Shaft Alignment Manual

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Effective December 6, 2006, this report has been made publicly available in accordance with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S. Export Administration Regulations. As a result of this publication, this report is subject to only copyright protection and does not require any license agreement from EPRI. This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication.

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EPRI Project Manager

TR-112449

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THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Rota-Tech, Inc.

ORDERING INFORMATION

Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box 23205, Pleasant Hill, CA 94523, (800) 313-3774.

Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. POWERING PROGRESS is a service mark of the Electric Power Research Institute, Inc.

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This report was prepared by Rota-Tech, Inc. 4104 Cindy Lane Denver, NC 28037 Principal Investigator J. Campbell

Nuclear Maintenance Applications Center (NMAC) 1300 W.T. Harris Blvd.

Charlotte, NC 28262

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner: Shaft Alignment Guide, EPRI, Palo Alto, CA: 1999. Report TR-112449.

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Shaft misalignment has long been recognized as a source of problems for machinery, operators, and owners. Experts agree that correct shaft alignment is one of the most important elements in improving machinery reliability. In an effort to reduce the frequency of machinery misalignment and improve machinery reliability, this guide provides users with an understanding of the

concept of proper shaft alignment through a discussion of the fundamentals of alignment. This guide also provides a limited discussion of machine problems, the impact of misalignment on the machine, and the consequences of misalignment on machine reliability.

Background

Within a nuclear power station, machinery shaft misalignment is responsible for major expenditures in the form of labor, machinery parts, and lost generation capacity. In response, large amounts of time and money are continually invested in state-of-the-art alignment systems, equipment, and training. Most of the training, however, focuses on operating the new systems with little or no training on the principles of proper shaft alignment. Consequently, fundamental problems and causes of misalignment continue to be overlooked, misalignment continues to occur, and machinery reliability is not improved—even after deployment of these costly systems.

Objective

To provide maintenance personnel with a thorough understanding of the fundamentals of proper shaft alignment in order to enhance the use of all shaft alignment systems.

Approach

This guide provides users with a tool to assist in decision making for improvement of reliability associated with rotating machinery, specifically improvements in shaft alignment practices. The document is geared to the “whys” rather than the “hows” of shaft alignment. With the age of most nuclear power plants and the training programs in place, the procedure for the actual task of performing shaft alignments is well documented. The guide could not be written without some specifics of certain alignment tasks; however, a blend of technical information has been the goal in the preparation of this document.

Results

The resulting guide provides a thorough discussion of the fundamental causes and effects of misalignment on machinery and how knowledge of the fundamentals of shaft alignment is crucial to performing consistent, correct shaft alignment.

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Within the power station, shaft misalignment is responsible for major expenditures in the form of lost generation capacity, as well as labor and machinery parts. This guide provides plant

maintenance personnel with fundamental information that will enhance their ability to achieve proper alignment using any shaft alignment system, thus increasing mean time between failures (MTBFs) and improving the reliability of all machinery with each alignment.

TR-112449 Keywords

Maintenance Shaft alignment Reliability

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ACKNOWLEDGMENTS

EPRI would like to recognize the contributions of the following individuals in the development and review of this guide:

William Bramlett Oconee Station/Duke Energy Corp. Deyton Brunson Brunson Instrument Company Michael Calistrat Michael Calistrat & Associates Alistair Campbell Bently Nevada Corp.

Pedro Cassanova Ludeca, Inc. Galen Evans Ludeca, Inc.

Bob Fulbright McGuire Station/Duke Energy Corp.

Jerry Garner Commanche Peak/Texas Utilities Electric Co. Frank Hale Catawba Station/Duke Energy Corp.

Charlie Jackson Consultant

Darron Jones Commanche Peak/Texas Utilities Electric Co.

Randy Kerr PECO Energy Co.

Jon Mancuso Kop-Flex Couplings Richard Massey A-Line Mfg.

Larry Pope Commanche Peak/Texas Utilities Electric Co. Kyle Russell Duke Engineering & Services

Tony Scheetz Commanche Peak/Texas Utilities Electric Co. Deiter Seidenthal Ludeca, Inc.

Dale Smith Smith Services Watson Tomlinson Duke Energy Corp.

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PREFACE

Shaft misalignment has long been recognized as a source of problems for machinery, operators, and owners. Within the power station, shaft misalignment is responsible for major expenditures in the form of labor, machinery parts, and lost generation.

Because experts agree that maintaining correct shaft alignment is essential to improving

machinery reliability, large amounts of time and money are continually invested in state-of-the-art laser alignment systems, equipment, and training. Unfortunately, this solution is much akin to the golfer who thinks “If only I could afford expensive clubs—they would make my game better,” while completely disregarding the fundamentals.

There is no argument that more accurate alignment results can be obtained through the use of these systems; however, the problems that caused the misalignment often continue to be

overlooked, just as they were before the systems were deployed. In light of this, the objective of this guide is to provide a thorough explanation of the fundamentals of proper shaft alignment and to give examples of how this knowledge can improve all shaft alignment practices. Because misalignment is a function of the behavior of the total machine train and system, this guide also includes a limited discussion of machine problems, the effects of misalignment, and the

consequences of misalignment on machine reliability.

The premise of this guide is that practicing shaft alignment with a thorough understanding of the fundamentals can enhance the effectiveness of all shaft alignment systems, resulting in improved machine reliability.

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Thorough Understanding of the Fundamentals of Shaft Alignment ... 2-2 Complete Analysis of the Machine From the Ground Up ... 2-3 Thorough Understanding of the Behavior of the Machine As Part of the System ... 2-3 Thorough Understanding of the Impact of Misalignment on the Machine... 2-3 Proper Sequence of Alignment Analysis... 2-4

3 EFFECTS OF MISALIGNMENT ... 3-1

Types of Machines Aligned in Nuclear Stations ... 3-1 Bearings ... 3-1 Alignment Tolerances... 3-6

4 MACHINE FRAME DISTORTION - SOFT FOOT... 4-1

Soft Foot... 4-1 Measuring Soft Foot Index... 4-2 Distinguishing Types of Soft Foot, Possible Causes, and Proper Correction

Techniques... 4-2 Parallel Air Gap... 4-3 Bent Foot or Angled Base ... 4-4 Taper Shims to Remove Soft Foot... 4-5 Baseplate and Foundation Irregularities ... 4-6 Deterioration ... 4-6 Machinery Vibration ... 4-6 Induced Soft Foot and Nozzle Loads... 4-7

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5 MACHINERY POSITION CHANGES... 5-1

Infrared Thermography... 5-7 Alignment and Preloads ... 5-8

6 SHAFT COUPLINGS AND POWER TRANSMISSION ... 6-1

Flexible Couplings ... 6-1 Restoring Forces and Moments... 6-5 Misalignment ... 6-6 Advantages and Disadvantages of Coupling Types... 6-9

