Simply put, permanent set in a compression spring is a condition where the spring is deflected (compressed) and does not return to its original free length. This occurs when the material is deflected beyond its elastic properties. The amount of material in the spring is insufficient to carry the load and "the spring gives up". While the spring may sometimes still work in the application, the load that it produces will be at a lower height and usually less than originally desired.

All too often this condition is the result of the designer leaving insufficient room to design a safe spring using the proper amount of material to carry the load and is then faced with the reality of having to redesign mating parts or an entire unit. Other alternatives include adding secondary operations to the spring such as presetting, heat setting or having to use a more costly special shaped material.

Working with a springmaker during the early phase of the design is always a good idea and can often save cost and help avoid unplanned disruptions in the production cycle.  -The 18th China(Guangzhou)Int’l Springs Industry Exhibition