# Dynamic Simulation of a 5 Degrees of Freedom Rotor Dropping on a Protective Bearing
## 2 Establishment of Dynamic Simulation Model for 5-Dof Rotor-Protective Bearing System  
The process of the high-speed rotor falling on the protective bearing is very complicated. Theoretical research and an experimental analysis of this highly nonlinear dynamic process have been carried out at home and abroad. Observed from the radial direction, the motion forms of the rotor falling on the protective bearing can generally be divided into two types: cylindrical motion and conical motion. In actual experimental conditions, conical motion is the main motion mode when the rotor falls, but in the theoretical research, the rotor falls on the protective bearing. The form of motion on the protective bearing is mainly cylindrical motion. At the same time, when the rotor falls on the protective bearing in the form of cylindrical motion, the radial contact force between the two is five times that of when conical motion occurs. Using multi-body dynamics, the motion of the rotor falling on the protective bearing can be directly analyzed. The motion process of the rotor falling on the protective bearing can be divided into four motion stages, namely free fall, impact collision, sliding, and free rolling. When the rotor starts to fall with a certain initial speed, the rotor is in the free fall stage before contacting the protective bearing; once the rotor contacts the protective bearing, the rotor begins to undergo multiple impact collisions within the gap circle and within the rotor and protective bearing. Under the action of friction between the rings, the rotor begins to slide on the inner ring of the protective bearing. Currently, the sliding includes two forms: sliding back and forth at the bottom of the gap circle and whirlpool sliding along the gap circle; finally, it continues to slide under the friction. Under the action of force, the speed of the rotor and the protective bearing at the contact point remains the same, and the roller performs pure rolling motion within the gap circle. But not every time the rotor falls on the protective bearing will the rollers undergo whirlpool sliding or pure rolling. From this, a preliminary analysis of the fall trajectory of the rotor falling on the protective bearing can be carried out, but only a preliminary analysis of the protective bearing can be carried out. The analysis of the normal contact force and tangential friction force is not in-depth. Due to the imbalance of the rotor mass, medium influencing factors have a great impact on the dynamic response of the entire system. The main axis of inertia of the rotor, the geometric center axis, and the geometric center of the bearing may not coincide. When the rigid rotor falls freely on the protective bearing and starts to move arbitrarily in space, then considering the influence of the eccentric moment on the drop response of the rotor-protective bearing system, when the rotor mass center and the centroid do not coincide, the dynamic model of each state of the rotor drop process is further described in detail. In this rotor-protective bearing system model, the rotor system in space has a total of five degrees of freedom, including axial degrees of freedom and radial degrees of freedom. The radial four degrees of freedom are the degrees of freedom of translation along the x and y axes and the degrees of freedom around the axis. The degrees of freedom of rotation are those around the x and y axes, and the axial degree of freedom is the degree of freedom of rotation around the \(z\)-axis. Since the coupling effect between the radial degree of freedom and the axial degree of freedom is small and can be ignored, the radial degree of freedom and the axial degree of freedom can be studied as two independent subsystems when modeling the system. Currently, the center of mass of the rotor is only affected by its own gravity. The distance between the protection bearing and the center of mass of the rotor is known. The rotor exerts radial support force and tangential friction force.