THE SOCIETY OF NAVAL ARCHITECTS AND MARINE ENGINEERSOne WorldTradeCenter,Suite1369,New York,N.Y,10048

Pawr 10 be Dre,mted at the Ship Wbr.ation SymposiumArlington, V..,October16-17,1978

Vibration Signature Analysis as aMaintenance Tool Aboard ShipJ. B. Catlin, Jr., Visitor, IRD Mechanalysis, Inc.,Columbus, OhioABSTRACT

This paper describes vibrationmeasurements of shipboard machines whichare now being utilized in a program ofpreventive maintenance as a supplement

*O standard shipboard maintenance pro-cedures. While similar in many respectsto vibration measurements used ashore,the shipboard program must take intoaccount environmental vibrations thatoccasionally mask the vibration of ship-board machines. Illustrations are shownof these environmental vibrations thatare caused primarily by propeller bladepassing frequencies and random turbu-lence from the propeller and hull.

Implementation of a shipboard prog-ram using two portable instruments,a vibration meter and an analyzer/XYrecorder, is briefly described. Thevalue of such a program results from itsability to detect machine deteriorationin its earliest stages while there issufficient time to correct problemsbefore they reach the critical stage.Examples of vibration signatures indi-cating machinery defects are given.

Measurement techniques and proce-dures are also discussed which have beendeveloped to pinpoint defects and sepa-rate environmental vibrations fromthose associated with the machines.

1NTRODUCTION

The use of vibration analysis as aroutine preventive maintenance toolaboard ship is only about ten years old,although there are a number of isolatedcases of its use prior to that time.Even today its widespread use is juststarting because, as with any new tech-nique, it has required time to gain theconfidence of both management and oper-ating personnel before being routinelyaccepted as a standard tool.

The marine industry’ s efforts inthe area of vibration analysis are notcompletely without precedent, sinceindustry ashore has been utilizing thispreventive maintenance technique foralmost 40 years. It is probably safeto say that most large industries

Preventive

thrcmghout the world, and countlesssmall, now rely on “ibration analysisas the backbone of their preventivemaintenance programs. The reason forthis is, of course, vibxation analysis’unique capability to diagnose machinerymechanical condition while a machine isoperating, and pinpoint e“en minor mech-anical problems (unbalance, defectivebearings) while they are still in theincipient stage.

There are, however, significant dif-ferences between the application ofvibration analysis aboard ship and thatashore, so it is not surprising that thetransfer of technology has not been morerapid.

Perhaps, the bigqest difference be-tween vibration analysis ashore andvibration analysis aboard ship is envir-onmental vibration. Although environ-mental vibration is occasionally encoun-tered in fixed machinery installationsashore, it is certainly not as pervasiveas aboard ship.

When it does occur aboard ship ittends to mask a machine’s self-generatedvibrations and thus obscure the truemechanical condition of the machine.It, therefore, requires a differentselection of measurement instruments,different measurement procedures, anddifferent interpretation techniques fromthose used ashore.

For purposes of this paper en”iron-- vibration can be defined, at leastas it relates to vibration analysis ofshipboard machines, as follows:

Environmental vibration is thatvibration, measured on a machine, whosesource is other than that directlyresulting from the operation of thatmachine.

For example: vibration of the mainH.P. turbine aboard ship which is causedby vibration from the propeller and mainreduction gears would be consideredmachine vibration associated with theoperation of the H.P. turbine. However,vibration of the ship service turbo-

generator which is caused by the propel-ler would constitute environmental“ibration, since the propeller is not

directly associated w~th- the operationo f the turbo-generator.

Aboard ship the predominant sources

of environmental vibration come fromthe blade passing frequencies and flowturbulence of the propeller Plus thewave action and flow turbulence imping-ing on the hull. To a lesser extent,

there can also be environmental vibra-tion caused by the transmission of onemachine 1 s vibration to another.

In the sections which follow,

instrumentation and measurement tech-niques are discussed which have beensuccessfully used to implement vibra-tion signature analysis as a preventivetool aboard ship. Examples are given~f vibration signatures which uncovereddefective machines. Further examplesshow cases where environmental vibra-tions of different types were encoun-tered, and methods used for analysesunder these conditions.

