محاضرة بعنوان Machinery Fault Diagnosis – A basic guide to understanding vibration analysis for machinery diagnosis

محاضرة بعنوان Machinery Fault Diagnosis – A basic guide to understanding vibration analysis for machinery diagnosis
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Machinery Fault Diagnosis – A basic guide to understanding vibration analysis for machinery diagnosis
Preface
This is a basic guide to understand vibration analysis for machinery diagnosis. In practice, many variables must be taken into account.
PRUFTECHNIK Condition Monitoring and/or LUDECA are not responsible for any incorrect assumptions based on this information.
©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Machinery Fault Diagnosis
A basic guide to understanding vibration analysis for machinery diagnosis.
1©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
This is a basic guide to understand vibration analysis for machinery
diagnosis. In practice, many variables must be taken into account.
PRUFTECHNIK Condition Monitoring and/or LUDECA are not
responsible for any incorrect assumptions based on this information.
2011 by PRÜFTECHNIK AG. ISO 9001:2008 certified. No copying or reproduction of this information, in any form whatsoever,
may be undertaken without express written permission of PRÜFTECHNIK AG or LUDECA Inc.
Preface
2©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Unbalance
Unbalance is the condition when the geometric centerline of a rotation axis doesn’t
coincide with the mass centerline.
F
unbalance = m d ω2
m
ω
MP MP
1X
A pure unbalance will generate a signal at the rotation speed and predominantly in
the radial direction.
Radial
3©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Static Unbalance
S
U
m
Static unbalance is caused by an unbalance mass out of
the gravity centerline.
The static unbalance is seen when the machine is not in
operation, the rotor will turn so the unbalance mass is
at the lowest position.
The static unbalance produces a vibration signal at 1X,
radial predominant, and in phase signals at both ends
of the rotor.
4©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Pure Couple Unbalance
m
S
U
-m
b
U = -U
Pure couple unbalance is caused by two identical
unbalance masses located at 180° in the transverse area of
the shaft.
Pure couple unbalance may be statically balanced.
When rotating pure couple unbalance produces a
vibration signal at 1X, radial predominant and in
opposite phase signals in both ends of the shaft.
5©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Dynamic Unbalance
S m
-m
U
U
Dynamic unbalance is static and couple unbalance at the
same time.
In practice, dynamic unbalance is the most common
form of unbalance found.
When rotating the dynamic unbalance produces a
vibration signal at 1X, radial predominant and the phase
will depend on the mass distribution along the axis.
6©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Documentation of balancing
Frequency spectra before/after balancing
and balancing diagram.
.. after balancing
before ..
Balancing diagram
7©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Overhung Rotors
A special case of dynamic unbalance can be found in
overhung rotors.
The unbalance creates a bending moment on the shaft.
Dynamic unbalance in overhung rotors causes high 1X
levels in radial and axial direction due to bending of the
shaft. The axial bearing signals in phase may confirm
this unbalance.
1X
Radial
Axial
8©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Unbalance location
The relative levels of 1X vibration
are dependant upon the location of
the unbalance mass.
9©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Misalignment
Misalignment is the condition when the geometric centerline of two
coupled shafts are not co-linear along the rotation axis of both shafts
at operating condition.
1X 2X
MP MP
A 1X and 2X vibration signal predominant in the axial direction is generally the
indicator of a misalignment between two coupled shafts.
Axial
10©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Angular Misalignment
Angular misalignment is seen when the shaft centerlines
coincide at one point along 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.
1X 2X 3X
Axial
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Parallel Misalignment
Parallel misalignment is produced when the centerlines
are parallel but offset .
The spectrum shows high radial vibration at 2X and a
lower 1X with 180° phase difference across the
coupling in the radial direction.
These signals may be also visible in the axial direction
in a lower amplitude and 180° phase difference across
the coupling in the axial direction.
1X 2X 3X
Radial
12©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Misalignment Diagnosis Tips
In practice, alignment measurements will show a
combination of parallel and angular misalignment.
Diagnosis may show both a 2X and an increased 1X signal
in the axial and radial readings.
The misalignment symptoms vary depending on the
machine and the misalignment conditions.
The misalignment assumptions can be often distinguished
from unbalance by:

  • Different speeds testing
  • Uncoupled motor testing
    Temperature effects caused by thermal growth should also
    be taken into account when assuming misalignment is the
    cause of increased vibration.
    13©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    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.
    14©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Shaft Bending
    A shaft bending is produced either by an axial asymmetry
    of the shaft or by external forces on the shaft producing
    the deformation.
    A bent shaft causes axial opposed forces on the bearings
    identified in the vibration spectrum as 1X in the axial
    vibration.
    2X and radial readings can also be visible.
    1X 2X
    Axial
    15©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Rotating Looseness
    Rotating looseness is caused by an excessive clearance between the rotor and the bearing
    Rotation
    frequency 1X
    and harmonics
    Radial
    Rotation frequency
    1X Harmonics and
    sub Harmonics.
    Radial
    Rolling element bearing:
    Journal bearing:
    16©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Structural Looseness
    Structural looseness occurs when the machine is not correctly supported by, or well
    fastened to its base.
    MP
    MP
    1X
    Radial
  • Poor mounting
  • Poor or cracked base
  • Poor base support
  • Warped base
    MP
    MP
    Structural looseness may increase vibration
    amplitudes in any measurement direction.
    Increases in any vibration amplitudes may
    indicate structural looseness.
    Measurements should be made on the bolts,
    feet and bases in order to see a change in the
    amplitude and phase. A change in amplitude
    and 180° phase difference will confirm this
    problem.
    17©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Resonance
    Resonance is a condition caused when a forcing frequency coincides (or is close) to the
    natural frequency of the machine’s structure. The result will be a high vibration.
