2016 3 rd International Conference on Mechanical, Industrial, and Manufacturing Engineering (MIME 2016) ISBN: 978-1-60595-313-7 Finite Element Modal Analysis of Twin Ball Screw Driving Linear Guide Feed Unit Table Xiangrong Xu, Xianchun Song, Hongkui Jiang, Yanfeng Li, Qunpeng Lu School of Mechanical and Electronic Engineering, Shandong Jianzhu University, Jinan,Shandong Province, P. R. China ABSTRACT: Twin ball screw driving linear feed system has good vibration resistance, high stiffness and fast system response, etc., can effectively decline vibration of NC machine tools generated in the case of high speed and high acceleration and deceleration, so it is used widely in the high-speed and high-precision NC machine tools. This article is focused on the completion of the finite element modal analysis of twin ball screw driving linear guide feed unit table. The modal of single ball screw driving linear guide feed unit and that of twin ball screw driving linear guide feed unit was analyzed, and the results were compared when nuts are located at the left, middle and right positions of twin ball screw driving linear guide feed unit. The results show that under the same vibration shape, the natural frequencies of twin ball screw driving linear guide feed unit are higher than those of single ball screw driving linear guide feed unit, the twin ball screw driving linear guide feed system has smaller vibration and better dynamic properties; different positions of nuts will affect the natural frequencies of twin ball screw driving linear guide feed unit, smaller in the middle position and larger on two ends, so the vibration on the middle position is larger than that on two ends. 1 INTRODUCTION With the development of high-speed, high-precision and heavy-load NC machine tools,lots of disadvantages of traditional single ball screw driving feed system in high-speed driving process have become more and more obvious,such as big noise,high temperature-rise and low precision. Twin ball screw driving feed system has the advantage of good vibration resistance, high stiffness and fast system response, and can effectively decline vibration of NC machine tools under the condition of high speed and high acceleration. Thus twin ball screw driving feed ststem is becoming more and more widely used in high-speed and high-precision NC machine tools. There are many literatures about the research on twin ball screw driving feed system.in such countries as the United States, Japan, Germany,etc., the technology in this field is relatively developed.but it is difficult to find the relative literatures because of technical security. He Wangyong et al. analyzed the coupling relationship between the loads and the axial and torsional coupling relationship between ball screw and nut of twin ball screw synchronous driving shaft[1]. Yang yong investigated the influence of drive at center of gravity and linear guide joint surface parameters on the dynamic performance of machine tool[2]. Zhou Yong established the finite element model of twin ball screw drive, analyzed its 11 dynamic characteristics, and verified the advantage of dual drive relative to the single drive[3]. Huang Xiaoyan used the finite element simulation software ANSYS Workbench to analyze the dynamic and static characteristics of a high-speed horizontal boring and milling machining center driven at center of gravity[4]. Ding Xihe et al. carried on dynamic characteristic analysis of twin ball screw driving feed system of NC machine tool under the different ball screw and guide span and found the influence law of different ball screw and guide span on the dynamic characteristics of a feed system[5]. Wang Renche et al. researched the influence of the position of center of gravity, the position of nut and the quality of the table on the natural frequency of feed system[6]. But the studies all above seldom included the support guide into the feed system. In this paper, considering the influence of the support guide, twin ball screw drive linear guide feed unit table is designed, and using ANSYS method, the modal analysis is carried on, the main modal parameters obtained, and so the basis can be provided for the further study of dynamic characteristics and influencing factors of twin ball screw driving linear guide feed unit.
2 TWIN BALL SCREW DRIVING LINEAR GUIDE FEED UNIT TABLE The structure of twin ball screw driving linear guide feed unit test table is shown in Figure. 1. It includes the base, twin ball screws, twin linear guides, twin support ball bearings and bearing supports, and table which does reciprocating linear motion along the ball screw pair. 3.2 Choosing the unit type, defining material properties and meshing (1) Selecting unit type In the ANSYS structural dynamics analysis, since Solid92 unit is 10 nodes hexahedron, its simulation of curve boundary is ideal, meshing is convenient by means of MeshTool and Refine mesh, it is the first selection for three-dimensional model of the irregular boundary. So 10 nodes SOLID92 unit is choosen as the unit type of the feed unit. (2) Defining the material properties The material properties of every component of the feed unit is shown in Table 1. Figure 1. Schematic diagram of feed unit test table. 3 FINITE ELEMENT ANALYSIS OF TWIN BALL SCREW DRIVING LINEAR GUIDE FEED UNIT TABLE Only linear behavior can be analyzed in the modal analysis of ANASY,while nonlinear units specified in the analysis usually are ignored and dealed with as linear units.moreover, the nature of the material in modal analysis must be linear and isotropic, therefore such linear features as the elastic modulus and density of the material must specified, and its nonlinear material properties ignored. The whole analysis process is as the following. 