INSTRUMENTATION AND PROCEDUSXS FOR ASHIPBOARD VIBBATION MEASUREMENTPRSVENTIVE MAINTENANCE PROGBAM

A complete shipboard vibrationpreventive maintenance program involves

the use of a handheld vibration meterand a portable narrow band vibrationanalyzer which can be connected to an

XY recorder to produce hard COPY vibra-tion signatures. Figures 1 and 2 show

examples of such instruments.

Fig. 1 Example of MarineVibration Meter

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Fig. 2 Example of VibrationAnalyzer with XY Recorder

In this paper the use of the hand-held vibration meter will not be speci-fically discussed; however, it should bementioned that the reason for utilizingtwo types of instruments is to minimizethe measurement time required while stillmaintaining the capability to detect andpinpoint machinery problems in theirincipient stages. The vibration meteris used for quick periodic checks of themachinery to determine whether it is ingood mechanical condition, or in need ofmaintenance and/or repair. On the otherhand, the analyzer is used to pinpointany defects detected by the vibrationmeter.

To startup a program aboard ship,it is necessary to take a complete setof vibration meter readings and a set ofvibration signatures with the analyzer.These may be taken in port or at seadepending upon the ship 1s schedule andthe effects of environmental vibration.This is a vital step which serves twopurposes:

First, it provides baseline dataagainst which future measurements can becompared. While all machines have cer-tain common predictable vibration ampli-tudes and frequencies, they also haveindividual characteristics which are theresult of manufacturing, assembly toler-ances, and their foundationing. By tak-ing baseline data at the start of aprogram, these individual characteris-tics can be accounted for, and greatlysimplify the task of pinpointing defects,should any occur at a later date.

Second, it provides a means ofevaluating the machinery mechanical con-dition at the start of the program.Although it is not necessary to runmachines at maximum speed or load forthese baseline measurements, it isimportant that any subsequent measure-ments be taken under the same operating

conditions. while this initial baselinedata will not provide the details ofmachinery condition because no previous“ibration measurements “ill be availableforthe

comparison;significant

nevertheless, most ofand even some minor

L-----

—.

problems can be caught with this data.

Once this set of measurements iscomplete, a schedule of periodic,routine vibration meter checks is setup to coincide with the ship’s operat-ing schedule. These checks are designedto catch any machinery mechanical prob-lems before they become serious, andprovide the lead time necessary toenable the ship to correct the problemat a convenient time.

No additional vibrat ion signaturesare required after the initial baselinesignatures are taken until such timethat the vibration meter indicates amechanical problem in the machine. Insuch case tbe analyzer and XY recorderare brought back aboard ship, and a newset of signatures obtained for themachine. These are then compared withthe baseline measurements for thatmac%ine to indicate the specific fre-quencies which have changed, thereby,pinpointing the source of the trouble.

As was implied above, it is onlynecessary to maintain the “ibraticmmeter aboard ship. The analyzer, be-cause of its relatively infrequent use,can be left ashore at some strategiclocation; and brought to the ship onlywhen necessary to analyze a problem.

Limiting the shipboard equipmentto a simple handheld meter is desirablefor several reasons. First, the crewsaboard ship have been reduced to apoint where there are very few manhoursavailable for special tasks. By usingthe vibration meter, measurementsrequire only a few seconds; hardly morethan the time it takes for a man comingon watch to place his hand on machinebearings to check their temper.at”re.Recording these measurements also takesonly a matter of minutes. Second, thesimplicity of meter operation and datainterpretation requires a very minimumof training, so it is not necessary forcrew members to attend any lengthytraining sessions.

The analyzer, on the other hand,which requires somewhat more skill inits operation and data interpretation,can be handled by sboreside personnel,such as a port engineer, who would bemore readily available for training.

At this point it might be worthnoting that while the primary benefitof vibration analysis aboard ship isderived from its use in preventivemaintenance, it can also prove valuablein a number of other areas, namely:

1. As an inspection tool for newconstruction to “erify mechani-cal condition of main andauxiliary machinery prior toship delivery.

2. As a troubleshooting techniqueto pinpoint tbe cause of sus-pected machine faults.

3. As a means of verifying thatmachinery repairs have beendone correctly.

4. As a pre-overhaul technique toaid in determining what repairwork is required.

5. As a post-overhaul techniquefor verification that repairwork has been properly accom-plished.

The specific signatures shown inthe figures of this paper which followwere for the most part generated with amanual lY tuned s“ept frequency narrowband (5 percent bandwidth) analyzer whichwas connected to an XY recorder to pro-vide the hard copy. All measurementswere made on the bearing housings of tbemachines with a velocity type transducer.These signatures were tbe result ofpreventive maintenance program startups,troubleshooting, as well as pre and postoverhaul measurement.

