128-Shaft Alignment Guide_e.pdf

GUIDANCE NOTES ON

PROPULSION SHAFTING ALIGNMENT

APRIL 2006

American Bureau of Shipping Incorporated by Act of Legislature of the State of New York 1862

Copyright 2006 American Bureau of Shipping ABS Plaza 16855 Northchase Drive Houston, TX 77060 USA

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ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 iii

Foreword

The mission of the American Bureau of Shipping (ABS or Bureau) is to serve the public interest, as well as the needs of its clients, by promoting the security of life, property and the natural environment primarily through the development and verification of standards for the design, construction and operational maintenance of marine-related facilities.

The Rules and Guides on which classification is predicated are established from theoretical and empirical principles of naval architecture, marine engineering and other engineering principles that have proven satisfactory by service experience and systematic analysis. The classification Rules are not intended to address every single aspect of the vessel design, but rather to indicate the minimum set of criteria which will ensure safety and functionality of all vital components of the vessel, and at the same time provide sufficient space to the industry to accommodate their practices and technologies with minimum constraints from regulatory bodies.

However, in situations where the complexity of the problem results in conflicting interpretation of regulations and when the consequence of this disparity results in damage to the equipment and affects vessel’s safety, additional regulation clarification and guidance may be necessary. The case of shaft alignment is an example of where the Bureau has noticed the need to provide a more detailed explanation on alignment design and practices, which has resulted in the development of the subject Guidance Notes. These Guidance Notes have been developed primarily to clarify the subject matter for the Bureau field inspectors and design review engineers to ensure consistency of the survey and plan approval process. Moreover, the subject guidelines may help the industry to improve its approach towards shaft alignment analyses and procedures.

Additionally, the Bureau has developed state of the art analytical tools primarily for the purpose of engineering analysis and design. The ABS shaft alignment program, combined with alignment optimization software, is capable of analyzing complex propulsion installations and, when used as design tool, may provide an optimal solution to the alignment problem.

We welcome your feedback. Comments or suggestions can be sent electronically to rsd@eagle.org.

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ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 v

GUIDANCE NOTES ON

PROPULSION SHAFTING ALIGNMENT

CONTENTS

SECTION 1 Introduction

1 Propulsion Shaft Alignment 2 Objective 3 The Alignment Problem

3.1 Solution to Alignment Problem

3.2 Analytical Support

4 Modern Vessel Design 5 Rule Requirements

SECTION 2 Shaft Alignment Design and Review

1 General 2 Review vs. Design 3 Review

3.1 Plans and Particulars Required

3.2 Shaft Alignment Model

3.3 Scope of Calculation

3.4 Results Verification

3.5 Documenting the Review

4 Design

4.1 General

4.2 Stern Tube Bearing

4.3 Stern Tube Bearing Contact Modeling

4.4 Crankshaft Modeling

4.5 Applying a Partial Equivalent Model of the

Crankshaft

4.6 Engine Bearing Misalignment

4.7 Bearing Clearance

4.8 Bearing Elasticity

4.9 Bearing Wear Down

4.10 Gear Meshes

TABLE 1 Influence Coefficient Matrix vi ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

FIGURE 1 Directly Coupled Propulsion Shafting – Example FIGURE 2 Bearing Reactions FIGURE 3 Nodal Slope and Deflection Curve FIGURE 4 Diesel Engine Output Flange Allowable Shear

Force and Bending Moment FIGURE 5 Tail Shaft Bearing Evaluation Program FIGURE 6 Ideal Contact Area on the Bearing Exerted by

Shaft (Misalignment Angle Zero) FIGURE 7 Tail Shaft Bearing Contact as a Function of

Alignment Design FIGURE 8 Crankshaft Outline FIGURE 9 Crankshaft – Equivalent Model for Shaft

Alignment FIGURE 10 FE Model of Half of the Crank FIGURE 11 Defining Equivalent Model FIGURE 12 Reduced Crankshaft Model – 2 M/E Bearings

Only FIGURE 13 Reduced Crankshaft Model – 4 M/E Bearings FIGURE 14 ABS Bearing Evaluation Interface FIGURE 15 Gear Driven Propulsion – Equal Gear Shaft

Bearing Reactions 0.21 mrad Gear Misalignment Angle

FIGURE 16 Gear Driven Propulsion – Uneven Gear Shaft Bearing Reactions Zero Misalignment Angle at Gear Wheel

SECTION 3 Shaft Alignment Procedure

1 General 2 Shaft Alignment Procedure 3 Sighting Through (Boresighting)

3.1 Piano Wire Application

4 Slope Boring – Bearing Inclination 5 Engine Bedplate Pre-sagging 6 Sag and Gap

6.1 Theoretical Background

7 Reactions Measurement 8 Bearing-Shaft Misalignment Measurement 9 Shaft Eccentricity 10 Intermediate Bearing Offset Adjustment

10.1 System with Forward S/T Bearing

10.2 System with No Forward S/T Bearing

10.3 Which Solution to Adopt

11 Diesel Engine Alignment

11.1 Crankshaft Deflections

12 FAQ – Problems and Solution

ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 vii

TABLE 1 Influence Coefficient Matrix – System with Forward Stern Tube Bearing

TABLE 2 Influence Coefficient Matrix – System without Forward Stern Tube Bearing

FIGURE 1 Example of Optical/Laser Sighting Through FIGURE 2 Piano Wire Application FIGURE 3 Slope Boring Arrangement FIGURE 4 Slope Boring Machine FIGURE 5 Bearing Inclination FIGURE 6 Bedplate Sagging Measurement Using Piano

