Vibration Monitoring
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Transcript of Vibration Monitoring
Machinery Fault DiagnosisA basic guide to understanding vibration analysis for machinery diagnosis.
1
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.
3
Vibration Monitoring
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.
4
Vibration Monitoring
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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Vibration Monitoring
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.
6
Vibration Monitoring
Documentation of balancing
Frequency spectra before/after balancing Balancing diagram
and balancing diagram.
.. after balancing
before ..
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Vibration Monitoring
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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Vibration Monitoring
Unbalance location
The relative levels of 1X vibrationare dependant upon the location ofthe unbalance mass.
9
Vibration Monitoring
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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Vibration Monitoring
Angular Misalignment
1X 2X 3X
Axial
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Vibration Monitoring
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.
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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Vibration Monitoring
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.
13
Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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.
17
Condition Monitoring Condition Monitoring
Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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.
22
Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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.
32
Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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
35
Vibration Monitoring
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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Vibration Monitoring
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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Vibration Monitoring
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"
38
Vibration Monitoring
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
39
Vibration Monitoring
Electrical Motors
Stator Eccentricity:
MP MP
1X 2X
Radial
2 fL
40
Vibration Monitoring
1X and 2X signalsfL without sidebandsRadial predominantHigh resolution should be used when analyzing two poles machines
Loose ironShorted stator laminationsSoft foot
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
41
Vibration Monitoring
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
42
Vibration Monitoring
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
43
Vibration Monitoring