guy -time and spectrum analysis paper - part i

8/13/2019 Guy -Time and Spectrum Analysis Paper - Part I

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Spectrum & Time Analysis ProceduresHow to Analyze Data

By

Kevin R. GuySenior Field Analyst

Delaware Analysis Services, Inc.P.O. Box 365

Francisco, Indiana 47649([email protected])

Abstract: Individuals assigned to vibration analysis programs struggle with analyzing collected vibrationdata for various reasons including, training and lack of confidence. The problem is not that people dontknow how to analyze data, but, rather they do not have a plan or procedure on how to systematically goabout analyzing the data. In many cases, the vibration analyst should rule out what is not the problem,instead of looking for what is the problem. This paper will present a plan of attack so that an individualwill have some guidance on what to look for when analyzing vibration data. Following the plan set forth inthis paper will give the inexperienced vibration analyst a procedure on how they should proceed whenanalyzing vibration data.

Key Words: Amplitude, Frequency, Time, Spectrum, Harmonic Vibration, Periodic Vibration; ImpulsiveVibration, Pulsating, Random Vibration, Sidebands, Synchronous, Non Synchronous, Subsynchronous,Electrical, Balance, Alignment, Looseness, Resonance, Vane Pass, Blade Pass, Gearmesh

Introduction:Analyzing data appears to be a daunting task. Many do not have experience examining atvibration data. The key is to go about the task with a plan to assist with the analysis. While each analystdevelops their own plan of how they analyze data, each analyst s plan contains many of the sameprocedures. Many inexperienced analysts only want to look at data in the spectrum plot because manyanalysis charts and tables present data in the frequency domain (Figure 1).

Figure 1Spectrum PlotSynchronous Vibration

1X

2X
mailto:[email protected]:[email protected]


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These same inexperienced analysts try to match the spectrum plot to a wall chart and expect the answerto jump out at them. A wall chart or other forms of troubleshooting tables are guidelines. The expectationis the exact same spectrum plot will be found and they will, therefore, have the solution to their analysis.

Data from the time plot will indicate what type of vibration is present. The five types of vibration areharmonic (Figure 2), periodic (Figure 3), beating (Figure 4), impulsive (Figure 5), or random (Figure 6).

Figure 2Time Plotperiodic Vibration

Figure 3Time PlotPeriodic Vibration


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Figure 4Time PlotPulsating or Beating Vibration

Figure 5Time PlotImpulsive Vibration


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Figure 6Time PlotRandom Vibration

When one determines the type of vibration from the time plot, they use the spectrum plot to find out whatgroup or frequency content the vibration frequencies in the spectrum fall under, they are: synchronous,non-synchronous, subsynchronous or electrical. While much of vibration analysis is pattern recognition,knowing the type of vibration from the time plot, and what groups the frequencies fall under in thespectrum plot, points the analyst to the vibration cause.

Ninety percent of our vibration problems are the big three: balance, alignment and looseness issues. If aforth problem was added to the list, it would be rolling element bearing faults. These four items areprobably ninetyfive percent of all vibration issues.

This paper will try to lay the foundation of rules for analyzing vibration issues by using techniquesdeveloped over the past thirty-three years of analyzing data. Vibration analysis does not have to bedifficult if you know how to read the data. Also, the vibration instrumentation does an excellent job atproviding information to make the task easier. The analyst has to have a plan and they have to know howto extract the information from the data.

Basics of Vibration Analysis: Before any discussion of analyzing data can begin, a discussion ofvocabulary must take place. The two most important words in the analysts vocabulary are amplitude andfrequency. Amplitude tells us the condition of the equipment and frequencies identify the problem. You,as an analyst, look at the frequencies present in the time and spectrum plots and determine the cause ofthe vibration. The type of frequencies present, help the analyst narrow down the cause of the vibration.

