A CASE STUDY ON Z-DRIVE PROPULSION SHAFTING SYSTEM FAILURE AND RESONANCE DUE TO GUIDE FORCE MOMENT

摘要

Installation space consideration is one aspect for the application of Z-drive propulsion shafting system on smaller and medium-sized specialized vessels. This propulsion configuration applies a diesel engine as prime mover connected to the intermediate shaft by means of a flexible coupling with reduction gear and propeller. Flexible coupling are employed to isolate the vibratory torque excited by the diesel engine or the excessive transient torque and moment that can occur as a result of an external impact. In this paper, a phenomenon leading to the repeated breakdown and damage of the flexible coupling during the vessel's sea trial was investigated. An unexpected resonance occurring between 600 ～ 700 r/min was identified and was regarded initially as due to the excessive vibration and transmission torque of the propeller. In order to precisely identify the cause of this incident, a series of comprehensive vibration measurements were carried-out and the design data of the related shaft system and engine were reviewed. In this stage, a lateral vibration was observed on the intermediate shaft causal to the elevated temperature of the flexible coupling. Further, a second vessel with similar propulsion system configuration was installed with different component specification. Similarly, a comprehensive vibration measurement and analysis was done to confirm the dynamic behaviour of its propulsion shafting system. Improved flexible coupling design and fitting of a flexible link were suggested as countermeasures to address the excessive deflection (x-, y-, and z-direction) during transient motion. Results also established the difference between theoretical calculation and actual measurement analysis of propulsion shafting dynamics and the occurrence of resonances may not be predicted accurately.