Tension Springs - Static stress

The calculation based on the stresses in the body of the spring (see Basics - Bases of calculus) does not take into account the curve of the wire (the spring is compared to a straight bar). The effect of curve induces in particular a significant length variation between the interior and the external fiber of the spring. Thus an irregularity of the distribution of the stresses in the section of the wire can be observed. The highest stress is on the internal envelope of the coils of the spring.

Distribution of the shear stresses

The theory of the springs being based on a simplified geometry of the spring, it is appropriate to use a stress correction coefficient to improve the precision of calculation. The difference in the levels of constraint is accentuated when the radius of curvature is small. The coefficient of correction k is thus defined according to the curvature rate. The literature provides many formulations of this coefficient. They give equivalent results.

DIN standards: k = (w + 0.5) / ( w - 0.75)

A tensoin spring must thus be designed so that the maximum corrected constraint

τ k2 = 8 D F2 k / ( π d3 )

does not exceed the acceptable maximum shear stress in the body of the spring τ zul.

usually : τ zul = 50% Rm for steel and τzul = 48%Rm for stainless steel.




Manuel Paredes