Research On Machining Method And Dynamic Performance Of Steep-lead Ball Screw Mechanism

Ball screw mechanisms have been widely used in commercial units, especially in the numerical control machine tools, due to its performance such as high accurate positioning, high stiffness, low friction, etc. With the development of mechanical production and NC machine oriented to high-speed, high-precision, multi-purpose and environmentalization, the feed speed of ball screw units is demanded higher and higher. The increase of feed speed of ball screw mechanism is realized by enlarging the lead rather the rotational speed because of the limited dN value (d-ball pitch circle diameter in mm, N-rotational speed of screw in rpm). With the increase of lead, the helical angle makes the proceeding more difficult, the dynamics performance different from the traditional ball screw mechanism. Therefore, it becomes an import thing to develop the present theory and proceeding method to improve the performance of ball screw mechanism, combining available tests.This thesis analyzes the kinematics and dynamics of ball screw mechanism, and research the problems in the designing and machining, the main content is as followed.1. The history and development trend of ball screw mechanism is presented, the present research work and problems are summarized, and the purpose and content of subject are introduced.2. The kinematics of the balls of the ball screw in track is systematically investigated, and the influency factors on the kinematics of the balls are analyzed. First, the Frenet-Serret coordinate is constructed based on the differential geometry theory. The number of balls in the ball screw mechanism is deduced correctly with concern of helical angle. The relationship between ball rolling speed and screw rotational speed is developed based on the compatibility condition of sliding velocity vectors. Finally, the motion of ball in the groove is analyzed with deformation. The conclusion is reached that the relative slip between the ball and groove is unavoidable, and with the increase of contact angle and lead angle, the revolution speed and spin speed is enlarged.3. The kinematics and dynamics of ball recirculation are analyzed combining theory and simulation.A method of return tube's optimum design is proposed. The impact contact model between the balls and return tube is presented in this paper based on the Newton law and Hertz theory, and the influencing factors of the contact impact are analyzed comparing the forces under different work conditions such as contact angle, material, construction of ball. The process of ball rolling in the return tube is simulated by ADAMS and Ansys-lsdyna software. The simulation result shows that factors such as entering angle, entering speed and error affect the ball's motion. The locations of the Brinell dent and fracture on the damaged return tube are in accordance with the contact point and the place stress concentration in the simulation result. The simulation result also shows that the cause of stress concentration and uneven impact is the uneven curvature of return tube. According to this conclusion, it is better to make the variance ratio of the return tube's curvature constant to make the ball's motion smoothly.4. The mechanical model of ball screw mechanism is developed, and the effect of the helical angle on the elastic deformation of the ball screw mechanism is analyzed. With concern of the factor of lead angle, the principal curvature of groove is deduced. According to Hertz theory, the elastic deformation between the ball and groove is get and the mechanical model is constructed considering the effect of uneven load distribution. In order to prove the presented mechanical model in this paper, a finite element model is constructed according to the real part. The simulation result is in accordance with the theory model. The analysis of the model shows that the axial elastic deformation of ball screw mechanism decreases with the lead angle and the torque vibration of ball screw mechanism are mainly caused by the impact contact between the ball and return tube 5. The dynamical performance of ball screw mechanism such as dynamic drag torque, vibration, noise and fatigue failure are measured. The experimental result shows that both the angle velocity of the ball and the friction between the ball and track increases with rise in helical angle. It also proves that the recirculation of ball mainly affect the ball screw's dynamical performance.6. In order to solve the problems such as low rigidity of the grinding wheel, small grinding domain in the processing of large-lead nut, a new processing method is investigated in this paper. At first models of the grinding process of the nut and dressing of grinding wheel are developed based on the meshing theory and geometry. The relationship between the setting angle of grinding wheel and axial profile are solved by numerical method, and the influencing factors of the grinding quality are investigated. The new method is proved to be able to increase the quality of processing and meet the production requirement by simulation and practice.This thesis provides theory and technology to support design and manufacture of high speed ball screw mechanism, and a practicable solution to develop high speed ball screw with independent intellectual property rights. All the work is of significance to improve the level of NC manufacturing.This project is supported by National Natural Science Foundation of China (Grant number 50675124).