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Vibration Case HistoriesVibration Case HistoriesBarry T. CeaseMeadWestvaco

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IntroductionIntroduction

Case History#1 – ExcessiveVibration Of Motor

Case History#2 – Strange Motor &Gearbox Vibration

Case History#3 – Repeat FanBearing Failures

Case History#4 – Pump BearingCage Failure

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Case History#1,Case History#1,Excessive Vibration Of MotorExcessive Vibration Of Motor

EQUIPMENT: Nash 9000 seriesvacuum pump driven by a singlereduction, parallel gearbox and a700 HP synchronous motor.

The vacuum pump is one of manyused to pull water out of the paper(dry it) as it moves thru theprocess.

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CH#1 – Equipment LayoutCH#1 – Equipment Layout

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CH#1 - ProblemCH#1 - Problem

Excessive & noisy vibration atmotor.

This problem could be heard & feltclearly at the motor.

It sounded like something wasrubbing and/or loose.

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CH#1 – Test Data & ObservationsCH#1 – Test Data & Observations(Motor)(Motor)

Motor speed was exactly 400 rpm. Pump speed was 267 rpm. Pump vibration data showed no

abnormal patterns or levels. Motor spectra showed many

harmonics of running speed and of133 cpm (1/3 x rpm).

Motor waveform showed impactingat 400 & 133 cpm.

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CH#1 - Motor SpectraCH#1 - Motor Spectra

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CH#1 – Motor Spectra ZoomCH#1 – Motor Spectra Zoom

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CH#1 – Motor WaveformCH#1 – Motor Waveform

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CH#1 – Test Data & ObservationsCH#1 – Test Data & Observations(Gearbox)(Gearbox) Gearbox input speed was 400 rpm. Gearbox output speed was 267 rpm

(1.5:1 ratio and 56T/84T). Gearbox spectra showed many

harmonics of both input & output speedand 133 cpm (1/2 x output).

Gearbox spectra didn’t show highgearmesh frequency at 22,400 cpm.

Gearbox waveform showed heavyimpacting at 400 cpm.

Gearbox Peakvue data showed manyharmonics of 400 cpm.

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CH#1 – Gearbox SpectraCH#1 – Gearbox Spectra

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CH#1 – Gearbox Spectra ZoomCH#1 – Gearbox Spectra Zoom

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CH#1 – Gearbox WaveformCH#1 – Gearbox Waveform

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CH#1 –CH#1 – PeakvuePeakvue SpectraSpectra

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CH#1 –CH#1 – PeakvuePeakvue WaveformWaveform

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CH#1 – Maintenance HistoryCH#1 – Maintenance History

The motor was changed out in7/02 and the gearbox was changedin 8/02.

After motor change in 7/02, motorvibration levels initially dropped,but eventually resumed previouslevels.

The gearbox installed in 8/02 hadbeen used before, but was the onlyspare available at the time.

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CH#1 – Conclusions &CH#1 – Conclusions &RecommendationsRecommendations

It was recommended that the gearboxbe changed out when possible due to aprobable gear fault.

The gearbox was changed in 5/03. An inspection of the gearbox showed

many gear teeth broken off on thepinion gear with significant wear onboth gears.

Follow-up data on both machines afterthe gearbox change showed much lowervibration levels – the problem vanished.

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CH#1 – Gearbox InspectionCH#1 – Gearbox Inspection

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CH#1 – Motor Data, B & ACH#1 – Motor Data, B & A

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CH#1 – Gearbox Spectra, B & ACH#1 – Gearbox Spectra, B & A

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CH#1 – Gearbox Wave, B & ACH#1 – Gearbox Wave, B & A

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Case History#2, Strange Motor &Case History#2, Strange Motor &Gearbox VibrationGearbox Vibration

EQUIPMENT: Agitator driven thrusingle reduction, parallel gearboxby an induction motor.

This agitator helps maintain theconsistency of the stock in ourhydrapulper tank.

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CH#2 - ProblemCH#2 - Problem

Strange pulsing noise comingfrom motor & gearbox.

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CH#2 – Test Data & ObservationsCH#2 – Test Data & Observations(Motor)(Motor)

Motor speed was exactly 1192 rpm. Agitator speed was 236 rpm (5:1 ratio). Motor spectra showed many harmonics

of running speed & closer inspectionshowed 48 cpm sidebands around eachharmonic.

