Mass unbalance and bent shafts both give force excitation at a frequency corresponding to once per revolution. Taking a LP B rotor of a 1000 MW USC steam turbine as an example, this paper studies the lateral vibration of the LP B rotor based on finite element. Responses of mass unbalance and bent shaft are analyzed and compared. | Journal of Science and Technology 54 (2) (2016) 266-275 DOI: COMPARISON BEHAVIOR OF A LARGE TURBO MACHINE VIBRATION BETWEEN UNBALANCE AND BENT SHAFT Ngo Van Thanh University of Transport and Communications, No 3, Cau Giay Road, Lang Thuong Ward, Dong Da District, Ha Noi, Vietnam Email: ngovanthanh@ Received: 8 May 2015; Accepted for publication: 21 September 2015 ABSTRACT Recently, large capacity steam turbines have come into wider use and have been designed to perform at even higher performance specifications. As we all know, it’s hard to maintain the concentricity of rotor-bearing system during operating, especially for multi-rotors within specified levels of vibration. Rotor vibration could be caused by many reasons. Mass unbalance and bent shaft are amongst the most frequently occurring of these reasons. Mass unbalance and bent shafts both give force excitation at a frequency corresponding to once per revolution. Taking a LP B rotor of a 1000 MW USC steam turbine as an example, this paper studies the lateral vibration of the LP B rotor based on finite element. Responses of mass unbalance and bent shaft are analyzed and compared. Keywords: rotor –bearing system, mass unbalance, bent shaft, LP B rotor, lateral vibration. 1. NTRODUCTION A shaft bent could be caused during shipping, installation, and operation. Bent shaft can either generate high vibration or create a lot of stress on other components during operation. Hence, catastrophic failure will result. It is therefore important to be able to recognize such a fault. For the vast majority of rotating machine, shaft bow (hog and sag) is negligible, and therefore it can be ignored for practice purposes. On long driving machines as turbo-generators in power plants or machines with long spacer shaft, bent shaft should be considered. Bend on shaft could be caused by such factors as: thermal distortion due to fast load changes-cooling or heating of the rotor, rubbing-the .
