프로펠러 가진에 의한 저속 디젤엔진을 갖는 추진축계의 종진동

Abstract

The axial vibration of propeller-induced excitation can be mostly seen in warships that have high-speed rotation propulsion shafting systems equipped with reduction gears. Coupled torsional and axial vibration is mainly focused on the excitation force that is induced by the cylinder’s gas pressure and the piston’s reciprocating mass in low-speed two-stroke diesel engines with seven or fewer cylinders; more complex vibration modes with one or more nodes can be seen in engines with eight or more cylinders. This thrust variation force caused by the axial vibration is the excitation force of the ship’s deckhouse in the longitudinal direction and has been researched extensively in the past. Recently, super-size containerships have been built that have more than 20 000 TEU. These ships have adopted low-speed diesel engines with 10 or more cylinders as their prime mover and the distance between the propeller and the main engine has increased significantly due to the streamlining of their hulls. As a result, there is the probability of resonance between the propeller’s axial excitation and the second node vibration. While this vibration frequency is rare, the possibility of resonance with the ship’s superstructure can create vibrations in the pipes and various accessories surrounding the main engine and stern tube. In addition, severe vibrations can occur if the natural frequency of these parts and that of the axial vibration are similar, which necessitates installing an additional damper on the intermediate shaft that resembles an axial vibration damper that is attached to the crankshaft free end. This paper intends to research the characteristics of axial vibration through theoretical analysis and measurement data with a focus on the 2nd order axial excitation of the propeller blade number in a propulsion shafting system with a 11G90ME engine as the research model.