Discussion about nonlinear boundaries for hydrodynamic forces in journal bearing

The study of rotating machines is usually carried out taking into account linearized hydrodynamic forces, considering dynamic coefficients of stiffness and damping, although a high order of nonlinearity can be significantly present in the system. To solve the nonlinear problem, the solution of Reynolds equation is practically mandatory for each time step in the numerical integration procedure, leading to high computational costs that often can make its application unavailable. In this paper, the validity of linear approximation for the oil film forces is discussed when the system operates under specific conditions, pointing out the influence of critical phenomena and dynamic parameters in rotordynamic analysis. Experimental tests are compared to numerical simulations for linear and nonlinear models of bearings in laboratory test rig in order to validate the analysis. Afterward, several simulations were accomplished, in time domain, for a rotor configuration more susceptible to critical operation, comparing the results for linear and nonlinear models. The main focus is on the influence of internal damping, gyroscopic effects, journal eccentricities, and excitation forces. The results demonstrate that the excitation force plays a fundamental role in nonlinearity degree of response, namely in extreme operation conditions under high excitation forces, the linear approach fails in representing the hydrodynamic bearings.