Wire FIGURE 7 Flange Arrangement in Sag and Gap Analysis FIGURE 8 Stern Tube Bearing Contact Condition Evaluation –

Sample Analysis FIGURE 9 System Sensitivity to Intermediate Shaft Bearing

Offset Change – with Forward Stern Tube Bearing

FIGURE 10 Bearing Reactions for Design Offset – with Forward Stern Tube Bearing

FIGURE 11 System Sensitivity to Intermediate Shaft Bearing Offset Change – without Forward Stern Tube Bearing

FIGURE 12 Bearing Reactions for Design Offset – without Forward Stern Tube Bearing

FIGURE 13 Crankshaft Installation in the Engine FIGURE 14 Diesel Engine Bearing Damage due to Edge

Loading FIGURE 15 Bearing Reactions for a Dry Dock Alignment with

Intentionally Unloaded Second Main Engine Bearing

FIGURE 16 Deflection Curve and Bearing Offset for Dry Dock Condition, which Resulted in Intentionally Unloaded Second Main Engine Bearing

FIGURE 17 Bearing Reactions for a Waterborne Vessel – Rectified by Hull Deflections and Bedplate Sag

FIGURE 18 Total Vertical Offset at the Bearings Including Prescribed Displacements, Hull Deflection Estimate and Bedplate Sag

FIGURE 19 Vessel in Dry Dock FIGURE 20 Vessel Waterborne – Hull Deflections Affect the

Propulsion FIGURE 21 Vessel Waterborne – Engine Sag Applied

SECTION 4 Shaft Alignment Survey

1 General 2 Alignment Acceptance Criteria

2.1 Attendance

2.2 Required Information

viii ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

2.3 Measurement Procedure

2.4 S/T Bearing Wear Down

2.5 Dry Dock Alignment

2.6 Noncompliance with Construction Completion

Requirements

2.7 Sag and Gap Acceptability

2.8 Number of Bearings to be Verified

2.9 Reaction Measurement Acceptability

2.10 Slope Boring

2.11 Dry Dock Alignment

2.12 Shaft Runout

2.13 Construction Practices

SECTION 5 Alignment Measurements

1 General 2 Bearing Reaction Measurements

2.1 Jack-up Method

2.2 Strain Gauge Method

3 Bearing Vertical Offset Measurements

3.1 Reverse Shafting Alignment Calculation of the Bearing

Offsets

4 Bearing Misalignment Measurements 5 Crankshaft Deflection Measurement 6 Gear Contact Misalignment Measurement 7 Sag and Gap Measurement 8 Eccentricity (Runout) Measurement of the Shaft

8.1 Dial Gauge Runout Measurements

8.2 Runout Measurement in Lathe

9 Stress Measurements

9.1 Stress in the Shafting

9.2 Stress in the Bearing

TABLE 1 Sample Influence Coefficient Matrix TABLE 2 Offset Vector Spectrum

FIGURE 1 Hydraulic Jack with Load Cell FIGURE 2 Digital Dial Gauge FIGURE 3 Reaction Measurement at Intermediate Shaft

Bearing FIGURE 4 Jack-up Measurement of the Bearing Reactions

Inside Diesel Engine FIGURE 5 Jack-up Curve FIGURE 6 Jack-up Curve for Unloaded Bearing FIGURE 7 Strain Gauge Installation FIGURE 8 Pair of Uniaxial Gauges FIGURE 9 Wheatstone Bridge

ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 ix

FIGURE 10 Bending Moments Measured at Nine Different Locations Along the Shaft Line

FIGURE 11 Reverse Analysis I/O Interface FIGURE 12 Dry dock – Bearing offset – Reverse Analysis vs.

As-designed Offset Comparison FIGURE 13 Ballast – Bearing offset – Reverse Analysis vs.

As-designed Offset Comparison FIGURE 14 Full Load – Bearing offset – Reverse Analysis vs.

As-designed Offset Comparison FIGURE 15 Intermediate Shaft Bottom Clearance and Runout

Measurement FIGURE 16 Crankshaft Deflection Measurements FIGURE 17 Gear Contact FIGURE 18 Sag and Gap Measurement FIGURE 19 Assembled Shafting – Condition Desired After

Sag and Gap is Verified FIGURE 20 Preassembly Shafting Setup – for Sag and Gap

Measurement

SECTION 6 Hull Girder Deflections

1 General 2 Analytical Approach 3 Hull Girder Deflection Measurements

3.1 Bending Moment Measurements

3.2 Bearing Reaction Measurements

3.3 Crankshaft Deflection Measurements

4 Example

4.1 Analytical Approach

4.2 Example - Hull Girder Deflection Measurements

5 Hull Deflection Application

FIGURE 1 Hull Girder Deflections Influence on Propulsion

System FIGURE 2 Strain Gauge Measurement FIGURE 3 Hydraulic Jack Locations for Reaction

Measurements on the Shafting and M/E Bearings

FIGURE 4 Bearing Reactions are Measurements Using Hydraulic Jacks

FIGURE 5 Vessel Deflections Change with Loading Condition

FIGURE 6 Large Container Vessel Shafting for Shaft Alignment Analysis Purpose

FIGURE 7 Shaft Alignment Design with No Hull Deflections Considered

FIGURE 8 Still-water Deflections of the Vessel x ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

FIGURE 9 Containership – Diesel Engine Bearing Reactions as a Function of Hull Deflections and Bedplate Sag

FIGURE 10 Still-water Hull Deflections – Ballast FIGURE 11 Still-water Hull Deflections – Laden