The word frequency can be broken down into four additional critical words that help the analyst indentify

the frequency content in the spectrum plot. Synchronous vibration is vibratory frequencies that are relatedto the operating frequency of the shaft (i.e. balance alignment, looseness, vane pass, blade pass,gearmesh). Subsynchronous vibration is vibration frequencies that occur at a frequency that is less thanthe frequency of the shaft (i.e. oil whirl, oil whip, fundamental train frequency, rubs). Non-synchronousvibration is vibration that is not related to the frequency of the shaft (i.e. rolling element bearing defectfrequencies, pump flow issues). Electrical vibration is vibration that is caused by line frequency, 60Hz inNorth American and 50 Hz around the rest of the world.


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The basis for mechanical vibration issues is shaft speed. The rotational (operating) speed of the shaft isexpressed in terms of revolutions per minute (rpm). The term frequency relates to cycles per unit time.Frequency can be expressed in cycles per minute (cpm) or cycles per second (cps Hz).

Using the following example, a machine operating at 3575 rpm (Figure 7) this vocabulary can be tiedtogether. If the operating speed of the shaft is 3575 rpm, then the shaft frequency is 3575 cpm or 59.58Hz. This is also the first order of the shaft. Any frequency that is an even multiple of the shaft frequencywould be synchronous vibration (Figure 7 - items 1 - 6). Vibration frequencies that occur at a frequencyless than the operating frequency are subsynchronous (Figure 7 item 7). Frequencies that are not aneven multiple or order of shaft frequency is non-synchronous vibration (Figure 7 item 8). Any vibrationthat is related to line frequency, 60 Hz (3600 cpm) or 50 Hz (3000 cpm) is electrically generated and iselectrical vibration (Figure 7 item 9). Table I contains a listing of vibration frequency groups and theirproblems.

When looking at how frequencies are identified in the spectrum plot, it is the choice of the individualanalyst on how they would like to view the frequency axis: cpm, Hz or orders. There is no right or wrongway to look at frequency in the spectrum. Many analysts like cycles per minute (cpm) because they relateit to the shaft speed in rpm. Seasoned and higher certified analysts tend to use cycles per second (Hz)due to needing the second unit when performing many vibration calculations. It is recommended to manynew inexperienced analysts that they use orders to assist with determining if the vibration is synchronous,

non-synchronous, subsynchronous or electrical. How your frequency span is setup is personnel. Manyswap back and forth between cpm, Hz and orders depending on what type of data is being analyzed.Order analysis, for the frequency units, works well when looking for rolling element defect frequencies.Defect frequencies will not be integers of operating speed.

One other important term is sidebands. Sidebands are evenly spaced frequencies that occur above andbelow a center frequency (Figure 7 Item 10). Sidebands are mainly found in three places, with motorshaving broken rotor bars, rolling element bearing frequencies signaling an imminent failure, andgearmesh. The sidebands for motor broken rotor bars are spaced at the number of poles times the slipspeed. Sidebands around gearmesh frequency are spaced at the frequency of the shaft, with the vibrationproblem. Rolling element bearing frequency sidebands are usually spaced at the frequency of the shaft orcage frequency. Table II has a list of locations where sidebands are found.

Figure 7Example of Spectral Frequency Terms


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Table IGenerated Frequencies versus Shaft Frequency

Subsynchronous Synchronous Non Synchronous Electrical

FTFGage Frequency Alignment Ball Spin Frequency Stator Problems

Oil Whirl Balance Ball Pass Outer Race

Oil Whip Looseness Ball Pass Inner Race

Belt Frequency Vane Pass Pump Flow IssueBlade Pass

Gearmesh

Table IISideband Frequency Locations

Sidebands Frequency Indication

Motors Number of Poles x Slip Speed Broken Rotor Bars

Gearboxes Spaced at Shaft Speed Indicates shaft with vibration problem

Rolling Element Bearings Bearing Defects Indicates imminent bearing failure

It is stated in many papers, that vibration analysis is nothing more than frequency matching of the spectraldatamatching frequencies, in the spectrum to specific components, of the machine train (Figure 8) andgrouping the frequencies, into synchronous, subsynchronous, non-subsynchronous and electrical.Vibration analysis is also evaluating the type of vibration from the time plots, in addition, to looking at thegrouping of the frequencies.

