Establishment of a contact stiffness matrix and its effect on the dynamic behavior of rod-fastening rotor bearing system

In this study, the mechanical model of the circumferentially distributed rods and the nonlinear contact stiffness matrix which characterizes the contact effect between the disks are established. The contact stiffness matrix is composed of seven stiffness coefficients that characterize the lateral stiffness, shear stiffness, bending stiffness, and torsional stiffness of the contact interface. Then the influence of pre-tightening load, rod diameter, contact surface roughness, deformation phase and uneven pre-tightening load on stiffness coefficients is analyzed. Finally, combined with the experimentally verified nonlinear oil film force model based on short bearing theory, the dynamic model of the rod-fastening rotor-bearing system is established. By means of spectrum cascades and Campbell diagrams, the influence of the pre-tightening load, rod diameter, contact surface roughness and uneven pre-tightening load on the nonlinear dynamic characteristics of the rod-fastening rotor-bearing system is numerically analyzed. The results show that increasing the pre-tightening load can increase the contact stiffness, and make the dynamic characteristics of the rod-fastening rotor bearing system closer to that of the complete rotor bearing system. The increase in the diameters of rods significantly delay the partial separation of the contact surfaces. The smooth contact surface provides higher contact stiffness, but it is not conducive to maintaining the full contact state of the contact surfaces. The stiffness coefficients fluctuate with the deformation of the rotor, and the process is significantly affected by uneven pre-tightening load. Moreover, a self-excited vibration frequency f_l accompanied by a combination frequency f_b= 3 f_l appears when the uneven pre-tightening load is caused by individual faulty rod. The number of rods, rod diameter, and contact surface roughness have similar effects on the dynamic characteristics of rod-fastening rotor bearing systems, which lead to fluent whirl and fluent whip appear in advance, reducing the stability of the system.