SECTION 7 Alignment Optimization

1 General 2 Optimization Example 3 Optimization

TABLE 1 Estimated Hull Girder Deflections TABLE 2 Optimal Solution TABLE 3 Dry Dock – Bearing Reactions for Prescribed

Offset TABLE 4 Ballast Vessel Hull Deflections – Bearing

Reactions and Total Bearing Offset TABLE 5 Laden Vessel Hull Deflections – Bearing

Reactions and Total Bearing Offset

FIGURE 1 Discrete Model of the Shafting FIGURE 2 Bearing Offset; Shaft Deflection Curve; Nodal

Slopes FIGURE 3 Bearing Reactions; Bending Moment; Shear

Forces FIGURE 4 Laden – Bearing Offset Disturbed by Hull

Deflections; Bearing Reactions – Unloaded M/E Bearing #2

FIGURE 5 Ballast – Bearing Offset Disturbed by Hull Deflections; Bearing Reactions – Unloaded M/E Bearing #2

FIGURE 6 GA Input Data and Output Showing Two of Ten Desired Solutions

SECTION 8 Glossary

1 Abbreviations 2 Definitions

ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 1

S E C T I O N 1 Introduction

1 Propulsion Shaft Alignment

The propulsion shafting alignment is a process which consists of two parts:

• The design and analysis

• The alignment procedure and measurements

The definition of the shaft alignment process and the practices in performing the alignment are not consistent in the industry. The terminology and requirements for the shaft alignment will vary depending on the machinery application, the propulsion system’s size, as well as on the perception of the alignment process itself1.

In order to avoid misunderstandings in interpretation and definition of the terms in these Guidance Notes, a definition of the propulsion shafting and the propulsion shafting alignment is provided below.

The definitions adopted here and the references made throughout these Guidance Notes, whether mentioned explicitly or not, are considered valid for the alignment of the ship propulsion machinery.

Propulsion shafting is a system of revolving rods that transmit power and motion from the main drive to the propeller. The shafting is supported by an appropriate number of bearings.

Propulsion shaft alignment is a static condition observed at the bearings supporting the propulsion shafts. In order for the propulsion shafting alignment to be properly defined, the following minimum set of parameters (whichever may be applicable) need to be confirmed as acceptable:

• Bearing vertical offset

• Bearing reactions

• Misalignment angles

• Crankshaft’s web deflections

• Gear misalignment

• Shaft and bearings’ strength

• Coupling bolts’ strength

1 The approaches to the alignment presently seen throughout the industry vary from defining the alignment as pure static and a combination of static and dynamic, with a variety of methods in modeling and analyzing the alignment.

Section 1 Introduction

2 ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

The alignment is considered to be satisfactory when it is possible to control the above parameters, and maintain them within the required limits under all operating conditions of the vessel. By “all operating conditions of the vessel”, the intent is that the alignment remains acceptable for:

• Vessel loading variation from ballast to laden

• Temperature variation affecting the propulsion shafting system

The change in loading of the vessel will result in variation of hull girder deflections, thus disturbing offset of the shaft-supporting bearings. Hull deflections will affect all bearings in the system simultaneously. Making the alignment satisfactory for all loading conditions will require several analyses to be conducted to verify alignment acceptability for all different vessel loadings.

The temperature rise or drop will also affect the bearing offset. However, unlike the hull deflections, the effect of temperature change may be local to the particular bearing or to the set of bearings (e.g., main engine, gear-box).

Basic propulsion shafting alignment criteria, requirements and limits are usually defined by Classification Societies, shipbuilders, designers or other regulatory bodies. The ABS criteria and requirements are defined in ABS Rules. For the reader’s convenience, appropriate paragraphs of the Rules are replicated, as deemed necessary.

Note: The above definition of the alignment made no reference to the dynamics and vibration. This is because the propulsion shaft alignment is calculated, conducted and verified when the system is at rest. Therefore, throughout these Guidance Notes, the propulsion shaft alignment will be regarded as a static condition.

However, the goal is to ensure that all of the activities on the alignment in the static condition will ensure an acceptable running condition of the shafting. It has been proven that a good static shafting alignment is a prerequisite for trouble-free dynamic operation of the propulsion shafting, as it will lower the probability of excessive whirling and lateral vibration of the shafting, as well as noise transmission

2 Objective

The objective of these Guidance Notes is to provide detailed information on the propulsion shafting alignment design and procedure in order to:

• Ensure safe operation of the propulsion shafting system on ABS-classed vessels.

• Improve the design review process and survey.

• Clarify the Bureau position with regard to the alignment problem in the industry and hope that this clarification may lead to a more unified approach in the alignment design, analysis and application.

The benefit of properly addressed alignment will be:

• Reduction in material damages (in particular) on the stern tube bearings and seals, main engine bearings, gear meshes and shafting coupling bolts

• Environmental pollution reduction

These Guidance Notes will primarily elaborate on the alignment problem in propulsion systems which are deemed alignment-sensitive, such as VLCC, ULCC and large bulk carriers. Accordingly, the following are specifically addressed:

• Directly driven propeller installations

• Low speed diesel installations

• Systems with relatively short and rigid shafting

• Vessels with a relatively flexible hull structure

ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 3

These Guidance Notes also provide insight into the alignment problems of gear-driven installations (turbine or electric) that fall into the above category of rigid shafts and flexible structures.

Large container vessels will be separately addressed, as these Guidance Notes discuss hull girder deflections in installations with single screw diesel propulsion.