Figure 8Spectrum PlotFrequency Matching

The type of vibration found in the time plot also helps the analyst identify vibration problems. Harmonicvibration (Figure 2) is indicative of unbalance problems and surprisingly looseness issues in sleevebearings.

Periodic vibration (Figure 3) is an indication of vibration that repeats over and over in the same timeinterval such as misalignment, vane pass, and gearmesh.

Pulsating or beating vibration (Figure 4) indicates two closely spaced frequencies that are adding andsubtracting in a beat cycle. In some cases, this can be more of a nuisance than a major issue. An

2X

1X

Vane Pass

Rotor Bar Pass

Slot Pass


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example would be the mechanical and electrical frequencies (2X running speed & 2X line frequency)beating in a two pole motor.

Impulsive vibration (Figure 5) indicates impacting and is found with rolling element bearing defects andgears having broken or cracked teeth. In severe looseness cases, impulsive vibration can also bepresent.

Random vibration (Figure 6) is vibration that comes and goes. It is not periodic and is usually associatedwith flow issues in piping or pumps.

Table IIITypes of Time Based Vibration and Locations

Type of Time Based Vibration Location Found

Harmonic Balance / Natural Frequency

Periodic Alignment / Looseness / Vane Pass / Blade Pass / Gearmesh

Pulsating or Beating Closely Spaced FrequenciesMechanical / Electrical or Both

Impulsive Roller Bearings / Broken Teeth Gears / Severe Looseness

Random Flow Problems in Pumps & Fans

Fault Analysis:While each analyst needs to develop their own thought process when analyzing vibrationproblems, several steps can assist with the process.

Steps to Analysis:1) When notified about a problem, talk to those who found it, and get his or her view on the

circumstances that lead up to the problem. Learn as much information about the equipment aspossible. It is best to talk to someone from operations or the shop floor as they normally havefirst hand knowledge of problems. Managers and supervisors normally obtain second or thirdhand information, and may not have all the technical information required to assist with theanalysis. Find the operating conditions during the vibration problems, which should include, butnot be limited to: shaft speed, temperatures, pressures, flows, or process changes.

2) Draw a schematic design of the equipment (Figure 9), and investigate its internals. Find out whattypes of bearings are used. This is important because the same type of bearing is normally notused throughout each equipment train. The number of teeth on the gears (Gearmesh), number ofblades on the impeller (Blade Pass), and bearing frequencies should be calculated. The locationof any shaft critical or structural resonance points should be identified. The analyst should have amental picture of the machine internals and know what the required function of each machine

component.

3) The analyst should also have some knowledge of the equipments application in the system. Thisinformation should include suction and discharge pressures (BEP), motor amps, flow, and anyother parameters that might point to a system operations problem rather than a mechanicalvibration problem.


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4) When collecting data used the right hand rule (Figure 9). Collect the horizontal data on the rightside of the machine and the vertical data 90 degrees left of the horizontal data or top deadcenter. If usingprox probes (Figure #10) collect the vertical data (Y) then the horizontal data (X).

Driver DrivenCoupling

Axial

Horizontal

Vertical

Figure 9Equipment Schematic

ProxProbe

Prox P

robe

Shaft

V H

Figure 10Prox Probe Locations

5) Collect data on the machine when it is out of service. This may sound odd; however, an analystneeds to know what frequencies are present when the machine is not operating. Any frequenciespresent when the machine is not operating, are not coming from the machine. They are beinggenerated by another machine and can be ignored during the analysis.

6) From the time plot (Figure 11) determine if five to eight rotations of the shaft are present and whattype of vibration is present. Based on the operating frequency and the time plot, there are 14.97cycles in the time plot. The time plot indicates impulsive vibration that is periodic in nature.

7) Determine if the frequency content present in the spectrum plot are synchronous, non-synchronous, subsynchronous or electrical. Almost all data collection systems will provide thisinformation in tabular form (Figure 12). The spectral data in figure 10 is 85.8% synchronous,meaning the vibration is order multiples of shaft speed.


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Figure11Time & Spectrum Plot

Figure 12Frequency Content Breakdown

Vibration Problems and Symptoms: The remainder of this paper will discuss the dominate vibrationissues that a vibration analyst will encounter along with the symptoms of each problem. Unfortunately notevery vibration issue can be covered in a short paper; but, the most common vibration problemsencountered will be discussed.

