Analytical, experimental and finite element analysis of elliptical cross-section helical spring with small helix angle under static load

This study aims in determining the stress distribution in elliptical cross-section helical springs with small helix angle (less than 10°, often termed as close-coiled springs) considering the effect of wire curvature, subjected to axial static load. We present both analytical and finite element analysis, validated by an experimental study conducted on an actual automotive valve spring. A novel analytical expression for the stress distribution in such springs is developed based on the theory of elasticity. In particular, this expression leads to the analytic formula for the maximum resultant shear stress and its location along the spring's cross-section as a function of the aspect ratio. Contrary to the case of the circular cross-section, this maximum shear stress location is not invariant. The proposed analytical expression agrees well with the computed FEA results and is in very good correlation with the experimentally obtained figures. As an additional outcome of this study, we describe a method to experimentally measure the shear stress in helical springs using strain gauges.