Research On Dynamic Characteristics Of Ball Screws Linear Guide Feed Unit

The development of high-speed, high-precision, high-efficiency and high-automation NC machine tools has a demand in higher and higher processing performance and dynamic characteristics. And among all the dynamic characteristics, vibration performance is the most important, that is to say, processing quality of machine tools depends mainly on the vibration of itself. Research shows that the process quality depends largely on the vibration of NC machine tools, especially to high-speed, high-precision NC machine tools. Therefore, vibration of machine tools is an important problem in the study of dynamic characteristics of machine tool.As a key functional part of NC machine tool feed system, ball screw, which is an elongated low rigidity component, easily produces vibration and noise in the external alternating force. Especially in large CNC machine tools, ball screw become longer with the longer travel of worktable, which results in a decline in such dynamic characteristics of feed system as stability, rigidity and vibration resistance, and so on, especially during heavy cutting, vibration becomes more serious, which both affects the machining precision and process quality of the surface and easily causes the rapid wear and damage of the tool.Linear rolling guide works as supporting and guiding device of the feed unit, large vibration may increase the dynamic load of the guide during the processing and thus reduce its life and accuracy retention. As a result, there is important theoretical and realistic value about research on the dynamic characteristics of Ball Screws Linear Guide Feed Unit.In this paper, by means of theoretical analysis and experimental research, on the basis of Dynamics theory, the Hertz contact theory, vibration theory and experimental modal analysis, the dynamic characteristics of Ball Screws Linear Guide Feed Unit is researched. The study contents are as follows:1) Considering the influence of ball screws, linear rolling guide and ball bearing joint interfaces of Ball Screws Linear Guide Feed Unit, the Dynamics model is built, and the vibration differential equation is derived on the base of Lagrange equation.2) Considering the influence of ball bearing in the end and on the basis of the Hertz contact theory, the contact deformation and stiffness analysis of ball screw feeding system under the external force is analyzed, and the structural influence factors on contact stiffness of the single and double nut ball screws is obtained and contrasted.3) The model of contact stiffness of linear rolling guide is built, which consider such influence factors as external force, the number and pre-deformation of balls, etc. on the contact stiffness. Taking linear rolling guide LSA-45G for example, contact stiffness value in Y and Z direction is calculated.4) By means of experimental modal analysis technique, experimental modal research on Ball Screws Linear Guide Feed Unit Test is investigated. Through the impulse test, a system main modal parameters are obtained, including modal mass, modal stiffness, modal damping and vibration shape.5) According to vibration theory and considering the elastic supporting of ball bearing at both ends of ball screw, frequency equation under this condition and the function of vibration shape is derived. Using the experimental modal parameters, vibration shape of Ball Screws Linear Guide Feed Unit is researched. The finite element modal analysis of Ball Screws Linear Guide Feed Unit and ball screw feed system unconsidering linear rolling guide are carried on and compared with the experimental result. The weak links of the system is obtained and some suggestions for improvement are put forward.Theoretical analysis and experimental study results are as follows: 1) The axial contact stiffness and the deformation of ball and raceway contact area have the direct influence on the axial dynamic characteristics of Ball Screws Linear Guide Feed Unit.Such design parameters as axial force, contact angle, helix angle, ball number, principal curvature, and so on, have an important influence on the axial contact stiffness and deformation on the ball and raceway contact area.2) Reasonable design of structure parameters of ball screw pair can improve the axial stiffness of ball screw pair.the increase of contact angle can result in the nonlinear decrease of the axial contact stiffness of the single nut ball screws. The increase of ball number can result in the linear decrease of the axial contact stiffness of the single nut ball screws. The change of helix angle has an unobvious influence on the axial contact stiffness of the single nut ball screws. But the axial contact stiffness of the single nut ball screws changes sensitively with principal curvature. The increase of axial force can result in the nonlinear increase of the axial contact stiffness of the single nut ball screws.3) Comparatively, the increase of axial force can result in the linear increase of the axial contact stiffness of the double nut ball screws, the increase of preload can result in the nonlinear increase of the axial contact stiffness of the double nut ball screws. Contact angle, ball number, helix angle and principal curvature have a similar influence on the axial contact stiffness of the double nut ball screws to the single nut ball screw, but under the same condition, the axial contact stiffness of the double nut ball screws is obviously higher than that of the single nut ball screws. Therefore, finding optimal contact angle, reducing the curvature ratio, increasing the number of balls and selecting the appropriate preload during the design of ball screw is conducive to improve the contact stiffness of the ball screw nut pair.4) Such design parameters as external force, pre-deformation and the number of balls,etc have influence on the contact stiffness of linear rolling guide. The increase of external force can result in the increase of contact stiffness in Y direction and the decrease of contact stiffness in Z direction of linear rolling guide. The increase of pre-deformation can increase the contact stiffness in Y direction and decrease the contact stiffness in Z direction of linear rolling guide.The increase of the number of balls can decrease the contact stiffness in Y direction and increase the contact stiffness in Z direction of linear rolling guide.The vibration shape of Ball Screws Linear Guide Feed Unit is mainly bending and twist motion.The weak links occurs mainly in the ball screw and the connection part of the connecting plate and ball screw.The natural frequencies of Ball Screws Linear Guide Feed Unit are affected by the support form of itself:natural frequencies of Ball Screws Linear Guide Feed Unit fixed at two ends are obviously higher than those of Ball Screws Linear Guide Feed Unit fixed at one end and simply supported. The natural frequencies of Ball Screws Linear Guide Feed Unit are affected by the location of the nut:The natural frequencies when the nut locates at two ends are higher than those when the nut locates at simply supported end, The natural frequencies when the nut locates at two ends are higher than those when the nut locates at middle.The natural frequencies of Ball Screws Linear Guide Feed Unit are more close to the experimental results than those of feed unit without linear rolling guide.Research results provide research direction for the decline of dynamic deformation and the increase of vibration resistance of Ball Screws Linear Guide Feed Unit, and have an important theoretical and realistic guiding value for the structure design, manufacturing and engineering application of Ball Screws Linear Guide Feed Unit.