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CasesTechno Fysica BV is a Dutchbased engineering company, highly specialised in solving all kinds of problems related to the
dynamic loading of machinery and installations. For this, several measuring and analysis techniques are used. In a series of
articles, we will give examples of what can go wrong in the engineering and operating of vessels, and what errors lie at the root of
the damages observed.
Example: Faulty assembly or installation of components
In a series of 10 general cargo ships, cracks developed in the hull at the location where the stern tube penetrates the aft ship. As
is common practice nowadays, the engine room is a s compact as possible to obtain a maximum of cargo space. This results in a
short, stiff shaft line, which makes certain engineering aspects, such as shaft alignment and thermal expansion, extra critical.
Aft ship of a compact, high powered general cargo ship with a short, stiff shaft
line.
In this particular design, in order to achieve an equal additional load due to the weight of the propeller shaft on both gearbox
bearings, the gearbox was placed above the centreline of the stern tube, i.e. 1.27 mm on the propeller side and 2.33 mm on the
engine side.
However, for this installation it was impossible with this alignment condition to ensure a positive load on the forward stern tube
bearing under cold and warm operating conditions and under zero pitch as well as under full pitch conditions.
The pitch setting has a big influence on the bending moment, and consequently the deflection, of the shaft due to the eccentricity
of the thrust.
This means that under certain operational conditions, contact between the forward stern tube bearing and the propeller shaft was
lost and bending moments were exerted on the aft stern tube bearing and the hull.
For the last ships of this series of 10, the manufacturer of the propeller shaft advised an alignment condition where the gear box is
placed below the centreline of the stern tube, i.e. –1.46 mm at the propeller side and – 1.98 mm at the engine side. This was
determined on the basis of a sag and gap alignment calculation.
Original alignment condition (top) and modified alignment condition (bottom)
In practice, this means that the gearbox is placed approximately 4 mm lower than in the original configuration.
The additional load on the gearbox bearings is not the same for both bearings, however it is still well within allowable limits and the
forward stern tube bearing carries a positive load under all possible operating conditions.
After this modification, no more cracks occurred in the stern tube connection to the hull.
For more info, please contact techno Fysica BV in the Netherlands at:
Techno Fysica BV
p.o. box 351
2990 AJ Barendrecht
The Netherlands
Tel. +31 180 620211
Fax. +31 180 620705
www.technofysica.nl
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