SOME EXAMFLES OF MACHINERY PROBLEMSPinpOinted BY SIGNATURS ANALYSIS

some marine engineers are able todetect defects in shipboard machines bychanges in sound or touch, although thiscapability requires considerable know-ledge and skill plus extended timeaboard the ship.

As an aid to the shipboard engineer,vibration siqnature analysis offers twoprimary advantages for machinery defectdetection. It provides a positive indi-cation of specific faults such as un-balance, misalignment, or defectivebearings which would be difficult todistinguish by sound or touch, and whichwould not be indicated by temperature orpressure. It also gives a quantitativenumber (i.e. , vibration level) which canbe used to evaluate the severity of thedefect, and aid in the decision as towhether to shut down or to run; and ifthe decision is to run, how long it willlast. Guidelines indicating acceptablemachinery vibration levels are availablefrem: machinery manufacturers, indus-trial associations, U.S. Navy (1) ,SNAME (2) , lRD Mechanalysis, lnC. (3) ,and others.

By using vibration signature analy-sis along with the traditional inputsof temperature, pressure, sound andtouch , the engineer is in a much betterposition to minimize operational prob-lems. This also gives the shipboardengineer the opportunity to reduce open-and-inspect work, as well as minimizingrepairs through knowledge of the spe-cific repairs which are needed.

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Given below are some case hist~rieswhich illustrate the use of vibrationsignature analysis as a preventivemaintenance tool.

Forced draft blowers are one of the

types of shipboard machines which aremore prone than others to mechanicalproblems. Figure 3 compares the “ibra-tion signatures for two of these “nitswhich were made during a pre-overhaulvibration check as an aid to machinery

overhaul planning. The one forced draft

7,6(.3)

;,

,ROTATION MEASURE pOSITlON “A”

_

) IK 2K 3K 4K 5K IOK FA3K

FREQUENCY - CYCLESIMIN

Fig. 3 Comparison of VibrationSignatures of TWO Forced

Draft Blowers

blower, unit A, was found to be in verygood mechanical condition with lowamplitude of about 1.8 nnn/s peak (.07

in/s peak) or 20.3 Jlm peak-peak (.8roils peak-peak) , at the rotationalfrequency and no other vibrations ofsignificance. The other blower, how-ever, shows a major vibration peak of12.4 n!m/s peak (.49 in/s peak) or 152.4

)m peak-peak (6 roils peak-peak), atrotational frequency caused by rotorunbalance. No other important vibra-tion peaks were found indicating that,except for the unbalance which wasrecommended for correction to avoidrapid wear of the bearings, the blowerwas in good mechanical condition.

On board ship during the same pre-o“erhaul vibration survey defecti”eball bearings were found on the turbinedriven tank washing pump. To show thevibration signature characteristics ofthese bad bearings, Figure 4 comparesthe “ibration on this pump bearing,

Unit A, with that of an identical pumpon a sister ship which had bearings ingood mechanical condition. As is often

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zz

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MEASURE

W

MEASURE POSITION”C”

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UNIT B

.IK 2K 3K 4K 5K IOK 50K

FREQUENCY- CYCLES/MINFig. 4 Vibration Signatures Show-ing Difference Between Good andBad Ball Bearings on Turbine

Driven Tank Washing Pump

tYPiCal of defective bearings, a highfrequency broadly peaked region of vibra-tion occurs. In this case it is seenfrom 40,000 to 60,000 cycles/rein. Thelower peak from 20,000 to 30,000 cycles/min is also attributed to the bearing.In contrast, the same pump bearing onUnit B shows no vibrations of signifi-cance in the frequency regions 10,000to 60,000 cycles/rein. This Unit B, how-ever, does exhibit moderate amount ofunbalance as indicated by the 6.1 mm/speak (.24 in/s peak) or 38.1 J!.mpeak-peak(1.5 roils peak-peak), vibration at pumprotational frequency.

As a further comparison, Figure 5shows the bearing housing of both theturbine and the pump. of particularnote is the fact that the bearing vibra-tions are not transmitted to the turbinewhich makes identification of the defec-tive bearings quite straight fon+ard.

