36
Machine diagnosis: Quick and easy through FFT analysis

VIB Booklet En

Embed Size (px)

DESCRIPTION

Vibration analysis

Citation preview

Page 1: VIB Booklet En

Machine diagnosis:Quick and easy through FFT analysis

Page 2: VIB Booklet En

Topic Page1. Introduction ....................................................................................... 22. Vibration spectra of a belt-driven exhaust fan ................................... 43. Machine condition trending ............................................................... 64. Level 1 / Level 2 Condition monitoring strategy ................................. 85. Vibration severity according to DIN ISO ............................................ 106. Motor components vulnerable to damage ....................................... 127. Rotor unbalance / Shaft misalignment ............................................. 148. Stator field asymmetry ..................................................................... 169. Armature field faults ........................................................................ 18

10. Practical vibration diagnosis: Rotor unbalance ................................. 2011. Practical vibration diagnosis: Shaft misalignment ............................. 2212. Practical vibration diagnosis: Field asymmetry .................................. 2413. Practical vibration diagnosis: Loose belt drive wheel ........................ 2614. Bearing evaluation parameters ......................................................... 2815. Normalization of shock pulse measurement ..................................... 3016. Anti-friction bearing damage diagnosis ........................................... 3217. Practical bearing diagnosis: inner race damage ................................ 34

Contents

Page 3: VIB Booklet En

1. Introduction

Vibration monitoring and vibration diagnosisof machines and aggregates has gained enor-mous importance during the past severalyears. Even smaller and medium-sized ma-chines are being included in vibration moni-toring strategies with increasing frequency.This is largely due to the fact that vibrationmeasurement equipment has reached a pricelevel that makes application of vibration mea-surement a viable alternative for these ma-chines as well.The interest in vibration technology and itssuccessful application in the electrician’s tradehas also increased dramatically during recentyears. On the one hand, machine operatorsincreasingly often demand a ‘vibration signa-ture’ record following installation or repair,while on the other hand, vibration monitoringand diagnosis offer considerable potential foradditional service business, especially through

consultancy to smaller operations that lackthe resources to pursue vibration measure-ment on their own. And of course, vibrationdiagnosis is a fantastic tool for localization ofdefects and causes of damage to machinesand aggregates, and one which can even beused as an objective defense against unjusti-fied warranty claims.

EDITION May 2010Order Number VIB 9.619G

Contents originally published as a presentation manu-script by M. Luft, PRÜFTECHNIK AG©Copyright 1998 PRÜFTECHNIK AG. All rights reserved.

Page 4: VIB Booklet En

4

Let us examine a simple practical example toillustrate the possibilities of vibration analysis:a belt-driven fan unit had failed due toexcessive vibration. Since the most severevibration level was measured on the drivemotor, the motor seemed the logical candi-date for examination. Vibration analysisshowed, however, that the extremely severevibration (15.2 mm/s) at the motor was occur-ring primarily at a frequency which was con-ducted to the motor via the belt drive. Whenthe belt drive wheel on the fan was balanced,vibration decreased to acceptable levels of2.3 mm/s on the fan and 3.2 mm/s on thedrive motor.

This case presents a typical method of opera-tion: a simple measurement of overall vibra-tion level allows the machine condition to berated as ‘good’, ‘satisfactory’, ‘unsatisfactory’and ‘unacceptable’. In the case of excessive

vibration, the root cause - drive belt wheelunbalance - was made clear by checking thefrequency peaks in the FFT vibration spec-trum.

2. Vibration spectra of a belt-driven exhaust fan

Page 5: VIB Booklet En

5

Vibration spectra of a belt-driven exhaust fan

Paint shop exhaust fan (P = 37 kW)

1. Parameter measurement

Vibration severity, vertical,measured at bearings

Motor: 1475 rpm = 24.58 Hz Fan: 820 rpm = 13.67 Hz

15.2 mm/s

11.3 mm/s

Fan bearing, radial/vertical

Fvert = 13.67 Hz

Fvert = 13.67 Hz

Motor bearing, radial/vertical

2. Signal analysis

FFT spectrum of vibration signal

Page 6: VIB Booklet En

6

3. Machine condition trending

A rational approach to successful and effec-tive condition monitoring is that of trendingthe development of characteristic overall val-ue measurements of machine condition overtime. The trend readings are plotted as shownhere and compared with appropriate warningand alarm thresholds. When thresholds areexceeded (and not before then), detailedvibration diagnosis is performed in order tolocate the exact source of trouble and todetermine the corresponding maintenanceremedy. Let us examine, then, the vibrationmonitoring and diagnosis techniques thathold particular relevance for electric motors.

