©2016 American Bureau of Shipping. All rights reserved.

Shaft Alignment Challenges –The Single Sterntube Bearing Design

Dr Chris LeontopoulosManager, Corporate Marine Technology

Athens, Greece 18th February 2016

The Auditorium of Maran and Alpha Tankers

Shaft Alignment Sensitivity Analysis

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Shaft Alignment Sensitivity Analysis

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This preliminary study aims to show the sensitivity of the design with a single 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 purposes

but 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 aboveor

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

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

Propeller Eccentric Thrust

How does the position of eccentric thrust influences the aft sterntube bearing misalignment angle ? Class Rules have an empirical limit of 0.3 mrad. Assumption: A misalignment angle of above 0.3 mrad may cause

damage to the aft sterntube bearing.

F

PROPVertical

orce

Dynamic Moment

Static slopemismatch tend to zero in ideal condition

L/2

L

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Single Sterntube Bearing Failures

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Single Sterntube Bearing Failures

Near-Optimum Shaft Alignment

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Propeller Eccentric Thrust

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Propeller Eccentric Thrust

Propeller Forces and Corresponding Loading Conditions Influencing the Shaftline

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Shaft Orbit in IB during Special Sea Trials for Shaft Alignment

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Intermediate Bearing and Engine Positioning

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

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Intermediate Bearing and Engine Positioning

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

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Traditional 2-STB Bearing Design versus Single STB Bearing Design

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Traditional 2-STB Bearing Design versus Single STB Bearing Design

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Traditional 2-STB Bearing Design versus Single STB Bearing Design

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Propeller Thrust Eccentricity versus Misalignment Angle

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Propeller Thrust Eccentricity versus Misalignment Angle

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IB and Engine Offset versus Misalignment Angle

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IB and Engine Offset versus Misalignment Angle

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Conclusions

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Through simplified computer shaft alignment models it has been shown that : The position of the propeller eccentric thrust influences the

misalignment angle much more when there is a single sterntube bearing.

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 forward sterntube bearing “corrects the slope” by taking the additional load.

Recommendations

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

In case of a propulsion installation with no forward stern tube bearing, the intermediate shaft bearing should be chocked and its offset not changed after the bore sighting is complete, (ABS SVR Rules 2015).

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

Due to the sensitivity of the design, further investigations can be conducted through systematic measurements on such powertrains.

ABS Experience of Contributing Factors to Related Failures

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Possible intermediate bearing and engine bearing offsets out of spec.

Potential residual lateral vibration from a calculated critical speed close to MCR.

Potentially additional forces from partially immersed propeller due to rough weather conditions.

Sensitivity of the design from potential unaccounted hull deflections.

Lubrication system with potentially insufficient cooling.

Afloat Re-Alignment

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Strain Gauge Measurements

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Shaft Alignment Measurements using Strain Gauges and the concept of Reverse Engineering.

Reverse Engineering

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Definition of Shaft Alignment : Shaft Alignment is the configuration of the shafts and bearings

relative to the centerlines of the bearings from the theoretical straight line condition, so as to achieve an acceptable bearing load distribution.

Definition of “Reverse Engineering Shaft Alignment” : Reverse Engineering Shaft Alignment is about performing a reverse

engineering calculation with the desired bearing load distribution as input or the measured shaft bending moment values at selected positions and determining a set of bearing offsets (usually more than one) 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 shaft bending moments and/or bearing reaction loads and the output is the bearing offsets that satisfy the desired bearing loads.

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

A special algorithm (which is called optimisation algorithm) is used to perform the reverse analysis through multiple iterations.

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Reverse Engineering Background Theory

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Reverse Engineering Background Theory

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Strain Gauge Measurements

Strain Gauge Installation Procedure

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Strain Gauge Measurements

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TELEMETRIC Strain Gauge Installation Procedure

Strain Gauge Measurements

TELEMETRIC Strain Gauge Installation Procedure

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Strain Gauge Measurements

Static Versus Dynamic (Telemetric) Strain Gauge measurements

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Strain Gauge Measurements

Static Versus Dynamic (Telemetric) Strain Gauge measurements

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Strain Gauge Measurements

Reverse Engineering through FE model

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Strain Gauge Measurements

Reverse Engineering through FE model

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Strain Gauge Measurements

Reverse Engineering through FE model

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Strain Gauge Measurements

Reverse Engineering through FE model

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Strain Gauge Measurements

Reverse Engineering through FE model

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Strain Gauge Measurements

Misalignment Angle in the aft sterntube bush

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Strain Gauge Measurements

Misalignment Angle in the aft sterntube bush – Dynamic Conditions

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Strain Gauge Measurements

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Telemetric strain gauge method is the BEST measurement method to demonstrate the shaft alignment status of a typical vessel shaftline under both static and dynamic (operating) conditions.

This is because it can provide with good accuracy the bearing offsets and hence the bearing reactions to verify that the shaftline bearings are not unloaded or overloaded at any time, particularly during operation.

Strain Gauge Measurements

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However, for SINGLE STERNTUBE BEARING shaftlines, there can be a margin of error when attempting to calculate the misalignment angle inside the aft sterntube 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 through assumptions due to the single point of shaft support in the sterntube and the shaft 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 theerror.

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 the

crankshaft bearing area.

Strain Gauge Measurements

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If the hull deflections are not known or available.

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

THEN It is recommended to install the light DSAM system, as either

additional or alternative measurement method.

The light DSAM system consists of a set of 2 pairs of displacement probes installed at the aft and forward part of the sterntube 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 system

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DSAM system

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DSAM system

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DSAM system

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Questions

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©2016 American Bureau of Shipping. All rights reserved.