ABS Shaft Alignment Challenges

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Shaft Alignment Challenges –

The Single Sterntube Bearing Design

The Auditorium of Maran and Alpha Tankers

Dr Chris LeontopoulosManager, Corporate Marine Technology

Athens, Greece18th February 2016

Shaft Alignment Sensitivity Analysis

This preliminary study aims to show the sensitivity of the design with asingle sterntube bearing.

Two computer shaft alignment models are used for this study: One with 2 sterntube bearings (traditional).

One with 1 sterntube bearing.

Both have similar shaft diameters and horsepower for comparison purposesbut of course, they are similar not identical systems.

Two variables are considered:

The eccentric propeller thrust position.

The movement of the intermediate bearing and engine at offsets above or

below the prescribed ones in the calculations. (This is to simulate theaccidental inaccuracies during the sighting process and unaccountedresidual hull deflections).

In both cases the objective function is the misalignment angle betweenthe shaft and the aft sterntube bearing bush.

Propeller Eccentric Thrust

How does the position of eccentric thrust influences the aftsterntube bearing misalignment angle ?

Class Rules have an empirical limit of 0.3 mrad.

Assumption: A misalignment angle of above 0.3 mrad may causedamage to the aft sterntube bearing.

Static slope mismatchtend to zero in idealcondition

Dynamic

PROPVertical

Dynamic

Moment

Single Sterntube Bearing Failures

Near-Optimum Shaft Alignment

Propeller Forces and Corresponding LoadingConditions Influencing the Shaftline

Shaft Orbit in IB during Special Sea Trials for ShaftAlignment

Intermediate Bearing and Engine Positioning

How does the position of the intermediate bearing and engineoffset affect the misalignment angle?

Intermediate Bearing and Engine Positioning

How does the position of the intermediate bearing and engineoffset affect the misalignment angle ?

Traditional 2-STB Bearing Design versus SingleSTB Bearing Design

Propeller Thrust Eccentricity versusMisalignment Angle

IB and Engine Offset versus MisalignmentAngle

Conclusions

Through simplified computer shaft alignment models it has beenshown that :

The position of the propeller eccentric thrust influences the

misalignment angle much more when there is a single sterntubebearing.

An accidental or inaccurate position of the intermediate bearing -engine has far more influence on the misalignment angle in the case

of a single sterntube bearing. Reaching negative misalignment angles is more readily possible with

single sterntube bearing designs. This is because the forwardsterntube bearing “corrects the slope” by taking the additional load.

Recommendations

It is recommended that a final optical sighting takes place with theengine and the superstructure already in place.

In case of a propulsion installation with no forward stern tubebearing, the intermediate shaft bearing should be chocked and itsoffset not changed after the bore sighting is complete, (ABS SVRRules 2015).

It is recommended that very light draft operations are re-considered in terms of power and RPM due to the sensitivity ofthe design on the downward bending moment from the propellereccentric thrust.

Due to the sensitivity of the design, further investigations can beconducted through systematic measurements on suchpowertrains.

ABS Experience of Contributing Factors toRelated Failures

Possible intermediate bearing and engine bearing offsets out ofspec.

Potential residual lateral vibration from a calculated critical speedclose to MCR.

Potentially additional forces from partially immersed propeller dueto rough weather conditions.

Sensitivity of the design from potential unaccounted hulldeflections.

Lubrication system with potentially insufficient cooling.

Afloat Re-Alignment

Strain Gauge Measurements

Shaft Alignment Measurements using Strain Gauges and theconcept of Reverse Engineering.

Reverse Engineering

Definition of Shaft Alignment :

Shaft Alignment is the configuration of the shafts and bearingsrelative to the centerlines of the bearings from the theoretical straight

line condition, so as to achieve an acceptable bearing loaddistribution.

Definition of “Reverse Engineering Shaft Alignment” :

Reverse Engineering Shaft Alignment is about performing a reverseengineering calculation with the desired bearing load distribution asinput or the measured shaft bending moment values at selectedpositions and determining a set of bearing offsets (usually more thanone) as output, which is to produce acceptable bearing loads under

the tested loading condition.

Reverse Engineering Background Theory

In the reverse engineering problem, the input is the desired shaftbending moments and/or bearing reaction loads and the output is thebearing offsets that satisfy the desired bearing loads.

The reverse problem is essentially a minimisation problem, where thequantity to be minimised is the difference between the desired and thecalculated quantity.

A special algorithm (which is called optimisation algorithm) is used to

perform the reverse analysis through multiple iterations.

Strain Gauge Installation Procedure

TELEMETRIC Strain Gauge Installation Procedure

Static Versus Dynamic (Telemetric) Strain Gauge measurements

Reverse Engineering through FE model

Misalignment Angle in the aft sterntube bush

Misalignment Angle in the aft sterntube bush – Dynamic Conditions

Telemetric strain gauge method is the BEST measurementmethod to demonstrate the shaft alignment status of a typicalvessel shaftline under both static and dynamic (operating)

conditions.

This is because it can provide with good accuracy the bearingoffsets and hence the bearing reactions to verify that the shaftlinebearings are not unloaded or overloaded at any time, particularlyduring operation.

S i G M

However, for SINGLE STERNTUBE BEARING shaftlines, there can be a marginof error when attempting to calculate the misalignment angle inside the aftsterntube bearing under certain loading conditions. This is due to the fact that

hull deflections involve a part of translation and a part of rotation of the sterntube

(as part of the flexing hull), of which the rotation can only be estimated throughassumptions due to the single point of shaft support in the sterntube and theshaft flexure between the aft sterntube and the intermediate bearing.

The above can be overcome if there is experience or actual knowledge and

accurate estimation of the hull deflections in order to minimise the error.

Other sources of error could include:

Approximate estimation of point of support in the stb aft bush.

Yielding of the stb bush white metal with a YS of around 50 MPa,

which spoils the system linearity.

Higher dependence on very sensitive influence coefficients at thecrankshaft bearing area.

St i G M t

If the hull deflections are not known or available.

If the remaining misalignment angle inside the aft sterntube bushis critical to be measured with the best possible accuracy,

It is recommended to install the light DSAM system, as eitheradditional or alternative measurement method.

The light DSAM system consists of a set of 2 pairs ofdisplacement probes installed at the aft and forward part of thesterntube bush that monitor continuously and with high accuracy

the shaft misalignment angle under all conditions. If the misalignment angle exceeds 0.3 mrad then the likelihood of

a bearing failure increases.

DSAM t

DSAM system

DSAM t

DSAM system

Questions

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ABS Shaft Alignment Challenges

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