Vibration Monitoring

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Machinery Fault Diagnosis sic guide to understanding vibration analysis for machinery diagnos 1

Transcript of Vibration Monitoring

Page 1: Vibration Monitoring

Machinery Fault DiagnosisA basic guide to understanding vibration analysis for machinery diagnosis.

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Unbalance

ω

m Unbalance is the condition when the geometric centerline of a rotation axis doesn’tcoincide with the mass centerline.

Funbalance = m d ω2

1X

RadialMP MP

A pure unbalance will generate a signal at the rotation speed and predominantly inthe radial direction.

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Static Unbalance

Static unbalance is caused by an unbalance mass out ofthe gravity centerline.

Sm

UThe static unbalance is seen when the machine is not inoperation, the rotor will turn so the unbalance mass isat the lowest position.

The static unbalance produces a vibration signal at 1X,radial predominant, and in phase signals at both endsof the rotor.

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Pure Couple Unbalance

U = -U

-m

Pure couple unbalance is caused by two identicalS

m

U b

unbalance masses located at 180° in the transverse area ofthe shaft.

Pure couple unbalance may be statically balanced.When rotating pure couple unbalance produces avibration signal at 1X, radial predominant and inopposite phase signals in both ends of the shaft.

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Dynamic Unbalance

U

Dynamic unbalance is static and couple unbalance at the-m same time.

S

m

U

In practice, dynamic unbalance is the most commonform of unbalance found.

When rotating the dynamic unbalance produces avibration signal at 1X, radial predominant and the phasewill depend on the mass distribution along the axis.

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Documentation of balancing

Frequency spectra before/after balancing Balancing diagram

and balancing diagram.

.. after balancing

before ..

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Overhung Rotors

A special case of dynamic unbalance can be found inoverhung rotors.

The unbalance creates a bending moment on the shaft.

1X

Radial

Dynamic un balance in overhung rotors causes high 1X

Axial

levels in radial and axial direction due to bending of theshaft. The axial bearing signals in phase may confirm

this unbalance.

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Unbalance location

The relative levels of 1X vibrationare dependant upon the location ofthe unbalance mass.

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Misalignment

Misalignment is the condition when the geometric centerline of twocoupled shafts are not co-linear along the rotation axis of both shaftsat operating condition.

1X 2XMP MP

Axial

A 1X and 2X vibration signal predominant in the axial direction is generally theindicator of a misalignment between two coupled shafts.

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Angular Misalignment

1X 2X 3X

Axial

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Angular misalignment is seen when the shaft centerlines coincide at one point a long the projected axis of both shafts.

The spectrum shows high axial vibration at 1X plus some 2X and 3X with 180° phase difference across the coupling in the axial direction.These signals may be also visible in the radial direction at a lower amplitude and in phase.

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Parallel Misalignment

Parallel misalignment is produced when the centerlinesare parallel but offset .

The spectrum shows high radial vibration at 2X and alower 1X with 180° phase difference across thecoupling in the radial direction.

These signals may be also visible in the axial directionin a lower amplitude and 180° phase difference acrossthe coupling in the axial direction.

1X 2X 3X

Radial

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Misalignment Diagnosis Tips

In practice, alignment measurements will show acombination of parallel and angular misalignment.

Diagnosis may show both a 2X and an increased 1X signalin the axial and radial readings.

The misalignment symptoms vary depending on themachine and the misalignment conditions.

The misalignment assumptions can be often distinguishedfrom unbalance by:

• Different speeds testing

• Uncoupled motor testing

Temperature effects caused by thermal growth should alsobe taken into account when assuming misalignment is thecause of increased vibration.

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Alignment Tolerance Table

The suggested alignment tolerances shown above are general values based upon experience and should not be exceeded.

They are to be used only if existing in-house standards or the manufacturer of the machine or coupling prescribe no other values.

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Shaft Bending

A shaft bending is produced either by an axial asymmetryof the shaft or by external forces on the shaft producingthe deformation.

