A CASE STUDY OF THRUST VARIATION FORCE DUE TO COUPLED TORSIONAL-AXIAL VIBRATION ON TWO STROKE LOW SPEED DIESEL ENGINES

摘要

Marine propulsion shafting system propelled either by modern super large two-stroke low speed engines or by engines exploiting higher stroke/bore ratios typically operates at lower speed range (mcr). Consequently, this results in better propulsive efficiency and higher thrust variation force as well. The thrust variation force due to axial and torsional vibration occurring on the propulsion shafting system is transmitted to the engine and ship structure via thrust bearing. This force may manifest as engine body vibration or ship super structure vibration in a longitudinal direction and can reduce the crankshaft fatigue strength by additional stress in the crankshaft pin and journal along with torsional vibration. Additionally, actual thrust variation force measurement is difficult to perform. In this paper, the axial vibration of propulsion shafting system with seven cylinder 7G80ME (24,400 kW@66 r/min) ultra-long stroke diesel engine and a fourteen cylinder 14RT-flex96C (80,080 kW@102 r/min) super-large diesel engine is ascertained by theoretical analysis and actual measurement. Both engines are installed with an axial damper of damping function and detuner of stiffness function respectively. Subsequently, the thrust variation force is estimated through this intermediate shaft axial amplitude on measurement and by correlating the thrust bearing stiffness and angular acceleration due to torsional vibration.