Recently, damages of the main engine aftmost bearing and the after stern tube bearing tend to increase, as the shafting system becomes stiffer due to the large engine power, whereas the hull structure becomes more flexible due to optimization by using high tensile thin steel plates.
Deferences of ship's draft condition and the thermal expansion are some common causative factor in the deformation of main engine bed and double bottom structure of engine room. And this is the reason that more sophisticated shaft alignments are required.
Therefore, to obtain the optimum status in shafting alignment at the design stage, it is strongly recommended that the change of bearing reaction force depending on ballast/load condition, the bending moment force occurred by propeller thrust, elastic deformation of bearing occurred by vertical load of shaft mass and etc., should be considered.
In this study, the optimum shafting alignment calculation was carried out, considering the number of main engine bearings, thermal expansion. and exploiting the sensitivity index, which indicates the reasonable position of forward intermediate shafting bearing for alignment. The optimal position of bearing in process for alignment is determined by sensitivity index, which is defined as follows.
Sensitivity index =
: Influence coefficient of intermediate bearing to ith bearing
: Number of total bearings taken into consideration
: Intermediate bearing number
According to the above process, the optimal bearing position of forward intermediate shafting was confirmed with smallest sensitivity index.
Finally, as the main subject in this study, the elastic deformation on main engine bearings occurred by vertical load of shaft mass were examined thoroughly and analysed allowable load of bearings, reaction influence numbers of all bearings.
As the result, a reliable optimum shafting alignment was derived theoretically. To verify these results, they were referred to the engine maker's technical information of main engine installation and being used shafting alignment programmes of both Korean Register of Shipping and Det Norske Veritas, their reliability were confirmed.