Shaft Alignment Challenges â The Single Sterntube Bearing Design
©2016 American Bureau of Shipping. All rights reserved.
Shaft Alignment Challenges – The Single Sterntube Bearing Design
Dr Chris Leontopoulos Manager, Corporate Marine Technology Athens, Greece 18th February 2016
The Auditorium of Maran and Alpha Tankers
Shaft Alignment Sensitivity Analysis
2
Shaft Alignment Sensitivity Analysis
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 above or 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. 3
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.
Dynamic Moment
PROP Vertical Force
Static slope mismatch tend to zero in ideal condition
L/2
L
4
Single Sterntube Bearing Failures
5
Single Sterntube Bearing Failures
Near-Optimum Shaft Alignment 6
Propeller Eccentric Thrust
7
Propeller Eccentric Thrust
8
Propeller Forces and Corresponding Loading Conditions Influencing the Shaftline
9
Shaft Orbit in IB during Special Sea Trials for Shaft Alignment
10
Intermediate Bearing and Engine Positioning
How does the position of the intermediate bearing and engine offset affect the misalignment angle?
11
Intermediate Bearing and Engine Positioning
How does the position of the intermediate bearing and engine offset affect the misalignment angle ?
12
Traditional 2-STB Bearing Design versus Single STB Bearing Design
13
Traditional 2-STB Bearing Design versus Single STB Bearing Design
14
Traditional 2-STB Bearing Design versus Single STB Bearing Design
15
Propeller Thrust Eccentricity versus Misalignment Angle
16
Propeller Thrust Eccentricity versus Misalignment Angle
17
IB and Engine Offset versus Misalignment Angle
18
IB and Engine Offset versus Misalignment Angle
19
Conclusions
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.
20
Recommendations
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 reconsidered 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. 21
ABS Experience of Contributing Factors to Related Failures
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.
22
Afloat Re-Alignment
23
Strain Gauge Measurements
Shaft Alignment Measurements using Strain Gauges and the concept of Reverse Engineering.
24
Reverse Engineering
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.
25
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.
26
Reverse Engineering Background Theory
27
Reverse Engineering Background Theory
28
Strain Gauge Measurements
Strain Gauge Installation Procedure
29
Strain Gauge Measurements
TELEMETRIC Strain Gauge Installation Procedure
30
Strain Gauge Measurements
TELEMETRIC Strain Gauge Installation Procedure
31
Strain Gauge Measurements
Static Versus Dynamic (Telemetric) Strain Gauge measurements
32
Strain Gauge Measurements
Static Versus Dynamic (Telemetric) Strain Gauge measurements
33
Strain Gauge Measurements
Reverse Engineering through FE model
34
Strain Gauge Measurements
Reverse Engineering through FE model
35
Strain Gauge Measurements
Reverse Engineering through FE model
36
Strain Gauge Measurements
Reverse Engineering through FE model
37
Strain Gauge Measurements
Reverse Engineering through FE model
38
Strain Gauge Measurements
Misalignment Angle in the aft sterntube bush
39
Strain Gauge Measurements
Misalignment Angle in the aft sterntube bush – Dynamic Conditions
40
Strain Gauge Measurements
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.
41
Strain Gauge Measurements
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 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 the crankshaft bearing area. 42
Strain Gauge Measurements
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. 43
DSAM system
44
DSAM system
45
DSAM system
46
DSAM system
47
Questions
48
©2016 American Bureau of Shipping. All rights reserved.
www.eagle.org
Shaft Alignment Challenges – The Single Sterntube Bearing Design
Dr Chris Leontopoulos Manager, Corporate Marine Technology Athens, Greece 18th February 2016
The Auditorium of Maran and Alpha Tankers
Shaft Alignment Sensitivity Analysis
2
Shaft Alignment Sensitivity Analysis
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 above or 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. 3
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.
Dynamic Moment
PROP Vertical Force
Static slope mismatch tend to zero in ideal condition
L/2
L
4
Single Sterntube Bearing Failures
5
Single Sterntube Bearing Failures
Near-Optimum Shaft Alignment 6
Propeller Eccentric Thrust
7
Propeller Eccentric Thrust
8
Propeller Forces and Corresponding Loading Conditions Influencing the Shaftline
9
Shaft Orbit in IB during Special Sea Trials for Shaft Alignment
10
Intermediate Bearing and Engine Positioning
How does the position of the intermediate bearing and engine offset affect the misalignment angle?
11
Intermediate Bearing and Engine Positioning
How does the position of the intermediate bearing and engine offset affect the misalignment angle ?
12
Traditional 2-STB Bearing Design versus Single STB Bearing Design
13
Traditional 2-STB Bearing Design versus Single STB Bearing Design
14
Traditional 2-STB Bearing Design versus Single STB Bearing Design
15
Propeller Thrust Eccentricity versus Misalignment Angle
16
Propeller Thrust Eccentricity versus Misalignment Angle
17
IB and Engine Offset versus Misalignment Angle
18
IB and Engine Offset versus Misalignment Angle
19
Conclusions
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.
20
Recommendations
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 reconsidered 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. 21
ABS Experience of Contributing Factors to Related Failures
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.
22
Afloat Re-Alignment
23
Strain Gauge Measurements
Shaft Alignment Measurements using Strain Gauges and the concept of Reverse Engineering.
24
Reverse Engineering
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.
25
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.
26
Reverse Engineering Background Theory
27
Reverse Engineering Background Theory
28
Strain Gauge Measurements
Strain Gauge Installation Procedure
29
Strain Gauge Measurements
TELEMETRIC Strain Gauge Installation Procedure
30
Strain Gauge Measurements
TELEMETRIC Strain Gauge Installation Procedure
31
Strain Gauge Measurements
Static Versus Dynamic (Telemetric) Strain Gauge measurements
32
Strain Gauge Measurements
Static Versus Dynamic (Telemetric) Strain Gauge measurements
33
Strain Gauge Measurements
Reverse Engineering through FE model
34
Strain Gauge Measurements
Reverse Engineering through FE model
35
Strain Gauge Measurements
Reverse Engineering through FE model
36
Strain Gauge Measurements
Reverse Engineering through FE model
37
Strain Gauge Measurements
Reverse Engineering through FE model
38
Strain Gauge Measurements
Misalignment Angle in the aft sterntube bush
39
Strain Gauge Measurements
Misalignment Angle in the aft sterntube bush – Dynamic Conditions
40
Strain Gauge Measurements
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.
41
Strain Gauge Measurements
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 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 the crankshaft bearing area. 42
Strain Gauge Measurements
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. 43
DSAM system
44
DSAM system
45
DSAM system
46
DSAM system
47
Questions
48
©2016 American Bureau of Shipping. All rights reserved.
www.eagle.org