7 VERTICAL MACHINES ... 7-1

Vertical Machines with Rigid Couplings ... 7-1 Vertical Machine Behavior ... 7-1 Causes of Misalignment ... 7-2 Alignment Procedure (Steps 1–9) ... 7-6 Pump Coupling Procedure (Steps A–G) ... 7-7 Alignment Procedure (continued)... 7-8 Conclusions... 7-9

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LIST OF FIGURES

Figure 1-1 Reverse Dial Setup ... 1-2 Figure 1-2 Typical Laser Alignment System ... 1-2 Figure 2-1 Colinear Alignment... 2-1 Figure 2-2 Offset Misalignment ... 2-1 Figure 2-3 Angular Misalignment... 2-2 Figure 2-4 Offset and Angularity... 2-2 Figure 3-1 Ball Bearing (Anti-Friction Bearing) ... 3-2 Figure 3-2 Laser Alignment System ... 3-4 Figure 3-3 Two Misaligned Shafts Using a Spacer Coupling ... 3-5 Figure 4-1 Parallel Air Gap... 4-3 Figure 4-2 Bent Foot or Angled Base ... 4-4 Figure 4-3 Step Shim ... 4-5 Figure 4-4 Pump Base Degradation Resulting From Transmitted Forces... 4-7 Figure 4-5 Examples of Induced Soft Foot ... 4-8 Figure 4-6 Pump Outboard Replacement Keys ... 4-10 Figure 4-7 Key Adjusting Bolts ... 4-11 Figure 5-1 Alignment of Shaft Centerline Heights... 5-2 Figure 5-2 Expansion Chart ... 5-3 Figure 5-3 Acculign Bars Measuring Movement of a Steam Generator Feed Pump ... 5-4 Figure 5-4 Laser Monitoring Movement Between Steam Generator Feed Pump and

Turbine ... 5-5 Figure 5-5 Dynalign (Dodd Bars) Being Used to Monitor Alignment Changes ... 5-5 Figure 5-6 Precision Sight Level Used for Optical Alignment Checks ... 5-6 Figure 5-7 Jig Transit Used for Measuring Alignment... 5-6 Figure 5-8 Scales Used With Jig Transits and Precision Levels ... 5-7 Figure 5-9 Shaft Orbits Acquired From Eddy Current Probes on a Sleeve Bearing

Machine... 5-8 Figure 6-1 Gear Coupling... 6-2 Figure 6-2 Grid Coupling ... 6-3

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Figure 6-6 Point of Moment... 6-7 Figure 6-7 Angular Misalignment... 6-7 Figure 6-8 Angular Misalignment and Offset at P2 ... 6-8 Figure 6-9 Angular Misalignment and Offset at P1 and P2 ... 6-8 Figure 7-1 Typical Stuffing Box, Noting Four Points Where Dimensional Runouts and

Concentricities Are To Be Measured ... 7-2 Figure 7-2 Typical Discharge Head (Motor Stand)... 7-3 Figure 7-3 Alignment Fixture Aligning Motor to Stuffing Box... 7-4 Figure 7-4 Adjustable Coupling Spacer and Nut in a Typical Pump Coupling... 7-5 Figure 7-5 Pump Coupling Indicator Positions... 7-6

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LIST OF TABLES

Table 3-1 Alignment Tolerances... 3-6 Table 6-1 Coupling Advantages and Disadvantages ... 6-9 Table 7-1 TIR Measurements From Installing the Pump Coupling... 7-7 Table 7-2 TIR Measurements From Installing the Pump Coupling... 7-8 Table 7-3 TIR Readings From the Vertical Pump Shaft Alignment Procedure ... 7-8 Table 7-4 TIR Readings From the Final Pump Shaft Alignment ... 7-9

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INTRODUCTION

Simply put, shaft alignment is placing two or more shafts along one axis or centerline. Although proper shaft alignment that increases machine reliability is much more complex, this simplistic approach is how alignment is often performed—with total disregard for the fundamental problems and causes of shaft misalignment.

The widespread use of this approach is shown by the increasing use of laser shaft alignment systems. Management personnel, acting on input from supervisors, technical personnel, and end users, often purchase these systems with the erroneous assumption that the laser shaft alignment system will eliminate their misalignment problems. In fact, it seems that anyone who can align machines relatively easily or quickly using these systems instead of other methods is now considered to have achieved total proficiency in performing shaft alignment, which greatly improves reliability.

In some cases, reliability is improved. This is normal where improper practices were previously performed with methods that have been used for a number of years. An example would be someone performing alignment using the rim and face method and not accounting for sag in the indicator.

In reality, however, this new-found knowledge gives the user and management the ability to perceive that good alignment has been performed by the fact that the given tolerances are quickly and sometimes easily reached. The result is that fundamental problems and causes of

misalignment continue to be overlooked, just as they were before these costly systems were deployed.

The most widely used methods for aligning machines with accuracy are the laser systems and the reverse dial methods. See the reverse dial method in Figure 1-1 and a typical laser alignment system in Figure 1-2.

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Introduction

Figure 1-1

Reverse Dial Setup

Figure 1-2

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Introduction

It is easy to understand the widespread use of laser alignment systems for these reasons:

• They are easy to set up.

• They record data accurately (when correct data are input).

• They give fast and accurate alignment corrections.

• They assist in making machinery adjustments to achieve alignment.

But, if the goal of proper shaft alignment is to improve machinery reliability, then the success of these systems should be measured by whether the mean time between failures (MTBF) increases with each alignment and, if so, by what percentage as compared to before the implementation of the system.

Misaligned machines can be divided into two categories:

• Machines with minor alignment problems

• Machines with major alignment problems

Machines with minor alignment problems exhibit satisfactory run time with a long mean time between failures. These are the machines that, when checked for shaft alignment deviations from specified targets and tolerances, are fairly close to being aligned from the last time they were aligned. These are also the machines that do not show signs of stress, that is, the foundation is in satisfactory condition and has been for some time. Removal of the machine does not reveal piping strain or loads. The bearing temperatures are within range, and vibration typically remains within acceptable ranges for long periods of time.

These machines have several advantages. In many cases, the machinery is not far from ambient temperature. Nozzle loads are small, and soft foot has been corrected if there was a problem. The foundation and grouting were installed properly, and periodic lubrication is performed at regular intervals and in a prescribed manner. These machines have a long run time between failures. The machines with major alignment problems are the ones that, when checked for alignment, are always far out of specifications when “as found” shaft alignment data are taken. If accurate records have been kept, the misalignment is always in the same direction perpendicular to the centerline of the shaft. The offsets and angularities are always somewhat close to the last data that were taken, if conditions were the same.

These machines typically operate under high load conditions, at elevated temperatures, and with piping loads. The foundation reveals signs of deterioration, and the mean time between failures is short. Most of the time used in alignment is spent to put the machine where the correct position is thought to be. Great energy and expenses are expended to correct the obvious problems, but the root cause of misalignment is ignored and the cycle continues. These are the machines that generally have high profits associated with them and the ones that this guide focuses on.

