Presetting and shot peening

Presetting   (or "scragging") and shot peening are two manufacturing operations which are carried out after coiling and heat treatment, but before putting a spring into service. They each confer a favourable residual stress distribution in the wire which increases the allowable operating loads.

The stresses in a torsion bar made from an elastic-perfectly plastic material during and after presetting are sketched here. A relatively large presetting load (torque) is applied to cause a band of material under the surface to become plastic while the core of the wire remains elastic ( b).

Presetting a spring, as opposed to a torsion bar, involves making the spring sufficiently long so that portion of the cross-section yields when the spring is compressed solid during preset. When the presetting load is removed the spring recovers elastically to a shorter free length. The residual stresses are similar to those in a torsion bar, but more complex due to direct shear and curvature-induced stress concentration. Presetting usually reduces the stiffness by 5-10%.When the presetting load is removed the bar recovers elastically but does not return to its original position since part of the wire cross-section has yielded - the bar remains permanently deformed. The corresponding residual stresses ( c ) are negative at the surface - negative in the sense of the stress induced by the applied load. When the bar is subsequently put into service these residual stresses are elastically subtractive from the stresses attributable to the load ( e), so that surface stresses are less than those which would have occurred in the absence of preset.

Shot peening involves bombarding the wire with high velocity pellets to impart a surface compression. Residual stresses are more surface- localised than those induced by presetting. Peening is particularly beneficial for fatigue in the presence of surface flaws (die marks, pits and seams), but is appropriate only for wire diameters exceeding 1.5 mm or thereabouts.
Springs destined for arduous duty are invariably preset and/or peened, however in the interests of simplicity we shall not make use of the significantly higher design stresses which these operations allow. The material strengths tabulated above refer to springs without preset or peening.