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A guide to shaft alignment

Achieving a satisfactory shaft alignment is paramount for the safe and reliableoperation of a ship during its lifetime. Lloyds Register EMEAs TechnicalInvestigations provides a comprehensive range of measurement and advisoryservices to help owners, operators and yards to help to ensure that the best

possible results are achieved during the process of shaft alignment.

Modern risk assessments and technical reviewsoften highlight the propulsion shafting system of aship as one of its most important assemblies,especially for single screw ships.

Powerful modern engines often feature large-diameter propulsion shafts and relatively stiffshafting systems which can adversely affect mainengines and gearboxes.

On the other hand, the more flexible shaftingsystems used to mitigate these effects can suffer

from whirling (lateral bending vibration modes). Ifwhirling frequencies coincide with rotationalspeeds, catastrophic resonant response results canoccur.

It is clear that a delicate balance must be struckduring the processes of shaft design, installation

and alignment in order to avoid costly repairs anddelays later in the ships life. An unexpected

shafting system problem, for example, failure of astern tube bearing due to misalignment, leading toan emergency dry-docking and consequent loss ofavailability, can cost well in excess of $250,000per incident for a small vessel.

More serious problems, for example, involving abent shaft and replacement for a larger vessel, cancost in excess of $1 million.

The risk of lost earnings and the potential threatto the safety of the ship make the issue of shaft

alignment a critical one for owners, operators andyards.

Methods

The four most popular methods of controlling shaftalignment are: the gap and sag method optical or laser sighting jacking

the strain gauge (bending moment) method.These methods can be used either singly or incombination.

The gap and sag method uses precalculatedflange measurements to establish the alignment,to be carried out whilst the flange couplings aredisconnected. The advantages of this methodinclude the simplicity of the measuring equipmentand the ease of control in both the horizontal andvertical directions. The disadvantages are limitedaccuracy and the lack of applicability in theservice condition. The latter is significant, as it is

advisable to check the alignment of the shaftingsystem whilst the machinery is hot.

The optical sighting or laser sighting methodis more accurate than the gap and sag method. Ituses high-quality sighting equipment and isgenerally used before the shafts are installed andcoupled up. Optical sighting is often used toestablish the reference line and the position of theengine supports and for determining how to borethe stern tube. Alignment and verification followsby means of the gap and sag method and finalalignment control by the jacking and/or strain

gauge method.

After the grounding ofthe Keymar in heavyweather, its ownersengaged LloydsRegister to check thevessels shaft alignmentduring each of themajor stages of thevesselss repair

Optical sighting is

one of four popular

tried-and-tested

methods used,

either singly or

together, to align

shafts.

Royal Belgian Institute of Marine Engineers

http://www.gallois.be/

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Thejacking method lifts the shaft line clear ofeach bearing in turn by means of a hydraulic jackand calibrated load cell. The shaft is lifted insteps, whilst deflections are recorded on a dialgauge and plotted against the applied load. Thismethod uses simple equipment and is employedwhen the shaft line is coupled up, ready foroperation.

However, it is not suitable for the control of

horizontal alignment and the yard may need toarrange for special supports for the jacks. Finally,as the jacks have to be positioned beside thebearings, adjustments have to be made to therelevant calculations to evaluate the true bearingload.

The strain gauge methodrequires a combination ofcomputation and strainmeasurements. If a shaft

line rests on a number of bearings, a theoreticaldistribution of bending stress may be calculated.If the bending stresses, determined from themeasured strains at an appropriate number ofstations, deviate from the theoretical, this is

taken to be caused by an alignment that differsfrom the theoretical straight-line case.Using the strain gauge method, both the

horizontal and the vertical direction may becontrolled. Loads on normally inaccessiblebearings can sometimes be determined. Readingscan easily be taken after the gauges are fitted,and the effects of oil film formation and propellerthrust may be studied.The disadvantages are that the method requiresthe skilled fitting and operation of strain gaugesand suitable data acquisition and analysissoftware and time is required for calculations

after taking the strain readings.

Checking alignment after grounding

It is not only during the installation stage thatshaft alignment is a critical issue. In the event of

a grounding, for instance, small movements atthe shaft bearings can impact bearing loads,leading to premature alignment failure. Oneowner who recognised the importance ofmonitoring shaft alignment was Stelmar,following the grounding of its aframax tankerKeymar.

In early 2003, the vessel was blown onto the

North African coast, along with three othertankers, by severe force 11 winds.

The grounding severely damaged 11 of the 16double-bottom tanks in the cargo region, leavingthe ship effectively floating on its tank tops. Thedamage eventually resulted in the replacement ofover 1,500 tons of steel, and a total repair bill inexcess of $10 million. In view of the significantsize of both the ship and the machinery it wasconsidered prudent by the owners to engageLloyds Register EMEAs Technical Investigationsto check the alignment at several stages during

the repair process.

A first alignment check was carried out prior toentering the shipyard to assess whether thedamage had significantly affected the shafting ornot. The results of strain gauging and jacking andengine crank web deflection measurementsshowed that the alignment was still in asatisfactory condition and that the shaft line wasunlikely to need major repair A second alignmentcheck, once the ship had settled on the blocks,showed that the significant change of stressing inthe hull during settling had not furtherjeopardised the shafting system and that shaftingrepair was unlikely to be necessary.

A final alignment check, after rebuild of the hulland floatation, led to some minor adjustments tooptimise the arrangements but again eventuallyconfirmed that the propulsion shafting alignmentwas satisfactory. The structural repairs andassociated hot work in the engine room had thepotential to significantly affect the alignment, butin this instance little change occurred. We havecarried out a large number of theoretical andpractical shaft alignments on a wide range

of ships over many years, says Peter Filcek,Technical Manager, Technical Investigations,Lloyds Register EMEA. The owner was keen toavoid the risk of shaft line failure, and we wereable to provide the necessary technical expertiseand capability to help to mitigate that risk.

Source: Horizons-05 July

For further information contact Peter Filcek, Technical Manager

Technical Investigations, LloydsRegister EMEAEmail: peter.filcek@lr.org

Lloyds Register isable to provideowners, operatorsand yards witheffective advice andguidance on shaftalignment.