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Introduction

A “failure” is defined as the following:

• “The condition or fact of not achieving the desired end or ends”

• “A cessation of proper functioning or performance”

• “Nonperformance of what is requested or expected”

All of the above definitions can be applied to failure of machinery at a power station. As related to shaft alignment, a failure can imply a degradation of any of the components or subcomponents within a machine or piece of equipment. These failures can damage or destroy couplings, rotors, mechanical seals, or bearings. With proper shaft alignment, machinery reliability is improved, MTBF is increased, and the cost and quantities of replacement parts are reduced.

The new “buzz word” is mean time between repairs (MTBR), which is essentially a MTBF because, if a repair is made, in a sense it is still a failure. Only preventive maintenance can be performed as a repair without a failure. Most maintenance, other than corrective, now being employed at power stations is either predictive maintenance (PDM) or reliability-centered maintenance (RCM).

Remember that, in order to have complete and satisfactory alignment, there must also be

satisfactory coupling alignment. The couplings joining two or more shafts are a part of the rotor systems; however, based on coupling manufacturers’ allowable misalignment, satisfactory coupling alignment does not mean satisfactory shaft alignment.

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2

FUNDAMENTALS OF A PROPER SHAFT ALIGNMENT

Proper shaft alignment is the aligning of two or more shafts to a “colinear position” at operating conditions, while ensuring that the shafts are operating with a minimum of forces applied to the individual shafts and across the coupling. See Figure 2-1 for an example of colinear alignment.

Figure 2-1

Colinear Alignment

How is shaft misalignment defined? There are three types of misalignment conditions:

• Offset - when two shafts are not coincidental to the same axis or centerline (see Figure 2-2)

• Angularity - when one or two shafts move away or toward the centerline as they approach or distance themselves from one or more machines (see Figure 2-3)

• A combination of the two (see Figure 2-4)

Figure 2-2

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Fundamentals of a Proper Shaft Alignment

Figure 2-3

Angular Misalignment

Figure 2-4

Offset and Angularity

Very seldom is one type of misalignment (offset or angularity) present without the other. Since this is the case, the term “offset” better illustrates misalignment not on the centerlines. Some authors prefer the term “parallel” misalignment, but this implies that the offsets are always parallel to each other. For the purposes of this document, misalignment not on the centerlines is referred to as “offset misalignment.”

The essential elements for performing proper shaft alignment include:

• Thorough understanding of the fundamentals of shaft alignment

• Complete analysis of the machine from the ground up

• Thorough understanding of the behavior of the machine as part of the system

• Thorough understanding of the impact of misalignment on the machine

Thorough Understanding of the Fundamentals of Shaft Alignment

Although proper training in the fundamentals of alignment is the first step to achieving satisfactory results, it is by no means the only step. Additional training should include:

• The basic mathematics involved with alignment

• The concept of “soft foot” and all of its variables

• Proper training on the alignment system being used

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• A basic understanding of coupling behavior for the various designs of couplings used

• An understanding how alignment impacts machines of different designs

Complete Analysis of the Machine From the Ground Up

While analysis in this sense does not necessarily mean the use of vibration analysis or infrared thermography, these and other types of data collection techniques do need to be used to gather data to analyze machine problems.

Visual inspection plays a very important part in machine analysis as it relates to shaft alignment. The condition of the baseplate and anchor bolts should be one of the first things inspected when approaching a machine for alignment or analyzing a problem machine where misalignment is the suspected cause of problems. Look closely at the condition of the grout under the base of a machine if this type of base is used.

Look for leaks of water, oil, or other fluids, noting any corrosion to the base that these fluids might have caused. If the machine train consists of a drive steam turbine, look for leaks around the gland steam sealing area. If the baseplate and grout are damaged or deteriorating, this will hinder satisfactory alignment and ensure that the alignment will have to be done again sooner than expected.

If the coupling is a lubricated coupling, look closely for a pattern of lubricant spraying or leaking from the coupling. Depending on the type of guard, sometimes machines using a gear-type coupling can be analyzed for coupling or alignment problems by placing a piece of white cloth or paper on or under the coupling guard. If a pattern of lubricant is visible, you should suspect a coupling problem or misalignment.

Testing shaft runout is an essential part of pre-alignment checks. With the widespread use of laser systems, sometimes shaft and coupling/coupling hub runouts are overlooked. The shafts should be rotated and all runouts taken before aligning the shafts.

Thorough Understanding of the Behavior of the Machine As Part of the System

Look closely at the way piping is routed to and from the machine. Visually inspect the piping support apparatus. Are they adjustable struts, rigid struts, or spring hangers? Getting a feel for the piping route and what the designer had in mind when calculating piping expansion and the direction of expansion can be very important. Is the machine being operated as it was intended?

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machine behavior? Most of the time, misalignment will show up at half speed, but it can also be seen at full speed. Additionally, misalignment can be seen in the axial direction.

Vibration is not proportional to misalignment. (For more information, see Alignment and Preloads in Section 5.) Forcing functions acting on a machine with misalignment can improve vibration levels.

Knowing the history of a machine is vitally important. What maintenance was performed during the last inspection or rebuild? Was the rotor balanced? Has there been a trend of increased

vibration, or was it a sudden change? Has the machine characteristically been a problem? If so, is the machine being operated at or near its design limits? Have conditions changed?

Proper Sequence of Alignment Analysis

All of these questions and many more require answers when diagnosing the cause of machinery problems. A good predictive maintenance program in conjunction with a good root cause analysis program goes a long way toward resolving alignment and machine problems.

Using the proper sequence in alignment is crucial to ensure that if any problems arise from the machine after alignment that they are not due to misalignment or improper alignment, or associated with areas that should have been checked during the alignment process.

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3

EFFECTS OF MISALIGNMENT

Most machines used in the nuclear industry, such as turbines, pumps, fans, compressors and other equipment, are similar to those used in other applications or other industries. This section details how shaft misalignment affects these machines and the consequences of these machines failing for any reason, but especially due to misalignment. Special consideration must be given to the equipment in the nuclear industry due to the circumstances under which it operates.

Types of Machines Aligned in Nuclear Stations

The machines in a nuclear station can be divided into three categories: power production

systems, safety systems, and support systems. Nuclear power production machines do not differ greatly from their counterparts in other power stations. They include:

• Feedwater pumps

• Booster pumps

• Condensate pumps

• Heater drain pumps

• Drive turbines

• Electric motors

• Steam turbines/generators

• Safety pumps

• Fans

It is obvious why failure in these machines is costly; lost generation capacity is the single largest cause of lost income at a power station.

Misalignment can and does affect the support system of machines. This support system consists of the components of the machine itself and the structure that supports the machine.

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Effects of Misalignment

anti-friction bearings are shown during analysis at some frequency associated with the bearing design, such as ball pass frequency or inner race frequency. For this to reveal itself, typically some damage has to occur to the bearing. Over time, the damage will continue to escalate to the detectable point.

