basics of alignment.doc

7/27/2019 Basics of Alignment.doc

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The Basicsof Alignmentand Balancing

Abstract

Misalignment and imbalance are two of the most common faultsfound in rotating equipment. Understanding how to properly diagnosisand correct for misalignment or imbalance in plant equipment andhow to deal with common pitfalls while out in the field is essential indoing the job right the first time.

This paper discusses the importance of precision shaft alignment andbalancing, including the negative effect on bearing life. Technical

information on balance and alignment theory, including the properdiagnosis of misalignment or imbalance, will be reviewed.Alignment BasicsThe alignment of shaft centerlines on coupled machines is one of themost important aspects of machine installation. Contrary to popularopinion, flexible couplings will not always compensate for evenmoderate amounts of shaft misalignment. Misalignment is anycondition in which the shaft centerlines are not in a straight lineduringoperation.Misalignment generates unnecessary forces. Precision alignmentremoves these forces resulting in reduced vibration and noise levels,minimized shaft bending and cyclic fatigue, reduced energy costs,and increased bearing, seal, and coupling life.

Shaft centerline misalignment can be classified as either angular oroffset (also called parallel). Angular misalignment occurs when theshaft centerlines meet at an angle. Offset misalignment occurs whenthe shafts are parallel, but offset from each other. The misalignmentmay be vertical, horizontal, or a combination of the two. Most shaftmisalignment is a combination of both angular and offsetmisalignment. Figure 1 graphically illustrates the alignment types.


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Figure 1: Types of Shaft Alignment

Another type of misalignment not associated with couplings is bearingmisalignment. The centerlines of two coupled shafts can be properlyaligned, but the bearings on one side of the coupling may bemisaligned. Bearings can be misaligned if they are not mounted in the

same plane; if they are not normal to the shaft, i.e. they are cockedrelative to the shaft; or because of machine distortion due to soft foot,an uneven base, or thermal growth.

Economics of Misalignment

There are a number of cost benefits of precision alignment. It canhelp reduce plant operating costs by reducing energy costs. Precisionalignment also results in increased maintenance savings throughreduced parts consumption and reduced overtime. Finally, it can helpdecrease equipment downtime and increase product quality.There has been much debate in recent years concerning the energycosts that alignment can save a facility. In theory, an aligned machine

needs less power to perform its job. Typically, a plant that moves toprecision alignment can reduce their energy costs from 3% to 10%.Energy savings can be calculated from the following formulas.


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Savings = Hours * kW * Cost

Pf = Motor Power FactorHours = Yearly Hours of Operation

Cost = Cost of Electricity

A recent study performed at the University of Tennessee found thateven small amounts of misalignment could significantly reducebearing life. The study found that if, on average, a motor was offsetmisaligned by 10% of the coupling manufacturer's allowable offset,there was a corresponding 10% reduction in inboard bearing life.Furthermore, if a motor was offset misaligned by 70% of the couplingmanufacturer's allowable offset, there was a corresponding 50%reduction in inboard bearing life (Hines et al) . The results of the studyare summarized in the following table.

Table 1: Offset Misalignment and Bearing Life (Hines)

Alignment TolerancesAlignment tolerances have often been treated with a halfhearted "just

get it close" attitude. But, alignment tolerances are actually themeasurement of a job well done and they provide the definition ofwhat close actually is.There are two reasons to use tolerances. The key reason is toestablish goals. If you do not have a goal, how do you know when the

job is finished. If there is not a goal, there cannot be a quality


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alignment. The second purpose of alignment tolerances is toestablish accountability. Accountability is the evaluation of alignmentquality. If there is no tolerance to compare an alignment to, how canthe quality of the alignment be judged? Accountability can createcompetition, driving a mechanic to get the job done better.

An early attempt to establish tolerances was probably Total IndicatorRunout, TIR. Many equipment manufacturers still use TIR in theirinstallation and maintenance manuals. However, TIR can beconfusing and misleading. As shown in Figure 2, 4 mils TIR is aworse alignment condition for Case A than for Case B because of thedistance between fixtures.

