Official Answer
Answer
'Permanent set' is an immediate plastic deformation that occurs the first time a spring is compressed to its work height or solid height if the internal stress exceeds the material's elastic limit. It results in a permanent loss of free height. 'Relaxation', on the other hand, is a time-dependent loss of height and load that occurs under constant stress, especially at elevated temperatures (creep). Both result in a lower-than-intended preload. To minimize permanent set, manufacturers often 'preset' the springs by compressing them to solid height during production. This induces beneficial residual stresses (autofrettage) that allow the spring to operate at higher loads without further deformation in the field.