Page 7: VIB Booklet En

7

Event-oriented

■■■■■ Parameter trend monitoring

■■■■■ Alarm notificationwhen tolerances are exceeded

■■■■■ Reference spectra (good condition)

■■■■■ Manual in-depth diagnosis /on-site analysis

Machine condition trending

Offline spectrumGood condition

SpectrumWarn

Offline signal analysisDiagnosis / Analysis

Alarm

Warning

Vibrationparameter

Tempo

SpectrumAlarm

Page 8: VIB Booklet En

8

Machine condition monitoring calls for mea-surement of suitable vibration characteristicoverall values, which allow the general vibra-tion condition of the machine to be estimat-ed. The trend development of these charac-teristic overall values points out conditiondeterioration, i.e. damage progression. Thistype of overall vibration measurement is char-acterized as ‘Level 1’ as shown here. It allowsmonitoring of many aggregates without im-posing high demands in terms of equipmentand manpower.

Characteristic overall value (Level 1) measure-ments such as these, however, are insufficientfor precise localization of defects, as thisrequires closer analysis of the machine spec-trum. Most types of damage can be detectedby their characteristic frequencies or typicalpattern of frequencies. ‘Level 2’ vibration

4. Level 1 / Level 2 condition monitoring strategy

diagnosis normally requires measurement ofvibration signals using an FFT vibration analyz-er by trained personnel who are experiencedin interpreting vibration spectra.

Page 9: VIB Booklet En

9

Level 1 / Level 2 condition monitoring strategy

Level 2: Vibration diagnosisfollowing alarm violation- Isolated- One-time- Specialist

Machine monitoringVibration loadBearing condition

ParametersVibration strength, displacement, accelerationShock pulse for bearing evaluationTemperatureRPMPump cavitation

Defect localization via spectrum analysisRotor unbalance, shaft misalignment, gear damage,turbulence, field faults, bearing diagnosis etc.

Signal analysisAmplitude spectrumEnvelope spectrumTime signalOrdinal analysisCepstrum

Level 1: Parameter trend monitoring- Comprehensive- Long-term- Less-skilled personnel

10

8

6

4

2

00 500 1000 1500

amm/s2

veffmm/s

tem-po

10

8

6

4

2

0

f in Hz

Page 10: VIB Booklet En

10

DIN ISO 10816-3 plays a veryimportant role for mainte-nance technicians in the eval-uation of machine vibrations.Part 3 of this standard, whichis the section that is of rele-vance to Condition Monitor-ing, has been revised. Groups3 and 4 of Part 3, which dealtwith pumps, have been re-moved. Instead, the standardwas expanded to include Part7 – namely, DIN ISO 10816-7.This new part deals entirelywith vibrations in centrifugalpumps.The new DIN ISO 10816-7 hasbeen in effect since August2009.

5. Vibration severity according to ISO standards

Page 11: VIB Booklet En

11

Vibration severity according to ISO standards

Page 12: VIB Booklet En

12

This illustration gives an overview of theelectric motor components most vulnerableto damage. Some types of damage exhibittypical vibration spectra patterns, and each ofthese phenomena shall now be explained indetail.

6. Motor components vulnerable to damage

Page 13: VIB Booklet En

13

Motor components vulnerable to damage

Bearing damage

Armature damage Stator damage

Coupling damage

Page 14: VIB Booklet En

14

Unbalance is understood to be an eccentricdistribution of rotor mass. When an unbal-anced rotor begins to rotate, the resultingrotating centrifugal force produces additionalforces on bearings and rotor vibration at theexact frequency of rotation. This characterizesthe spectrum of an unbalanced machine, i.e.the rotation frequency appears as a ‘peak’with elevated amplitude, and this can signifi-cantly degrade the overall vibration conditionof the machine. The necessary redistributionof rotor mass is achieved by balancing themotor rotor either with a balancing machinefollowing disassembly or on-site using a vibra-tion-based balancing instrument. Reference#3 indicates acceptable residual unbalancefor rigid rotors.

Shaft misalignment of directly coupled ma-chines results primarily in elevated vibration at

twice the shaft rotation frequency, sometimeswith the peak at shaft rotation frequencyelevated as well. If the radial misalignment(i.e. shaft offset) dominates, then this peak ismost pronounced for measurements taken inradial direction (perpendicular to the shafts).If angular misalignment (coupling gap) ispredominant, then vibration elevation will bemost noticeable in frequency spectra of axialmeasurements. Many manufacturers and op-erators of electric machines have adopted theuse of modern laser-optical shaft alignmentsystems such as OPTALIGN® to correct exces-sive shaft misalignment. Recommended align-ment tolerances are outlined in Note #4.

