Rocks & mirror

Encyclopedia of Marine and Energy Technology

Shaft alignment


A static condition observed at the bearings supporting the propulsion shafts. It is calculated, conducted and verified when the system is at rest. A good static alignment is required for trouble-free dynamic operation of the propulsion shafting. The objective is that the alignment remains acceptable for vessel loading variation as well as temperature variation aff ecting the shafting system.

The basic procedure for determining vertical alignment of shafting and gears is to calculate, for the cold condition at time of aligning, a shape of shaft line that will produce in the hot operating condition essentially equal loads on the main gear bearings and reasonably equal loads on line bearing. Two methods used for establishing the desired alignment are the gap and sag method and the bearing load measurement method. The alignment is checked during installation by a taut wire, a sighting telescope or a laser beam.

Further reading: ABS Publication ”Guidance Notes on Propulsion Shafting Alignment”

(2004), can be downloaded from

Alignment procedure – An executable part of the alignment process where alignment is performed in accordance with the defined requirements.

Alignment process – Alignment process consists of the design and analysis, the alignment procedure and measurements.

Bearing clearance – Radial gap between the shaft and the bearing shell.

Bearing off set – Vertical displacement of the contact face of the bearing from the optically- established central line of the shafting.

Gap and sag method – The procedure of verification of the alignment condition prior to shafting assembly. Sag and gap is verified between mating flanges and has to comply with the appropriate, analytically obtained values. Sag and gap procedure should be conducted in the dry dock. If sag and gap is conducted on a waterborne vessel, then the accuracy of analysis may be in question, as the hull deflection effect needs to be considered. The yard should be able to fully control the alignment.

Bearing load measurement method – In this method, the desired bearings loads are obtained directly by measuring the actual bearing reactions.

Jack-up method – Jack-up method is a direct way to check bearing reactions. Measurements are conducted by hydraulic jacks which are placed in close proximity to the bearing of which reaction is to be measured. It is strongly recommended to use hydraulic jacks in combination with the load cell, as the measurement accuracy will significantly improve.

Download the Encyclopedia