

Knowing the answers to the following questions will greatly assist the spring designer.

• Will the spring operate under static or cyclic conditions? If cyclic, what are the minimum and maximum operating loads, deflections, or heights? What is the desired life?
• What is the operating environment?
• What is the operating temperature?
• Does the assembly include physical stops to limit spring deflection? If so, what are the limits?