Bifurcations of Unbalanced Flexible Rotor Subjected to Several Nonlinear Forces
Cao Shuqian,Chen Yushu
(Department of Mechanics, Tianjin University, Tianjin, PR China, 300072)

Abstract: The nonlinear dynamic properties of a 4DOF unbalanced flexible rotor with nonlinear oil film forces, nonlinear internal damping, nonlinear elastic forces and linear external damping are studied using DIM (Direct Integral Method). The results show that when only the oil film forces are applied to the rotor, there only exists the period-doubling bifurcation arising from the instability of the lubricant in the bearings. While both oil film nonlinear forces and internal damping are applied to the rotor, the rotor firstly loses its stability due to the oil film forces and gives the period-doubling motion. Along with increasing of the rotational speed, the rotor loses its stability again through internal damping in the form of quasi-period motion. The bifurcation crises are found in the above analysis. This type of instability of flexible rotor with internal damping due to oil film whirling is a new one which is different from some types of instability of internal damping introduced in existing papers and more accordant with the engineering practice. The two typical motions of rotors arising from instability of oil film and internal damping respectively can be used to determine the failure modes of rotors.
Keywords: unbalanced flexible rotor, nonlinear forces, period-doubling bifurcation, quasi-period bifurcation, bifurcation crisis.