The vibration based diagnostics of ship propulsion systems

The article has been worked out on the basis of the report devoted to the conducted diagnostic investigations of the ship main propulsion unit’s mechanical system. Gdansk University of Technology has been ordered with such investigations by the repair Shipyard carrying out the ship’s overhaul. Diagnostic tests involved measurements and analyses of the vibration signals generated in selected constructional kinematic pairs of the identical (twin) starboard and portside marine propulsion lines. The vibration signals have been registered at the representative load ranges. By this way the stability of the considered mechanical unit has been estimated both in a global and local sense. Appearing at that time the resonance phenomena which result in a kinetic energy dissipation of the ship main propulsion unit’s masses being in a rotational movement have been identified. There have been also localized places of the largest growth of the changeable internal tensions’ amplitudes, what considerably constrains cycles number of the load alterations at which elements transmitting the propulsion torque from engines to propellers crack, as a result of the material fatigue phenomenon. The most probable reasons of the mechanical unit’s enlarged vibration level have been pointed out on the basis of a carried out analysis of the alignment results, the specifity of the applied main engine running on the partial loads as well as the range of constructional changes that had been made during the propulsion unit’s overhaul. Results of the repeated diagnostic sea trials, carried out after the reduction gears’ modification recommended by the Authors, confirmed a relevance of the earlier formulated diagnosis. The modification works aimed to perform a resonance offset by the damping correction of hydraulic devices additionally founded on the gears frames. A technology of the marine propulsion shaft line’s transverse and longitudinal vibration measurements carried out by means of the portable vibration register as well as a method of the vibration spectral analysis have been focused within the program description of the performed diagnostic investigations. They represented the base for a diagnostic inference about the considered mechanical unit’s dynamic state