Measurement method for dynamic shaft bearing loads resulting from axial vibrations in a propulsion shaft system of a megayacht

Abstract

In this study, a measurement method concerning dynamic shaft bearing loads resulting from axial vibrations in a propulsion shaft system of a megayacht was developed. Axial shaft vibration measurement set-up was designed, materials of the experiment were selected and the steps of the measurement were defined.

The main aim of the thesis is to determine the forces coming from the propeller. A new method was developed to calculate these forces. This method is based on modal analysis and mobility function which is the back bone of the modal analysis. Therefore an experimental set-up and the steps of the experiment were developed uniquely in order to obtain the mobility function of the shaft line for megayachts in motion.

An example experiment was designed for a particular motor yacht. All the materials and the tools were selected according to particulars of this megayacht. However the steps of the experiment can be applied to any yacht and are given in a table in conclusion.

The main challenges of the thesis are applying the excitation force and finding suitable tools for the experiment. The measurement of vibration is desired to be performed on a yacht as built. Laboratory devices and conditions are not valid any longer. The spaces around the shaft, real working conditions of the yacht and the lack of convenient excitation methods are all obstacles for this experiment. There are many different types of excitation methods like pressurized air, acoustics, electromagnetic excitation and laser. They are all non-contact type excitation methods but they have different drawbacks and are not suitable to be used in this measurement.

On the other hand an impact hammer which is a contact type excitation method and also dangerous to be used on rotating shaft can be a solution. It has also disadvantages and is not a perfect solution, but it was decided to be used with special modifications and additional devices. It is a special impact hammer which can be controlled automatically.

Hitting in axial direction to the 34 ton of propeller and the shaft which is rotating with 188 rpm is the particular case considered here. Moreover there is just one suitable place to hit the shaft. It is a flange which is full of bolts. Therefore two different solutions were developed to enable to hit the flange: covering the bolted face of the flange by additional steel plate and attaching the impact hammer to the shaft. The first one needs extra design of plates. In the second option, an automated impact hammer is attached to the shaft and can be remote controlled. These designs reduce many drawbacks of the impact hammer.

The thesis was developed as theoretical basis because of the lack of an on-going megayacht project in the shipyard during the thesis. Therefore performing the measurement and confirming of the method can be a future study.