7. Rotor unbalance / Shaft misalignment

3 ISO 3945Mechanical vibration of large rotating machines withspeed range from 10 to 200 rev/s; Measurement andevaluation of vibration severity in situ, 12/19854 OPTALIGN® PLUSOperating instructions and alignment handbook,PRÜFTECHNIK AG, Ismaning, Germany, 03/1997

Page 15: VIB Booklet En

15

Unbalance

Amplitude of fn too high

■■■■■ Rotation frequency fn = rpm/60■■■■■ Evaluation standard: ISO 2372, ISO/DIS 10816-3

Shaft misalignment

Twice (2x) rotation frequency 2fn

■■■■■ Radial: radial misalignment■■■■■ Axial: axial misalignment

f in Hzfn2fn

mm/s

f in Hz

mm/s

fn

Rotor unbalance / Shaft misalignment

Page 16: VIB Booklet En

16

Field asymmetry of electric motors can becaused by stator or rotor (armature) defects.The most common faults are

• Motor core short circuiting from armaturerubbing or burnout

• Asymmetrical winding

• Asymmetrical power feed and

• Eccentric armature position.

Stator field defects can be recognized in thevibration spectrum as peaks occurring attwice the mains frequency, without side-bands.

8. Stator field asymmetry

Page 17: VIB Booklet En

17

Stator field asymmetry

2fMains

Twice mains frequency 2fMains visible

Mains frequency fMains = 50 or 60 Hz

Exception: rectifier drives

No sidebands visible around 2fMains

2-pole machines:2x rotation frequency lies just below 2fMains

99.0 101.0f in Hzfn

mm/s

f in Hz2fn 2fMains

mm/s

Stator field asymmetry

■■■■■ Core burnout, short circuit■■■■■ Eccentric armature position■■■■■ Asymmetric power feed■■■■■ Asymmetric winding

Page 18: VIB Booklet En

18

Rotor field asymmetry is caused by:

• Damaged bars (breakage/fracturing, loose-ness) or

• Short circuited bars or

• Short circuited rings (breakage/fracturing)or

• Short circuited armature packs (e.g. byoverloading at excessive speed)

These faults can be detected in the vibrationspectrum by the evidence of

• Bar passing frequency with sidebands attwice the mains frequency and

• Mains frequency with sidebands at slippingfrequency.

The only possible remedy here is usuallycomplete replacement of the armature.

9. Armature field faults

Page 19: VIB Booklet En

19

Armature field faults

Bar passing frequency fbar with sidebandsvisible at 2fMains intervals

Bar passing frequency fbar = fn x nbarwith rotation frequency fnand nbar = number of armature bars

Mains frequency: fMains = 50 or 60 Hz

Sidebands visible around 2fMains at fslip intervals

with slip frequency fslip = 2fMains/p - fnand p = number of stator poles

99.0 101.0 f in Hz2fn2fMains(100 Hz)

mm/s

fbar f in Hzfn 2fMains

mm/s

Armature field fault

■■■■■ Bar breakage■■■■■ Bar fracture■■■■■ Bar looseness

Page 20: VIB Booklet En

20

The vibration spectrum exhibits a typical un-balance pattern. The levels of vibration severi-ty measured at several locations on the ma-chine point indicate that the source of excita-tion lies near the coupling. Simple rotor bal-ancing of the brake disk reduced motorvibration to 3.5 mm/s and gearbox vibrationto 3.1 mm/s.

10. Practical vibration diagnosis: Rotor unbalance

Page 21: VIB Booklet En

21

Practical vibration diagnosis: Rotor unbalance

Belt conveyor gearboxP = 600 kWn = 996 rpm (fn = 16.6 Hz)

Vibration severity Motor GearboxA, RH in mm/s 3.1 -A, RV 7.8 9.2A, AX 5.3 6.2B, RH 4.4 -B, RV 6.8 -

Cause: Brake disk unbalance

Gearbox, inboard bearing, vertical Gearbox, inboard bearing, axial

GearboxBrakeMotor

fn = 16.6 Hz (unbalance)

fn = 16.6 Hz (unbalance)

Page 22: VIB Booklet En

22

The vibration spectrum shows a distinct peakat twice the shaft rotation frequency, whichclearly indicates shaft misalignment. Follow-ing shaft alignment, the peak has disap-peared, but the rotor unbalance evident in theprevious spectrum remains to be corrected.

