Numerical Modeling of the Dynamic Behavior of Shredder Hammers and Correlation With Material Damage

Pre-forming and fragmentation of the ferrous scrap used into the electric arc furnace for the melting process is a relevant activity for a steelmaking plant. Shredding machines are applied to suitably reduce the size of scrap. A set of hammers is connected to a main rotor. Rotation converts the high kinetic energy of each hammer into a strong impact against the scrap. Metallic parts are crushed and fed into the electric arc furnace. Damage of the hammer material is due to impact, vibration, wear and temperature. In addition fatigue affects its life. An effective prediction of the damage location as well as of its propagation in the hammer is rather difficult. A resident health monitoring system cannot be easily applied. Therefore a preliminary model was built to predict the dynamic behavior of each hammer in rotation and to compute the applied stress, while the impact is occurring. A rotor-dynamic analysis was performed by means of a Multi Body Dynamics and a Finite Element code, respectively. Magnitude, direction and frequency of the dynamic loads were first computed by the Multi Body Dynamics code. Stress exciting the hammer material was then computed by the Finite Element Method. Nonlinearities are crucial for the design operation. Friction among the materials, clearance between the pin and the hammer and the nonlinear behavior of materials are all relevant for the nonlinear dynamic response of the hammer. Numerical results were compared to some preliminary observations performed on an industrial plant. They allowed motivating the occurrence of cracks and wear effects in some critical points of the hammer. Some design criteria were defined and successfully tested to improve the performance of materials.