Balancing Best Practices 6329A1

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Balancing Best Practices Page 1

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Balancing Best PracticesBalancing Best PracticesBalancing Best Practices

About the Bently Nevada Balancing Fundamentals course:

This course is a 3-day introduction to the basic concepts of machinery balancing. It incorporates many

established topics from the Machinery Diagnostics course, but concentrates especially on those topics

which apply directly to machinery balancing.

This Best Practices topic introduces suggestions that have been proven to be helpful for people

performing machinery balancing. As always, it is up to each individual participant to determine the most

appropriate way to apply these Best Practices, if at all, at his or her own operation.

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Page 2 Balancing Fundamentals

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CautionCaution

Balancing a machine can be a very complexBalancing a machine can be a very complexprocess. A complete description of everyprocess. A complete description of every

possible situation is beyond the scope of thispossible situation is beyond the scope of thisintroductory course.introductory course.

For the best understanding of your uniqueFor the best understanding of your uniquesituation, it is recommended that you contact asituation, it is recommended that you contact aMachinery Diagnostic Engineer through yourMachinery Diagnostic Engineer through yourlocal Bently Nevada office.local Bently Nevada office.

It is impossible for a short, introductory course such as this one to qualify its participants as experts. It is

certainly not intended to supply all of the answers that can possibly arise during a machinery balancing job.

Nevertheless, the Balancing Fundamentals course is expected to provide its students with a better

understanding of the basics of machinery balancing - including some guidelines on when to ask for expert

assistance.

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Topics

Topics

!! Evaluating Machine ConditionEvaluating Machine Condition

!! DecisionmakingDecisionmaking

!! Understanding Engineering AssumptionsUnderstanding Engineering Assumptions

!! Collecting Transient DataCollecting Transient Data

!! Recognizing ResonanceRecognizing Resonance

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Topics (continued)To

pics (continued)

!! Considering Mode ShapesConsidering Mode Shapes

!! Compensating for RunoutCompensating for Runout

!! Applying the Ten Percent RuleApplying the Ten Percent Rule

!! Machine Balancing TechniquesMachine Balancing Techniques

!! Documenting ResultsDocumenting Results

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Evaluating Machine ConditionEvaluating Machine Condition

Before attempting to balance a rotor,Before attempting to balance a rotor,remember that an increasing 1X vibrationremember that an increasing 1X vibrationmay be caused by changes other thanmay be caused by changes other thanunbalance.unbalance.

Loose footing?

Rub?

Misalignment?

Shaft Crack?

Before attempting to balance a machine, you need to understand the balancing process and be aware of the

potential problems involved in misrepresenting balance conditions, or providing incorrect information forcalculations.

Caution: It is VERY important to determine if high 1X vibration is actually a result of unbalance before

adding or removing correction weights. Attempting to balance machinery that has other problems may

result in unpredictable and possibly catastrophic results.

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Evaluating Machine ConditionEvaluating Machine Condition

!! Some causes of unbalance in anSome causes of unbalance in an

established machine:established machine:

Broken blade or vaneBroken blade or vane Fouling with dirt, corrosion, or process materialFouling with dirt, corrosion, or process material

Differential thermal expansion (thermal bow)Differential thermal expansion (thermal bow)

Loose rotating componentLoose rotating component

Unbalance does not happen only to newly-installed or recently-repaired rotors. Even a machine that has

been running smoothly for months or years may suddenly develop an unbalanced condition. Here are someof the more common causes:

Broken blade or vane

Fouling with dirt, corrosion, or process material

Differential thermal expansion (thermal bow)

Loose rotating component

For conditions such as these, the best solution to the unbalance is to correct the root cause of the problem

before balancing the machine. Consider a loose part, for instance. Balancing the machine to compensate for

the part would only be effective as long as the loose part remained fixed in position. If the part were to

shift, the rotor would once again be unbalanced.