11. Practical vibration diagnosis: Shaft misalignment

Page 23: VIB Booklet En

23

Practical vibration diagnosis: Shaft misalignment

Hydroturbine generatorP = 55 kWn = 1000 rpm (fn = 16.67 Hz)

Vibration severity Generator Gearbox

inboard, RH 9.5 1.5 mm/sinboard, RV 4.1 -inboard, AX 4.4 -

Vertical alignment correction Before After

Angularity (Ø = 170 mm) 0.42 mm - 0.02 mmOffset 0.44 mm 0.05 mm

Cause: Shaft misalignment

Generator, inboard bearing, original condition Following shaft alignment

fGen.

2fGen. = misalignmentfGen.

2fGen. = good alignment

GearboxGenerator

Page 24: VIB Booklet En

24

The motor had drawn attention due to elevat-ed vibration which also occurred with thecoupling removed. The unusually high peak attwice the line frequency pointed toward sta-tor damage. Disassembly revealed that thestator packet had burned out due to localshort circuiting of the core. The motor had tobe completely replaced.

12. Practical vibration diagnosis: Field asymmetry

Page 25: VIB Booklet En

25

Practical vibration diagnosis: Field asymmetry

Steel mill exhaust blowerP = 250 kWn = 2999 rpm (fn = 50 Hz)

Vibration severity

Motor inboard, RH 4.8 mm/s

Cause: Stator core burnout

Motor, inboard, radial horizontal Zoomed view, 100 Hz peak

2fMains

Field asymmetry

2fMains

Field asymmetry

MotorBlower

Page 26: VIB Booklet En

26

A press drive motor had developed severevibration and was producing unusual noisesthat had become more pronounced from oneday to the next. In stark contrast to the usualvibration spectrum, the rotation frequencywas hardly visible at all, but multiples of therotation frequency were quite obvious. Thesesymptoms remained unchanged when thedrive belt was removed from the motor. Thesource was found to be loose mounting of thebelt drive wheel on the motor shaft. Theproblem was resolved by remachining themotor shaft and reattaching the belt drivewheel.

13. Practical vibration diagnosis: Loose belt drive wheel

Page 27: VIB Booklet En

27

Practical vibration diagnosis: Loose belt drive wheel

Press driveP = 200 kWMotor: 1486 rpm = 24.77 Hz

Vibration severity

Motor inboard 6.9 mm/sMotor outboard 7.1 mm/s

Cause: Excessive play in motor shaftbelt drive wheel

Motor inboard, before repair Following repair

fmotor = 24.77 Hz

Flywheel

Drive belt

fmotor = 24.77 Hz

Page 28: VIB Booklet En

28

As a rule, bearing race damage cannot bedetected by elevated levels of low-frequencyvibration parameters until damage is quitesevere. The reason for this is that when therolling elements pass over a damaged area ofthe race, a shock pulse is created that can bedetected only in the high-frequency range atfirst. This is why special bearing characteristicoverall values were developed for anti-frictionbearing monitoring; there is no international-ly-accepted standard for these so far, and so avariety of different characteristic overall val-ues can be found in use today.

This illustration lists the most well-known ofthese bearing parameters. In Germany, forexample, the shock pulse method has estab-lished itself over the past 25 years as an easy-to-use and reliable measurement techniquefor monitoring anti-friction bearings. In con-trast to all other bearing parameters, this

method uses two parameters for evaluation.The shock pulse maximum value dBm, whichindicates the severity of shocks in the rollingbehavior of the bearing, is useful in detectinginitial damage to bearing races. The ‘carpetlevel’ of shock pulses, dBc, indicates the basenoise level of the bearing, which increasesprimarily due to lubrication problems, generalwear of races, insufficient bearing clearanceor residual stress due to improper installation.

One typical characteristic of all anti-frictionbearing parameters is the dependency of theirlevels upon various influences such as rollingvelocity, i.e. bearing size and rpm, signaldamping, bearing load and lubrication. This iswhy it is practically always necessary to take acomparative measurement in good conditionor to normalize readings relative to goodcondition.

