This whitepaper considers stress, set and weight as important considerations in establishing a custom spring design which need to be understood at the outset. The paper expands on this general discussion with many additional formulae and tables, along with detail considerations required for correct specification of compression springs.
Chris Petts explains that in considering compression spring stress, the dimensions, along with the load and deflection requirements, determine the stresses in the spring. When a compression spring is loaded, the coiled wire is stressed in torsion. The stress is greatest at the surface of the wire; as the spring is deflected, the load varies, producing a range of operating stress. Stress and stress range govern the life of the spring. The higher the stress range, the lower the maximum stress must be to attain comparable life. Relatively high stresses may be used when the stress range is low or if the spring is subjected solely to static loads.
Other factors of importance include compression spring set - which describes when a compression spring is compressed and released, how it is supposed to return to its original height and, on further compressions, the load at any given point should remain constant at least within the load limits specified. When a spring is made and then compressed the first time, if the stress in the wire is high enough at the point the spring is compressed to, the spring will not return to its original height (i.e., it will get shorter). This is referred to as 'taking a set', or 'setting'. When a custom spring is supplied longer than specified to compensate for length loss when compressed in assembly, this is referred to as 'Allow for Set'.
View the full whitepaper below.