3.1 Three-dimensional solid model In this paper,the structure of the feed unit researched is complicated, so the direct modeling is difficult in ANSYS. So the model is established in SolidWorks software, and then imported into the ANSYS to be analyzed so that the difficulty of modeling the complex model in ANSYS can be greatly reduced. Some small structure features of feed unit, such as chamfer, thread, etc., will increase the number of units after the meshing and impact the efficiency of analysis and calculation,but the effects on the dynamic characteristics of the whole structure are little. As a result, the model is simplified and imported into ANSYS as shown in Figure. 2. Figure 2. Simplified model. 12 Table 1. Material properties of every component of feed unit. Properties Values Elastic modulus 2.07 10 11 pa Poisson's ratio 0.3 Density 7.85 10 3 kg/cm 3 Material damping 0.1 (3) Meshing There are many kinds of meshing methods in ANSYS, such as free meshing, mapped meshing, sweep meshing, and hybrid meshing, etc. In the paper,mapped meshing is adopted for ball screws and free meshing is used to the rest part of twin ball screw drive linear guide feed unit, and the result of meshing is shown in Figure 3. 3.3 Loading and solution Both ends fixed installation is choosen for the ball screw pair, so all constraints of three s are loaded to two fixed ends and the base. After boundary conditions and loading is completed and then modal analysis type is set in the solver, the first vibration modes when the nut is located at the middle position of the feed unit are shown in Figure 4. 6 order modals are extracted using the Block Lanczos method. 3.4 Model anlysis results In this paper, modal analysis is carried on respectively when the nut is located at the left end, middle and right end position of feed unit, and the results of the natural frequency analysis in different positions are shown in Table 2. Table 2. Natural frequency and vibration shape description of feed unit. Position Modal Natural frequency and vibration shape description Middle 1 33.261 the first-order bending of ball screws in Y 2 33.320 the first-order bending
Right end Left end 3 39.381 the first-order rotation of table around X Axle 4 134.89 the first-order bending of table in Z 5 141.74 the second-order bending 6 317.32 the second-order torsion of table around Y Axle 1 54.783 the first-order bending 2 75.041 the first-order bending of ball screws in Y 3 81.166 the first-order rotation of table around X Axle 4 132.53 the first-order bending of table in Z 5 162.10 the second-order bending 6 241.30 the second-order torsion of table around Y Axle 1 82.184 the first-order bending of ball screws in the same Z 2 150.03 the first-order bending of ball screw in different Z 3 172.38 the first-order bending of ball screws in different Y 4 172.90 the first-order bending of ball screws in the same Y 5 181.97 the second-order bending of ball screws in the same Z 6 207.12 the second-order bending of ball screws in different Z Figure 3. Meshing result of feed unit. (c) 3rd order 13
(d) 4th order (c) 3rd order Figure 4. Modal analysis results when the nut is located at the middle position of the feed unit. Vibration modes when the nut is located at the right end of the feed unit are shown in Figure 5. 14
position of single ball screw driving feed unit are shown in Table 3. and Figure 7. (d) 4th order Figure 5. Modal analysis results when the nut is located at the left end of the feed unit. (c) 3rd order Vibration modes when the nut is located at the left end of the feed unit are shown in Figure 6. The natural frequencies and vibration shape description when the nut is located at the middle 15
Table 3. Natural frequency and vibration shape description of single ball screw driving feed unit. Modal Natural frequency Vibration shape description 1 28.994 the first-order bending of ball screw in Z 2 32.369 the first-order bending of ball screw in Y 3 45.888 the first-order bending of table in Z 4 112.44 the first-order rotation of table around Z 5 201.30 the second-order bending of ball screw in Z 6 329.76 the second-order bending of ball screw in Y (d) 4th order Figure 6. Modal analysis results when the nut is located at the right end of the feed unit. 16
(c) 3rd order Figure 7. Modal analysis results when the nut is located at the middle position of single ball screw driving feed system. 4 CONCLUSIONS The above analysis results show that under the same vibration shape, the natural frequencies of twin ball screw driving feed unit are larger than those of single ball screw driving feed unit, and the position of nut of twin ball screw drive linear guide feed unit has obvious influence on the dynamic characteristics of the feed unit, and each order natural frequency at two fixed ends is significantly higher than that in the middle position. (d) 4th order 5 ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51205235) and the Doctoral Funds of Shandong Jianzhu University (XNBS1246). REFERENCES [1] He Wangyong, Tang Xiaoqi & Li Yongbo.2010.Dynamics modeling and analysis of a motion axis driven by a synchronous dual ball screw based on FEA. Manufacturing Technology & Machine Tool (12):83-92. [2] Yang yong.2010.influnence of drive at center of gravity and guide joint surface parameters on dynamic characteristics of machine tool. Nanjing:China. [3] Zhouyong.2008.Dynamic characteristics analysis and motion control research of high speed feed drive system.wuhan: China. [4] Huangxiaoyan & Lizhuang.2011. Application of the principle of drive at center of gravity to the structure design of key components of high-speed horizontal boring and milling machining center. Dalian HuaGen machinery co., LTD:China. [5] Ding Xihe.2014.Dynamic characteristics analysis of dual ball screw drive linear feed system. Modular Machine Tool & Automatic Manufacturing Technique (3):26-32. [6] Wang Renche, Zhao Tong, Ye Peiqing & Liu Yan. 2014.Three-Dimensional modeling for predicting the vibration modes of twin ball screw driving table. Chinese Journal of Mechanical Engineering 27(1): 211-218. 17