Research On Dynamic Characteristics Of High-speed Ball Screw With Nut Driven

Numerical control machine tools of large-scale and high-speed require not only a continuous increase in rotating speed of spindle but also a more rapid feed speed. Conventional ball screw pair, especially the large stroke screw pair of large-scale, heavy-duty and high-end numerical control machine tools, is much heavier than the nut. Therefore, screw rotary inertia force can cause the rising of heat, deformation and severe energy consumption.The deformed screws will produce vibration and noise during the rotation reducing the operation accuracy of the equipment. This remains an open question in the field. Therefore, according to the relativity of movement, the study puts forward a new-type of "nut-driven ball screw pair" and changes the traditional "screw-driven mode" to "nut-driven mode".With the aid of mathematical modeling, mechanical analysis, computational analysis software, experimental study and other methods, the high-speed transmission performance of nut-driven ball screw pair on the basis of establishing rolling body kinematics, contact model and feed system structural dynamics model was studied in this thesis. The contents of the study are as follows:1. According to kinematical theory, the rolling body of high-speed but-driven ball screw pair was analyzed. The study has clarified the kinematical relationship between balls rolling speed and drive nut axis revolving speed, deduced the revolution and rotation angular speed of the rolling body and acquired the contact point position and sliding speed of the rolling body in double nut. A conclusion is drawn based on theoretical analysis that the speed and direction of rolling body in driving nut of nut-driven ball screw pair are different from those of rolling body in driving nut of screw-driven ball screw pair; at the same driving speed, the revolution and rotation angular speeds of rolling body of nut-driven ball screw pair are lower than those of rolling body of conventional screw-driven ball screw pair.2. According to Hertz’s contact theory, analysis on kinematics and dynamics rolling element contact characteristics of nut driven ball screw pair under high-speed was performed. In an effort to establish contact analysis model for high-speed nut-driven ball screw pair under joint action of axial load, inertial effect, displacement constraint and apply iteration method to solve non-linear simultaneous equations. At the same time, the change rule of ball screw pair contact parameters under different revolving speed and axial force has been analyzed as well. Feed acceleration was obtained on the basis of theoretical calculation. The results show that when ball screw pair is at a high speed, contact angle, axial load and elastic deformation in the nut assembly change with speed and axial load. Compared with screw-driven type, it is driven by the motor with the same torque; nut-driven ball screw reduces rotational inertia of drive system, thereby reducing energy consumption of system driver; nut-driven feed system can produce great feed acceleration and has high speed.3. The dynamical analysis on feed system of double nut-driven ball screw pair with adjustable pre-tightening force was carried out; firstly, by establishing the structural dynamical model of ball screw pair, both the effects of changes in pre-tightening force on natural frequency and that of the changes in major design parameters on the low order natural frequency of the system have been analyzed. The results show that the natural frequency of feed system has time-varying characteristics. Compared with conventional screw-driven type, it adopts nut-driven ball screw pair, which improves the low order natural frequency of feed system, causes the system to have a wider bandwidth, improves the speed loop gain and position loop gain of the system, and thus enhances rapid response capacity and machining accuracy of the feed system.4. The study established the analysis model of friction torque to analyze the generating mechanism of friction torque of the ball screw pair. The influences of manufacturing errors and the axial load on the friction torque were analyzed. The reasons of low-frequency fluctuations of friction torque were studied. It applies the elastic hydrodynamic lubrication theory to analyze the lubrication conditions of ball screw pair. The results show that the axial load and rolling path waviness have a significant impact on fluctuation of friction torque.5. The nut-driven ball screw pair was designed and put into practical application, and relied on super long axis keyway milling machine to carry out vibration response test and temperature test on nut-driven ball screw pair and screw-driven ball screw pair. Experimental studies show the first-order natural frequency of nut-driven ball screw pair system is higher than that of screw-driven ball screw pair; under the same operating environment, the temperature rise of screw-driven ball screw pair is higher than that of nut-driven ball screw pair. The study provides the theoretical and technical supports for design and manufacture of ball screw pairs with high speed and high precision, indicating a practical way to the research and development of high-speed and high-precision ball screw pair with completely independent intellectual property rights, which showed theoretical and practical significance to improve the manufacturing level of CNC machine tools in China.