On still another ship a main cir-culating pump was ob.ser”ed to be “ibrat-ing excess iwsly. A vibration signatureanalysis re”ealed that it was not “nbal-ance as was suspected, but misalignment,since the large “ibration peak was occ”r-

—.

‘Ow3?$ r MEASURE POSITION “ B“

<ROTATION

FREQUENCY- CYCLES/MINFig. 5 Comparison of Vibration

Signature on Turbinewith that on Pump

\ ring at twice rotational frequency ratherthan at rotational. This is shown inFigure 6. The effect of this misalign-ment was aggravated by the fact that the

MEASURE POSITION”A”

MEASURE FCSITION”A”

!OTATION

i

) IK 2K 3K 4K 5K IOK 30KFREQUENCY. CYCLES/MIN

Fig. 6 Example of Main CirculatingPump with Shaft MisalignmentProblem which was Aggravated

by a Resonant Foundation

machine foundation had a natural fre-quency which coincided with twice rota-

tional frequency. This frequency waschecked by “bump” test in which thefoundation was bumped and resultingnatural frequency automatically regis-tered on the analyzer.

The very high level of about 20.3

mm/s peak (.80 in/s peak) , or 254,Limpeak-peak (10 roils peak-peak), calledfor priority action to avoid excessivewear on the coupling and bearings. P rOp-

er alignment was recommended to reducethe vibrations to an acceptable level,although it was also suggested that thefoundation be stiffened in the athwart-ship direction to eliminate the resonantcondition. Again, for comparison p“r--poses, an identical pump and motor unit,with the exception of the foundationwhich had considerably less height, isshown as Unit “B” in Figure 6. In thisunit, no important vibrations are presentindicating that it is in good operatingcondition.

Another forced draft blower whichwas checked is shown here in Figure 7 as

MEASURE P3SITION”E”

1f’ROTATION _

l/2 x ROTATION

)JLhh-&_SEARINGS//\

FREQUENCY- CYCLES/MINFig. 7 Forced Draft Blower withMultiple Problems: Misalignment,Looseness, Bearing Deterioration

an illustration of a unit having multipleproblems. This measurement was on apillow block bearing supporting theforced draft blower shaft. The twicerotational frequency peak which is great-er than the rotational is indicative ofa small amount of misalignment, whilethe multiple harmonics of rotationalshows some looseness in the bearing. Inaddition, the broad-band vibration ex-tending from below 5,0OO cycles/rein to

P-5~

above 20,000 cycles/rein shows some minorbearing deterioration. While this analy-sis indicated that the beari”q couldprobably operate for some time, it wasrecommended that “ibration checks be madeat frequent intervals to watch for fur-ther deteriorations, and plans made tochange bearings at a convenient time inport .

In the sections which follo”, theeffect of environmental “ibration frompropeller, hull and adjacent machineswhich occasionally interfere “ithmachinery measurement is discussed.

HULL VIBRATION GENERATED BY SHAFT ROTA-TION AND PROPELLER BLADE FREQUENCIES

To obtain a true indication of amachine ‘ s mechanical condition it isnecessary to eliminate the effects of anysignificant environmental .?ibration fromthe vibration measurement. It is worth-

while, therefore, to identify the charac-teristics of those environmental vibra-tions which may affect vibration readings .

As a result of vibration rneasure-

Lments on a nunOJer of ships, scone tentat-ive observations ca” be made concerninghull environmental “ibration. Theseobservations are limited primarily tovibration measurements on auxiliarymachinery, and not the hull girder itself.

The “ost significant of the hullenvironmental vibrations is generallycaused by the nropeller blade frequency(i.e. number of propeller blades timesshaft RPM) and its hazmonics. on presentmerchant ships fundamental blade fre-quencies typically run fvmn a low ofabout 32o cycles/rein to a high of 600cycles/rein. The higher harmonics, asmeasured at the thrust bearing, qenerallydrop off in amplitude, with the secondharmonic, perhaps, one-half, or less,the amplitude of the f“ndamental and thehigher harmonics still lCJWI-. An exampleof this is shown in Figure 8 for a com-mercial ship at full speed in calm seas.