Figure 1: TIR Tolerance

Another commonly used tolerance is machine moves. Using machinemoves as a tolerance can also be very confusing. Even though themove at one of the feet in Case A in Figure 3 is double digits, theoffset at the coupling is 1 mil. In the second case, the machine movesare 3 mils at both feet but the coupling offset is 7 mils. The angle isthe same in both cases. The alignment in Case B is worse than thealignment in Case A.


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Figure 3: Machine Moves Tolerance

A more accurate tolerance is offset and angle. The offset is measuredas close to the power plane as possible. Offset and angle are notdependent on the size of the machine or on the

position of the fixtures. This is not true for TIR or machine moves.

True alignment consistency can be achieved because one set oftolerances can be used plant-wide.Another type of tolerance is jackshaft tolerances. This toleranceshould be used when a machine has a jackshaft or spool piece orwhen the alignment fixtures are more than 20 inches apart. Often,applications that use jackshaft tolerances have two distinct points ofpower transmission and it is not practical to measure offset at eachplane. Jackshaft tolerances consist of two angles, as shown in Figure4.


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Figure 4: Jackshaft Tolerances

Regardless of the type of tolerance used, the tolerance levels should

vary with speed. Faster running machines should have tightertolerances.

Diagnosing MisalignmentThere are a number on non-intrusive methods of diagnosingmisalignment including vibration analysis and infrared thermography.

The traditional rules for diagnosing misalignment include highvibration at 1xRPM and/or 2xRPM and 180 phase shifts across the

coupling. Specifically, when a machine has angular misalignment, thecoupling effect pulls on the shafts, producing movement anddistortion of the shafts as they attempt to align themselves. Theshafts are forced in the axial direction once each revolution and moveradially as the coupling pulls on them. In spectral data, we expect tosee a peak at shaft turning speed in the axial direction. The radialdirections, horizontal and vertical, will also have a peak at shaftturning speed.With offset misalignment, the shafts tend to move or bump the sensortwice during each rotation in the radial directions. There is very littlemovement in the axial direction. In spectral data, we expect to see apeak at two times shaft turning speed in the radial directions.


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But, how accurate are these traditional rules? A study was performedon five coupling types to investigate these rules. A summary of theamplitude trends for the various coupling types is shown in Table 2.

Table 2: Amplitude Summary (Nower)

For the grid coupling, misalignment was easy to track. The 4xRPMamplitude was the best to track; it also had the highest amplitude.The 3xRPM peak was the best amplitude to track misalignment forthe jaw coupling. Misalignment was easy to track when using the3xRPM peak. For the bun coupling, the 2xRPM peak was the best totrack. Horizontal misalignment was difficult to track. Misalignment

was difficult to track with the rubber gear coupling because theamplitudes were very low. The 6xRPM peak was the best to trackmisalignment but it was only correct 50% of the time. The 2xRPMpeaks had the highest amplitudes but only trended correctly 40% ofthe time. Misalignment was very difficult to track for the shimcoupling. 6xRPM was the best amplitude to track but was only correct30% of the time. 2xRPM had the highest amplitude but did not trendcorrectly. To further emphasize the effect of the coupling on thevibration characteristics of a machine, figure 5 shows a similarmisalignment condition for each coupling.


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Figure 5: Spectra from each Coupling, Similar Misalignment (Nower)

The traditional rules state that the phase relationship across thecoupling can be an indication of shaft misalignment. Compare thehorizontal phase readings taken on the bearing on one side of thecoupling with those taken on the opposite side bearing. If there isapproximately 180 shift between readings, the shafts are moving in

opposite directions. Phase readings made in the axial directionacross the coupling can also be used to detect misalignment.Readings approximately 180 apart mean the machines are movingopposite each other and indicate angular misalignment. Thisassumes that the sensors are facing the same direction whenreadings are taken. The study found that, like vibrationcharacteristics, the phase characteristics of a machine are alsocoupling-dependent. The jaw, shim and bun couplings followed thetraditional rules; however, the grid and rubber gear couplings did not.

