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Oil film bearings are applied for large rotational machineries owing to their large load capacity and damping effect. However, a swirling flow of the lubricant induces self-excitation at high rotational speed. The stability margin changes during many years due to the mechanical wear and fatigue. Thus, stability diagnosis of the rotor-oil film bearing systems is required. In this paper, we focus on the onset rotational speed of self-excitation as a stability index. Firstly, a 2-DOF rotor-oil film bearing model is constructed using the mode synthesis method and Bently/Muszynska model for oil film bearings. Then, an estimation method of the onset rotational speed using frequency responses measured at lower rotational speeds than the onset one is derived from the model. Finally, a test rig consisting of a rotor supported with two oil film bearings and a radial magnetic bearing is fabricated and frequency response functions with forward circular excitation are measured by utilizing the magnetic bearing. The experimental result shows that the predicted onset rotational speed using the frequency responses at 30, 40 and 50 rps is 60.4 rps, and the error between the estimated and measured onset rotational speeds is less than 4 rps (6%).

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