Such is not always the case for propellerblade frequencies meaa”red on the a“xili-ary machines. The second, third, or e“enfourth propeller blade harmonics can ha”eamplitudes almost as high or higher thanthe fundamental. The measurement datasuggests that this is caused by thenatural frequencies of the auxiliarymachines and their foundations, “hichaPPar~ntlY are often in the range ofthe higher harmonics rather than thatof the fundamental.

While the amplitudes of these fre-quencies nary from point to point becauseof different structural transmissionpaths and local natural frequencies , forengine rooms located at the stern ampli-tudes appear to be typically highest cmmachines farthest aft, and decrease withdistance forward and upward in the ship.

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(.3) MEASURE FuSITION “A”

~

I /

‘XBLADERATE d

! 3.8 ,2x BLADE RATE FWD./ ,., ,

4Un

<

=’-”1I

A,/3xBLAOE RATE

:oL500 IK 2K 3K 4K 5K IOK 50K

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FREQUENCY. CYCLES/MINFig. 8 HuI1 Girder Vibration As

Measured at Thrust Bearinq

This observation is based upon limited

data, ho”ever, and it is recognized thatthere are cases where low vibrations of

the hull can be magnified at the bridge *or other areas of a ship.

From the standpoint of reduced en-vironmental “ibration, the engine roomlocated at some distance from the sternoffers considerable ad”antaqe; hows”er,most present designs place the engineroom as far aft as possible to increasecargo carrying capability.

Many other factors also affect themachinery vibration amplitudes of thepropeller blade frequency vibrations onthe auxiliary and propulsion machines.Ship displacement, horsepower, hull form,stern design, hull “ibration modes , wakevelocity profile in way of the propellerand the draft .a”d trim are some of themore important of these.

In proceeding with a machinery “ib-ration measurement program, there aremany ways in which tbe effects of propel-ler blade frequency “ibrations can beeliminated.

1. It should first be stated thatin many instances the propellerblade vibrations are of suffi-ciently 10” amplitude as to beof no consequence in the machin-ery measurement. Figure 9 com-pares the vibration siqnature ofan SSTG set in port with thatobtained underway with the shipat full speed. The in-port andat-sea loads were 160 kw and190 kW, respectively. For thisparticular case the “ibrationsignatures are almost identical.Propeller “ibration frequencies

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are completely absent.

MEASURE POSITION “C” ~— GEN, ROT. : y y

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160KW IN PORT

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A T6:(.03) MEASURE POSITION “C”

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& ~h‘GENROTEmma$j UNIT A

z=

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IIi190KW AT SEA

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—TURB. ROT,

i r-2 xTURB. ROT.

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00 IOK 20K 30K 40K 30K

FREQUENCY CYCLES/MIN

Fig. 9 Comparison of In-Port andAt-Sea Vibrations of a ShipService Turbogenerator Se’c

2. It is also often the case thatthe propeller blade frequenciesof significant amplitude arebelow the frequency range ofinterest for the machine beingmeasured. The majority ofshipboard machines have rota-tional speeds above 1400 r/rein,while propeller blade frequen-cies of significance are usuallyof 1200 cycles/rein and below.Under these conditions an instru-ment measuring overall vibrationwhich includes a suitable highpass filter can be used toeliminate tbe propeller vibra-tions and measure only thosevibrations associated with themachine itself. The vibrationsignature will also be unaffec-ted by such propeller vibra-tions, as shown in Figure 10.

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MEASURE POSITION”A”

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‘Ix BLOWER ROTATION

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FREQUENCY CYCLES/MINFig. 10 Vibration Signatures Show-ing Case Where Propeller Blade Fre-quencies are Lower Than Frequencies

Associated with the Machine

3. Where significant propellerblade frequencies extend upinto the frequency region ofmachine vibrations, it is oftenstill possible to separate thetwo when the propeller vibra-

tions occur at different fre-quencies from those of themachine. In addition, propel-ler bIade frequencies typicallyhawe a distinctively differentcharacteristic appearance thanthose generated by machineryproblems such as unbalance andmisalignment when analyzed bya narrow band slowly sweepingfrequency analyzer. It shouldbe mentioned that a fast sweep-ing analyzer will not showthese differences. This prO-Videz a further means of iden-tifying and separating the twosources. Even when propellerblade frequencies and machinefrequencies coincide, the dif-ference in the characteristicsstill enables the operator toobtain an indication as to whichof the two vibrations is domin-ant. Figure 11 illustrates tbedifferent appearance of thesetwo types of vibrations. Thisdifference is caused by the factthat the propeller blade fre-quency vibrations are generatedby a random (flow) forcing func-tion “hich produces a “hashy”vibration peak, whereas machineunbalance and misalignment arecaused by mechanical forceswhich are typically almost sinu-soidal and produce a smooth

P-7

vibration peak.

