Key factors that affect finger spring fatigue strength

The fatigue strength of a finger spring depends on more than the wire grade alone. It is set by the yield strength of the material and by the condition of the spring surface, which carries the highest service stresses. Yield strength, surface state, part size and metallurgical quality together determine how many cycles the spring survives before fatigue fracture.

Yield strength

The yield strength of the material correlates directly with its fatigue strength: the higher the yield strength, the higher the fatigue limit. To improve the fatigue strength of a finger spring, the material should therefore be selected with the highest practical yield strength and yield-to-tensile ratio. For the same material, a fine-grained structure gives a higher yield strength than a coarse-grained one.

Surface condition

Service stresses concentrate at the surface of the wire, so surface quality has a strong influence on fatigue life. Cracks, defects and scratches introduced during rolling, drawing and forming are common starting points for fatigue fracture. The lower the surface roughness, the smaller the stress concentration and the higher the fatigue strength; as surface roughness increases, fatigue life falls.

Steel grade and manufacturing method also matter. A cold-drawn spring loses less fatigue strength than a hot-formed one, because heating during hot forming and heat treatment roughens the wire surface and causes decarburization by oxidation, both of which lower fatigue strength. Grinding, shot blasting and surface rolling of the wire all increase fatigue strength.

Size effect

The larger the part, the greater the chance of defects arising in hot and cold working and of surface defects in general, and fatigue performance falls as size grows. The size effect should therefore be taken into account when calculating the fatigue strength of a spring.

Metallurgical defects

Metallurgical defects include non-metallic inclusions, gas bubbles and element segregation in the material. An inclusion near the surface acts as a stress concentration site and can start a fatigue crack prematurely at the interface between the inclusion and the base metal. Vacuum melting and vacuum casting improve steel quality and reduce this risk.