Though the focus is primarily on the vessels which for one reason or another fall into the category of alignment-sensitive installations, it is not intended to disregard possible problems in the other propulsion installations. Accordingly, the discussion concentrates on the propulsion systems which are deemed highly sensitive to small disturbances in bearing offsets. The principles and practices discussed below shall certainly apply (partially or fully) to the other vessels’ designs as well.

These Guidance Notes will address the following:

• Shaft alignment design and review

General alignment considerations

Robust alignment design

Tail shaft bearing problems

Diesel engine bearing problem

Intermediate shaft bearing offset adjustment

Crankshaft equivalent model

Bearing clearance

Gear meshes

Acceptance criteria

• Shaft alignment procedure

Shipbuilder practices

Construction tolerances

Construction priorities

Sighting through

Slope boring or stern tube bearing casting

• Alignment measurement

Sag and gap

Jack-up

Strain gauge

• Hull girder deflections

Hull deflection measurement

Hull deflection prediction

• Alignment optimization

4 ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

3 The Alignment Problem

As the vessel’s cargo capacity, and consequently, the installed power of the main drive increase, the propulsion shafting alignments are increasingly more sensitive to disturbances affecting vertical offset of the bearings. These disturbances primarily result from hull deflections and temperature change.

However, another very significant influence on the quality of the alignment is the accuracy of the shaft alignment analyses and the alignment procedure itself. All of these problems will be specifically addressed in these Guidance Notes.

The shaft alignment problem can be summarized as follows:

• High sensitivity of the shaft alignment to small disturbances in the bearing vertical position

• Disparity between highly flexible hull girder structure and the rigid propulsion shafting

• Difficulties in hull girder deflection evaluation

• Problems in maintaining the desired accuracy of the shaft alignment analysis

• Inconsistency and inaccuracies in conducting the alignment procedure

The shaft alignment allowable tolerances are measured in fractions of millimeters, as opposed to the vessel, which is constructed with tens of millimeters of acceptable error margin. This disparity would eventually not be a problem (required analytical accuracy is practically achievable), if the alignment procedure itself was conducted after a majority of the vessel’s structure was in place and welded, i.e., no significant structural changes were made during and after the shaft alignment procedure was completed. However, major structural works are often not finished when the alignment is performed.

There are a number of reasons for that, some of which will be addressed below.

Ideally, a shaft alignment analysis should be performed for maximum allowable alignment tolerances, resulting in acceptable bearing reactions and misalignment angles under all operating conditions of the vessel (loaded, ballast, hot and cold). This implies that hull deflections should be accounted for initially.

The hull girder deflections are not easy to predict, calculate or measure. It is therefore difficult for the industry to comply with the requirement that hull deflections be accounted for in alignment calculations.

The alignment process is rather inconsistent among shipbuilders, involving different practices that builders have adopted in vessel construction, which results in a spectrum of different approaches to the alignment. These Guidance Notes will not try to investigate all of the different practices, but will to a certain extent address some of the extreme cases, which if not thoroughly controlled may result in propulsion installation damage and failure.

The alignment sensitivity is measured by bearing reaction response to the bearing offset change. On sensitive installations such as VLCC, ULCC and large bulk-carriers (which are addressed in particular in these Guidance Notes), a fraction of a millimeter disturbance in vertical bearing offset may cause a significant change in bearing reaction.

3.1 Solution to Alignment Problem

The preferable condition for performing the propulsion shafting alignment procedure would be in the dry dock, just before the vessel is launched. At that stage, the vessel’s structural work is nearly complete. Thus, a minimum disturbance on the alignment resulting from the construction work after the launching may be expected.

However, to comfortably rely on dry dock alignment, the hull deflections need to be predicted with relatively high confidence. Prediction of hull deflections would enable the design of a dry dock alignment robust enough to prevent other than hull deflection disturbances from adversely affecting the alignment. ABS achieved this goal by conducting comprehensive hull deflection measurements on

ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 5

a series of different vessels (various types and sizes), which now constitutes a database of expected hull deflections for certain categories of vessels. ABS now has the ability to relatively accurately estimate the hull deflections (for more details, consult Section 6) and to conduct optimization of the alignment, utilizing optimization software (Section 7).

3.2 Analytical Support

There are a number of computer programs in the industry which can address the alignment problem accurately and comprehensively.

The ABS shaft alignment software, however, has several features that distinguish it from most of the other similar computer programs. Namely, in addition to the common analysis which provides bearing reactions, misalignment angles, sag and gap data, influence coefficient matrix, the ABS alignment software is further capable of:

• Estimating hull deflections for the given ship type and the basic ship parameters

• Evaluating the stern tube bearing contact and misalignment condition, and

• Optimizing the alignment within given constraints (hull deflections, thermal growth, etc.).

More details on shaft alignment optimization are given in Section 7.

Basic ABS shaft alignment analysis software is available for distribution to third parties. The software package includes sample shaft alignment models, user’s manual and theoretical backgrounds. On specific demand, software application training can be provided as well.

4 Modern Vessel Design

The propulsion system of a modern merchant vessel commonly consists of a diesel engine driving a propeller through directly-coupled2 shafting. As mentioned, such a design results in an increased disparity in flexibilities between the hull girder structure and the shafting. Namely:

• As the demand for power rises with the increase in the vessel’s size and with scantling optimization, the vessel’s hull becomes more flexible.

• The shafting diameters become larger and the shafts stiffer.

Consequently, the alignment of the propulsion system becomes increasingly sensitive to small deviations in bearing offsets resulting in difficulties in analyzing the alignment and conducting the alignment procedure.