Whatever the cause of a high vibration problem, always analyze and diagnose your available machineryinformation before attempting to balance a machine

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DecisionmakingDecisionmaking

!! Should the machine be allowed to continueShould the machine be allowed to continue

running, or should it be stopped now andrunning, or should it be stopped now and

repaired before it suffers damage?repaired before it suffers damage? BNC Machinery Diagnostic Engineers can provideBNC Machinery Diagnostic Engineers can provide

guidance and help you understand your options.guidance and help you understand your options.

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!! If the machine really has a balance problem:If the machine really has a balance problem:

Is it appropriate to leave the rotor as it is?Is it appropriate to leave the rotor as it is?

Does the rotor need an in-place trim balance?Does the rotor need an in-place trim balance? Does the rotor need to be removed for balanceDoes the rotor need to be removed for balance

correction?correction?

Does the rotor need major rebuilding orDoes the rotor need major rebuilding or

replacement?replacement?

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!! Is another malfunction causing high 1XIs another malfunction causing high 1X

vibration?vibration?

thermal bows or load vectorthermal bows or load vector rub conditionrub condition

cracked shaftcracked shaft

internal or external misalignmentinternal or external misalignment

fluid-induced instabilityfluid-induced instability

incorrect bearing clearanceincorrect bearing clearance

degraded bearing supportsdegraded bearing supports

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Understanding EngineeringAssumptions


!! Linearity of Machine ResponseLinearity of Machine Response

Actual response may be non-linear if shaftActual response may be non-linear if shaftdisplacement is excessive.displacement is excessive.

Bently Balance methodology assumes that machinery response is fundamentally a linear function. If the

machine is operating under a range of conditions where response is highly non-linear, it may be verydifficult to achieve a predictable balance solution. A machine's response is greatly affected by the support

stiffness, such as with a bearing. The system stiffness and therefore response is directly related to the rotor

position inside a bearing. Bearing stiffness includes the effects of foundations and supports as well as the

lubricating film of the bearing itself. Attempting to balance a machine under highly non-linear areas of

response such as at or near a resonance, may result in unpredictable results.

Fluid film stiffness is reasonably linear over normal small ranges of rotor displacement, but can become

quite nonlinear if the shaft displacement is excessive. As the shaft approaches the bearing surface, it

squeezes the viscous lubricating fluid into a very thin film--causing stiffness to increase rapidly, as shown

in the bathtub graph.

Bearing supports and the rotor itself have a more linear response, since they are designed to be loaded well

below the proportional limit for the materials from which they are made.

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!! Repeatability of Machine ResponseRepeatability of Machine Response

Influence Vectors onlyInfluence Vectors only changechangewhen somewhen somecharacteristic of the machine is modified.characteristic of the machine is modified.

If Influence Vectors change, investigate!If Influence Vectors change, investigate!

H = C/WH = C/W

H = ((O+C)-O)/WH = ((O+C)-O)/W

Bently Balance methodology assumes that the machine response to the addition or subtraction of a

weight is repeatable. Results that are not repeatable might signify that the problem is not a simpleunbalance situation.

One of the most powerful features of Bently Balance is its ability to use historical Influence Vectors

(IVs). But Influence Vectors can change if something has happened to the rotor since they were last

calculated. Before using historical IVs, ensure that nothing has changed which could significantly affect the

machine's response. If influence vectors have changed, this is an indication that the machine conditions

have changed. Before proceeding, it is best to identify the reasons for such changes. If Influence vectors

have changed significantly, new influence vectors should be calculated.

Review of Influence Vector Calculation:

Recall the calculation of influence vectors from the earlier explanation in Section 5 of your Student Manual

(Single Plane Balance Response topic, page 7):

H = C/W, orH = ((O+C)-O)/W

Where: H is the influence vector, with units of observed displacement per amount of mass in the calibration

weight. C is the response due to the calibration weight. W is the angular location and mass of the

calibration weight. O is the original vibration response of the machine before the calibration weight was

added.