76EASURE POSITION “B” O ~ B A

,,,

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FREQUENCY. CYCLES/MINFig. 11 Illustration Showing theDifference in Appearance BetweenPropeller Blade Vibrations andVibrations Caused by MachineUnbalance and Misalignment

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4. When, as it infrequently occurs,the propeller blade vibrationsare coincident and/or dominatethe machine vibration spectrum,the machinery vibration measure-ments should be made only atreduced ship speed (with conse-quently lower propeller bladevibrations) , or in port. Severalexamples which follow comparemachine vibration signaturestaken at full ship speed withthose taken in port.

Figure 12 compares the at-sea within-port signatures of a large verticalshaft ballast pump. The at-sea measure-ment was with the ship at full speed,calm sea and normal loaded draft. Theat-sea signature is dominated by thefirst, second, third and fourth harmonicsof the propeller blade frequencies. Inthe in-port signature it is interestingto note that the frequency at the secondharmonic is still present, and is, infact, not primarily caused by the propel-ler, but by tbe pump’s own turbulent flowwhich is exciting a natural frequency inthe pump and its foundation.

Figure 13 shows the in-port and at-sea signatures for a main circulatingwater pump. Again, the measurement atsea was taken with the ship at full speedin calm seas with normal load draft. Thefirst and fourth propeller blade fre-quencies control the at-sea signature tmaking it necessary to measure this unitin port in order to determine themachine’s true vibration signature.

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MEASUREFOSITION ‘8A”

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—A

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,0 ,500 IK 2K 3K 4K 5K IOK 50K

FREQUENCY. CYCLES/MINFig. 12 Comparison of In-Port and

At-Sea Vibrations of LargeVertical Shaft Ballast Pump

5.3,.,lr.,=...,0=OnCITIfW,’A” A+~

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a 3.8~(.15) “ (1 ~~:y

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FREQUENCY CYCLES/MINFig. 13 At-Sea and In-Port VibrationSignatures of Main Circulating WaterPump Showing Presence of Propeller

Blade Frequencies

r

HULL vIBRAT1ON GENE~TED By HULL ANDPRDPELLER RANDOM FLOW TURBULENCE

Machine vibration caused by hulland propeller generated random flow tur-

bulence is not often a problem in machin-

ery vibration measurement but can OCCaS -s~onally result in masking a machine’svibration signature. Impact type vibra-

tions can also occur which are causedprimarily by wave action, but these aregenerally intermittent and do not have asignificant effect on the vibration sig-natures. It is the random turbulence

generated vibration which is not easy tomeasure and identify because it lacksthe distinctive discrete frequencycharacteristics which are associated withmachine vibrations and those of the prop-eller blade frequencies. Identificationis primarily by deduction through theprocess of elimination of identifiablefrequencies. This process is aidedsomewhat by the fact that it’srandombroad band nature produces a distributed“hashy” type signature when plotted by aslow swept narrow band frequency analyzer.This distributed energy often forms intobroadly peaked frequency regions, andoccasionally is exhibited as fairly&arply pronounced frequency peaks. Thispeaking appears to be the result of weak,or strong excitation of local hull and/orfoundation structural natural frequencies.The extent of the peaking is felt to berelated to the degree of damping in thestructure, although some periodic flowvortex generation could be a contributingf actor.

As might be expected, flow turbu-

lence appears to be most pronouncedtoward the stern; and, therefore, whenit is seen it generally affects machin-

e~ :nstalled close to the stern andparticularly those mounted low in tbeship.