Imbalance is a Synchronous Vibration: The problem of imbalance (Figure 13) is characterized by ahigh operating speed vibration (1X) in the softest direction on the machine which is normally horizontal.The symptom is referred to as one per rev due to the unbalance force passing the vibration sensor onceevery revolution. Horizontal and vertical data will be 90 degrees apart due to the positioning of thesensors (Figure 14). The time plot data will be a fairly harmonic signal. Correction of the problem requiresbalancing of the machine component.

One issue many have with balancing, is no one wants to believe a new piece of equipment or a recently

balanced piece of equipment can have a balance issue when it is installed and put back in service.Equipment that is balanced in a low speed balance machine is balanced for shaft rigid modes. If theshaft, when installed runs near or above the first critical, the shaft will become flexible and may still needto be balanced.

Many times, plants will have a motor and a fan balanced separately. The motor and fan will be installed,coupled up, aligned and placed in service. Analysis of this installed equipment will show a balanceproblem because the motor and pump were balanced separately. Now they are running as a unit, theyexhibit imbalance issues because they need to be balanced as one component.

Impulsive Vibration


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Additionally, many pieces of equipment are driven with variable frequency drives (VFD). These machinesmay operate with low levels of vibration at low speeds and then have imbalance issue when operating atfull speed. Vibration analysts must understand that whenever the speed of a machine doubles the forceof the vibration, not the vibration amplitude, goes up by a factor of four. This causes the rotor to operatewith a balance problem. When balancing machinery driven by a VFD, balance it at the top of the speedrange. If it is balanced at full speed it will be balance at low speed.

Figure 13SpectrumImbalance ProblemHigh 1X Vibration Frequency

Figure 14TimeImbalance ProblemHorizontal & Vertical Data

1X

Horizontal & Vertical signals 90 degrees apart


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Misalignment is a Synchronous Vibration: Alignment issues are maybe the number one cause ofmachinery vibration problems and they are also the toughest to convince people they have alignmentproblems. The problem is not that people dont know how to align equipment, the problem is people donot use the correct cold offset.

Lets face it, when equipment is aligned, it is actually misaligned, so that when it thermally grows the shaftwill grow so that it is in a straight line. To get accurate alignment, the thermal growth needs to becalculated and plotted (Figure 15) or the equipment needs to be optically measured from cold to hot orhot to cold so the actual thermal growth can be determined. Once the actual growth is known orcalculated, the correct cold offset can be determined.

Figure 15TimeThermal Growth for Shaft Elevations5 Inches to 25 inches

Alignments issues are indicated by operating speed (1X) and twice operating speed (2X) vibration in thehorizontal and vertical directions (Figure 16). Axial vibration will only show operating speed (1X) vibrationcomponents (Figure 17).

Figure 16Spectrum PlotHorizontal1X & 2X Vibration Frequencies

1X 2X


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Figure 17Spectrum PlotAxial1X Vibration Frequencies

One additional symptom of severe misalignment is a figure eight (8) orbit (Figure #18) which is datacollected from dual prox probes. Normally orbits are collected from two prox probes; however, orbits can

also be collected from two accelerometers. Figure 19 is an orbit from a horizontal and verticalaccelerometer mounted on a 4000 horsepower four pole motor, also showing severe misalignment.

Figure 18Prox Probe OrbitFigure 8 Severe Misalignment

Figure 19Accelerometer OrbitFigure 8 Severe Misalignment

1X


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Looseness is a Synchronous Vibration Problem and in Severe Cases Subsynchronous: It is fairlystraight forward to identify looseness issues from spectrum and time plots; however, the cause of thelooseness maybe difficult to identify. Everyone views looseness as someone forgetting to tighten downthe equipment hold-down bolts. The fact is this is rarely the problem, when the bolts are loose the timeplot is very impulsive (Figure 11). Looseness issues are usually internal clearance and fit issues.Additionally, looseness problems can be the result of foundation cracks, grout crumbling and other issueswith the mounting of machinery. The bottom line is looseness is usually many issues combined causingone large problem. There is not a smoking gun; but, rather a number of smaller issues that add up to alarger problem.