The results of the phase analysis are summarized in table 3.


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Table 3: Phase Results (Nower)

The traditional rules also state that temperature can be used as anindication of misalignment. Misaligned machines run hotter thanaligned machines. The coupling study found that as with the vibration

and phase characteristics the temperature response varied with thecoupling type used. The coupling temperature showed the mostconsistency during the test. Temperatures were also taken on themotor and generator bearings. The grid coupling showed an upwardtrend in coupling temperature 75% of the time. However, overall thegrid coupling showed an upward trend in temperature 50% of thetime. When the shim coupling was used, the machine temperaturesdid not trend upward. The results are summarized in table 4.

Table 4: Temperature Results

The Alignment ProcessMachinery alignment is a process. The process begins with Pre-Shutdown and Pre-Measurement preparation. Then, you takealignment data and reposition the machine. Once the alignment iswithin tolerance, return the machine to service and take follow updata to verify the alignment.


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The pre-shutdown preparation should include an inspection formachinery problems. Any problem that can affect the quality of thealignment should be corrected before the alignment is attempted.This inspection should include:

Complete visual inspection of the machineincluding the foundation, baseplate, bolts,welds, etc.

Vibration and phase analysis of the machine. Motor electrical data for later efficiency

calculations. Thermal temperature readings or infrared

thermography on the machine.

Also, during pre-shutdown, gather all tools and supplies neededincluding the alignment kit and stainless steel shims and assign thenecessary personnel. Finally, follow all safety procedures forequipment lockout.Once the machine is shutdown and properly locked out, there are anumber of items that should be completed. First, remove the couplingguard and inspect the coupling. Check for rubber powder forcouplings with rubber inserts or for leaking grease for lubricatedcouplings as a symptom of coupling looseness and wear. Also, check

for looseness and play between the coupling halves. Inspect thekeyway. A loose key is a sign of a loose coupling.

Foundations, grout, and baseplates should be checked for cracks,bows and any other weaknesses that may hinder the alignment.Ensure that the baseplate and machine feet are clean, and are burrand corrosion free. The original shim pack should be inspected. It isimportant to know the amount each foot has been shimmed so thatyou know how much a machine can be lowered. Clean the shims and

replace any shims that are in bad condition. There should be no morethan five shims under each foot because stiffness of the footdecreases as the number of shims increases. If several shims are tobe used, sandwich the thinner shims between the thicker ones. Next, measure the shaft and coupling runout. If the coupling runout isgreater than 5 mils, replace it. It is especially important to have a true


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coupling when using the Face-Rim method. Check for and correctany soft foot that exists in the system. The machine will run betterand you will have an easier time aligning the machine to thetolerances if you correct the soft foot.

Piping strain is any condition where pipe flanges do not freely make aperfect match, but must be forced to make a match. This distorts andweakens both the pipe and pump. The connecting piping can forcethe pump and motor into misalignment. Check for and correct anypiping strain.Once all the pre-measurement checks have been performed, it istime to measure the misalignment. First, set the required end float onthe coupling and perform the rough alignment. Finally, measure the

initial misalignment.

Once you have measured the initial misalignment, calculate therequired machine moves and reposition the machine. Acquire newdata to check the alignment condition. If the alignment is withintolerance, prepare to return the machine to service. Otherwise,calculate the new machine moves and reposition the machine.Continue taking data and repositioning the machine until thealignment is within tolerance.Once the alignment is within tolerance, follow all safety procedures toreturn the machine to service. Once the machine has reachedoperating load, speed, and temperature, acquire follow-up data toverify the thermal growth parameters and the alignment condition.

Balancing BasicsImbalance is one of the most common faults found in rotatingequipment. Imbalance is the force created by a rotating object whenits center of mass does not coincide with the center of rotation.

Simply put, there is a heavy spot on the object. As the object rotates,this force causes vibration. Imbalance can be caused by a number ofsituations including wear and corrosion, incorrect assembly, structuraldamage, and build up of foreign matter.