The alignment-related damages are attributed to a number of factors which are addressed in detail in these Guidance Notes. However, in order to prevent and reduce alignment-related problems, it is important to ensure that:

• The shaft alignment analysis provides the necessary (accurate and applicable) data to support each stage of the actual alignment process.

• The alignment procedure is conducted with the required accuracy in accordance with Class regulations and good marine practices.

2 “Directly coupled installation” means a diesel engine connected to propeller through propulsion shafting only (no elastic couplings or reduction gears are used in the system).

6 ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

5 Rule Requirements The propulsion shaft alignment is a static process where acting loads are static forces and moments.

Dynamic factors are not normally considered, as the primary purpose of the analysis is to support the static shafting alignment procedure. Accordingly, these Guidance Notes apply to the static design of the propulsion shafting alignment, as well as shafts, couplings, clutches and other power transmitting components for propulsion purposes.

It has been recognized that accurate alignment at the stern tube bearing is crucial in order to avoid the occurrence of bearing failure problems. Slope boring (or bearing inclination) has been used in the marine industry to achieve the required level of alignment.

It is normally required that the shaft alignment be carried out in the presence of a Surveyor. It is desired that sighting-through and, when applicable, Sag and Gap measurements be verified after the superstructure is put in place and all major welding works are completed. Final alignment should be verified in the afloat condition. When alignment calculations are required to be submitted in accordance with 4-3-2/7.3 of the Rules for Building and Classing Steel Vessels, the calculated alignment data are to be verified and recorded by appropriate measurement procedures in the presence and to the satisfaction of a Surveyor.

Shafts and associated components used for transmission of power essential for the propulsion of the vessel are to be designed and constructed so as to withstand the maximum working stresses to which they may be subjected in all service conditions.

In addition to the design requirements addressed in the ABS Rules for Building and Classing Steel Vessels, consideration is to be given to additional stresses in the shafting system given rise to by shaft alignment.

In general, shaft alignment calculations and a shaft alignment procedure are to be submitted for reference. Specifically, they are to be submitted for review for the following alignment-sensitive type of installations:

i) Propulsion shafting of diameter larger than 400 mm (15.75 in.)

ii) Propulsion shafting with reduction gears (refer to 4-3-1/1.5 of the ABS Rules for Building and

Classing Steel Vessels) where the bull gear is driven by two or more ahead pinions.

iii) Propulsion shafting with power take-off or with booster power arrangements.

iv) Propulsion shafting for which the tail shaft bearings are to be bored sloped.

The alignment calculations are to include bearing reactions, shear forces and bending moments along the shafting, slope boring details (if applicable) and detailed description of the alignment procedure.

The alignment calculations are to be performed for theoretically aligned cold and hot conditions of the shaft with specified alignment tolerances.

Calculations are to be performed for the maximum allowable alignment tolerances and are to show that:

• Bearing loads under all operating conditions are within the acceptable limits specified by the bearing manufacturer.

• Bearing reactions are always positive (i.e., supporting the shaft).

• Shear forces and bending moments on the shaft are within acceptable limits in association with other stresses in the shaft.

• Forces and moments on propulsion equipment are within the limits specified by the machinery manufacturers.

In general, if calculated relative misalignment slope between the shaft and the tail shaft bearing is greater than 0.3 × 10-3 rad, then consideration is to be given to reducing the relative misalignment slope by means of slope boring or bearing inclination.

ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 7

S E C T I O N 2 Shaft Alignment Design and Review

1 General A shaft alignment designer has to ensure, and the reviewer has to verify, that the strength of the designed parts (bearings, shafts, coupling bolts, couplings) is sufficient to prevent the stress exerted by the acting loads to damage the same.

In particular, the alignment design should satisfy the following:

• Bearing condition:

Acceptable reaction load Even load distribution throughout the bearing

• Shaft strength

• Satisfactory crankshaft deflections

• Acceptable gear contact condition

• Satisfactory coupling bolts strength

• Acceptable clutches and flexible coupling misalignment tolerances

2 Review vs. Design Analytical models do not always represent the propulsion systems accurately and may not always provide sufficient information to ensure an “error free” alignment procedure. This Section is intended to address the incompatibility problem between the analyses and the alignment procedures, discuss possible solutions and provide advice, where possible. First, differences between the review and the design process will be defined.

The review process serves to verify soundness of an existing design, and it has to thoroughly follow the alignment criteria and guidelines, as defined in the Class Rules. These criteria are mostly strength of material related to specific concerns for safety of life, equipment and environment.

The design process is more complex than the review itself. It requires experienced personnel and is a time-consuming effort with a goal of defining a satisfactory set of parameters to comply with all alignment criteria. The parameters that primarily need to be defined are bearing offset (vertical position) and bearing location (horizontal position). Moreover, it is to be verified that alignment complies with shaft geometry, material properties, installation constraints and other requirements related to the propulsion shafting interaction with surrounding systems.

The design process, if conducted properly, should essentially optimize propulsion shafting for the given parameters3.

3 By optimization here, we do not only imply application of a particular optimization tool or software, but also an educated engineering process in combining the design parameter in an acceptable solution which further complies with imposed regulations and requirements.

Section 2 Shaft Alignment Design and Review

8 ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

The design and review requirements are detailed primarily for the installations involving direct driven propulsion systems (Section 2, Figure 1). Other designs may have slightly different requirements but, in general, a similar approach will apply.

FIGURE 1

Directly Coupled Propulsion Shafting – Example

3 Review

Overall, the plan review during and after construction is conducted by the Bureau to verify to itself and its committees that a vessel, structure, item of material, equipment or machinery is in compliance with the Rules, Guides, standards or other applicable criteria.

