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  Shaft alignment calculations

Using the Sag and Gap method - More than just laser aligning




The proper alignment of the propeller shaft is of vital importance for marine propulsion installation. For this, Techno Fysica uses the sag and gap method of calculation. This method ensures and acceptable distribution of the bearing load over all bearings in the installation over all operating conditions (from cold start to operating in hot engine rooms). Furthermore, it is often the only method which makes it possible to accurately predict resulting bearing reaction forces on the gearbox bearings, for which manufacturers often give stringent specifications.



Why is proper shaft alignment important?

Proper shaft alignment increases the reliability of your installation. Not only will a more even distribution of resulting gravity loads result in longer bearing life, it may even prevent critical failures due to bearing overload, bending fatigue cracking of shafts or even gear failures.


Fatigue failure due to cyclic bending of a propeller shaft

How does it work?

On the basis of available data on shaft, couplings, propeller, gearbox etcetera, a computer model is prepared of the complete propulsion shaft installation. Using a zero offset reference line, bearing reaction forces and elastic line of the shaft are calculated. This “zero” condition (to be compared to a laser-line alignment) is very often a far from optimal condition.

Then, using unit displacements for each bearing, a sensitivity matrix is obtained, which enables the calculation of optimal bearing positions. Usually, this can be fully achieved by virtually displacing the gearbox bearings (plan-parallel or tilted). The resulting ideal alignment is then corrected for thermal expansion of the gearbox during its increase in temperature from cold start to operating temperature.
When the ideal bearing positions are established, the flanged connection(-s) are opened. The shaft will then bent as a result of gravity forces applied on the installation. The resulting values for sag and gap are determined.

When these values are applied during alignment, the installation will behave as predicted in the calculation. The result: optimal shaft alignment and highest reliability!





Photo on right side:
bronze stern tube bearings, locally burnt due to too much slope of shaft through the bearing



Why Techno Fysica

Originally part of a shipyard in the Rotterdam port area, Techno Fysica has a long history in the field of consultancy in ship’s propulsion installations. This large experience has always been coupled to the use of state of the art equipment and, as is common these days, by advanced and dedicated computer soft- and hardware.
Our approach is not only theoretical, but also practical. Furthermore, as we understand the urgency of answers in short notice, we will do the most in order to help you got you answers or solutions in the shortest time possible. And: right the first time!


Related services:

Apart from aligning calculations, Techno Fysica performs several other calculations. Examples are:

  • Torsional vibration calculations

  • Lateral vibration calculations (whirling)

  • Finite element calculation of stress and frequencies

  • Operational shape deflection calculations

  For more information:

Please visit www.technofysica.nl

or sent an e-mail to