Mechanical design of bimaterial helical springs with circular cross-section

This paper presents an approximate theoretical model for the mechanical behaviour of helical springs with circular cross-section formed by an inner elastic core encased in an outer annulus of dissimilar elastic properties. Closed-form equations are developed for stresses and deflection in the spring undergoing either bending or axial end loads. For both loading conditions, the model takes into account the stress concentrations arising in the cross-section due to curvature of the spring axis. The disclosed equations are specialized for bimaterial springs with a polymer core and a thin nanometal cladding, a solution reflecting a unique technology recently brought onto the market by a leading polymer manufacturer. In this special case, the cladding performs as an efficient thin-walled tube under torsion with the soft core preventing the danger of wall instability. A design procedure is exemplified, showing that this construction leads to lighter and smaller springs than all-metal or all-polymer counterparts.