Engineers need to confirm that all information required for review is received:

• Shaft alignment model

• Scope of submitted calculation

• Results of analysis

• Shaft alignment procedure

After the review is completed, the reviewer needs to document the result of this review.

The review of the submitted shaft alignment analysis and procedure is to be conducted by inspecting the results of the alignment analysis and by conducing check analysis using the ABS shaft alignment software.

3.1 Plans and Particulars Required

Plans and particulars to be submitted for review most commonly include the following:

3.1.1 For Propulsion Shafting

Shafting arrangement

Rated power of main engine and shaft rpm

Thrust, line, tube and tail shafts, as applicable

Couplings – integral, demountable, keyed, or shrink-fit, coupling bolts and keys

ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 9

Shaft bearings

Stern tube bearings detailed drawings

Allowable bearing load

Shaft seals

Power take-off to shaft generators, propulsion boosters, or similar equipment, rated 100 kW (135 hp) and over, as applicable

Material properties of the shafts and the bearings (modulus of elasticity and density)

Propeller mass and material

3.1.2 For Cardan Shafts

Dimensions of all torque-transmitting components and their materials

Rated power of main engine and shaft rpm

Engineering analyses

3.1.3 Calculations and Procedures

Propulsion shaft alignment calculations and detailed shaft alignment procedure where propulsion shaft is sensitive to alignment (see ABS Rules for Building and Classing Steel Vessels 4-3-2/7.3).

3.1.4 Diesel Engine

Crankshaft equivalent model

Location and mass of the following equipment, when applicable: flywheel, chain drive tightening load, cam-drive gear, torsional vibration damper

Detailed drawings, material properties and location of the main engine bearings

Allowable bearing load

3.1.5 Reduction Gear

Detailed drawings of the gear shafts and main gears

Mass and location of the gears

Material properties

Location and material of the gear shaft bearings

Allowable bearing load

3.1.6 Shaft Alignment Procedure

Detailed description of the shaft alignment procedure

3.2 Shaft Alignment Model

By exercising sound judgment, the engineer should verify that the submitted discrete model represents the actual propulsion system with sufficient accuracy:

The engineer should verify that the line shaft model and reduction gear model (where applicable) correspond to the respective design drawings. The diesel engine equivalent model shall be evaluated by confirming that the engine equivalent model complies with engine design particulars (engine type, diameters, location of the timing gear, etc.). Bearing offsets shall be verified to include hot and cold conditions.

10 ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

3.3 Scope of Calculation

A goal of the shafting alignment calculation is to provide data to the ship production personnel in order to ensure satisfactory alignment under all operating conditions of the vessel (from ballast to full-load). Accordingly, the submitted calculations shall be conducted and verified for:

• Dry dock condition

• Waterborne vessel, hot and cold engine or gear box.

As the alignment procedure starts in the dry dock (positioning of the bearings, slope boring, etc.), the calculation needs to provide sufficient information to the production personnel for the dry dock procedures. It may be beneficial to conduct most of the alignment procedures (sag and gap, and bearing reaction load verification) in the dry dock just before launching of the vessel, as one can take advantage of the fact that the alignment analysis can be quite accurately confirmed for the dry-dock condition, as the alignment is not influenced by hull deflections which are difficult to predict.

Once the vessel is launched, it is also important to evaluate the alignment’s sensitivity to hull deflections. In cases where hull deflections are not available (neither analysis of hull deflection nor the measurements from sister vessels are available), the assessment of alignment sensitivity to hull deflections may be attempted by inspecting the influence coefficient matrix (see 2/3.4.1). Since the influence coefficient matrix inspection alone is not sufficient to confirm shafting interaction with the hull structure, some knowledge of the vessel’s basic structural data is also needed.

Explanation: At least one analysis shall be provided, considering vessel afloat and hot engine condition. This condition would imply a fully or partially submerged propeller, and must confirm that all bearing reactions are positive and all measured reactions are within reasonable tolerances from their calculated counterparts (see 2/4.7).

Separate analyses are normally provided for the Sag and Gap procedure. Sag and Gap measurements alone shall not be accepted as adequate for alignment acceptance. Sag and Gap analytical data is normally given for dry dock condition. Bearing reactions are often measured after the Sag and Gap procedure is finished and shafting assembled. However, if reactions are measured in the dry dock, at least one additional reaction measurement shall be conducted with the vessel afloat.

Note: The shaft alignment procedure is a part of a static process, thus the analysis shall provide adequate information to support the procedure. Dynamic loading may not be appropriate to apply if the verification is to be conducted under the static condition.

3.4 Results Verification

The verification shall include, but not be limited to the following:

• Influence coefficient matrix

• Bearing reactions

• Deflection curvature

• Stern tube bearing slope boring requirements

• Angular inclination at the main gear wheel

• Shear forces and bending moments

• Allowable loads on all bearings

ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006 11

3.4.1 Influence Coefficient Matrix

The influence coefficient matrix tabulates a relationship among relative reactions in bearings and the unit offset change at each particular bearing (Section 2, Table 1). Accordingly, the influence coefficient matrix can be used to evaluate shafting sensitivity to possible disturbances in the bearing offset and assess changes in the bearing reactions. The disturbances of concern are:

i) Hull deflections

ii) Thermal deviations

iii) Bearing offset adjustment

On the main diagonal of Section 2, Table 1, the influence coefficient matrix also provides information on the gradient of the bearing jack-up curvature. For detailed elaboration on jack-up measurements, see 5/2.1.

