In cyclic applications, the-load-carrying ability of a compression spring is limited by material fatigue strength. Velocity of end coils is low compared with the natural frequency. To select the optimum stress level, it is necessary to balance spring cost vs. reliability. Reducing operating stresses increases spring reliability as well as cost. A complete knowledge of operating environment, expected life, stress range, frequency of operation, speed of operation and permissible levels of stress relaxation are required in order to make the best choice between cost and reliability.
Because maximum stress is at the wire surface, any surface defects, such as pits or seams, severely reduce fatigue life. Shot peening improves fatigue life and minimizes the harmful effect of surface defects, but it does not totally remove them.
Maximum allowable design stresses for fatigue applications should be calculated using a “maximum percentage of tensile strength before set removed” correction factor. Note that shot peening increases the fatigue strength by as much as 20% at lives of 10 million cycles.
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