Looseness is identified by a large operating speed (1X) vibration along with multiples of operating speedthat are random in amplitude. This means each multiple of shaft speed is not lower in amplitude than theprevious frequency. The amplitudes are up and down (Figure 20). If the looseness is severe enough therecould also be 1/2X orders of vibration.

Figure 20Spectrum Plot - Looseness

The interesting item about looseness is the time plot (Figure 21) can be harmonic. Many analysts look atthis and believe the dominant vibration is operating speed (1X) and it can be balanced. You cant balance

looseness problems, even if the dominant vibration is 1X.

Figure 21Time PlotLooseness

One trick to help in the analysis of looseness problems is the phase relationship between the horizontaland vertical vibration data. With a looseness problem, the phase relationship between the horizontal andvertical data points will be 0 degrees or 180 degrees. Physically the horizontal and vertical data points

1X

Orders of 1X

Lower Amplitude


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are 90 degrees apart; however, due to the nonlinearity of looseness the phase angle differential will notbe 90 degrees.

With looseness problems; if a balance issue is also present, the looseness problem must be corrected,before the imbalance can be addressed.

Many looseness issues are the result of clearance issues with fluid film bearings (Babbitt Bearings).Bearing clearances for journals where the shaft journal is less than 5.00 inches in diameter, the rule is:

Bearing clearances for journals where the shaft journal is greater than 5.00 inches in diameter, the rule is:

If there are severe looseness issues, such as a rolling element bearing loose on a shaft or loose in thehousing, very impulsive vibration is found in the time plot with difference frequencies in the spectrum(Figure 22). The spacing of the difference frequencies in the spectrum and the frequency of the impact intime plot is normally the speed of the shaft suffering the looseness problem.

Figure 22Time and Spectrum PlotLooseness with Impacting

Rolling Element Bearing Defects are Non-synchronous with the Cage Frequency beingSubsynchronous:Bearing defect analysis can be helped by setting the spectrum plot to show frequency

in orders or using the equipment software to provide the frequency content in the spectrum plot (Figure23 and Figure 24). The frequencies present in the spectrum, other than operating speed, are multiples ofthe ball pass frequency outer race. Additionally; the non synchronous BPFO (1X and Multiples) havesidebands spaced at 12.7 Hz which is the cage frequency (FTF) of the bearing.

Normal vibration amplitudes for severe bearing defects are normally less than .05 in/sec (0-pk) and thesefrequencies are over .30 in/sec (0-pk). Bearing defect frequencies will deteriorate in the axial directionfirst, than progress to the radial directions.


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Figure 23Spectrum PlotNon - synchronousBearing Defect Vibration

Figure 24Spectrum Frequency Content of Figure 22

The time plot from a bearing defect is expected to be impulsive and in most cases it is impulsive.However, if the bearing damage is severe, it is difficult to see the impulsive vibration in the time plot(Figure 25). While the impacts are present (Figure 25) it may be difficult to identify their frequency. In thiscase, it is recommended to only look at a piece of the time plot. This spreads the time plot out (Figure 26).

In Figure 26, the time based data has been spread out (expanded) to better analyze the data forimpacting. Looking at the period of vibration for four cycles of impacting, the period of vibration is .0197seconds which equates to a frequency of 50.72 Hz, however, there were four cycles of vibration utilized.Therefore, the 50.72 Hz needs to be multiplied by four, resulting in an actual frequency of 202.88 Hz,which is the first order if the BPFO defect frequency (Figure 23).

When there are many frequencies present in a time plot, the number of bits of data that makeup eachcycle of vibration may be very small. Using only one cycle of vibration may not provide an accuratefrequency from the time plot. To ensure accurate frequencies analysis from a time plot, it is advisable touse five to ten cycles of vibration to get accurate frequency identification.