Motor waveform showed pulsations ormodulation at a period of approx. 1.25sec or 48 cpm.

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CH#2 – Motor DataCH#2 – Motor Data

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CH#2 – Motor ZoomCH#2 – Motor Zoom

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CH#2 – Test Data & ObservationsCH#2 – Test Data & Observations(Gearbox), Part 1(Gearbox), Part 1

Gearbox speed was 1192 rpm input &236 rpm output (single reduction @ 5:1ratio).

Gearbox spectra showed high gearmeshfrequency (26,200 cpm) with sidebandsat 945 cpm.

Gearmesh sidebands usually relate toone of the gear speeds, but 945 cpmdidn’t correlate to either speed (1192 or236 rpm).

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CH#2 – Test Data & ObservationsCH#2 – Test Data & Observations(Gearbox), Part 2(Gearbox), Part 2

Gearbox waveform showedimpacting or modulation at 945cpm.

Agitator data showed nothingabnormal.

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CH#2 – Gearbox DataCH#2 – Gearbox Data

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CH#2 – Gearbox ZoomCH#2 – Gearbox Zoom

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CH#2 – Conclusions &CH#2 – Conclusions &Recommendations (Motor), P1Recommendations (Motor), P1 It was concluded that the strange

vibration data on the motor was theresult of broken rotor bars.

The 48 cpm sidebands around themotor harmonics related directly to itspole pass frequency.

The classic spectral pattern of brokenrotor bars is running speed harmonicswith sidebands at pole pass frequency.

The classic waveform pattern of brokenrotor bars is pulsations at pole passfrequency.

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CH#2 – Conclusions &CH#2 – Conclusions &Recommendations (Motor), P2Recommendations (Motor), P2

Pole Pass Frequency =(Theoretical RPM – True RPM) * #Poles. PPF = (1200 – 1192) * 6 = 48 cpm. The recommendation was made to

changeout the motor at the next outage. Later inspection by a motor repair shop

showed many broken rotor bars.

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CH#2 – Conclusions &CH#2 – Conclusions &Recommendations (Gearbox)Recommendations (Gearbox) After consultation with the gearbox

vendor, it was concluded the thestrange data from the gearbox waslikely due to the 4-yoke design ofthe bull gear.

The 4-yokes in the gear hub resultin minor deviations from the gearpitch circle causing modulationeach time these teeth move in andout of the mesh.

4 * 236 rpm = 945 cpm.

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CH#2 - 4-Yoke GearCH#2 - 4-Yoke Gear

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CH#2 – Conclusions &CH#2 – Conclusions &Recommendations (Gearbox),Recommendations (Gearbox),Part 2Part 2

The vendor indicated it shouldn’tbe a problem, but recommendedan annual gear inspection &continued vibration monitoringlooking for any change incondition.

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Case History #3, Repeat FanCase History #3, Repeat FanBearing FailuresBearing Failures

EQUIPMENT: Overhung,centrifugal fan belt-driven by a 60HP induction motor.

This is a critical fan necessary tothe process of winding the paperinto customer-specified sizes.

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CH#3 – Equipment LayoutCH#3 – Equipment Layout

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CH#3 - ProblemCH#3 - Problem

Repeat fan bearing failures. In one instance, vibration detected

bearing faults on this fan less than amonth after changeout.

Predictive maintenance was able todetect these failures early enough toschedule repairs during outages, butafter three fan bearing changeouts in 12months, we knew something had to bedone differently.

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CH#3 – Test Data & Observations,CH#3 – Test Data & Observations,Part 1Part 1

Motor speed was 1786 rpm Fan speed was 1985 rpm Motor spectra showed running speed &

harmonics, fan speed & harmonics, beltfrequencies & little else.

Fan spectra was similar to motor data,but also showed fan bearing defectfrequencies (BPFO & harmonics).

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CH#3 – Fan SpectraCH#3 – Fan Spectra

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CH#3 – Test Data & Observations,CH#3 – Test Data & Observations,Part 2Part 2

Fan trend data showed initial dropwhen bearings were changed, butsoon jumped up to previous highlevels days or weeks afterchangeout.

