Vibration Case Histories

1 9/19/20061 Vibration case HistoriesVibration case HistoriesBarry T. CeaseMeadWestvaco9/19/20062 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 Failure9/19/20063 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 #1 Equipment LayoutCH#1 Equipment Layout9/19/20065CH#1 - ProblemCH#1 - Problem Excessive & noisy Vibration atmotor. This problem could be heard & feltclearly at the motor.

2 It sounded like something wasrubbing and/or #1 Test Data & ObservationsCH#1 Test Data & Observations(Motor)(Motor) Motor speed wasexactly400 rpm. Pump speed was 267 rpm. Pump Vibration data showed noabnormal patterns or levels. Motor spectra showed manyharmonics of running speed and of133cpm(1/3 x rpm). Motor waveform showed impactingat 400 & #1 - Motor SpectraCH#1 - Motor Spectra9/19/20068CH#1 Motor Spectra ZoomCH#1 Motor Spectra Zoom9/19/20069CH#1 Motor WaveformCH#1 Motor Waveform9/19/200610CH#1 Test Data & ObservationsCH#1 Test Data & Observations(Gearbox)(Gearbox) Gearbox input speed was 400 rpm. Gearbox output speed was 267 rpm( :1 ratio and 56T/84T). Gearbox spectra showed manyharmonics of both input & output speedand 133cpm(1/2 x output).

3 Gearbox spectradidn tshow highgearmeshfrequency at 22,400cpm. Gearbox waveform showed heavyimpacting at 400cpm. GearboxPeakvuedata showed manyharmonics of #1 Gearbox SpectraCH#1 Gearbox Spectra9/19/200612CH#1 Gearbox Spectra ZoomCH#1 Gearbox Spectra Zoom9/19/200613CH#1 Gearbox WaveformCH#1 Gearbox Waveform9/19/200614CH#1 CH#1 PeakvuePeakvueSpectraSpectra9/19/200615C H#1 CH#1 PeakvuePeakvueWaveformWaveform9/19/20061 6CH#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 #1 Conclusions &CH#1 Conclusions &RecommendationsRecommendations It was recommended that the gearboxbe changed out when possible due to aprobable gear fault.

4 The gearbox was changed in 5/03. An inspection of the gearbox showedmany 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 #1 Gearbox InspectionCH#1 Gearbox Inspection9/19/200619CH#1 Motor Data, B & ACH#1 Motor Data, B & A9/19/200620CH#1 Gearbox Spectra, B & ACH#1 Gearbox Spectra, B & A9/19/200621CH#1 Gearbox Wave, B & ACH#1 Gearbox Wave, B & A9/19/200622 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 #2 - ProblemCH#2 - Problem Strange pulsing noise comingfrom motor & #2 Test Data & ObservationsCH#2 Test Data & Observations(Motor)(Motor) Motor speed was exactly 1192 rpm.

5 Agitator speed was 236 rpm (5:1 ratio). Motor spectra showed many harmonicsof running speed & closer inspectionshowed 48cpmsidebands around eachharmonic. Motor waveform showed pulsations ormodulation at a period of approx. or #2 Motor DataCH#2 Motor Data9/19/200626CH#2 Motor ZoomCH#2 Motor Zoom9/19/200627CH#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 highgearmeshfrequency (26,200cpm) with sidebandsat 945cpm. Gearmeshsidebands usually relate toone of the gear speeds, but 945cpmdidn t correlate to either speed (1192 or236 rpm).9/19/200628CH#2 Test Data & ObservationsCH#2 Test Data & Observations(Gearbox), Part 2(Gearbox), Part 2 Gearbox waveform showedimpacting or modulation at 945cpm.

6 Agitator data showed #2 Gearbox DataCH#2 Gearbox Data9/19/200630CH#2 Gearbox ZoomCH#2 Gearbox Zoom9/19/200631CH#2 Conclusions &CH#2 Conclusions &Recommendations (Motor), P1 Recommendations (Motor), P1 It was concluded that the strangevibration data on the motor was theresult of broken rotor bars. The 48cpmsidebands 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 #2 Conclusions &CH#2 Conclusions &Recommendations (Motor), P2 Recommendations (Motor), P2 Pole Pass Frequency =(Theoretical RPM True RPM) * #Poles. PPF = (1200 1192) * 6 = 48cpm.

7 The recommendation was made tochangeoutthe motor at the next outage. Later inspection by a motor repair shopshowed many broken rotor #2 Conclusions &CH#2 Conclusions &Recommendations (Gearbox)Recommendations (Gearbox) After consultation with the gearboxvendor, it was concluded thethestrange 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 = #2 - 4-Yoke GearCH#2 - 4-Yoke Gear9/19/200635CH#2 Conclusions &CH#2 Conclusions &Recommendations (Gearbox),Recommendations (Gearbox),Part 2 Part 2 The vendor indicated it shouldn tbe a problem, but recommendedan annual gear inspection &continued Vibration monitoringlooking for any change History #3, Repeat FanCase History #3, Repeat FanBearing FailuresBearing Failures EQUIPMENT: Overhung,centrifugal fan belt-driven by a 60HP induction motor.

8 This is a critical fan necessary tothe process of winding the paperinto customer-specified #3 Equipment LayoutCH#3 Equipment Layout9/19/200638CH#3 - ProblemCH#3 - Problem Repeat fan bearing failures. In one instance, Vibration detectedbearing faults on this fan less than amonth afterchangeout. Predictive maintenance was able todetect these failures early enough toschedule repairs during outages, butafter three fan bearingchangeoutsin 12months, we knew something had to bedone #3 Test Data & Observations,CH#3 Test Data & Observations,Part 1 Part 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).

9 9/19/200640CH#3 Fan SpectraCH#3 Fan Spectra9/19/200641CH#3 Test Data & Observations,CH#3 Test Data & Observations,Part 2 Part 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 .9/19/200642CH#3 Fan Trend DataCH#3 Fan Trend Data9/19/200643CH#3 Test Data & Observations,CH#3 Test Data & Observations,Part 3 Part 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 #3 Conclusion &CH#3 Conclusion &Recommendations, Part 1 Recommendations, Part 1 The relatively tight alignment tolerancesof the existing tapered roller bearingscombined with the poor condition of thefan base made for short bearing life.

10 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 #3 Conclusions &CH#3 Conclusions &Recommendations, Part 2 Recommendations, 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 #3 Conclusions &CH#3 Conclusions &Recommendations, Part 3 Recommendations, Part 3 This case history is a good example ofboth predictive & proactivemaintenance.