1X

1X BPFO


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Figure 25Time PlotBearing Defect

Figure 26Time Plot Expended from Figure #25Bearing Defect

Pump Vane Pass is a Synchronous Vibration Problem caused by Low Flow Conditions orPumping Dense Liquids:Vane Pass is equal to the number of vanes on the impeller times the shaftfrequency. The majority of pump vibration issues are not caused by mechanical problems; but, ratheroperating conditions. The best troubleshooting reference you can supply yourself when analyzing a pumpvibration problem, is a pump curve (Figure 27). For the pump to run reliably and without vibration issuesthe pump needs to be within +/- ten percent of the best efficiency point (BEP).

4 CyclesImpacting


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Figure 27Pump Operating Curve

If operation of the pump gets over 110% of the BEP, the pump will be pumping more water than it isdesigned to pump, and it will start cavitating and have a low discharge pressure and a flow much higherthan designed. The spectrum plot will have broadband noise generated by the flow disturbances in thepump (Figure 28). The time plot will contain random vibration due to the flow noise.

Figure 28Spectrum Plot - Pump Operating on Right Side of the BEP

If the pump flow drops below 65% of the BEP, the pump will operate in a low flow condition with higherdischarge pressure and lower flow then design.

The spectrum plot will be dominated by vane pass (Figure 29). The time plot will contain random vibrationdue to the flow noise. The vane pass is generated by the force vector of the pumping (Figure 30).

When one uses a fixed speed pump and it is operating on the BEP, the shaft speed and pump flowgenerate a resultant discharge vector (green) of water that leaves the pump without restriction. If the flowrequirements of the system drop, the shaft speed remains constant and the discharge vector ( red)

Broadband Noise - Lift Off


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becomes steeper. This results in the fluid being discharged from the impeller to be slammed against thepump housing causing a hammering that results in a vane pass frequency being generated.

Additionally, a hydraulic instability occurs, called low flow shuttling. This causes an axial thrusting of theshaft to occur.

Figure 29Spectrum PlotPump Operating in a Low Flow ConditionLeft Side of BEP

Figure 30Pump Force Vectors

Vane pass can also be caused by pumping very dense fluids such as limestone slurry used in powerplant scrubbers (Figure 31). In these cases, the vibration maybe more of a nuisance issue then an actualproblem. Additionally, pumping against a high head pressure will also cause vane pass.

The forces on the shaft due to the vane pass vibration can cause a bowing of the rotor and this will result

in a high operating speed vibration along with the vane pass frequency in the spectra plot.

6 vanes on impeller


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Figure 31Spectrum PlotVane Pass due to Fluid Density

Blade Pass on fans is a Synchronous Problem caused by Aerodynamic Issues: Blade passvibration on fans is essentially the same problem as vane pass on a pump. This is a synchronousvibration. Blade pass is the number of blades on the fan wheel times the shaft frequency (Figure 32). Thecause of blade pass is a discharge damper that is pinched closed and is causing backpressure on thefan, much as running a pump on the left side of the BEP point.

Figure 32Time and Spectrum PlotBlade pass on FanImproper Damper Position

Gearmesh vibration is a Design Induced Synchronous Vibration:Gearmesh vibration is the numberof teeth on a gear times the shaft frequency. This is a synchronous vibration. Gearmesh vibration isusually a dominate vibration in the spectrum plot. The most common problems with gearing are broken orcracked teeth (Figure 33). The time plot data is very impulsive due to the impacting of the broken,cracked or chipped teeth.

Identifying the shaft causing the problem is done by looking at the spacing of the sidebands around thegearmesh frequency or the spacing of the frequencies in the spectrum plot (Figure 33). This data comesfrom a single reduction gearbox with an 18 tooth pinion gear meshing with a 94 tooth bull gear. Thegearmesh frequency is 535.5 Hz having an input frequency of 29.75 Hz and an output frequency of 5.68Hz.

1X

Blade Pass


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Figure 33Time and Spectrum PlotCracked ToothImpulsive Vibration

The vibration is being caused by the input shaft to the gearbox based on the spacing of the sidebandsaround gearmesh and the impact frequency in the time plot.