One of the mechanics involved inthe bearing change told us, “it tookus over an hour to get the bearingsaligned to where the shaft wouldeven turn”.

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CH#3 – Fan Trend DataCH#3 – Fan Trend Data

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CH#3 – Test Data & Observations,CH#3 – Test Data & Observations,Part 3Part 3

The fan bearings were standardpillow block style housings withtapered roller bearings inside.

We assumed these were self-aligning bearings as most pillowblocks are, but this assumptionturned out to be false.

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CH#3 – Conclusion &CH#3 – Conclusion &Recommendations, Part 1Recommendations, Part 1 The relatively tight alignment tolerances

of the existing tapered roller bearingscombined with the poor condition of thefan base made for short bearing life.

We asked our bearing supplier for areplacement bearing type that wouldcarry the same load, but be moreforgiving for misalignment.

We also had our machine shop fabricatea new fan base that was machined flat& line bored to perfectly fit the newpillow block bolt pattern.

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CH#3 – Conclusions &CH#3 – Conclusions &Recommendations, Part 2Recommendations, Part 2

After installation of the new typebearings & new fan base we have nothad another bearing failure.

Our bearing life has went from anaverage of 4-months to 26 months andcounting.

Further inspection of maintenancehistory showed a fan speed increasewhich corresponded quite well to ourincreased rate of failure.

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CH#3 – Conclusions &CH#3 – Conclusions &Recommendations, Part 3Recommendations, Part 3

This case history is a good example ofboth predictive & proactivemaintenance.

Predictive maintenance allowed us toavoid catastrophic fan bearing failuresand perform repairs during scheduledoutages.

Proactive maintenance extended the life& reliability of the fan bearings.

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Case History#4, Pump OutboardCase History#4, Pump OutboardBearing Cage FailureBearing Cage Failure

Equipment: Double-suction, centrifugalpump driven by a 1250 HP synchronousmotor turning at 514 rpm.

This is the most critical pump in theprocess directly providing the product(stock) used to make paper on the papermachine.

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CH#4 - ProblemCH#4 - Problem

Increasing HFD vibration levels at pumpoutboard. Current levels had exceededmaximum of long term trend.

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CH#4 – Test Data & Observations,CH#4 – Test Data & Observations,Part 1Part 1

Since the bearing was oil lubricated, ananalysis of the outboard bearing oil wasrequested.

Oil analysis results showed very highcopper levels indicating cage wear.

Vibration data showed very high HFDlevels on outboard bearing.

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CH#4 – HFD TrendCH#4 – HFD Trend

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CH#4 – Test Data & Observations,CH#4 – Test Data & Observations,Part 2Part 2

Peakvue spectra showed harmonics offundamental train frequency (cage).

An 10/01 paper by J. Robinson & J. Berryrecommends a Peakvue fault level of 4.0g’s pk-to-pk for a 500 rpm machine.

Peakvue waveform on outboard bearingshowed levels at 11.3 g’s pk-to-pk!!!

Maintenance history showed a continuingproblem of a leaking pump outboard seal.

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CH#4 –CH#4 – PeakvuePeakvue Spectra, P1Spectra, P1

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CH#4 –CH#4 – PeakvuePeakvue Spectra, P2Spectra, P2

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CH#4 –CH#4 – PeakvuePeakvue WaveformWaveform

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CH#4 – Conclusion &CH#4 – Conclusion &Recommendations, Part 1Recommendations, Part 1

The recommendation was made tochangeout the pump outboard bearing.

Later inspection of the bearing showed thecage worn badly particularly in the areaseparating the rolling elements from oneanother.

During the bearing change, shims werefound under the bearing making up the fitwith the housing.

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CH#4 – Conclusion &CH#4 – Conclusion &Recommendations, Part 2Recommendations, Part 2 Failure analysis of the bearing found lube

contamination and skidding as the primarycauses of failure. Corrective actions were asfollows:

1) Replace the outboard bearing housing, 2) Repair or replace the pump outboard packing

gland, 3) Replace the pump rotor which was causing

thrust loading, and 4) Upgrade the lube from an ISO 68 to an ISO

150 weight oil (temporarily use grease untilpacking gland repaired).