Abstract: 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 carried out on a vertical axis ballscrew feed system of a three axis vertical machining center, which showed fairly good agreement with the theoretical results.

Key words: vertical axis feed system;without counterweight;variable coefficient;whole stroke;dynamic characteristics