Prior to shaft alignment, all the shaft were set in a line to keep flange coupling from misalignment and paralled to each other. However, several noticeable problems such as abnormal wear of bearing, no load on intermediate shaft bearing, excessive heat in the bearing, abnormal on reduction gear, and damage in the bearing has occurred. To solve these problems theories on optimal positioning of shaft bearings were developed and applied to general ships. From 1960s to 1970s companies like large shipyards and classification society started to show promising improvements on solving many of the problems. Recently, there are increasing reports on the after stern tube bearings for the engine of heavy 2-stroke ships being damaged due to incorrect wrong shaft alignment. Most shaft damages are caused due to insufficient analysis,design change of the ship in the design process, lake of shaft alignment experiences, and undefined analytical standards. To prevent furthermore damages, the classification societies have issued separate regulations for shaft alignment.
Currently shaft alignment is done by simple technical study or hypothesis to find the position of the reaction point. However, reaction point is dependent on many factors such as external force, elasticity of the shaft, lubrication, and pressure distribution. Due to these factors, predicting the reaction point becomes very complicated.
In addition when calculating the reaction force on after stern tube bearing, the classification society's recommendation is to use 1/3 and 1/2 position of bearing diameter from end of bearing for static condition. However, this recommendation was established a long time ago, and due to current implementation of EEDI (Energy Efficient Design Index), ships are required to use heavier propellers. Thus, investigation for new calculation method is on demand. The objective of this paper is to check and calculate a suitable reaction point in the after stern tube bearing considering the stiffness of bearing supporter, oil film and bearing.