Fatigue analysis of propulsion shafting system with bending-torsion coupled loads
Date of Issue2017-05-31
School of Mechanical and Aerospace Engineering
With development of conventional VLOG, VLCC and 10000 TEU Container ship, the distance between shaft bearings increases and thus leads to the decrease of shaft stiffness, which has a negative effect on shaft stress. Increased propeller excitation force, induced by improved thrust power, also leads to the worse shaft vibration. In addition, a heavy propeller has a negative effect on shaft line. Therefore, it is necessary to investigate on the fatigue life of propulsion shaft, which holds a key to ship design and ship safety. In this paper, 57000t bulk cargo is considered as the research object. The main purpose is to analyses fatigue of Propulsion Shafting System with bending-torsion coupled loads, and achieve the design of total life cycle. The equivalent shaft model is built and the harmonic response analysis is performed to figure out resonance rotating speed, which is used to compare with experimental data and thus verify the correctness of the damping and excitation force applied. Based on ANSYS Workbench, the bending - torsion coupled models are created to obtain the transient FEA result, which is imported to ANSYS Ncode DesignLife to complete subsequent fatigue analysis. The main contents in this paper are as follows: 1) Research on the method of harmonic response: Modal superposition and Full method. In addition, 5 kinds of damping models and their application ranges are discussed, and how to apply local damping is elaborated. 2) Research on the method to apply gas excitation and propeller excitation, and the validity of this method is verified based on the experimental data. 3) The workflow and theory of fatigue analysis is elaborated, which includes how to convert non-symmetric cycle loading to symmetric cycle loading, and the method of stress combination, converting complex stress to scalar quantity. This can help to proceed subsequent fatigue analysis based on S-N curve. 4) To balance efficiency and accuracy of calculation, the stress concentration factor is introduced and methods of simplifying models are also discussed, which include equivalent crank model and kinematic pairs.