Abstract: Spindle system on ultra-high speed grinding supported by liquid hybrid bearing is special. In the process of machine working, spindle rotates in high speeds, and the supporting stiffness of hybrid bearing changes dynamically with the change of rotation speed of spindle. In order to understand the dynamic characteristics of the spindle system, Flunent is used to calculate the dynamic stiffness of the liquid hybrid bearing. The 3D finite element model of spindle system on ultra-high speed grinder is established, and the natural frequencies and mode shapes are obtained by utilizing ANSYS. Through setting different stiffness in different rotational speeds, the results of dynamic modal analysis are obtained, and the critical speeds of the spindle system are obtained through graphical method. The results show that in high speed rotation state，the centrifugal stiffening effect occurs due to the changes in structural stiffness of the system. With the rise of rotational speeds the differences become more significant. The simulation analysis is verified to be reliable through vibration experiment. the errors between experiment and simulation mainly come from the simplification for support model.

Key words: Critical speed, Modal analysis, Spindle system, Stiffness, Ultra-high speed grinding, additive manufacturing, electron beam, scanning method, selective melting, surface topography