Bearing temperature is a good indication that a problem exists. Unfortunately, direct reading temperature instrumentation is typically not installed on anti-friction bearing machines unless they are in a critical application. Even then you are getting only part of the picture because the probe is usually contacting the outer race, and temperature equalization and heat transfer can give some misleading information. If the area of concern or damage is directly on the

temperature probe, indications of a problem are easily seen.

Figure 3-1

Ball Bearing (Anti-Friction Bearing)

The bearing in Figure 3-1 is typical of the radial bearings installed in a majority of pumps that use anti-friction bearings. This bearing and the associated data are for a 6312 bearing.

The amount of clearance in a bearing has a relationship to the amount of misalignment present and the amount of external preload on the bearing and shaft, which is also impacted as the misalignment adds a bending moment to the shaft. Depending on the amount of shaft deflection and the position along the shaft where this deflection occurs, the bearing acts accordingly. Using the above bearing as an example, if shaft deflection or bowing from misalignment or other sources results in the deflection occurring close to the bearing, the bearing must flex a certain amount with the shaft.

The 6312 bearing above, utilizing a C3 fit, has an internal clearance of 23–43 micrometers (µm), or .0009–.0017 inches. This internal clearance is the total distance through which one bearing ring can be moved relative to the other. Dividing this clearance in half gives the radial clearance between the ball and the inner or outer race. This not only includes the radial direction of

clearance, but also the axial direction of clearance. This says that if shaft deflection occurs that causes the inner race to try to skew within the outer race, then on one side of the bearing, the

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clearance is closed, and diagonally across the bearing, the clearance is closed. This generates higher temperatures, the ability to lubricate the bearing has been diminished, and wear occurs at an accelerated rate.

Mechanical seals fail for several reasons that can be associated with misalignment. First, the bearings begin to fail, and then the shaft is allowed to move in relation to the stationary seal face. Exactly how this happens depends on the type of misalignment present and the state of the bearings. Nozzle loads typically affect the stationary seal faces and their concentricity and perpendicularity to the shaft centerline. Nozzle loads can affect the bearing housing, causing deflection of the shaft and the rotating portion of the seal. Trying to discover which seal component is being affected is very difficult. Therefore, when the loads need to be reduced anyway, removing nozzle loads on the machine is very important and is the best method of solving the problem without major analysis and time expenditure.

In theory, the hysteresis— the failure of a property to return to its original value once an applied external force is no longer applied—of the seal component, particularly a pusher or multiple-spring rotating face, is evident as it tries to maintain contact with the stationary face at a given machine speed. This contact and flatness are of utmost importance. Seals are lapped flat, using a measurement of light bands. If seals did not need to be flat to seal, then shaft movement would be irrelevant. The shaft could bend, move axially a great amount, move radially a great amount, and the seal still would not leak. But shaft deflection in the range of .00001 of an inch (0.254 micron) is far greater than the light band range to which the seal is lapped. Surface speed

(surface feet per minute or SFPM) is a factor that, when added to the equation, reduces seal life. This is why smaller shafts at higher speeds can use a seal without leakage better than a machine with a larger shaft.

Most alignments performed on horizontal machines use some sort of laser system. Figure 3-2 below shows a typical laser alignment tool attached to a shaft.

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Figure 3-2

Laser Alignment System

Methods of measuring misalignment and correcting it are varied. Shaft alignment technology has progressed over the years, and systems are now very sophisticated. This does not imply that some of the older methods are inadequate or unacceptable under certain situations.

Some tools commonly used to align shafts are the following:

• Straight edge

• Feeler gage

• Parallel blocks (plain or adjustable)

• Micrometers

• Calipers

• Dial indicators

• Lasers

All of these methods are good, based on the following criteria:

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• Criticality of equipment

• Time involved in completing the job

• The resolution required to achieve satisfactory results

To determine how much time should be spent on shaft alignment in order to achieve satisfactory results, the following questions must be answered:

• How important is the machine to the overall operations and profit of the plant?

• Has the machine been performing satisfactorily in the past, and are records available to prove the reliability of the machine?

• Has the machine been thoroughly analyzed to determine what has an effect on misalignment?

• What is the cost of maintenance for the machine?

• Are parts expensive, and is the machine labor intensive?

• Is the machine presently off-line for maintenance?

• Is the repair or alignment on the critical path?

Figure 3-3, showing two misaligned shafts using a spacer coupling, is indicative of the type of problems encountered in the field.

Figure 3-3

Two Misaligned Shafts Using a Spacer Coupling

NOTE: This diagram is exaggerated for the purposes of illustration.

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coupling angle is just as important because the behavior of the rotors of machines is probably impacted more by the coupling angle than by the shaft misalignment angle.

Alignment Tolerances

Much of the discussion about shaft alignment centers on alignment tolerances. What is close enough to achieve the desired results?

Because of the age of most nuclear power plants, the manufacturer’s literature is sometimes sketchy. Depending on the date of publication, the Operations & Maintenance (O&M) manuals provide alignment targets or tolerances are based on a zero offset and a zero angularity

alignment. Many O&M manuals state something similar to the following:

“The machine shall be aligned within .002” on the rim (offset), and .001” on the face (angularity).“

Usually, no information on the length of spacer is included.

The alignment tolerance information in Table 3-1 applies to both vertical and horizontal shafts. These suggested tolerances are the maximum allowable deviations from desired values (targets), whether such values are zero or nonzero. Use them in the absence of in-house specifications or tighter tolerances from the machinery manufacturer. Normally, the columns labeled Excellent apply to all alignment work. The exception is rough machinery designed to vibrate, such as ball mills, shaker screens, hammer mills, and so on. For such machinery, the information in the Acceptable columns can be used.

Table 3-1

Alignment Tolerances

Courtesy of Ludeca, Inc.

Tolerances for Shaft Alignment

Short Couplings Spacer

Shafts

Excellent Acceptable Exc. Accpt.

Offset Angularity Offset Angularity RPM mils mils per

inch

mils per 10"

mils mils per inch mils per 10" mils per inch mils per inch 600 5.0 1.0 10.0 9.0 1.5 15.0 1.8 3.0 900 3.0 0.7 7.0 6.0 1.0 10.0 1.2 2.0 1200 2.5 0.5 5.0 4.0 0.8 8.0 0.9 1.5 1800 2.0 0.3 3.0 3.0 0.5 5.0 0.6 1.0 3600 1.0 0.2 2.0 1.5 0.3 3.0 0.3 0.5 7200 0.5 0.1 1.0 1.0 0.2 2.0 0.15 0.25

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4

MACHINE FRAME DISTORTION - SOFT FOOT

Machine frame distortion or casing deflection is a topic that deserves considerable discussion and is the most overlooked area during the process of shaft alignment. It is also the area that can pay the highest returns for reliability of machinery when associated with proper shaft alignment. Frame distortion can be divided into three categories:

• Soft foot

• Baseplate and foundation irregularities

• Induced soft foot and nozzle loads

Soft Foot

Soft foot in the classic sense is the flexing or bending of the frame foot when it is tightened to the base. Any gap that exists is reduced with this tightening, and forces are applied to the frame or casing. These forces result in casing deformation and in some interaction between the stationary and rotating parts of the machine.

