Also in cruise ships and LNG carriers electric propulsion systems are becoming popular due to operational flexibility, reduced noise emissions and closely matched engine power produced to engine power demanded. A self excited torsional vibration phenomenon occurring at the lower speed range of a special purpose vessel has been investigated in an electric propulsion shafting system consisting of a motor, a coupling and a propeller, and supported by a thrust bearing. During operation at speeds lower than 30 rpm, a bouncing phenomenon was detected on the flexible coupling. This phenomenon resonated, adding to the increased vibratory torque amplitude at a particular frequency. It resulted in deformation on the circular rubber segment of the flexible coupling. The phenomenon is perceived to be caused during the damping process by the alternating tension and compression action of the coupling's rubber elements. Further, the bouncing phenomenon produced a variable thrust force on the thrust bearing and generated underwater radiated noise. On the other hand, at operating speeds above 30 rpm, the bouncing phenomenon diminished. The investigation sought possible countermeasures with regard to flexible coupling performance and critical speed avoidance. Countermeasures recommended to address this self excited vibration phenomenon are: (1) To change the flexible coupling in which the contact face of the rubber element can receive simultaneous compression-shear torque, since the flexible coupling rubber element cannot fully dissipate or absorb the generated torque in a certain speed range. This leads to the rebound or 'bouncing' of the rubber element and generates the self excited torsional vibration. (2) To pass rapidly through the critical speed of 18 to 30 rpm, since the resonance was seen at 21 rpm and reducing the passage time at this speed range will therefore minimise the manifestation of the self excited torsional vibration.
Self excited vibration in a specialised electric propulsion system
Motortechnische Zeitschrift ; 75 , Special Edition September: industrial ; 54-59
2014
6 Seiten, 10 Bilder, 11 Quellen
Aufsatz (Zeitschrift)
Englisch
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