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!! Repeatability of Slow Roll VectorsRepeatability of Slow Roll Vectors

Slow Roll Vectors should remain constant unlessSlow Roll Vectors should remain constant unlesssomething happens to change runout.something happens to change runout.

If slow roll changes, it is time to investigate theIf slow roll changes, it is time to investigate the

rotor more closely.rotor more closely.

Slow Roll Vectors should remain relatively constant. The slow roll vectors effectively represent 1x runout

that is not associated with unbalance and should be subtracted from vibration data as long as the slow rollvalues remain constant. Any changes in slow roll vectors are an indication of changing machine conditions,

such as:

mechanical damage to observed rotor surface

magnetization of observed rotor surface

corrosion of observed rotor surface

change in rotor bow, either permanent or thermal

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!! Distribution of rotor mass affects rotorDistribution of rotor mass affects rotor

mode shapes:mode shapes: Evenly-distributed mass such as a long, flexibleEvenly-distributed mass such as a long, flexible

generator rotor acts like several small masses.generator rotor acts like several small masses.

Concentrated mass such as a short, massive diskConcentrated mass such as a short, massive disk

acts as a single weight plane.acts as a single weight plane.

The distribution of rotor mass will have a significant affect on rotor mode shapes. The mass distribution

should be considered:

evenly-distributed mass such as a long generator rotor

concentrated mass such as a short, massive disk.

Individual Rotor Modeling

If you have a unique machine situation that could benefit from exact rotor modeling, contact your Bently

Nevada Service Representative. Bently Nevada Engineers can perform this service on a case-by-case basis.

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Collecting Transient DataCollecting Transient Data

!! Transient DataTransient Data

Gives accurate picture of the mode shapesGives accurate picture of the mode shapes

Used in polar plots which help in determiningUsed in polar plots which help in determiningrequired correction weight placementrequired correction weight placement

Allows examining influence of the weightAllows examining influence of the weight

changes over the entire speed rangechanges over the entire speed range

Although a machine may be balanced based on measurements taken at a single operating speed, it is much

better to collect measurements over a transient event--such as a startup or shutdown--whenever possible.With Bently Balance, the ability to import ADRE for Windows databases greatly facilitates the use of

transient vibration data. Transient data collection is important to machinery balancing for several reasons:

Transient data is necessary to give an accurate picture of the mode shapes of the shaft.

Polar plots derived from the transient data give an accurate presentation of the vibration and help you

determine the required correction weight placement.

Using transient data, the influence of the weight changes can be examined over the entire range of

startup/shutdown data--ensuring that there are no problems, particularly near resonance(s).

Two of the most commonly-used transient data plots are the Polar Plot and the Bode Plot. Both plots

display the same vector data in different ways. Polar plots are used in Bently Balance to help the

diagnostician identify mode shapes, resonant speed ranges, slow roll vectors, structural resonances, and

high spot/heavy spot locations. In addition, predicted results are also presented on the polar plots. Transientdata provides a visual characterization of machine response over the entire speed/operating condition range.

The user can derive the synchronous amplification factor as well from these transient data formats.

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Recognizing ResonanceRecognizing Resonance

!! Occurs at ~90 degrees phase lag from theOccurs at ~90 degrees phase lag from the

angle of the heavy spot. Indicated by:angle of the heavy spot. Indicated by:

Peak in 1X vibration amplitudePeak in 1X vibration amplitude Rapid change in 1X vibration phase lag angleRapid change in 1X vibration phase lag angle

!! Caution: Balancing aCaution: Balancing a

machine for operation closemachine for operation close

to a resonance speed isto a resonance speed is

risky and should berisky and should be

avoided.avoided.

Resonance occurs at a shaft rotative speed (or speed range) equal to a lateral natural frequency of the rotor

system. Due to rotor unbalance, when the speed increases or decreases in this range, the observed vibrationcharacteristics are:

a peak in the 1X Amplitude, and

a more rapid change in the 1X vibration phase lag angle.