Resonance Excitation of a Natural Frequency: All vibration analysts are taught early on that younever operate machinery within +/- 15% of a natural frequency. To have a resonance, you must have aforcing function close to a natural frequency. When the forcing function is within fifteen percent of anatural frequency, the natural frequency will be excited and the vibration amplitudes will be amplified. Onfixed speed equipment it is fairly straight forward to be able to design the shaft and bearings so themachine will operate away from a natural frequency.

The natural frequency of a system is based on stiffness and mass. Increasing mass lowers a naturalfrequency, while increasing stiffness, increases a natural frequency.

(1)

[()()] () (2) [()()] (3)M = Mass (lb-sec

2/in)

K = Stiffness (lb/in)fn= Natural Frequency (Hz)

As a general rule, large equipment has a low natural frequency due to its mass and small light weightequipment has a high natural frequency.

Symptoms of resonant condition include balance sensitivity, high operating speed (1X) vibration and theamplitude in one radial direction six times higher than the other direction. Balance sensitivity is a conditionwhere a piece of equipment always requires balancing, indicating the equipment is operating close to anatural frequency.

Figure 34 is a resonance issue on a fixed speed motor driving a single stage pump. The operating speedof the pump is 3565 rpm (59.41 Hz). The vibration data indicates a high spectral component at 58.44 HZwith a somewhat harmonic time plot. Impact tests confirmed this natural frequency at 58.45 Hz.Operating speed is within 15% of the natural frequency and it is exciting the natural frequency causing aresonant condition.

Sidebands

Frequenciesspacing at input

speed

Impacting at input speed


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Figure 34Time and SpectrumFixed Speed Resonance Excitation

Variable speed equipment is very susceptible to operating close to a natural frequency due to the widespeed range over which they operate. Figure 35 is data from a VFD driven motor pump combinationoperating at 1643 rpm. The same equipment (Figure 36) operating at 1190 rpm has vibration amplitudes

that have increased tenfold due to operation near a natural frequency. Figure 37 is a cascade plot ofroute data showing the amplitude changes when the shaft speed is close to its natural frequency.Figure 38 is an impact test of this VFD driven pump.

Figure 35Spectrum Plot VFD driven MotorOperating at 1643 rpm

Figure 36Spectrum Plot VFD driven MotorOperating at 1190 rpm


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Figure 37Cascade plot of Monthly reading showing amplitude when operating at Natural Frequency

Figure 38Impact Test of VFD Motor Pump in Figure 35 and Figure 36

Resonance due to VFD electrical excitation:The electrical excitation frequency of the VFD can be aforcing function if it is close to the equipment natural frequency. This electrical excitation frequency canbe calculated from the following formula:

Figure 39 contains data from a gearbox driven by a VFD motor when the electrical excitation is 31.00 Hzand the shaft speed is 612 cpm (10.20 Hz). This is operating away from its natural frequency. The samemachine operating when the electrical excitation frequency is 33.33 Hz and the shaft speed is 652 cpm(10.86 Hz) can be observed in Figure 40. The electrical excitation frequency is exciting the naturalfrequency of the structure supporting the gearbox. Confirmation of the excitation from the electricalfrequency can be found in Figure 41. When the power is cut, the vibration dies immediately and when themotor is turned back on the vibration immediately returns.


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Figure 39Electrical Excitation away from Natural Frequency

Figure 40Electrical Excitation at Natural Frequency

Figure 41Compressed Time Plot - Vibration Data at Electrical Cut Off


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Conclusion:Vibration analysis does not have to be difficult, if the analyst takes a systematic approachand rules out what is not the problem, rather than trying to analyze for what is the problem. Look atthe type of vibration that is present in the time plot: Harmonic, Periodic, Impulsive, Pulsating orRandom vibration. Then group the frequency content present in the spectrum into: Synchronous, Non-synchronous, Subsynchronous or Electrical.

Use the type of time plot vibration and the frequency content groups to narrow down the vibration cause.Analyzing vibration is the enjoyable part of being involved in the vibration field. Use the information thedata provides to identify the cause of the vibration.


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Important Troubleshooting Equations:

() ()

() ()

() ()

() ()

()()() ()

guy -time and spectrum analysis paper - part i

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