14. Bearing evaluation characteristic overall values

Page 29: VIB Booklet En

29

■■■■■ Shock pulse

■■■■■ K(t) method

■■■■■ Spike energy

■■■■■ BCU value

■■■■■ Curtosis factor

■■■■■ GSE factor

■■■■■ SEE factor

■■■■■ Accel. crest factor

Bearing evaluation parameters

Regardless of characteristic overall value:reliable condition evaluation still requires

Initial value? Tolerances?

Rate of increase over time?

?

?

Page 30: VIB Booklet En

30

This illustration shows the normalization pro-cedure that PRÜFTECHNIK instruments useduring shock pulse measurement to compen-sate the influence of rolling velocity differenc-es. The initial level, and in turn the adjustedinitial value dBia, are determined by taking acomparative measurement in good condition.This serves as the reference for relative levelmeasurement of maximum shock pulse valuedBm and the shock pulse carpet value dBc.This procedure allows measurements fromdifferent bearings to be compared using thesame level scale so that tolerances do nothave to be set individually for every singlemeasurement location.

15. Normalization of shock pulse measurement

Page 31: VIB Booklet En

31

Normalization of shock pulse measurement

Non-normalized measurement

Shock pulse peak value dBm and carpet value dBcas absolute level in dBsv

Normalized measurement

Shock pulse max. value dBm and carpet value dBcas relative level in dBsv referenced to dBia value

■■■■■ Threshold (limit) values set individuallyfor every single location

dBm

dBC

Normalization

dBm

dBC

dBia

Alarm

Warning

Alarm

Warning

■■■■■ dBia value includes influence factors such as rollingvelocity, signal damping, bearing load

■■■■■ Different threshold limits are linked to the setup dBiavalue; the same predefined threshold values are usedfor all locations

dBn

40

dBsv

70

0

0

Page 32: VIB Booklet En

32

Similarly to vibration diagnosis via frequencyspectrum measurement, in-depth diagnosis ofanti-friction bearings may be performedthrough analysis of the signal ‘envelope’.

The illustrations here explain the envelopeanalysis procedure, which begins with filter-ing out the appropriate range of frequenciesthat contain the signal emitted by the bearingduring operation. This signal component isexamined for the pulses that arise whenbearing elements roll over damaged loca-tions. Demodulation is used to calculate acurve that ‘envelops’ the bearing signal. If thetime interval between periodically-occurringpeaks in the envelope curve match one of thecritical frequencies characteristic of bearingdamage, then the corresponding bearingcomponent can be assumed to be damaged.

16. Anti-friction bearing damage diagnosis

This procedure allows extremely accurate di-agnosis of damage to anti-friction bearings,even in cases where extraneous signal compo-nents such as gear meshing noise tend tocover up the actual bearing signal. It doesrequire knowledge of certain geometric dataof the bearing, including the bearing diame-ter, the number and diameter of rolling ele-ments, the load angle and the operatingspeed.

Page 33: VIB Booklet En

33

Anti-friction bearing damage diagnosis

Time signal Time signal

Ta

f in Hz

DamageNo damage

a, m/s²

a, m/s²

a, m/s²

Envelope curve

Envelope curve

Envelope curve spectrum

t in st in s

Envelope curve spectruma, m/s²

fa 2fa etc. f in Hz

Damage frequency fa = 1/Ta

Page 34: VIB Booklet En

34

This shows an example of advanced damageto the inner race. The great increase in shockpulse levels, especially that of peak value dBm

from 18 to 48 dBsv, signified serious bearingdamage. Envelope spectrum analysis indicat-ed a pattern typical of inner race damage,which was then confirmed following bearingreplacement: one of the two races of theinner ring already exhibited a damaged sur-face area of about 15 x 15 mm / 5/8” x 5/8”.

17. Practical bearing diagnosis: inner race damage

Page 35: VIB Booklet En

35

Practical bearing diagnosis: inner race damage

fi = inner race damage frequency Inner race intact

Inboard bearing A Outboard bearing BEnvelope spectrum Envelope spectrum

Paint shop exhaust fanP = 110 kWMotor: 1307 rpm = 21.78 HzFan: 908 rpm = 35.75 Hz

Bearing: 22218 tapered roller bearing

Shock pulse readings dBm dBc

Inboard bearing A 48 29 dBSVOutboard bearing B 18 7 dBSV

Cause: Severe inner race damage on inboard bearing

Inner race damage

A B

Page 36: VIB Booklet En

36

PRÜFTECHNIK AGOskar-Messter-Str. 19-2185737 Ismaning, GermanyTel.: +49 89 99616-0Fax: +49 89 99616-300eMail: [email protected]

Productive maintenance technology

A member of the PRÜFTECHNIK Group