A bent shaft causes axial opposed forces on the bearingsidentified in the vibration spectrum as 1X in the axialvibration.

2X and radial readings can also be visible.

1X 2X

Axial

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Rotating Looseness

Rotating looseness is caused by an excessive clearance between the rotor and the bearing

Rolling element bearing:Rotation

frequency 1X

and harmonics

Radial

Journal bearing:

Rotation frequency1X Harmonics andsub Harmonics.

Radial

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Structural Looseness

Structural looseness occurs when the machine is not correctly supported by, or wellfastened to its base.

MP

• Poor mounting

• Poor or cracked baseMP

MP MP

1X

Radial

• Poor base support

• Warped base

Structural looseness may increase vibrationamplitudes in any measurement direction.Increases in any vibration amplitudes mayindicate structural looseness.

Measurements should be made on the bolts,feet and bases in order to see a change in theamplitude and phase. A change in amplitudeand 180° phase difference will confirm this

problem.

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Condition Monitoring Condition Monitoring

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Resonance

Resonance is a condition caused when a forcing frequency coincides (or is close) to thenatural frequency of the machine’s structure. The result will be a high vibration.

1st form of natural 2nd form of natural flexure 3rd form of natural flexureflexure

v v v

tx tx

t tx t t

t

Shaft 1st, 2nd and 3rd critical speeds cause aresonance state when operation is near these

critical speeds. f1st nat, flexuref2nd nat, flexuref3rd nat, flexuref

no harmonic relationship

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Resonance

• Resonance can be confused with other common problems in machinery.

• Resonance requires some additional testing to be diagnosed.

Grad

MP MP

1 φ1= 240°

1

1.O.

Amplitude at rotation frequency fn by residualunbalance on rigid rotor.

1 2ResonanceStep-up

rev/min

Phase jumpby 180°

rev/min

22 φ2= 60...80°

1.O.

Strong increase in amplitude of the rotationfrequency fn at the point of resonance, step-updependent on the excitation (unbalanced condition)and damping.

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Resonance Diagnosing Tests

Run Up or Coast Down Test:

• Performed when the machine isturned on or turned off.

• Series of spectra at different RPM.

• Vibration signals tracking mayreveal a resonance.

The use of tachometer is optionaland the data collector must supportthis kind of tests.

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Resonance Diagnosing Tests

Bump Test:

3

Excitation - force pulse

F/a

tDouble beat5 ms

Shock component, natural vibration, vertical

t

s 1 2

1

Frequency response, vertical

Natural frequency,vertical

Response - component vibration

3

t

2 Decaying function

Frequency response, horizontal

2

nd mod.

1st mod.Natural frequency,horizontal

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Journal Bearings

Journal bearings provides a very low friction surface to support and guide a rotor through a cylinderthat surrounds the shaft and is filled with a lubricant preventing metal to metal contact.

High vibration damping due to the oil film:

• High frequencies signals may not be transmitted.

• Displacement sensor and continuous monitoringrecommended

Clearance problems (rotating mechanical looseness).

0,3-0,5X 1X

RadialOil whirl

• Oil-film stability problems.

• May cause 0.3-0.5X component in the spectrum.

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Rolling Element Bearings

1. Wear:

Wear

• Lifetime exceeded

• Bearing overload

• Incorrect assembly

• Manufacturing error

• Insufficient lubricationWear

The vibration spectrum has a highernoise level and bearing characteristicfrequencies can be identified.

Increased level of shock pulses.