Since the various pumping systemsaboard ship can develop flow excitedvibrations in themselves as a resultof the fluids passing throuqh the pumpsand piping, it is often difficult toseparate these vibrations from thosegenerated by the hull and propeller flow.In such cases, where it is suspectedthat hull and/or propeller may be contri-buting to the vibrations measured on apumping system, it is necessary to takemeasurements in port or at reduced shipspeed where hull and propeller flow werefelt to be contributing , as shown inthe following examples:

This first example, shown in Figure14, involves vibration measurements on a

general service pump located on the lowerlevel of a ship with a stern configuredengine room. The two signatures showthe same measurement point for the pumpoperating at slow speed under calm at-sea conditions with the ship at fullspeed, and in-port also with the pump at

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(.21) MEASURE POSITION “A”

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UNIT A

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—A

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FREQUENCY CYCLES/MINFig. 14 Comparison of At-Sea andIn-Port Signatures of GeneralService Pump Which Shows theCharacteristics of Hull andPrope her Random Turbulence

Flow Excited Vibrations

slow speed. While there are some indi-cations of propeller blade frequencies,

the broad band nature of the at-seasignature is attributed to the hull andpropeller random turbulence flow excited“ibrations.

The second example, shown in Figure15, illustrates the effect of hull andpropeller generated flow vibrations ona main circulating pump. Again, thispump was installed low in the ship inan aft located engine room. The twoconditions under which the vibrationmeasurements were taken were: undercalm sea state with ship at full speed,and in-port. It can be seen that thevibration signature of the at-sea mea-surement from 600 cycles/rein to about4,000 cycles/rein was dominated by prop-eller and hull vibrations which includeboth the propeller blade frequenciesand the random turbulence.

NACHINE VIBRATION CAUSED BY TRANSMISSIDNOF VIBRATION FROM ADJACENT MACHINES

A ship’s ,hull and associatedmachine foundationing would appear tobe an ideal transmission path for vibra-tory energy, and that vibrations occur-

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‘REQUENCY. CYCLES/MINFig. 15 At-Sea and In-Port Signa-tures of Main Circulating PumpWhose At-Sea Signature was Domi-nated bv Propeller Blade Fre-

quenci~s as-well as RandomTurbulence

ring in one machine would be readilytransmitted to adjacent machines withresulting difficulty i“ vibrationmeasurement. Surprisingly, however, forthe conditions encountered in machinevibration measurements, this does not

aPPear tO be a problem in the majorityof cases. Several reasons can be sug-gested for this:

1. Adjacent machines often involvespared units, which means thatonly one unit is running at onetime.

2. The inherent design of tbefoundations makes the structuralpath a relatively poor tranmnit-ter because of changes in struc-tural impedance; auxiliarymachine foundations generallyhave a relatively low mechanicalimpedance in comparison to theship frames, longitudinal andcolumns .

3. Machine vibratory energy isdissipated by transmission inmany directions, rather than bybeing transmitted solely to anadjacent machine.

Of course there are occasional cases

where there is noticeable coupling: butthis usually occurs where a machine isvibrating excessively because of amechanical problem, which is readilyrecognized and pinpointed.

One specific example of vibrationtransmission involved adjacent ForcedDraft Fans. These Fans were mounted onstructural foundations as shown inFigure 16. Both were required to be

“%A

Fig. 16 Diagram showing the Struc-tural Foundations of Two

Force Draft Fans

operating at the same time under normalsteaming conditions. The two adjacentsupport columns at position (A) wereclose together and supported by. the samestructural member. The increased stiff-ness at this point was not sufficient toprevent tbe transmission of the fanvibration between the two F.D. fans.Measurements on both fans indicatedhigh vibration levels at rotationalspeed indicating major rotor unbalance.This is shown in Figures 17 and 18.

21.3_(.84)

[A

— ROTATIONMEASURE POSITION “O”

[~~,,,

: ,.

-M- FWO

2K 3K 4K 5K OK 50K

z FREQUENCY CYCLES/MINFig. 17 Signature of Forced DraftFan #1 Whose Unbalance Vibration

Caused a Beat with Fan #3P-lo

?----

16.6~(.66)

$ 4.’sr

L (.!6)

I

J,-ROTATION

MEASURE POSITION “D”

J\ -

UNIT 3

AT SEA

o t500 IK 2K 3K 4K 5K IOK 50K

FREQUENCYCYCLES/MINFib. 18 Signature of Forced Draft

Fan #3

What indicated the vibration transmis-sion between the two units, bowever, wasthe wide, slowly varying amplitude atrotational speed on each machine indi-cating a “beat” between the vibrationsof both machines. When each machine wasrun individually, (i.e. one at a time) ,the beat completely disappeared, andthe mechanical condition of each machinecould then be evaluated. As seen inFigures 17 and 18, machine #1 with alevel of 21.3 mm/s peak (.84 in/s peak)at rotational speed had significantlymbre unbalance than machine #3 with anunbalance level of 16.8 mm/s peak (.66in/s peak) .