The resonance speed is at the point that is 90 degrees phase lag from the angle of the heavy spot for that

resonance mode. This speed may be slightly different from the rotative speed peak amplitude point. These

changes may not happen at the same frequency due to nonlinearity, damping, and/or asymmetry in the

system stiffness.

As a rotating machine changes speeds (such as during a startup) it may pass through one, two, or even more

speed ranges where vibration increases substantially above normal values for steady state operation. When

this effect is caused by reaching a natural vibration frequency of the entire rotor, it is known as a "balanceresonance." The speed corresponding to this effect has historically been called a "critical" speed.

Note: Some machines may have other important resonances besides those of the rotor itself. For example,

mounting pedestals, piping systems, and other attached structures all have their own unique natural

frequencies. It is possible that machine vibration could excite these structural resonant frequencies as well

as those associated with the machine rotor.

The mode shape of a rotor changes when entering a speed range corresponding to a balance resonance

frequency of the rotor. The Polar and Bode plots show a vibration amplitude peak at resonance and a

change in the direction of the vibration vector--which lags by approximately 90 at resonance and up to

180 when the speed is well above the resonance.

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Considering Mode ShapesConsiderin

g Mode Shapes

!! Rotor mode shapesRotor mode shapes

change when passingchange when passing

through balance resonance.through balance resonance. Long, flexible rotors canLong, flexible rotors can

have more mode shapes.have more mode shapes.

Short, rigid rotors haveShort, rigid rotors have

fewer mode shapes.fewer mode shapes.

Although rotors appear rigid, they actually bend slightly. Depending on their stiffness, distribution of mass,

and the frequency of the "forcing function" caused by rotation speed, rotors set up longitudinal standingwaves with characteristic shapes. These vibrational modes can affect both the vibration response of the

rotor and the measurements taken by proximity probes.

The first mode for a flexible rotor is a single bow, arching between two bearings and shown in the

"Cylindrical Translational Mode" diagram above. This shape corresponds to a vibrating string on a musical

instrument such as a cello, or a jump rope being spun slowly. The machine rotor takes this shape when

operating in the range of speeds associated with the first balance resonance.

The second mode is a double bow, forming an "S" shape between two bearings, and shown in the

Pivotal/Conical Mode diagram. This shape corresponds to a vibrating musical instrument string that is

being touched lightly at midspan. The machine rotor takes this shape when operating in the range of speeds

associated with the second balance resonance.

Some machines with long, flexible rotors and very high operating speeds may actually encounter the third

mode or even higher modes. But the majority of rotating machines operate in either the first or second

mode.

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Considering Mode ShapesConsiderin

g Mode Shapes

!! Vibration transducers are subject to theVibration transducers are subject to the

following effects as the rotor mode changes:following effects as the rotor mode changes:

measured vibration will be small if the transducermeasured vibration will be small if the transduceris near a nodal point and larger if the transduceris near a nodal point and larger if the transducer

some distance away from the nodal point.some distance away from the nodal point.

measured vibration will be out of phase whenmeasured vibration will be out of phase when

comparing two transducers on opposite sides ofcomparing two transducers on opposite sides of

a nodal point, due to the "rocking" effect of thea nodal point, due to the "rocking" effect of therotor.rotor.

Nodal Points are locations of zero rotor displacement. For a first mode vibration, the two nodal points are

both somewhere in or near the bearings supporting the rotor. For a second mode, two of the nodal pointswould be in or near the bearings and the third node would be somewhere between the two bearings. Nodal

points are affected by system stiffness and mass distribution.

The mode shape of a rotor changes when passing through a balance resonance during a startup or

shutdown. Resonance is characterized by an amplitude peak and a corresponding 90 degree lagging change

in phase angle. The Polar Plot will show a vibration amplitude peak at resonance and a change in the

direction of the vibration vector-- typically lagging by 180 once the system is well above the resonance.

Once the machine has passed through a balance resonance, a close study of the vibration readings will help

determine the new rotor mode shape.