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Rolling Element Bearings

2. Race Damage:4

3

Roller bearing geometry and damage frequencies:

Angle of contactDw D Arc diameter

d Rolling element diameterDpw Z Number of rolling elements

n Shaft RPM

1 - Outer race damage2

2 - Inner race damage

3 - Rolling element damage

4 - Cage damage

1

Example of rollover frequencies:

Ball bearing SKF 6211

RPM, n = 2998 rev/min

Ball pass frequency, outer race: BPFO =

Ball pass frequency, inner race: BPFI =

Ball spin frequency: BSF =

Fundamental train frequency: TFT =

Z n

260Z n260D nd60n2 60

d( 1- cos ) DimensionsD

d( 1+ cos ) d =77.50 mm

D

d 2 D =14.29 mm( 1- cos )

D

d Z = 10( 1- cos )D

= 0

Rollover frequencies

BPFO = n / 60 4.0781 = 203.77 Hz

BPFI = n / 60 5.9220 = 295.90 Hz

2.fw = n / 60 5.2390 = 261,77 Hz

fK = n / 60 0.4079 = 20.38 Hz

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Rolling Element Bearings

Outer race damage:(Ball passing frequency, outer range BPFO)

BPFO 2 BPFO 3 BPFO 4 BPFO

Outer race damage frequency BPFO as well asharmonics clearly visible

Inner race damage:(Ball passing frequency, inner range BPFI)

fn

Sidebands at intervals of 1X

BPFI 2 BPFI 3 BPFI 4 BPFI

Inner race damage frequency BPFI as well asnumerous sidebands at intervals of 1X.

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Rolling Element Bearings

Rolling element damage:(Ball spin frequency BSF)

Sidebands in intervals of FTF

2 BSF 4 BSF 6 BSF 8 BSF

Rolling elements rollover frequency BSF withharmonics as well as sidebands in intervals of FTF

Cage damage:(Fundamental train frequency FTF)

FTF and 2nd, 3rd, 4th harmonics

Cage rotation frequency FTF and harmonicsvisible

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Rolling Element Bearings

Lubrication Problems:Major fluctuation in

Lubricant contamination

Insufficient lubrication

Over-greasing

• Race damage

• Defective sealing

• Contaminated lubricant used

• Insufficient lubricant

• Underrating

• Maintenance error

• Defective greaseregulator

• Grease nipple blocked

level of shock pulsesand damage

frequencies

Subsequentsmall temperature

increase

Large temperatureincrease after

lubrication

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Rolling Element Bearings

Incorrect mounting.

Shock pulse

Bearing rings out of round, deformed.

• Incorrect installation

• Wrong bearing storage

• Shaft manufacturing error

• Bearing housing overtorqued.

Bearing forces on floating bearing.

• Incorrect installation

• Wrong housing calculation

• Manufacturing error in bearinghousing

Cocked bearing.

Air gap

Dirt

FixedFloatingbearing bearing

Damagefrequencies

envelope

Severe vibrationBearing temperature

increases

Axial 1X, 2Xand 3X.

• Incorrect installation

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Blade and Vanes

MPA blade or vane generates a signal frequency called

blade pass frequency, fBP:

MP f BP = B n N

Identify and trend fBP.

Bn = # of blades or vanesN = rotor speed in rpm

An increase in it and/or its harmonics may be asymptom of a problem like blade-diffuser or voluteair gap differences.

fBPF

RadialExample characteristic frequency:

3 struts in the intake; x=3.

9 blades; Bn=9.

fBP x = N Bn x

Characteristic frequency = N27

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Aerodynamics and Hydraulic Forces

MP MP

There are two basic moving fluidproblems diagnosed with vibrationanalysis:

• Turbulence

• Cavitation

Cavitation: Turbulence:

1X fBPF RandomRandom 1X

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Belt Drive Faults

MP

MP

Ø1 ω 1

MP

MP

Ø2

ω2

Belt transmission a common drive system inindustry consisting of:

• Driver Pulley

• Driven Pulley

• Belt

The dynamic relation is: Ø1 ω1 = Ø2 ω2

Belt frequency:

3,1416 ω1 Ø1fBl

l: belt length

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Belt Drive Faults

Belt Worn:fn

The belt frequency fB and first two

1X,2X,3XfB

Pulley Misalignment:

Offset

Radial

Angular Twisted

(or even three) harmonics arevisible in the spectrum.

2 fB generally dominates the spectrum

1X of diver or driven pulley visible andpredominant in the axial reading.

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Belt Drive Faults

Eccentric Pulleys:

The geometric center doesn’t coincide with the rotatingcenter of the pulley.