lnterestinglYr measurements Of twoidentical forced draft fans, with the

same foundationing, located on the oPPo-site side of the ship, had somewhat lowervibration levels and had no indicationsof a beat frequency. The signatures ofthese machines are shown in Figures 19and 20. This showed that by balancing

6,1(,24)- MEASURE POSITION“D”

— FWO

~

wah ATSEA\

= 500 IK 2K 3K 4K 5K IOK 50K

FREQUENCYCYCLES/MINFig. 19 Signature of Forced DraftFan (1f3entical to #1 in Figure 17)

Which Exhibited No Beating

A B c D

~ (%- MEASURE FOSITION “O”—ROTATION

-FWO

wn

<3,6 - UNIT 4

z (.15) AT SEA=

300 IK 2K 3K 4K 5K IOK 50K

FREQUENCY CYCLES/MIN

/ I 1 \ [//////////////)

Fig. 20 Si9natUre of Forced DraftFan (Identical to #3 in Figure 18)

Which Exhibited No Beating

the machines “ith tbe high vibrationlevels that the problem could be solved;and that it would not be necessary tostiffen the foundationing at point A,although this might be desirable toreduce the vibration coupling betweenthe machines. It also showed that forlow vibration levels the structural pathwas not sufficient to interfere withpreventive maintenance vibration checksof each machine, even when both wererunning.

SUMMARY

The use of vibration measurementaboard ship for preventive maintenanceis only about ten years old, but hasproved to be a valuable supplement toother standard shipboard maintenanceprocedures. Vibration Measurementaboard ship differs from that ashorebecause of environmental vibration whicharises primarily from propeller bladepassing frequencies, random turbulencegenerated by the propeller and hull,wave action and, to a limited extent,transmission of vibration from onemachine to another.

A shipboard program can be imple-mented “ith two portable instruments: ahandheld vibration meter which is usedfor quick periodic checks of machinecondition, and a vibration analyzer/XYrecorder which provides graphic vibra-tion signatures for each machine topinpoint defects detected by the vibra-tion meter.

Many different mechanical problemscan be caught in their incipient stageusing “ibration measurement which thenpermits planned corrective maintenance,rather than emergency shutdowns. Someof the more important of these include:unbalance, misalignment, defectivebearings, looseness, and gear problems.

P-ll-

J---

—- ———

Under some conditions environmentalvibrations can interfere with the truemeasurement of machine vibrations. Anumber of procedures are a“ailable fordealing with this. Special electronicfiltering in the vibration meter willreduce the effect of low.frecf”encypropeller vibrations, and careful sig-nature analysis can separate environmen-tal vibrations from those of the meichine.For some machines in areas of highenvironmental vibration it nay be neces-sary to carry o“t the measurements inport or at low ship speeds. Where en-vironmental vibrations are caused byvibration transmission from adjacentmachines, foundation stiffening may berequired, but more often the problem canbe solved by reducing the vibrations ofthe offending machine by balancing, oras otherwise required. This impro”esthe machine 1.snwcb.anical condition andeliminates the environmental “ibrationproblem.

Machinery preventive maintenanceand a knowledge of each machine, s condi-tion is of vital concern aboard ship forseveral reasons: 1) ships are often atlocations where it is difficult to obtainspare parts and repair work, 2) ship-board machinery is forced to functionin a hostile environment of corrosionand “ibration, and 3) a machineryfailure at sea is always a serious andsometimes dangerous occurance. Againstthis , vibration measurement provides onemore preventive maintenance tool for theshipboard engineer.

REFERENCES

1. “Military Standard MechanicalVibrations of ShipboardEquipment, ” MIL-sTD-167B(SHIPS), August, 1969.

2. “Acceptable Vibration of MarineSteam and Heavy-duty GasTurbine Main and AuxiliaryMachinery Plants, ,’T & R CodeC-5, SNANE, September, 1976.

3. R. L. Baxter and D. L. Bernhard,“Vibration Tolerances forIndustry, ” ASME 67-PEM-14 ,April, 1967. (Available asReprint No. 985 from lRDMechanalysis, Inc. , Columbus,

Ohio. )

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