无平衡重的竖直轴进给系统动态特性变化规律研究

Research on the Variation Rule of the Dynamic Characteristics of a Vertical Feed System without Counterweight

针对小型立式加工中心的竖直轴滚珠丝杠进给系统,由于主轴箱等移动部件质量较小而未安装平衡重装置,此时移动部件的重力将直接作用到滚珠丝杠的传动系统上,在重力和初始预紧力综合作用下,丝杠螺母副和轴承副的实际状态将发生变化,进而影响该动结合部的接触刚度。主轴箱等移动部件处于全行程内不同位置时,即丝杠的受力作用点会发生变化,从而改变该系统的传动刚度,最终影响该系统的动态性能。以传统的竖直轴滚珠丝杠进给系统为研究对象,采用集中参数法进行动力学模型等效,建立考虑传动刚度变化的变系数动力学方程。理论计算移动部件重力作用下丝杠螺母副、轴承副的等效轴向刚度,进一步分析考虑丝杠受力作用点改变时传动刚度的变化规律,最终研究进给系统的固有频率和加速度频响在全行程内的变化规律。最后以某三轴立式加工中心的竖直轴进给系统为案例,进行了模态试验,理论与试验结果具有良好的一致性。

The counterweight device is not installed on the vertical axis feed system of some vertical machine centers because the masses of the headstock and other moving components are smaller. Thus, the headstock and other moving components exert the gravity on the ball screw feed system directly. Under the effects of the gravity and the initial preload of the kinematic joints, the actual condition of the screw-nuts joints and bearings joints will be changed. Furthermore, the stiffness of the kinematic joints is affected. The stress points of the screw in whole stroke would chang when the headstock and the moving components are at the different positions in the whole stroke. Thus, affect the transmission stiffness of the system, as well as the dynamics. Taking a typical ball screw feed system as an example, an equivalent dynamic model is established using the lumped parameter method, and then a variable-coefficient dynamic equation is also established considering the variation of the transmission stiffness. The equal axial stiffness of the screw-nut joints and bearing joints under the gravity is analyzed theoretically, as well as the variation of the transmission stiffness in whole stroke considering the change stress points of the screw. Meanwhile, the variation of the natural frequency and acceleration vibration response of the feed system is studied in whole stroke. Finally, the modal experiment is carded out on a vertical axis ballscrew feed system of a three axis vertical machining center, which showed fairly good agreement with the theoretical results.