In most cases, a complete check for soft foot is not done. The machine to be moved (MTBM) is usually checked, but the stationary machine is seldom checked for soft foot. This, in turn, continues to result in problems with the stationary machine. For example, consider the comparison of turbine-driven feedwater pumps to motor-driven pumps at the power station. When performing alignment on turbine-driven pumps, typically above 10,000 HP, there is a tendency to move the pump. In doing so, it quickly becomes evident just how much soft foot is present due to classic soft foot problems or piping-induced soft foot. When aligning motor-driven machines and the motor is the MTBM, the machine is seldom checked for soft foot. Rarely is the machine ever unbolted from the pedestal or base.

This is an area where much progress could be made in improving the reliability of the machines. Instead, often the distortion due to piping-induced soft foot on the driven machine is mistaken for other problems. One such problem is the contact of wear rings in overhung horizontal pumps, such as American National Standards Institute (ANSI) or American Petroleum Institute (API) pumps. Often radial reaction of the rotor and impeller are blamed for this when it might be due to distortion, particularly at higher temperatures.

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Machine Frame Distortion - Soft Foot

Measuring Soft Foot Index

It is common to measure soft foot by mounting an indicator on the machine base so that the stem is on top of the foot, but this method has numerous drawbacks. Depending on how the foot is bent, the indicator can under indicate the soft foot or even read zero when there is significant soft foot. This will leave the machinery installer with the mistaken idea that the bolt and foot are okay, when, in fact, harmful distortion exists in the machine frame.

The indicator can also indicate soft foot that does not move the shaft centerline and, therefore, can be ignored. This is usually the case when the shims and feet are very large compared to the size and the load area of the bolt. If the load area is correctly supported, the rest of the foot can move a considerable distance vertically without any distortion passed to the machine frame and bearings. The success of the indicator-on-top-of-the-foot method is highly dependent on machine geometry. For example, a foot movement of 0.002 inches (0.05 mm) has entirely different

consequences for machines with only 6 inches (15.24 cm) between the feet than for those with 40 inches (101.6 cm) between the feet.

In short, the indicator-on-top-of-the-foot method misses significant soft feet, gives false alarms, is highly affected by machine size, and is, in general, a poor method of measuring machine frame distortion. A much more indicative value can be obtained from measurements taken at the shaft. Because soft foot really means machine frame distortion, any system that purports to measure it must some how quantify distortion in the machine frame. One simple, yet effective, way to do this is to determine if the bearings moved when the hold-down bolts were tightened. If the bearings did not move when the hold-down bolts were tightened, then there was little or no machine frame distortion and certainly not enough to influence the rotor. On the other hand, if the bearings did move due to the hold-down bolts being tightened (or loosened), then the frame is sufficiently distorted to affect the running position and, subsequently, the condition of the rotor.

For most, if not all, industrial machines, it is geometrically impossible for a single bolt to distort a machine frame in such a way that both bearings move, resulting in a shaft movement that is pure parallel displacement. Furthermore, if only one bolt is inspected at a time, any motion in the nearest bearing will be largely vertical. In fact, purely horizontal movement of a bearing due to tightening a single base bolt is geometrically impossible. To summarize this:

• Machine frame distortion from tightening (or loosening) a single bolt always induces a change in the vertical shaft angle.

• Machine frame distortion can be measured by quantifying change in the vertical shaft angle.

Distinguishing Types of Soft Foot, Possible Causes, and Proper Correction Techniques

Not all soft feet are the same. They are caused by a variety of conditions, some of which might not even be related to the machine itself. However, as a rule, all soft feet are bad and should be

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no general purpose, one-kind-cures-all soft foot correction. If your soft foot check indicates a soft foot, fix it. However, be careful to analyze the type and cause of the soft foot before doing anything. Indiscriminate shimming or trial and error will most often makes things worse. Although a high-resolution laser system can be used for gauging the amount and effect of the soft feet, it cannot determine the cause nor the corrective action. The soft foot mode of a high-resolution laser system senses shaft deflection caused by soft foot, accurately and reliably. These systems display a soft foot index that is an absolutely reliable indication of machine frame distortion (soft foot). They do not display the correction for the amount of machine frame distortion detected.

There is no device yet made that can measure soft foot at the shaft or on top of the foot and accurately analyze the necessary correction. Shaft-mounted devices, even the best laser systems with soft foot functions, are not “gap meters,” nor are they “shim meters.” This is a limitation of all measuring systems that are mounted anywhere except between the bottom of the foot and the top of the base. In other words, if you are not measuring in the gap under the foot, you are not measuring the gap under the foot.

Cause and corrective action can be determined by using feeler gages, which are essential for removing soft foot. Proper feeler gage technique for a single foot is to measure the gap under all four corners of the same foot. From these four readings, an excellent idea of the shape of the gap can be developed. The maximum allowable soft foot typically is 0.002 inch (50 µm), although a real effort should be made to obtain zeros.

Parallel Air Gap

Condition: This is the mental picture most often associated with the word “soft foot.” It is where

three feet sit solid and flat, but one foot does not touch (see Figure 4-1). A feeler gage will find an equal gap at all four corners of the foot. Contrary to common assumption, this type of soft foot is quite rare. Note that the foot diagonally opposite will show soft foot, but a smaller amount. It is impossible to have three parallel air gaps on one four-footed machine. Likewise, it is impossible to have two air gap feet side by side. They must be diagonally opposite each other.

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Causes:

• One leg is too short.

• One baseplate mounting pad is not coplanar with the other three.

• Inadequate shims are under one foot.

Correction: Add shims to equal the amount shown on the feeler gage. Do not fall into the often

unproductive trap of attempting to divide the shims with the diagonally opposite foot. The laser system readings of the four feet indicate relative coplanarity of the feet. This, in accordance with the feeler gage results, will often show that three of the feet are largely coplanar, and the fourth foot is clearly the one to be shimmed. If the laser system shows two diagonal soft feet with the same value and the feeler gage gaps are the same across the diagonal, then shim both feet. With experience, both diagonally opposed soft feet can sometimes be shimmed according to the coplanarity of the four feet, but this is not recommended. It is far better to shim one foot and take the readings again. You will often find that the problem is solved.

Bent Foot or Angled Base

Condition: The bottom of the foot is not coplanar with the base. It has feeler gage readings that

clearly show a slope from one corner of the foot to another. Often, but not always, one corner or one side of the foot is touching the base, and the foot acts as a lever when bolted down (see Figure 4-2). Because of this, the bent foot usually induces soft foot in the two opposing feet and sometimes in the fourth foot as well. This gives the machine the appearance of having three or four soft feet, but they will all go away when the bent foot is corrected.

Figure 4-2

Bent Foot or Angled Base

Causes:

• Machinery that has been dropped or handled roughly

• Bent or poorly machined baseplates

• Severe angularity misalignment

• Feet that have been welded

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Correction: Re-machine the feet, the base, or both; or build a step shim. Step shims are a

field-proven method that is both safe and effective. The idea is to build a set of steps that match the slope of the foot (see Figure 4-3).

