| This project comes from the National Science and Technology Major Project "high-end CNC machine tools and basic manufacturing equipment"(item No.:2010ZX04001-032)-Moving Beam without Ram Vertical Milling-lathe Complex Machining Center. The machining accuracy of CNC machine tool’s feed system is greatly determined by its transmission accuracy and positioning accuracy. Ball screw, as one of the main feed mechanisms from Vertical Milling-lathe Complex Machining Center’s feed system, possesses advantages of high transmission efficiency, small friction resistance, high sensitivity, smooth movement, low speed without crawling, long service life, accuracy of good stability, high positioning accuracy and repeated positioning accuracy, fine synchronization, etc.. So it is mainly used in the feed mechanisms of precision machine tools, CNC machine tools and others.The feed mechanism’s dynamic performance is a key indicator affecting the high speed vertical machining center working performance and its workpiece quality. The center’s high cutting speed, rapid moving acceleration and deceleration also causes the machine heat, the large inertia of the moving parts. It leads to the machine moving parts’thermal deformation and vibration and will ultimately affect the workpiece’s working accuracy, surface quality and machine tools’precision of stability and tool durability. Therefore, we can conclude the machine tool feed mechanism’s basic requirements under high-speed machining. First, the "three high", namely high static stiffness, dynamic stiffness and thermal rigidity; second, the moving parts should be lightweight to minimize drive system’s inertia.This paper mainly analyzes and studies the dual-drive ball screw using computer modeling and finite element simulation technology. Then study its finite element modeling, structural static analysis, structural dynamics analysis, thermodynamic analysis, size optimization design and so on. Finally, we propose a solution to the ball screw’s size optimization providing certain principles for the design and choice of ball screw in machining center. Specific research contents are as follows:(1) Approximate the screw two ends bearing group’s supporting role by the elastic support constraints and represent the elastic support’s foundation stiffness with radial stiffness of the bearing. Considering the dual-drive ball screw’s two ends bearing joints may affect its static characteristics. we establish a reasonable and simple dual-drive feed system’s finite element model, make its structural static analysis and get a dual-drive ball screw static deformation cloud chart in typical milling conditions. Simulation results show that the dual-drive ball screw static maximum deformation volume is within5μm satisfying the dual-drive feed system’s static stiffness design requirements.(2) Calculate the ball screw’s modal analysis based on modal analysis theory. We get its first six natural frequencies and modal shapes. Simulation results show its maximum speed is obviously lower than the critical speed of the first order. Thus it will not resonate within the effective speed range in milling conditions; Do harmonic response analysis under sinusoidal excitation to the screw and get its harmonic response amplitude-frequency characteristic curve. Results show the maximum amplitude is at a frequency of55-60Hz under sinusoidal excitation force. Therefore, in actual cutting conditions, the machine tool should avoid the dynamic loads under this frequency to avoid greater vibration affecting the workpiece machining accuracy.(3) Analyze the main heat source of high-speed hollow ball screw and calculate of its heat value according to relevant thermodynamic theory. Meanwhile, compute its thermal boundary conditions and simulate its temperature field distribution in actual conditions. Then apply the steady-state temperature field distribution acting as a "hot load " on the ball screw, make its heat-structure coupling analysis and get its thermal displacement cloud chart. Results show the screw nut has the highest temperature about65.374℃. the joints of screw ends and bearing has about49.65℃secondary to the screw nut. The temperature is slightly higher only near the nut to the whole screw threaded section, the rest sections’close to room temperature20℃and the maximum does not exceed21℃. The screw’s maximum hot displacement volume is33.535μm in conditions. Finally, find out a hollow ball screw’s pretension should be8275.53～10399.31N premising the proper amount of pretension.(4) Optimize the size of the ball screw’s relevant parameters by its relevant size design theory. Simulation results show the screw can obtain the optimal design variables when its static, dynamic and thermal rigidity were all fully optimized. We conclude the optimized model characteristics have been enhanced in static, dynamic and thermal compared with the original model. Then the rigidity of the dual-drive feed system is increased and it may also reduce the synchronization the transmission error of the dual-drive system to some extent. |
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