There are two types of imbalance in rotating equipment. In staticimbalance, there is a single heavy spot on the rotor. In a dynamic


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imbalance situation, there are two heavy spots on the rotor. Coupleimbalance is a specific type of dynamic imbalance with two equalheavy spots space 180 apart on the rotor. The following figuregraphically illustrates the types of imbalance.

Figure 6: Types of Imbalance

There are many benefits to machine balancing. Precision balancingprolongs the life of the machine's bearings and other components.The imbalance force is transferred to the bearings, housing, piping,

and other fixed machine positions. This added force has anexponential effect on the life of the bearings and has a similar effecton the other machine components. Balancing also translates toreduced maintenance cost through reduced spare parts inventory andreduced labor costs. Machinery downtime is decreased and productquality is increased.

Diagnosing ImbalanceBefore attempting to balance a machine it is important to verify that

the fault is imbalance. Since imbalance is believed to be the mostcommon machine fault, balancing is often attempted with littlevibration analysis. The vibration due to imbalance will have the samecharacteristics as the force that caused the vibration. The dominantvibration will occur at the shaft turning speed (1xRPM). The vibrationwill be highest in the radial directions. Check the characteristics of the1xRPM peak. The peak should not be a double peak. The vibration


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must be from the shaft under analysis not transmitted from othersources. Axial vibration should be low except for overhung rotors.Overhung rotors will have high axial vibration at shaft turning speed.This vibration is due to the rocking of the structure in response to theimbalance force, which is outboard to the supporting structure.

The vibration amplitude should be steady and repeatable. Ifharmonics of shaft turning speed are present, they should have verylow amplitudes. Figure 7 illustrates a typical spectrum due to animbalance problem. Furthermore, 1xRPM peaks in the radialdirections should have similar amplitudes. If the vibration in onedirection, either horizontal or vertical, is greater than two times theother direction, other causes of the vibration should be investigated.

Figure 7: Typical Imbalance Spectrum

In addition to the vibration spectrum, the waveform data should also

be checked. The waveform should be symmetric at 1xRPM. It shouldnot have any truncations or sharp discontinuities. Beats in thewaveform indicate closely spaced frequencies. The cause of thesefrequencies should be investigated before attempting to balance themachine. A typical waveform is shown in the following figure.


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Figure 8: Typical Imbalance Waveform

The phase angle of the vibration should be steady and repeatable.The phase shift from horizontal to vertical on a bearing housingshould be approximately 90 (30). If the phase shift is near 0 or180, system resonance should be suspected. If the imbalancecondition is a static imbalance condition, the phase relationship

between horizontal end-to-end readings should be the same as thevertical end-to-end readings. For dynamic imbalance, the phaserelationship between the end-to-end horizontal and vertical readingswill depend on the relationship of the heavy spots.

Imbalance always causes vibration at shaft turning speed. But, a1xRPM peak is not always caused by imbalance. There are manymachine faults that can cause a 1xRPM peak in the spectrum. Shaftor bearing misalignment will often cause vibration at shaft turningspeed. However, harmonics of running speed are usually present inthe spectrum. A bent or bowed shaft also causes vibration at shaftrunning speed. The vibration is similar to misalignment. Because ofthe bend or bow, a significant amount of imbalance also exists. If thevibration is steady, balancing can be attempted. However, acceptablelevels of vibration are rarely achieved. Eccentric machinecomponents generate vibration that is identical to the vibration


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caused by an imbalance condition. The force and resulting vibrationfrom eccentricity is often strongly directional. If the vibration in onedirection, either horizontal or vertical, is greater than two times thevibration in the other direction, test the system for eccentricity orresonance before attempting to balance the machine. When amachine operates near a system resonance, high vibration at runningspeed results from small amounts of imbalance. While this vibrationappears to be pure imbalance, slight changes in machine speedcause changes to the vibration amplitude and phase. These changesmake balancing the machine very difficult. Mechanical looseness,electrical faults, and v-belts can also generate vibration near shaftturning speed. It is important to check for these other faults beforeattempting to balance a machine.

