Modeling and experimental verification of helical square wire spring for ICE (Internal Combustion Engine) application

Automotive springs are subjected to dynamic loads that results critical for their fatigue life. In this thesis, developed in collaboration with Dayco, a torsional helical spring of an Overrunning Alternator Damper was studied. An analytical model was implemented by means of a spreadsheet to calculate static characteristics and stresses of this spring, presenting a trapezoidal cross-section, loaded simultaneously by compression and torsion. After a quick description of the production process and heat treatment and the benefit that an OAD has on the performance of the Front End Accessory Drive System (FEADS), the analytical model is presented. The analytical model is based on the formulas of A.M. Wahl. “Mechanical Springs”, and the warping of the trapezoidal section under torsion is taken into account through the numerical calculation of the shape factor through static simulations of a trapezoidal beam, varying the geometry of the section. The results are then compared with experimental tests and FEA. Fatigue tests have been conducted and a Wohler diagram is extrapolated, the relative Basquin equation coefficients are inserted into the model to be used for possible cumulative damage calculations. A coaxiality error of the test bench produced some early ruptures of the tested springs, this led to an analysis of the influence of the coaxiality error on the maximum stress in the spring in ANSYS. Some of the fracture surfaces of broken springs are then analyzed by means of an optical microscope. In the end, the main goal to build an analytical model to bypass the FEM calculation is reached, since the results are almost overlapped, with a maximum relative difference of 5%.