High 1X of the eccentric pulley visible in the spectrum,

Beltpredominant in the radial direction.

direction

Easy to confuse with unbalance, but:

• Measurement phase in vertical an horizontal directionsmay be 0° or 180°.

• The vibration may be higher in the direction of the belts.

Belt Resonance:

If the belt natural frequency coincides with either thedriver or driven 1X, this frequency may be visible in thespectrum.

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Gear Faults

Spur Gear:

Driving gear

Gear (wheel)

Planet Gear:

gear wheel

Driven geargear wheel pair

gear train

Pinion

Ring (cone)

Sun gear

Planet gear

Carrier

Worm Gear:

Gear

Worm gear

Bevel Gear:

Bevel gear

Bevel gear

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Gear Faults

Gear Meshing:

Gear meshing is the contact pattern of the pinionand wheel teeth when transmitting power.

Left flank

89

Right flank5

6

Starting point oftooth meshing

End point of toothmeshing

Working flank

85

8688 4 87 3

2

1

Pitch point Non working flank

Pitch line

The red dotted line is the contact path where themeshing teeth will be in contact during the

rotation.

Flank line Top land

Toothspace Tip edge

Pitch surface

Gear mesh frequency fZ can be calculated:

Fz = z fn

Where z is the number of teeth of the gear rotatingat fn.

Tooth Root flank

Root mantel flank

Flank profile

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Gear Faults

Incorrect tooth meshing

MP

fn1

fz fz 2 fz 3 fz 4 fz

z2

z1

MPfn2

Wear

fz 2 fz 3 fz

MP MP Detail of X:X

fz

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Gear Faults

Incorrect tooth shapefz

MP

fz Detail of X:

X

MP

Tooth break-out

MP MP fz and

X Detail of X: harmonics

z1 z2 Sidebands

fz

fn1 fn2

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Gear Faults

Eccentricity, bent shafts

MP MP Detail of X:X

fz

“Ghost frequencies" or machine frequencies

Gearwheel beingmanufactured

Cutting tool

Worm drive part of thegear cutting machine

zM

fz andharmonicsidebands

fz f M “Ghost frequency"

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Electrical Motors

Electromagnetic forces vibrations:

Twice line frequency vibration: 2 fL

Bar meshing frequency: fbar = fn nbar

Synchronous frequency: fsyn = 2 fL / p

Slip Frequency: fslip = fsyn - fn

Pole pass frequency: fp=p fslip

fL: line frequency

nbar: number of rotor bars

p: number of poles

• Stator eccentricity

• Eccentric rotor

• Rotor problems

• Loose connections

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Electrical Motors

Stator Eccentricity:

MP MP

1X 2X

Radial

2 fL

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1X and 2X signalsfL without sidebandsRadial predominantHigh resolution should be used when analyzing two poles machines

Loose ironShorted stator laminationsSoft foot

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Electrical Motors

Eccentric Rotor:

Rotor offset

Misalignment

Poor base

MP

fp 1X 2X

R

adial

2 fL

fp, 1X, 2X and 2fL signals.

1X and 2fL with sidebands at fP.

Radial predominant.

High resolution needed.

MP

Tslippage

t [ms]

Modulation of the vibration time signal with the slipfrequency fslip

Tslip 2-5 s

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Electrical Motors

Rotor Problems:

1. Rotor thermal bow:1X

Radial

Unbalanced rotor bar current

Unbalance rotor conditions

Observable after some operation time

2. Broken or cracked rotor bars:

1X 2X 3X 4X

f [Hz]

1X and harmonics with sidebands at fP

Radial High resolution spectrum needed

Possible beating signal

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Electrical Motors

3. Loose rotor bar:

1X 2X

Loose connections:

fn 2f L

fbar 2f bar

Radial

f [Hz]

Radial

fbar and 2fbar with 2fL sidebands

2fbar can be higher

1X and 2X can appear

2fL excessive signal with sidebands at 1/3 fL

Electrical phase problem

Correction must be done immediately

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