The Field Balancing ProcessField balancing is an efficient, and often necessary, practice toachieve an acceptable running condition for a machine. Fieldbalancing is simply balancing a machine after it has been installed.The balance condition of the rotor may change when put into servicebecause of factors like stress relieving, erosion, and buildup.Vibration specifications, which were met in the shop, may no longerbe satisfied in the final running condition of the machine.

There are several advantages to field balancing. The rotor isbalanced in its own bearings and at its normal operating speed andload. Also, the rotor is driven in the same manner that it will be drivenduring normal operation. Tear down of the machine is not necessary.

And therefore re-assembly and re-alignment are not necessary either.Finally, due to all of these factors, downtime is greatly reduced.However, starting and stopping the machine during the balance jobcan be difficult. Also, adding or removing the correction weights canbe very difficult.

Dynamic balancing is a process. The process begins with pre-shutdown and pre-measurement preparation. Then, you measure theimbalance, perform the trial run(s) and apply the correction weight(s)and check the results. If the results are not within tolerance, performa trim run. Once the vibration is within tolerance, return the machineto service and take follow up data to verify the balance results.


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The pre-shutdown preparation should include an inspection for othermachinery problems. Any problem that can affect the balance jobshould be corrected before the balancing is attempted. Thisinspection should include a complete visual inspection of the machineincluding the foundation, baseplate, bolts, and welds, and vibrationand phase analysis of the machine. Also, during pre-shutdown,gather all tools and supplies needed including the balancing kit andassign the necessary personnel.

After completing the pre-shutdown inspection and verify that theproblem is imbalance, a number of items should be completed beforebeginning the balancing job. First, if the rotor dirty with process orenvironmental material, clean the rotor. In many cases, cleaning therotor will result in an acceptable balance condition. Next, set up the machine for the balance job. First, determine thenumber of weight planes needed for the job. A weight plane is a crosssection though the rotor where weight can be added or removed. Todetermine the number of weight planes, use the ratio of the diameterto the width of the rotor as a guideline. If the ratio is greater than 4, asingle weight plane can be used. Figure 9 illustrates the weight planeratio. Remember that the ratio is a guide; there are always exceptionsto the rule.


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Figure 9: Weight Planes

Also, determine the number of measurement points. A measurementpoint is the point where the vibration of a plane is measured. Eachweight planes requires at least one measurement point. Using

multiple sensors, even for a one-plane job, provides better data frommore consistent readings, better results because vibration isminimized across the entire machine not just at one location andhelps identify other faults that can complicate the balance job.

After determining the number of weight planes and measurementpoints, set up the machine. Mount the transducers at the bearings.

Avoid moving the transducers during the balancing process. Set upthe phototach to read phase; take the phase readings from one shaftreference throughout the job. Check that the machine speed is steadyand repeatable. Also, determine where the correction weights will beplaced and how they will be attached to the rotor. Finally, gather thecorrection weights, a scale, and all tools needed to apply the weights.

Once the equipment has been set up, acquire the reference data.Perform the trial runs and calculate the needed correction weight andlocation. Permanently apply or remove the correction weight. Check


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the results of the balance job. If the vibration is not within tolerance,perform the necessary trim runs.Be sure to follow all safetyprocedures during the balance job.

SummaryMisalignment and imbalance are among the most common faultsfound in rotating equipment. Because of their frequency ofoccurrence, machines are often aligned or balanced without takingthe time to properly diagnose the machine fault. Diagnosingmisalignment in a machine can be difficult because the vibration,phase, and temperature characteristics are dependent on the type of

coupling used. Diagnosing imbalance can be complicated by othermachine faults. Misalignment and imbalance lead to reduced bearing,seal and coupling life. Precision alignment and balancing reduceplant operating costs through reduced maintenance and energy costsas well as reduced equipment downtime.

basics of alignment.doc

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