Prestressing

Prestressing (sometimes called scragging or removing permanent set) is a process which will not only improve the relation performance of a spring but if the spring stresses are in the pre-stress 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 (pre-stressed) it will not return to its original length. This length loss has to be factored in by the manufacturer so that the length is correct after pre-stressing and does add complexity however if done correctly the springs relaxation performance over time is significantly improved. Best practice is to pre-stress the spring a minimum of 5 times to ensure that it has stabilised.

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).