Figure 4-3 Step Shim

The procedure to build a step shim is as follows:

1. Fill any gap that exists under the entire foot so that one corner or edge of the foot is touching the shim.

2. Measure the largest remaining gap.

3. Divide this gap by 4, 5, or 6 (the number of steps) to obtain the step thickness.

4. Select 4, 5, or 6 shims of the step thickness and insert them one step at a time. Without lifting the machine, insert them by hand only until they are snug.

Some adapting of the method is required for feet that are bent diagonally or skewed. Each vertical shim correction to the foot requires the steps to be rebuilt. Do not expect the steps to fit back in exactly the same way after shimming because vertical angularity corrections affect the slope of the feet. After the final alignment, trim and discard the excess portion of the step shims, as shown in Figure 4-3.

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with the step shimming method. However, there are also disadvantages associated with this method. First, if you are trying to keep the number of shims to a minimum, the taper shim adds to the total shim thickness being used and must be accounted for on all feet. A minimum shim thickness of 1/16 inch (1.6 mm) is usually required to fabricate a step shim. Second, the shim typically requires fabrication from carbon steel in order to be clamped to a magnetic chuck on a surface grinder.

Baseplate and Foundation Irregularities

Baseplate or pedestal condition and the foundation play an integral part in the ability of machines to remain in an aligned condition. The baseplate to foundation interface, which is typically grout, is an area that deserves scrutiny. Because many problems can occur within baseplates and

foundations that affect the operability of the machine, baseplate conditions play an integral role in shaft alignment.

Deterioration

The baseplate and foundation might be subject to certain environmental conditions that cause them to become unstable or deteriorate over time. The grout can begin to flake, erode, or

crumble due to the environment to which the foundation is subjected. Deterioration is most often found in chemical plants, but it can occur in almost any situation. Continuous cleaning with large quantities of water can also have an adverse effect on the grout and metal portions of baseplates. Over time, rust and erosion of the grout can weaken the baseplate or foundation. If forces are present from piping-induced loads, the baseplate reacts to these forces and can move

accordingly. Piping-induced or nozzle loads can cause the grout to crack.

Machinery Vibration

The problems associated with vibration are twofold:

• The first problem deals with the vibration caused by shaft misalignment. Vibration does not always reveal misalignment. Several factors are involved, such as stiffness of the bearings and the machine support structure. The external and internal preloads imposed on the machine can dampen the vibration.

Soft foot, regardless of the type, can cause vibration. This is easily revealed when a soft foot bolt is loosened and the vibration decreases.

• The second problem involving vibration is in the baseplate and supporting structure. Baseplate looseness and grout cracking or dusting is evidence of high nozzle loads or high cycle fatigue. An example of high cycle fatigue would be predominantly high vane passing frequencies on a centrifugal pump. This high frequency vibration for an extended period of time begins to fatigue the grout and baseplate. Shaft alignment is hard to maintain or correct without going to the root cause of the problem.

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The fatigue can manifest itself in the form of broken parts on the machine or even destruction of the baseplate over time. The grout is usually one of the first areas to show damage

(see Figure 4-4), such as dusting or crumbling under vibratory loads.

Figure 4-4

Pump Base Degradation Resulting From Transmitted Forces

Induced Soft Foot and Nozzle Loads

For the purpose of this guide, nozzle loads are divided into two categories: induced soft foot which creates nozzle loads in the vertical direction, and nozzle loads for forces and moments in the horizontal direction.

Condition: A high-resolution laser alignment system shows soft foot, usually two feet on same

side or same end of machine, and a feeler gage finds a gap, usually parallel or nearly parallel. A secondary symptom is that the foot does not get better (it becomes worse) or another foot becomes much worse after shimming the gap amount (See Figure 4-5).

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Figure 4-5

Examples of Induced Soft Foot

Causes: External forces on the machine. Coupling strain and pipe strain are the two most

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• Overhung machines or attachments

• Belt or chain loads

• Gears

• Any overhung or extended shafts

• Hoses

• Flex conduit

• Structural bracing attached to the machine

• Jacking bolts inadvertently left tight

Correction: Remove the source of the force. Note that a high-resolution laser alignment system

makes a good tool for testing for pipe strain during construction. Attach the laser alignment system and enter soft foot mode before attaching any pipe, but with all base bolts tight. Remove any soft foot from the machine and leave the laser alignment system set up to read one foot (any foot). Tighten the bolts on the piping. If the laser alignment system records more than 1.5 mils (37 µm) movement while the flange bolts are being tightened, the piping is straining the

machine. As further proof, test again for soft foot while the piping is tight, and compare the data to pre-piped soft foot conditions.

Nozzle Loads

Typically, nozzle loads are on the driven machine, unless the driver is a steam turbine (forces from the piping are transmitted into the pump casing or other type machines and continue on into the foundation or baseplate). Remembering that for every action there is a reaction, it is easy to see why the large forces applied through the piping can destroy a baseplate quickly. As the baseplate gives up some of its strength, the loads imposed on the machine tend to move the machine more freely. The result is misalignment and the possibility of catastrophic failure of all machines in the train. Large, high-energy pumps, such as feedwater pumps, can use keys to limit movement of the casing. This does not reduce the stresses in the casing or reduce distortion from nozzle loads or piping strain. It changes only the location where the forces and stresses enter (and leave) the casing.

The forces and moments in the horizontal aligning direction are a very important area of concern associated with nozzle loads. These forces tend to move the machine in the horizontal direction in much the same manner as the induced soft foot moves the machine in the vertical direction. In most cases, a moment is also associated with these forces.

When trying to align a machine with this problem, moving the machine requires a great deal of force to bring the machine into alignment. By applying this force against the opposing forces, casing distortion is added to the problem. As temperatures increase in the machine, the machine

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Over time, these forces manifest themselves in the form of rubs internal to the machine, pedestal and baseplate damage, and grout failure. These horizontal forces and moments are best detected by loosening all hold-down bolts, removing casing keys if they are present on the subject machine, and loosening on all jacking bolts if they have been tightened in an effort to maintain alignment. By observing where the machine moves and the forces required to move the pump back into position, information can be gained about the forces present.

Large pumps, such as feedwater pumps, often use keys to limit movement of the pump and direct thermal growth in a particular direction. The typical design uses a pin on the coupling end of the pump and a rectangular key on the thrust end of the pump. The pin on the coupling end provides for very limited movement in the horizontal and axial direction. The key on the opposite end of the pump allows for thermal growth in the axial direction and limited movement, usually less than 5 mils (0.1mm) in the horizontal lateral direction.

When pumps such as these are loosened from the baseplate in an effort to detect lateral forces and moments about the axis of the nozzles, these keys must be loosened or removed. Many times, this requires grinding welds that hold the keys or key blocks in place. Figure 4-6 shows one station’s resolution to achieving horizontal moves and remedying lateral or horizontal forces and a “twist” in the pump. Figure 4-6 shows the modified key block used with adjustments, to be made after alignment, and the welded jacking brackets.