3.4.1(a) Hull Deflections. ABS established a method (Section 6 and Section 7) to account for a hull deflection effect on the shafting alignment for several categories of ships (tankers, bulk carriers and container vessels) which will allow designers to estimate hull deflection of those vessels with relatively high confidence. On the vessels where no hull deflection data base is available, the following approach is suggested:

The influence coefficient matrix can be used to assess hull deflection influence on the propulsion shafting. The problem is that the influence coefficient matrix provides information on sensitivity of the shafting, but it gives no indication of the supporting hull structure behavior. Without knowing the deflections of the structure below the shaft line, the propulsion system’s behavior cannot really be evaluated. In other words, some relative value or ratio between the stiffness of the shafting and the structural stiffness of the hull structure is needed, as the interaction of the two will define propulsion system behavior; i.e.:

i) Scenario 1 – Compliant System: Proportionally rigid shafting and hull:

• If the stiffness of the shafting is proportional to the stiffness of the hull structure, theoretically, it is expected for the shafting to follow the flexing of the hull without changing the alignment condition on the bearings.

• Rigid shafting will have large influence coefficients and will be sensitive to very small deviations in the bearing offsets. However, at the same time, the proportionally rigid hull structure will not disturb bearing offset as the deflections of the stern part of the vessel are also expected to be in the negligible range.

This scenario may be a typical case for a smaller vessel.

ii) Scenario 2 – Noncompliant Shafting: Rigid shafting and elastic structure:

This condition is not desired.

• Stiff shafting and a relatively elastic structure will most probably have as a consequence a propulsion system that is highly sensitive to hull deflections.

• Stiff shafting (accompanied with large influence coefficients) will not be able to accommodate relatively large hull deflections and will change reactions significantly as the relatively elastic hull flexes. As the hull bends, the shaft stays relatively undeformed, thus detaching itself from the bearings.

This condition may represent large vessels with very short shafting of a very high power. Arrangements like this, although not desired from the point of view of alignment, are very energy efficient and are most common in VLCC, ULCC and large bulk carrier design.

12 ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

iii) Scenario 3 – Compliant Shafting: Elastic shafting and rigid structure:

This is the desired condition.

• Elastic shafting and a relatively rigid structure will result in a very submissive shafting, which no matter how the hull flexes, remains in contact with the bearings.

If no hull deflection analysis or measurements are available, an engineer can assess structural flexibility through:

• Experience from similar installations

• Basic vessel data (type, length, breath, block coefficient, double/single hull, etc.)

Explanation: Past experience has shown that common propulsion shafting design on VLCCs as well as ULCCs, and similar size bulk carriers of 100,000 dwt and larger, with directly coupled diesel engine propulsion and single propeller have alignments that are very sensitive to hull deflections.

For Scenarios 1 and 3, it is also important to note that even though the shaft remained in contact, the bearing may become damaged if the relative misalignment angle between the shaft and the bearing increases so as to result in edge bearing contact. The edge bearing contact implies a high contact pressure being exerted by the shaft, and a possible dynamic problem as the oil film development may happen at much higher revolutions or not at all in extreme cases.

Moreover, the diesel engine sensitivity to the alignment problem will not change much with vessel design.

A sample influence coefficient matrix, which corresponds to sensitive alignment of a large bulk carrier (per Shaft Alignment Program’s Users Manual), is shown in Section 2, Table 1 below.

3.4.1(b) Thermal Deviations. The effect of thermal deviations is typically local. Temperature change affects a particular bearing offset as the structure that the bearing is attached to is exposed to heating or cooling. The influence coefficient matrix can be directly applied as an investigative tool to assess how this local change affects the bearing reactions.

Example: An oil tank, if located beneath the bearing, may cause a local structural deflection which may change the bearing’s offset. As the offset changes, the bearing reactions may be adversely affected.

This is particularly true in systems where influence coefficients are high.

Temperature changes in the diesel engine and the reduction gear foundation will result in simultaneous offset changes in all of the engine and gearbox bearings. It is often assumed that the thermal condition change will affect all bearings with equal intensity. This is, however, not completely true, as for example, the midsection of the diesel engine will expand more with temperature increase than the forward and after engine structure. One way to compensate for this thermal discrepancy is that a prescribed sagging of the diesel engine bedplate may be applied (see also Subsection 3/5).

3.4.1(c) Bearing Offset Adjustment. Correction of the alignment condition may be necessary when:

• The bearing reaction measurement shows a large deviation from the calculated values, or

• The diesel engine crankshaft deflections do not comply with the engine designer’s requirements, or

• Gear tooth contact is smaller than required by the Rules.

The bearing offset is often adjusted at the intermediate shaft bearing(s). This adjustment has a local effect similar to the thermal changes explained above.

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The application of the influence coefficients is very significant in cases like these as it can provide a very fast tool to verify the effects of bearing offset adjustment on not only the bearing to be corrected, but also on the adjacent bearings.

3.4.2 How to Read Influence Coefficients

As mentioned before, the influence coefficient matrix tabulates the relationship among relative bearing reactions due to the unit offset change at each particular bearing/support. A sample influence coefficient matrix (per Shaft Alignment Program’s Users Manual; Sample Example_01.sha) is shown below:

A sample propulsion shafting system has 12 bearings, one of which (bearing #5) is the thrust (axial) bearing, which is not of particular interest for alignment purposes.