As the rotor mode shape changes, the internal clearances of the machine will also change. Also, the Polar

plots of vibration will be strongly affected. The Polar plots are a prime source of information about the

mode shape of the rotor. Vibration transducers are subject to the following effects as the rotor modechanges:

measured vibration will be small if the transducer is near a nodal point and larger if the transducer some

distance away from the nodal point.

measured vibration will be out of phase when comparing two transducers on opposite sides of a nodal

point, due to the "rocking" effect of the rotor.

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Compensating for Runout


!! Two types of runout:Two types of runout:

MechanicalMechanical

ElectricalElectrical

!! Measured at slow roll conditions:Measured at slow roll conditions:

Slow enough so dynamic motion (vibration)Slow enough so dynamic motion (vibration)

effects are negligible.effects are negligible.

Typically below 10% to 20% of the first balanceTypically below 10% to 20% of the first balance

resonance speed.resonance speed.

Rotor runout (sometimes called "glitch"), as measured with a proximity probe, exists in two forms,

mechanical and electrical. Either or both of these effects may introduce errors into measured rotorvibration:

Mechanical runout generates an A.C. output signal from a shaft relative transducer caused by a change in

the gap between the transducer and the rotor and not from either a change in shaft centerline position or

from vibration. Common sources include out-of-round shafts, scratches, hoist chain marks, dents, rust or

other conductive buildup on the shaft, punched stencil marks, flat spots and engravings.

Electrical runout generates an A.C. output signal from a proximity probe transducer caused by effects other

than a change in the gap between the probe and rotor. Electrical runout is the result of non-uniform shaft

material properties such as electrical conductivity, resistivity, or permeability or a local (spot) magnetic

field at the point being observed on the shaft. Electrical noise, poor grounding, capacitive coupling between

power and signal lines can all add to electrical runout. Rotor synchronous runout is measured at machine

"slow roll" speed and should be repeatable, by definition.

Slow roll speed is defined as a machine rotative speed low enough so that dynamic motion (vibration)

effects from such forces as unbalance are negligible. On most machines, slow roll speed typically occurs

below 10% to 20% of the first balance resonance frequency (first "critical" speed).

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!! Document for future useDocument for future use

Manually or electronicallyManually or electronically

!! Compensate plotsCompensate plots Manually or electronicallyManually or electronically

Runout Documentation

For balancing calculations, it is important that synchronous runout be verified as repeatable. Runout should

be documented on each balancing run--usually during slow roll following a machine shutdown from

normal operating conditions--to ensure that it has not changed due to thermal effects, "gravity" bow,

alignment or other effects. When using ADRE for Windows, slow roll data may be captured electronically.

However, when recording manual measurements, this data must be taken manually.

Applying Runout Compensation

In terms of balancing, the vibration signal error introduced by runout that is synchronous with rotative

speed (1X) must be taken into account during the balancing calculations. Synchronous runout is a

repeatable vector quantity (amplitude and phase) that has a frequency equal to the shaft rotative frequency

(1X) and can be vectorially subtracted from the 1X vibration signal. Synchronous runout should be factored

out of balance measurements by vector diagrams or vector arithmetic (for manual calculations), or byelectronic compensation (when using ADRE for Windows data).

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Applying the 10% RuleA

pplying the 10% Rule

!! If you have no prior knowledge of theIf you have no prior knowledge of themachine:machine:

Use a calibration weight which willUse a calibration weight which willgenerate a force less than 10% of thegenerate a force less than 10% of therotors mass.rotors mass.

If you have reliable prior balancing data for a machine, you can assign Calibration Weights based on that

information. However, if you do not have such information--and particularly if the machine operates athigh speed (above 6000 rpm), you should follow this general guideline:

Caution: The first calibration weight installed on the machine should not yield a centrifugal force greater

than 10% of the weight of the rotor. Once the response is known, additional weights can be modified to the

needed size to balance the unit.

Bently Balance calculates centrifugal force automatically, and provides a warning message if a proposed

balance weight will exceed the 10% Rule.