Figure 4-6

Pump Outboard Replacement Keys

Jacking Bolts or Taper Pins

Machine frame distortion can also be caused by dowel or taper pins used to limit the movement of the machine. Jacking bolts are used occasionally in alignment to limit the movement of

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Figure 4-7

Key Adjusting Bolts

In many instances, the taper pin (or in some cases straight type fitted dowels) is used improperly to limit the movement of machines. Analysis of the total axial growth of machines should be performed to determine the fixed end of the machine, and the taper pins should be installed on that end of the machine. Installing taper pins diagonally from one end of the machine to the other increases the probability of casing distortion at elevated temperatures. The manufacturer or a reliable consultant should be consulted if there are questions concerning doweling the feet of a machine.

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Gaps Without Soft Foot

Condition: Sometimes, a foot that does not show soft foot by high-resolution laser alignment

system readings will have a rather large gap under it when the bolt is loose and no gap when the bolt is tight. Another version of this same phenomenon is that the laser alignment system indicates a relatively small soft foot, but the feeler gages find a much larger gap.

Causes:

• The base is moving.

• The foot is bending without bending the machine (weak or flimsy feet).

• The base or machine is cracked, loose, or otherwise defective.

Correction: Even given the absence of soft foot in this condition, most people choose to shim

the gap, although it rarely improves the running condition of the machine. The base or machine must be repaired or rebuilt to eliminate this condition. Note that this condition often has the side effect of making vertical alignment corrections very unpredictable or even impossible. If the machine can be aligned (it responds to vertical corrections) and is not loose or broken, then the gap can often be ignored. If the tightening and loosening of the foot’s base bolt does not affect the shaft centerline (no soft foot reading), then the bearings are not being moved or distorted, so no harm to the rotor, bearings, or coupling occurs.

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5

MACHINERY POSITION CHANGES

The running alignment position of machines is the aligned position of two or more machines relative to each other at running conditions, which differs from the cold or shutdown aligned position. Often, machines are misaligned in a shutdown condition in hopes that the alignment during operation (or under “running conditions”) is within the acceptable tolerances for that machine. While this is often referred to as “hot alignment,” there are others who define hot alignment as mounting alignment equipment and capturing data immediately after machines are taken off line. Although this method is probably better than nothing if certain rules are followed, it is not very accurate and is about the same as guessing where the alignment of the machines should be.

There are several methods that provide alignment criteria for machines to be misaligned in the cold condition and achieve alignment during operation. These methods are listed below in order of least to most accurate:

• Guess where the machines should be aligned

• Shut down and perform the alignment

• Use the manufacturer’s recommendations

• Monitor the machines from one condition to the other

The manufacturer of a boiler feedwater pump and the manufacturer of a drive turbine tend to give information based on their respective machines. This information is based on either monitored or calculated data, and this data is typical for “thermal growth” considerations of the respective machines. Most often, you will get numbers from the manufacturers that represent some vertical change in the machines relative to ground or to another machine. Some horizontal change might also be provided.

More often than not, however, the horizontal misalignment targets are far from being what the manufacturer of the machines provides or what can be calculated at the power station.

Monitoring alignment changes allows you to determine targets in the horizontal direction. Horizontal types of misalignment are generally due to piping forces (either static or dynamic) that prevent the pump from being aligned where desired or move the pump after startup.

Large pumps present problems in this area. Many large pumps have keys or a combination of pin and key along the bottom of the pump casing. Some pumps even have keys radially projecting

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Machinery Position Changes

When targets reveal that a pump must have a given amount of horizontal angularity or horizontal offset, provisions must be made to modify these keys from their as-built configuration (See Section 4).

Figure 5-1 is a graphical representation supplied by one pump manufacturer of the calculated thermal growth of a motor-driven pump with a gear box.

Figure 5-1

Alignment of Shaft Centerline Heights

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Figure 5-2 Expansion Chart

Typically, the horizontal changes supplied by the manufacturer are not close to what is encountered in the field when the machines in question are acting together with the entire system. Although the vertical changes may occasionally be within the range provided by the manufacturer, the horizontal changes seldom are.

The terms “hot alignment” and “thermal growth” do not disclose the complete story of running position alignment. The most accurate terminology is “transient alignment monitoring” because it best describes what running position alignment is about. You must monitor the alignment changes under all conditions to establish an understanding of the behavior of the machines. Capturing alignment changes within all operating parameters gives you an opportunity to explore these changes.

Machines can be monitored starting with the machine cold (at shutdown) and monitoring the changes as the machine reaches its operating condition. Monitoring can also be performed starting with the machine at operating conditions and monitoring to shutdown. Monitoring can also take place anywhere in between, if certain parameters are to be observed without

determining the full amount of changes of the alignment.

The preferred method is to monitor the machine from operating conditions to shutdown. This enables you to analyze and implement the alignment data during shutdown and observe the

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There are two concepts involved in monitoring machine movement and arriving at the alignment targets. For the purpose of clarity and convenience, these methods can be referred to as absolute and relative monitoring.

Absolute monitoring involves the technique of measuring one machine from a fixed reference point from the ground, such as a column, wall, or floor. The types of monitoring equipment that do this include precision sight levels and jig transits, Acculign bars, and Jackson cold water stands.

The relative alignment methods monitor the changes in alignment between two or more machines. The equipment used for relative monitoring includes Dynalign or Dodd Bars, Permalign lasers, and rotating Vernier gages. When absolute measurements are compared between two or more machines, these measurements can also be referred to as relative. See Figures 5-3 through 5-8 for pictures of various types of monitoring equipment.

Figure 5-3

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Figure 5-4

Laser Monitoring Movement Between Steam Generator Feed Pump and Turbine

Figure 5-5

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Figure 5-6

Precision Sight Level Used for Optical Alignment Checks Source: Brunson Instrument Company

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Figure 5-8

Scales Used With Jig Transits and Precision Levels Source: Brunson Instrument Company

Infrared Thermography

Infrared thermography can play a very important part in analyzing misalignment. Thermography can detect problems through temperature rises in the couplings or bearings. It cannot distinguish the amount of misalignment, only the results of misalignment. In many cases, this is just as important as measuring the amount of misalignment.

Used together, data from both infrared thermography and transient alignment monitoring systems can be very informative to the technician, as well as to management personnel who may need to see more evidence of problems in order to allocate resources necessary to resolve the problems. Past studies involving alignment analysis have determined that some previously held beliefs about misalignment are not necessarily true, including the following:

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Both of these have been used in the past as major selling points for alignment hardware companies.

Alignment and Preloads

A preload is a directional load or force applied to the rotating shaft. Two categories of preloads are internal and external. Internal preloads deal with forces generated from within the machine and go far beyond the scope of this guide.

