Vibration Analysis of Crank Shaft for Long Stroke Diesel Engine using Component Mode Synthesis

Abstract

Recently there is a trend to install long stroke type diesel engines with a small number of cylinders on ships for saving energy and simpler maintenance, but the engines cause to increase ship vibration problems induced by shafting vibrations, due to less stiffness of crankshaft and increased exciting forces of diesel engine. For estimating coupled longitudinal and torsional vibration response of long stroke type crankshaft by conventional methods, problems for modelization have been still left to be solved. So, the authors carried out detailed static and dynamic analysis of total shaft system concerning IHI-SULZER 6RTA58 diesel engine, by the use of 3-dimensional finite element method with solid elements applying component mode synthesis and image element method to reduce a large amount of computer time. These calculated results have agreed well with tested results of longitudinal stiffness of actual crankshaft and with the results of vibration measurements of shafting on board ships, and have shown great improvements of accuracy especially for the coupling vibration characteristic between longitudinal and torsional vibrations compared with the conventional methods. From these studies, the authors also obtained improved simplified formula for estimating such as equivalent longitudinal stiffness, equivalent longitudinal exciting force converted from radial gass force.