STRESS APPROXIMATION TECHNIQUE FOR HELICAL COMPRESSION SPRINGS SUBJECTED TO LATERAL LOADING

Helical compression springs are commonly used devices capable of storing kinetic energy. Typical applications vary in simplicity, ranging from low stress amplitudes and in favorable environments, e.g. ball point pen spring at room temperature, to millions of cycles in elevated temperatures, e.g. valve train spring in IC engines. Regardless of the load or environment, springs are able to use the intrinsic elasticity of the material and the initial geometry to resist plastic deformation, all while allowing for the transfer of load over various distances. Generally, these loads are parallel to the axis of the spring; however, as more complex designs arise, these uniaxial springs are gaining popularity in a variety of off-axis loading situations, e.g. flexible shaft couplings, invalidating traditional stress/strain equations. As such, equivalent stress and strain equations have been developed capable of fast, real-time calculations based upon visual inspection of the bent helix. Coupled with the initial dimensions and material of the spring, the state of equivalent stress/strain can be resolved at any position within the wire. Experiments were conducted on several off-the-shelf steel springs (conforming to ASTM A229), then compared to FEA and analytical solutions. Ultimately, it was observed that through an approximation of the bent helix, the equivalent stress and strain can be determined at any location within the wire, allowing for the approximation of life and crack initiation locations of the spring.