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!! Reasons for the 10% RuleReasons for the 10% Rule

The unfamiliar machine may have anThe unfamiliar machine may have an

unexpectedly severe resonance response duringunexpectedly severe resonance response duringstartup.startup.

If vibration becomes severe during a startup,If vibration becomes severe during a startup,

there may not be adequate time to shutdown thethere may not be adequate time to shutdown themachine before damage is done.machine before damage is done.

Reasons for the 10% Rule

The unfamiliar machine may have an unexpectedly severe resonance response during startup. This

vibration at resonance may be made much worse by the newly-installed weight.

If vibration becomes severe during a startup, there may not be adequate time to shutdown the machine

before damage is done. This is especially likely for a machine such as an electric motor, which accelerates

very quickly.

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!! Calculating Centrifugal Force:Calculating Centrifugal Force:

2=

rMFc

Fc= Centrifugal ForceM = Mass of weightr = Radius of weight location = Shaft speed22

The centrifugal force created by a Balance Weight is directly proportional to the mass of the weight, its

radial distance from the center of the rotor, and the square of the rotor speed. Calculate the force resultingfrom a balance weight by using the following equation:

Fc= Mr2, or M = Fc/r

2

Fc= centrifugal force exerted by the balance weight

M = mass of the balance weight

r = radius of the balance weight location on the weight plane (from the center of the rotor)

= rotor speed in radians per second

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Applying the 10% Rule


!Metric (SI) Units Example - based on a largeinduction motor with the following characteristics:

M = 907 kg

r = 25.4 cm,

speed = 3585 rpm

! First, find rotor weight: F = MGF = (Mrotor)(G) = (907 kg)(9.806 m/s

2) = 8894 kg-m/s2 or 8894 N.

Note: Newtons (N) of force areequivalent to units of kg-m/s2.

Metric (SI) Units Example - based on a large induction motor with the following characteristics:

Rotor Mass = 907 kg, Weight plane slot radius = 25.4 cm,Rotor speed = 3585 rpm, Gravitational acceleration, G = 9.806 m/s2

Substituting these values into the equations will allow calculating the mass for a calibration weight that

satisfies the "10% Rule.

First, find the rotor weight: F = MG. F = (Mrotor)(G) = (907 kg)(9.806 m/s2) = 8894 kg-m/s2or 8894 N.

Note: Newtons (N) of force are equivalent to units of kg-m/s2.

Now find the force equal to 10% of the rotor weight:

(10%)(8894 N) = 889.4 N or 889.4 kg-m/s2.

Substituting this force into the equation M = Fc/r2will give the maximum size of the balance weight that

meets the "10% Rule".

Convert rpm to radians/second and substitute into the equation as :

(3585 rpm) / (9.55 rpm/(rad/s)) = 375.4 rad/s.

M = Fc/r2= (889.4 kg-m/s2)/(0.254 m)(375.4 rad/s)2

M = 0.0248 kg or 24.8 g

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! Now find the force equal to 10% of rotorweight: (10%)(8894 N) = 889.4 N or 889.4 kg-m/s2.

Substitute this force into the equation M = Fc/r2 to findthe maximum allowable size of the balance weight.

! Convert rpm to radians/second and substitute intothe equation as : (3585 rpm) / (9.55 rpm/(rad/s)) = 375.4 rad/s.

M = Fc/r 2 = (889.4 kg-m/s2)/(0.254 m)(375.4 rad/s)2

M = 0.0248 kg or 24.8 g

Metric (SI) Units Example - based on a large induction motor with the following characteristics:

Rotor Mass = 907 kg, Weight plane slot radius = 25.4 cm,Rotor speed = 3585 rpm, Gravitational acceleration, G = 9.806 m/s2

Substituting these values into the equations will allow calculating the mass for a calibration weight that

satisfies the "10% Rule.

First, find the rotor weight: F = MG. F = (Mrotor)(G) = (907 kg)(9.806 m/s2) = 8894 kg-m/s2or 8894 N.

