실험계획법을 이용한 차량 리프스프링의 최적설계

Abstract

Relations between deflection and load in the leaf spring of the suspension system have inherent nonlinearity caused by the inter leaf friction. The leaf spring affects the riding comfort and handling performance of vehicles. Therefore, it is important that the static and dynamic hysteresis of leaf spring be properly described in order to improve the reliability of its design and analysis. However, as the theoretical approach of the leaf spring is difficult and complex, its design procedures remain rather an art than science. We are looking forward to optimize the design process of the leaf spring by applying the design method of experiment. In this thesis, the necessity and validity in the finite element modeling of the leaf spring are observed with considering the inter leaf contact and friction. The theoretical and experimental methods are compared through the load-deflection hysteresis which indicates the static and dynamic characteristics of leaf spring. The static and dynamic characteristics of leaf spring are predicted by MARC software used in finite element analysis. The leaf spring applied in the design method of experiment is based on FEA results. The design method for the optimization is proposed by approaching the characteristic of the FRP leaf spring(Carbon/Epoxy and E-glass/Epoxy) into the characteristic of the steel leaf spring. Following the above study, conclusions are drawn as below; 1. The hysteresis of the leaf spring is ignored in the unfolding and judgment methods because these theoretical analyses don't consider the inter leaf friction. But, the finite element analysis used in this thesis takes account of the load-deflection hysteresis of the leaf spring. 2. In results of experiment and FEM which apply theory of the leaf spring, the static spring constant in the judgment method and the finite element method exists within a acceptable range, ±10%. It is verified that these conventional methods are not suitable for describing the spring constant according to load and the hysteresis. Finite element method which enables the variation of shape and friction to express is required. 3. The more initial load increases under same amplitude, the more spring constant increases. If the amplitude is increase, the spring constant decreases. A numerical difference between experiment and FEA is 0.44~4.30%. So, results of the finite element analysis agree well with results of experiment. 4. The reliability of optimum design is acceptable because error is about 0.33~0.87% as shown in results on the static spring constant of steel leaf spring and FRP leaf spring. Easy design method of leaf spring is proposed by using design method of experiment. 5. Steel leaf spring has the hysteresis caused by the inter leaf friction. But, FRP leaf spring shows the linear behaviors without the inter leaf friction. 6. The design method for the optimization of FRP leaf spring is proposed by applying design method of experiment in order to improve the characteristics of steel leaf spring. It is expected that the excellent design such as the simple and reduction of design process leads to extend the applications in machines.