In a great many installations, care is given to obtaining the optimal alignment of the various parts in relation to each other. In particular the fact that specialist equipment such as lasers are becoming available more and more cheaply, make aligning the various components of a propulsion installation into a child’s game.
However, being aligned is not always a guarantee for optimal load distribution across the interdependent components! Sometimes it is even necessary to purposely move a part – and thus un-align! – in order to avoid damage or premature failure through wear. The need to do something like that mostly involves in installations with long shafts and overhanging weights, such as the propeller shaft installation in sea or inland waterway ships.
Techno Fysica uses the Sag & Gap method to calculate the optimal alignment of an installation. This method is a prerequisite for some suppliers of gear boxes to provide warrantees.
It is only via such calculations that both bearings of an output shaft of a gearbox end up with a load of equal measure (and direction). This method is, for that matter, fully accepted and sometimes even required by classification societies.
In this method, the reaction forces on all bearings are calculated, with regards to the complete shaft line. In this calculation, the fact that a gearbox is warm or cold, or even the effect of the possible thrust eccentricity through an adverse flow of water to the propeller (if known), can be considered.
With the help of this calculation model the optimal positions of bearings in the installation are determined. It may often be necessary to reposition the gearbox up or down, or even at an angle. This way the optimal weight distribution can be determined, as well as determining whether the deflection shape of a shaft and, in particular its bending, does not disagree with the available clearance in the bearings. This can lead to seizure in the aft propeller shaft bearings!
Once the optimal distribution is known, the flanges of all couples will be disengaged from each other in the calculation model.
As a result, sag and angular differences of the flanges will occur in relation to each other, which leads to a gap between the flanges. These sag & gap values can then be used as the starting point for alignment on board or in the workshop.
The result: an optimal distribution of bearing forces, so that the longevity of the bearings and thus a high operating reliability can be reached.