Note: Newtons (N) of force are equivalent to units of kg-m/s2.

Now find the force equal to 10% of the rotor weight:

(10%)(8894 N) = 889.4 N or 889.4 kg-m/s2.

Substituting this force into the equation M = Fc/r2will give the maximum size of the balance weight that

meets the "10% Rule".

Convert rpm to radians/second and substitute into the equation as :

(3585 rpm) / (9.55 rpm/(rad/s)) = 375.4 rad/s.

M = Fc/r2= (889.4 kg-m/s2)/(0.254 m)(375.4 rad/s)2

M = 0.0248 kg or 24.8 g

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Machine Balancing TechniquesMachine Balancin

g Techniques

!! Balancing Process:Balancing Process:

Measure Baseline ResponseMeasure Baseline Response

Perform Calibration Run(s)Perform Calibration Run(s) Evaluate Machine ResponseEvaluate Machine Response

Calculate Influence Vectors (IVs)Calculate Influence Vectors (IVs)

Calculate SolutionCalculate Solution

Apply Correction WeightsApply Correction Weights

Verify SolutionVerify Solution

Document ResultsDocument Results

The basic concept behind modern machinery balancing is that it is possible to find Influence Vectors (IVs)

by measuring the machine's response to a known Calibration Weight installed in a known location on aWeight Plane. Once the IVs are known, this information may be used to calculate the mass and location for

Solution Weight(s) to reduce vibration to a minimum. This approach greatly reduces the "trial and error"

methods that were used in years past.

Basic Balancing Procedure

(1) Measure Baseline Response - Perform a "Reference Run" and record the machine's vibration response

before the installation of a Calibration Weight. Transient conditions such as startup or shutdown provide

the most useful information.

(2) Perform Calibration Run(s) - Install a known Calibration Weight at a known location on a specific

Weight Plane. Record the machine's vibration response with the Calibration Weight installed. Apart from

the addition of a Calibration Weight, the more closely you can duplicate the conditions that existed during

the Reference Run, the better.(3) Evaluate machines response - from transient data, if available (polar, bode).

(4) Calculate Influence Vectors (IVs) - Compare the machine's response BEFORE and AFTER the

Calibration Weight was installed to determine the Influence Vectors.

(5) Calculate Solution - Use the IVs to determine Solution Weight(s) to minimize the machine's vibration

response. Ensure the solution makes sense before installing correction weights.

(6) Apply Correction Weights to Machine - Install (or remove) Correction Weights as calculated based on

the IVs.

(7) Verify the Solution - Take another set of vibration samples during startup to verify the machine

vibration has changed as expected.

(8) Document Results - Capture all of the pertinent information for posterity. You never know who

might need it - or when!

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g Techniques

!! Measure Baseline ResponseMeasure Baseline Response

At normal operating conditionsAt normal operating conditions

!! Perform Calibration Run(s)Perform Calibration Run(s)

On as many weight planes as practicalOn as many weight planes as practical

Measuring Baseline Response

The most important consideration is to take your representative baseline measurements at conditions that

are as close as possible to normal operating conditions of speed, load, temperature, flowrate, etc. This way,

the calculated influence vectors will be useful for calculating a solution at these same conditions.

Performing Calibration Runs

Once again, machine conditions should be as close as possible to normal operating conditions of speed,

load, temperature, flowrate, etc.

Hypothetically, a separate calibration run should be made for each individual weight plane. Realistically,

the weight plane(s) closest to the measurement points of concern are the ones which usually have the

biggest effect. Since there may be only a limited window of opportunity during a particular outage, you

may need to select a small number of calibration runs which will produce the most useful information.

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g Techniques

!! Evaluate Machine ResponseEvaluate Machine Response

Transient Plots (polar, bode)Transient Plots (polar, bode)

!! Calculate Influence Vectors (IVs)Calculate Influence Vectors (IVs)

Manually or using softwareManually or using software

Compare with previous valuesCompare with previous values

!! Calculate SolutionCalculate Solution

Consider optimal alternativesConsider optimal alternatives

Evaluate Machine Response

Always use transient data, if available, as it includes valuable information (such as resonance speeds) thatis not available from data taken at a single operating point.