Only the external type of preload exists for shaft misalignment. There are other types of external preloads that interact with or impact the structure or casing of the machine, including piping loads (forces and moments) and soft foot. The immediate effect of a preload due to misalignment is to force the shaft into one sector of a bearing. A strong indication of preloads, both magnitude and direction, can be determined with the use of proximity probes (x and y) close to the bearing. These preload data are in the form of shaft orbits.

The use of bearing metal thermocouples in conjunction with shaft orbits and infrared

thermography can yield excellent results in determining if misalignment exists. This is easily detected and brought to light in a machine, such as turbine generator train, or a high-energy pump, such as a feedwater pump. The vibration might be low on one bearing accompanied by a high temperature, while the adjacent bearing will have a higher vibration and lower bearing temperature.

The amount of preload can be related directly to the amount of misalignment. Spring-type couplings, such as a diaphragm coupling, exhibit the least amount of preloads on a bearing and its supporting structures, while a rigid-type coupling will impose the most preload.

In Figure 5-9, a circle or ellipse, as shown in the first two orbits, is the norm when no

unidirectional loading or preloads are present. As you move across the page, greater preloads are encountered. The last orbit is where the shaft is located in the bottom of the bearing due to a large amount of misalignment, and the results can show up as twice the shaft speed. An elevation in bearing temperatures can also accompany this scenario. Remember, there are other things that can cause increased vibration. Misalignment occurs perpendicular to the shaft orbit and forces the orbit to flatten; thus, the sensors perceive this as twice the running speed.

Figure 5-9

Shaft Orbits Acquired From Eddy Current Probes on a Sleeve Bearing Machine

A phase difference of 90 degrees between x and y probes should be theoretically true; however, two probes can show a phase difference of 180 degrees. A steady-state preload will cause the shaft to move eccentric to a position within the bearing. This type of orbit is seen most often in

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lower temperature in one bearing and creating an opportunity for an unstable shaft, which may result in oil whirl.

These external preloads can also be due to the misalignment itself. Piping strain and soft foot pose another problem with casing deformation. You have a choice of where you want these forces to enter the pump. They can enter through the piping or through the keys and supporting structures of the pump casing. These forces can also be transmitted into the structure supporting the pump, such as the base and grout of the machine (see Figure 4-4).

Smaller machines with anti-friction bearings pose special problems with preloads. Detection might need to be performed with infrared thermography, as well as vibration analysis, to detect preloads that have an effect on alignment and reliability of the machines. Machines that use a gearbox for speed changes will have preloads associated internally with the machine, which act upon alignment while the machine is in operation.

While piping strain impacts alignment, it also impacts the wear of parts. Piping strain as it relates to misalignment is often overlooked. This is particularly true in the horizontal direction. What appears as a minor amount of piping growth to the piping designer can be a major amount to rotating machinery personnel. Piping growth due to heat can have a severe impact on the misalignment of machines. Some of this misalignment can be accounted for with transient alignment monitoring and corrections.

Cold piping strain in the horizontal direction must be accounted for and remedied as stated in the Induced Loads section. Radial and axial pump keys under the casing do not eliminate these loads; they just enter the casing from another location. The forces against the keys constrain the pump, but they add to the loads on the casing just the same. These keys may require modification from the original installed position.

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6

SHAFT COUPLINGS AND POWER TRANSMISSION

Shaft couplings and power transmission go together with shaft alignment. Shaft alignment is seldom performed without the opportunity at least to look at the flexible shaft coupling that connects the machines being aligned. During shaft alignment, machines with lubricated couplings are generally inspected or preventive maintenance is performed. Dry-type couplings also require inspection for damage such as fatigue or cracking.

This guide does not attempt to discuss all the details of shaft couplings and how they are

designed, applied, and used. Instead, they are briefly discussed in this section in the areas where they play an important role in the process of aligning shafts and in the behavior of machines due to shaft and coupling misalignment.

Flexible Couplings

A flexible coupling transfers or transmits power from one machine to another and makes accommodation for some shaft misalignment. There are two types of flexible couplings: one allows for misalignment by sliding, the other by flexing.

Typical couplings that allow for misalignment through sliding are gear-type couplings and flexible grid-type couplings. The misalignment of two shafts is accommodated during rotation of the shafts by the sliding of gear meshes (see Figure 6-1) or, in the case of the grid coupling, the grid to the grooves in the hubs. The grid coupling also has some bending involved, but this is used for torsional loading.

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Shaft Couplings and Power Transmission

Figure 6-1 Gear Coupling

Courtesy of Falk Corp.

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Figure 6-2 Grid Coupling

Courtesy of Falk Corp

Couplings that allow for misalignment due to bending are flexible diaphragm couplings and flexible disk couplings. The diaphragm coupling can use a single steel diaphragm or a

convoluted diaphragm made up of several layers of thin flexible steel plates. Figure 6-3 shows a diaphragm coupling.

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Figure 6-3

Diaphragm Coupling

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Figure 6-4

Flexible Disk Coupling

Restoring forces are very important because all couplings resist being misaligned. The coupling tends to try to run in a straight direction, and this imposes preloads on the shaft, trying to force the shaft into a particular sector of the bearing.

Restoring Forces and Moments

You should be aware of coupling behavior in misalignment. All couplings resist being

misaligned and try to operate in a non-misaligned condition (hence the term “restoring forces”). These forces act on the shafts in the form of a moment arm trying to bend the shaft and, in doing so, adding stresses to the shaft. Misalignment under these conditions can fatigue a shaft (and/or coupling) and eventually result in failure (see Figure 6-5).

Resistance to being misaligned occurs only under conditions where torque is transmitted and not in a standstill condition. When machines are borderline aligned and shafts move into a region or area of misalignment while torque is being transmitted, coupling lockup can occur on gear-type couplings.

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Figure 6-5

Stub Shaft Replacement

Misalignment

Coupling misalignment differs from shaft misalignment. Coupling alignment or misalignment is the angle in degrees from the axis of one shaft to the axis of another shaft. The coupling

manufacturer usually provides allowable coupling misalignment in terms of degrees of

misalignment. If the manufacturer gives a number in thousandths of an inch (0.001=25 µm) for offset, it is the measurement of the distance between flex planes of the coupling times the tangent of the allowable angle of misalignment.

Taking into consideration the restoring forces of the coupling and the bending moments acting on the shafts, a rule of thumb can be proposed. If the misaligned shafts are graphed on paper with the proper scaling and a line is extended from the centerline of one shaft to a point of intersection on the opposing shaft, this is the point where the moment occurs (see Figure 6-6).

Shaft Alignment Manual

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ACKNOWLEDGMENTS

PREFACE

CONTENTS

LIST OF FIGURES

LIST OF TABLES

1

INTRODUCTION

2

FUNDAMENTALS OF A PROPER SHAFT ALIGNMENT

3

EFFECTS OF MISALIGNMENT

4

MACHINE FRAME DISTORTION - SOFT FOOT

5

MACHINERY POSITION CHANGES

6

SHAFT COUPLINGS AND POWER TRANSMISSION