Element size effect on the analysis of heavy-duty machine cross-rail

Abstract

Heavy-duty gantry machine plays an important role in the energy, shipbuilding, aerospace, transportation and military industry and other pillar industries, and is a manifestation of national equipment manufacturing capacity. Cross-rail is one of the key structural components, and its natural frequency affects the dynamic performance of the machine. It is difficult to obtain the accurate analysis of natural frequency by using the analytical method because of the complex internal structure. The finite element method is a general method to obtain natural frequency, but, is also difficult to obtain the precise result, because the analysis result depends on many factors, such as meshing manners, the constraint conditions, joint parameters, damping, and so on. The density of mesh is an important factor to affect the analysis result especially. Theoretically, the smaller element, the closer to the true result. But more dense grid could sharply increase the degree of freedom, and result in the insufferable solving speed in the analysis of the cross-rail. So to find a balance point between the speed and the accuracy becomes very meaningful. In this paper, the modal analysis was carried out on the cross-rail. The paper focus on exploring the influence of different element size on the results of the analysis. In order to verify the correctness of the modeling, the modal testing is carried on the cross-rail, and the experimental result is adopted to prove the correctness of finite element modeling. 1. Cross-rail free modal finite element analysis 1.1 The cross-rail profile: Cross-rail on this study is about 15 meters, and the quality is 99.716 tons, as shown in figure 1-1.The global coordinate system is as following. Anti-gravity is the Z direction, Y is the longitudinal direction, front and rear direction is X direction. Fig. 1-1 CNC heavy-duty gantry machine and global coordinates The cross-rail material is QT600. The property of the cross-rail material is as showing in table 1-1 Table 1-1 Cross-rail material parameters Density (kg/m) Young's modulus (GP) Poisson's ratio 7200 174 0.275 4th International Conference on Machinery, Materials and Computing Technology (ICMMCT 2016) © 2016. The authors Published by Atlantis Press 964 1.2 FEM element selection: The analysis tool of FEM is Ansys. Solid185 element is adopted to mesh the cross-rail. Solid185 entity structure applicable to the general three-dimensional structure modeling, it can be to prism, tetrahedron and pyramid for degradation, the element is shown in figure 1-2. Fig.1-2 Solid185 element and degradation Solid185 element is defined by eight nodes, and each node has the X, Y, Z three directions of the translational degrees of freedom. The element has a super elasticity, stress players, creep, large deformation and large strain capabilities. Stress output of Solid185 elements as shown in figure 1-3. Fig. 1-3 Stress output direction of the Solid185 element (SX, SY, SZ) 1.3 Effect of element size on modal analysis: Adopting the automatic meshing method. In the condition of completely free-free, modal analysis was carried out on the cross-rail, not imposed any constraint. Since cross-rail is in a free-free state, the first six orders vibration belongs to the rigid body motion, so the natural frequency is zero. Beginning with 7th frequency values, namely that the 7th order as the first order modal. Studies have shown that the size of the element will largely influence the finite element simulation result. It is necessary to control the grid size to achieve ideal result. At first , taking the element size of 30 mm , and the first 5 order modal simulation result is shown in figure1-4. a) First-order mode shape b) Second-order mode shape c) Third-order mode shape d) Fourth-order mode shape