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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 allpolymer counterparts

This article deals with the numerical and experimental validation of a theoretical elastic model for bimaterial helical springs developed by the authors in a recently published article. The numerical validation is performed on finite element models involving one half turn of several springs identified by three spring indices (c=D/d= 3, 5, 10) and three section types (solid homogeneous, solid bimaterial and thin hollow). The experimental validation involves compression tests on two polymer (acrylonitrile butadiene styrene) spring configurations produced by rapid prototyping and cladded by ionic infiltration with CrNiCo alloy. For the larger prototype spring, the stresses are measured on the outside of the coil by means of miniature strain gauges. The numerical results confirm the theoretical stress concentration factors within an error of 5%. The experimental results closely agree with the predicted spring rates of all springs, either fully polymeric or bimaterial. In addition, the strain gauge measurements on the instrumented spring correlate well with the theoretical stresses calculated for that particular geometry

The helical spring is a widely used element in suspension systems. Traditionally, the springs have been wound from solid round wire. Significant weight savings can be achieved by fabricating helical springs from hollow tubing. For suspension systems, weight savings result in significant transportation fuel savings. This paper uses previously published equations to calculate the maximum shear stress and deflection of the hollow helical spring. Since the equations are complex, solving them on a computer or spreadsheet would require a trial-and-error method. As a design aid to avoid this problem, this paper gives nomograms for the design of lightweight hollow helical springs. The nomograms are graphical solutions to the maximum stress and deflection equations. Example suspension spring designs show that significant weight savings (of the order of 50% or more) can be achieved using hollow springs. Hollow springs could also be used in extreme temperature situations. Heating or cooling fluids can be circulated through the hollow spring.