TABLE 1

Influence Coefficient Matrix

RELATIVE BEARING REACTIONS [kN] -> R[0.1-offset]-R[0-0ffset] Due to 0.1[mm] OFFSET relative to the ZERO bearing Offset Node | < 7> < 14> < 27> < 41> < 45> < 46> < 48> < 50> < 52> < 54> < 56> < 58> Supp.| 1 2 3 4 5 6 7 8 9 10 11 12 < 7> 1 | 4.289 -7.896 4.768 -3.222 0.000 2.120 -0.069 0.014 -0.003 0.001 -0.000 0.000 < 14> 2 | -7.896 15.425 -11.112 9.951 0.000 -6.546 0.215 -0.044 0.009 -0.002 0.000 -0.000 < 27> 3 | 4.768 -11.112 13.421 -25.421 0.000 18.858 -0.618 0.125 -0.025 0.005 -0.001 0.000 < 41> 4 | -3.222 9.951 -25.421 166.594 0.000 -250.824 123.820 -25.141 5.104 -1.035 0.202 -0.028 < 45> 5 | 0.000 0.000 0.000 -0.000 0.000 0.000 0.000 -0.000 -0.000 -0.000 -0.000 -0.000 < 46> 6 | 2.120 -6.546 18.858 -250.824 0.000 511.825 -379.402 125.081 -25.395 5.148 -1.004 0.141 < 48> 7 | -0.069 0.215 -0.618 123.820 0.000 -379.402 461.940 -295.733 108.086 -21.910 4.273 -0.599 < 50> 8 | 0.014 -0.044 0.125 -25.141 0.000 125.081 -295.733 389.320 -280.955 104.921 -20.460 2.870 < 52> 9 | -0.003 0.009 -0.025 5.104 0.000 -25.395 108.086 -280.955 386.156 -279.504 100.648 -14.120 < 54> 10 | 0.001 -0.002 0.005 -1.035 0.000 5.148 -21.910 104.921 -279.504 381.883 -259.045 69.538 < 56> 11 | -0.000 0.000 -0.001 0.202 0.000 -1.004 4.273 -20.460 100.648 -259.045 281.235 -105.848 < 58> 12 | 0.000 -0.000 0.000 -0.028 0.000 0.141 -0.599 2.870 -14.120 69.538 -105.848 48.046

Each column provides bearing reactions change corresponding to the relating bearing rise of 0.1[mm] at respective bearing (e.g. column #2 are bearing reactions to 0.1[mm] offset at bearing #2)

Support designation for this particular model:

#1 Aft stern tube brg; #2 Fwd. Stern tube brg; #3 Intermediate shaft brg; #4 to #12 main engine bearing;

Brg #5 is axial brg having all coefficients zero.

The larger the influence coefficient number, the more sensitive a particular bearing will be to the offset change at the respective bearing/support.

The relationship between the offset and the reaction load is linearly proportional, therefore, to obtain the same matrix for 1 mm offset change (instead of 0.1 mm as in the case above) all matrix coefficients shall be multiplies by factor 10.

Experience has shown that the shafting model, as shown in this example, represents a propulsion shafting system which is relatively sensitive to the offset variation (hull deflection changes, thermal influence, etc.). Sensitivity of the alignment shall be judged relative to the stiffness of the structure that is holding the bearings. Namely, flexibility of the double bottom structure may be expected to be higher below the intermediate shaft bearings than below the main engine bearings.

The particularly sensitive area to be evaluated is the interface between the shafting and the main engine. The reason for this sensitivity is the sudden change in the stiffness of the supporting structure between the two.

14 ABS GUIDANCE NOTES ON PROPULSION SHAFTING ALIGNMENT . 2006

Influence coefficients determined for the engine bearings are very high, which leads to the conclusion that the engine is much more sensitive to bearing offset variations than the shafting. This is a correct statement. However, the structure supporting the crankshaft (engine block) is also of a relatively large degree of stiffness compared to the structure of the double bottom below the shafting. Thus, the engine bearings will not change offset significantly as the structure below them is relatively rigid.

Therefore, even though the influence coefficient factors for the engine bearings are high, the deflections within the engine will be smaller for the shafting. Accordingly, the sensitivity to hull deflections may be lower than for shafting. Similarly, for the shafting where influence coefficients are small, the bearing offset change is expected to be high due to a more flexible double bottom, thus, it may result in higher sensitivity to hull deflections.

Why is the engine still very sensitive to hull deflections? Most of the time, it is not because of the engine itself, but rather because of the discrepancy in stiffness between the engine structure and the double bottom structure below the shafting. This may consequently result in the two aftmost main engine bearings (Section 2, Table 1: #4 and #6) being very sensitive to change in intermediate shaft bearing (Section 2, Table 1: #3) offset deviation (e.g., in Section 2, Table 1, the offset raise of 1 mm at the intermediate shaft bearing would unload M/E bearing #4 for 254 kN, and add an additional 166 kN at bearing #6).

Note: Column 5 and Row 5 of the above influence coefficient matrix (Section 2, Table 1) are all zero because the subject bearing represents the axial (thrust) bearing which reaction is not affected by small changes in vertical offset on any of the bearings in the system.

3.4.3 Bearing Reactions

Satisfactory bearing reactions are one of the primary criteria for alignment acceptance. It is difficult to establish an acceptability margin as the factors influencing reaction load are very difficult to predict accurately. Essentially, alignment is acceptable as long as the bearing reactions are always positive (under all operating/loading conditions) and no bearing is unloaded. Any positive static load is therefore acceptable. However, for practical reasons, at least 10% of the allowable load would be desired on the bearing in order to prevent unloading due to unaccounted-for disturbances.

Reaction loads are not the only criteria that are important for alignment acceptance. Relative misalignment between the shaft and the bearings has at least the same importance.

FIGURE 2

Bearing Reactions

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3.4.4…