PRESTRESSING

PRESTRESSING (sometimes called scragging or removing permanent set) is a process that will not only improve the relation performance of a spring, but if the spring stresses are in the prestress range will in fact improve the fatigue life as it increases the allowable stress limit of the material.

PRESTRESSING is where the spring is made longer than required so that when compressed to solid height (prestressed), it will not return to its original length. This length loss must be factored in by the manufacturer so the length is correct after PRESTRESSING and does add complexity; however, if done correctly, the springs relaxation performance over time is significantly improved. Best practice is to prestress the spring a minimum of five times to ensure it has stabilized.

Prestressing can be applied to compression, extension, and torsion springs which (if the material is in the prestress range) will all benefit from the procedure.

How to: To check if your spring has been correctly pre-stressed measure the free length, then compress the spring carefully to solid height (avoid crushing) between two parallel plates, then measure the free length again. If the spring loses height then it has not been properly pre-stressed (scragged).