Otimização paramétrica de rolo polimérico de correia transportadora de minério

Abstract

Mining is an economically significant activity in Brazil. The ore transportation, performed by belt conveyors, begins with the sieving, crushing, and beneficiation operations and ends with the supply of ships and railway wagons. The roller is one of the fundamental elements of belt conveyors since it supports, guides, and directs the material on the belt. This component is basically composed of a body that rotates around a fixed shaft in metallic structures called easels. Rollers (tube and shaft) used in ore conveyor belts are mostly made of steel, which consequently leads to this part having a high mass. The severity of ore transport, associated with weathering and the high volume of material loaded on the belts, often causes this mechanical component to fail prematurely. Equipment maintenance is carried out manually by operators, hindering roller replacement. Aiming to achieve a mass reduction in this mechanical component, this work presents a parametric structural optimization study of a polymeric roller. To accomplish this, a finite element method (FEM) model is built using Ansys Workbench software. The roller considered was initially composed of an external tube made of high-density polyethylene (HDPE), bearing seats of polyamide 6 (PA.6), and a shaft made of steel. To characterize the polymeric materials (HDPE and PA.6), stress relaxation tests were executed in specimens. The results of the shear modulus variation over time were incorporated into the model to calculate the Prony series terms, thus accounting for the viscoelastic effect. The optimization of the structure aimed at minimizing mass was conducted using surrogate models of radial basis functions (RBF) and the Globalized Bounded Nelder-Mead (GBNM) algorithm, with the shaft radius and tube inner radius as design variables. In the optimization problem constraint’s definition, the design loads, and also the stress and deflection angle limits imposed by the ABNT NBR 6678:2017 were considered. The optimization process was divided into two cases. In the first one, which regards the roller’s initial material settings (i.e., PEAD tube and bearing seats of PA.6), the optimum point violated the constraints and could not reduce the mass of the system. In the second case, which uses the PA.6 in both the bearing seats and the tube, it was found a minimum point that respected all the constraints and reduced the roller’s mass by 15,5%, which is equivalent to 5,15kg. The primary reason for the divergence between the cases was the difference in stiffness between the polymeric materials. In the stress relaxation tests, PA.6 exhibited a longitudinal elasticity modulus significantly higher than that of PEAD. This mechanical property directly influences the misalignment angle between the shaft and the bearings, as well as the maximum deflection of both the shaft and tube. Additionally, the graphical results indicate that the curves of structural responses in the viscoelastic simulations resemble the Maxwell-Voigt model, which describes viscoelastic phenomena. Although incorporating viscoelasticity increases computational costs, it offers a more accurate representation of real mechanical behavior compared to a linear-elastic model.