Calculating Influence Vectors

When comparing the before and after data from your calibration runs, it is important to ensure that the

calibration weights were the only variables which changed. Speed, load, temperature, flowrate, and other

process parameters should be as consistent as possible between runs.

Compare the new IV values with any historical influence vectors you may have available. If these values

are inconsistent, further investigation may be required. It is possible that undetected damage or other

changes may have occurred to the machine since the last time IV values were determined.

Calculating Solution

When calculating a solution, it is important to consider the most typical operating conditions for themachine. For instance, if the machine spends a large amount of time at two very different operating

conditions, an optimal solution should be determined which works well with both of these operating

conditions.

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g Techniques

!! Apply Correction WeightsApply Correction Weights

Slot type, Hole type, Other...Slot type, Hole type, Other...

Grinding/WeldingGrinding/Welding

!! Verify SolutionVerify Solution

Capture the machine's response during the firstCapture the machine's response during the first

startup after installationstartup after installation

Applying Correction Weights

Once a solution is calculated, Correction Weights are added or removed on Weight Planes. For large

machines, this usually involves inserting threaded cylindrical weights (in a hole type weight plane) or

inserting a sliding weight and clamping it into place (in a slot type weight plane). On some machines, this

may involve adding or removing washers on a coupling bolt, or even bolting on lead plates (such as on the

blades of large cooling tower fans).

Smaller machines may not be designed for the addition of balance weights. Instead, their rotors may need

to have weight added by weld buildup or removed by drilling or grinding (or even by a laser cutter).

Note: When adding or removing material from a small rotor which does not incorporate a balance weight

plane, it is extremely important to keep track of the angular orientation of the rotor - especially if the rotor

will be removed from its casing for the mass adjustment. It may be necessary to mark the shaft with a

phase reference mark before the rotor is removed.

Verify Effects of Correction Weights

Capture the machine's response during the first startup after installation of the Correction Weights in order

to verify that the solution was adequate.

Class Notes:


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!! Document Results!Document Results!

Vibration Vectors Runout Vectors (Slow Roll)

Influence Vectors

Transducer type and orientation

Angular reference frame for machine and Weight Planes

Calibration Weights and Correction Weights

Speed/load and Process conditions

Balance calculations and graphs

Documenting Results

It is very important that you document the steps you performed to balance the machine. This documentation

may include hard copy reports as well as electronic information such as an ADRE for Windows

database. If the rotor ever needs rebalancing in the future, the documentation you create now will be

extremely valuable. The following is a list of minimum suggested information to be recorded:

All vibration vectors

Synchronous runout vectors (taken at Slow Roll conditions)

Influence Vectors

Transducer type and orientation

Angular reference frame for machine and Weight Planes

All Calibration Weights and Correction Weights used

Speed/load and process conditions during machine Runs

Copies of any balance calculations and graphs

-and-

Any other pertinent machine or process information you think might be useful!

Note: You can use the New Event Wizard in Bently Balance to add written notes to your balance

database.

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!!

Document Results!Document Results! Machine Train layout

Documenting Results

Machine Train layout - A machine train drawing summarizes a large amount of useful information in a very

small space. Create a drawing for the machine being balanced if one does not exist already. The example

shown above was created in Bently Balance for a 75 MW steam turbine generating set that is part of a

combined cycle cogeneration plant. The following components are shown in the drawing:

High Pressure / Intermediate Pressure Steam Turbine

Low Pressure Steam Turbine

2-Pole Generator (3600 RPM @ 60 Hz output)

Five fluid film radial bearings

Two rigid couplings

Two Balance Weight Planes (on the generator) Five pairs of X-Y displacement probes

One Keyphasor phase reference probe


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Class Notes:

Balancing Best Practices 6329A1

Documents

Transcript of Balancing Best Practices 6329A1