    1st form of natural
    flexure
    2nd form of natural flexure 3rd form of natural flexure
    t
    v
    tx
    t
    v
    tx t
    v
    tx
    f f
    1st nat, flexuref2nd nat, flexuref3rd nat, flexure
    no harmonic relationship
    t
    Shaft 1st, 2nd and 3rd critical speeds cause a
    resonance state when operation is near these
    critical speeds.
    18©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Resonance
  • Resonance can be confused with other common problems in machinery.
  • Resonance requires some additional testing to be diagnosed.
    2
    Strong increase in amplitude of the rotation
    frequency fn at the point of resonance, step-up
    dependent on the excitation (unbalanced condition)
    and damping.
    1 = 240° 2 = 60…80°
    1.
    O.
    MP MP
    1
    1.
    O.
    Phase jump
    by 180°
    Resonance
    Step-up
    Grad
    rev/min
    1 2
    rev/min
    Amplitude at rotation frequency fn by residual
    unbalance on rigid rotor.
    19©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Resonance Diagnosing Tests
    Run Up or Coast Down Test:
  • Performed when the machine is
    turned on or turned off.
  • Series of spectra at different RPM.
  • Vibration signals tracking may
    reveal a resonance.
    The use of tachometer is optional
    and the data collector must support
    this kind of tests.
    20©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Resonance Diagnosing Tests
    Bump Test:
    Response – component vibration
    t
    F/a
    5 ms
    Double beat
    s
    1 2
    3
    t
    1
    2
    3
    Decaying function
    Excitation – force pulse
    Shock component, natural vibration, vertical
    t
    Frequency response, vertical
    Natural frequency,
    vertical
    Frequency response, horizontal
    Natural frequency,
    horizontal
    1st mod.
    2nd mod.
    21©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Journal Bearings
    Journal bearings provides a very low friction surface to support and guide a rotor through a cylinder
    that surrounds the shaft and is filled with a lubricant preventing metal to metal contact.
    Clearance problems (rotating mechanical looseness).
    Oil whirl
  • Oil-film stability problems.
  • May cause 0.3-0.5X component in the spectrum.
    0,3-0,5X 1X
    Radial
    High vibration damping due to the oil film:
  • High frequencies signals may not be transmitted.
  • Displacement sensor and continuous monitoring
    recommended
    22©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Rolling Element Bearings
  • Lifetime exceeded
  • Bearing overload
  • Incorrect assembly
  • Manufacturing error
  • Insufficient lubrication
    Wear
    Wear
  1. Wear:
    The vibration spectrum has a higher
    noise level and bearing characteristic
    frequencies can be identified.
    Increased level of shock pulses.
    23©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    3
    2
    1
    4
    Angle of contact
    D Arc diameter
    d Rolling element diameter
    Z Number of rolling elements
    n Shaft RPM
    Rolling Element Bearings
    Roller bearing geometry and damage frequencies: 1 – Outer race damage
    2 – Inner race damage
    3 – Rolling element damage
    4 – Cage damage
    Dimensions
    d =77.50 mm
    D =14.29 mm
    Z = 10
    = 0
    Rollover frequencies
    BPFO = n / 60 4.0781 = 203.77 Hz
    BPFI = n / 60 5.9220 = 295.90 Hz
    2.f
    w = n / 60 5.2390 = 261,77 Hz
    f
    K = n / 60 0.4079 = 20.38 Hz
    Example of rollover frequencies:
    Ball bearing SKF 6211
    RPM, n = 2998 rev/min
    Dw
    D
    pw
  2. Race Damage:
    BPFO = ( 1- cos )
    BPFI = ( 1+ cos )
    BSF = ( 1- cos )
    TFT = ( 1- cos )
    Z n
    2 60
    Z n
    2 60
    n
    2 60
    D n
    d 60
    d D
    2
    d D
    d D
    d D
    Ball pass frequency, outer race:
    Ball pass frequency, inner race:
    Ball spin frequency:
    Fundamental train frequency:
    24©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Rolling Element Bearings
    Outer race damage frequency BPFO as well as
    harmonics clearly visible
    Outer race damage:
    (Ball passing frequency, outer range BPFO)
    Inner race damage frequency BPFI as well as
    numerous sidebands at intervals of 1X.
    Inner race damage:
    (Ball passing frequency, inner range BPFI)
    BPFO 2 BPFO 3 BPFO 4 BPFO
    f
    n
    BPFI 2 BPFI 3 BPFI 4 BPFI
    Sidebands at intervals of 1X
    25©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Rolling Element Bearings
    Rolling element damage:
    (Ball spin frequency BSF)
    FTF and 2nd, 3rd, 4th harmonics
    2 BSF 4 BSF 6 BSF 8 BSF
    Sidebands in intervals of FTF
    Rolling elements rollover frequency BSF with
    harmonics as well as sidebands in intervals of FTF
    Cage rotation frequency FTF and harmonics
    visible
    Cage damage:
    (Fundamental train frequency FTF)
    26©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Rolling Element Bearings
    Lubrication Problems:
    Lubricant contamination
  • Race damage
  • Defective sealing
  • Contaminated lubricant used
    Major fluctuation in
    level of shock pulses
    and damage
    frequencies
    Insufficient lubrication
    Subsequent
    small temperature
    increase
  • Insufficient lubricant
  • Underrating
    Over-greasing
    Large temperature
    increase after
    lubrication
  • Maintenance error
  • Defective grease
    regulator
  • Grease nipple blocked
    27©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Rolling Element Bearings
    Incorrect mounting.
    Bearing rings out of round, deformed.
  • Incorrect installation
  • Wrong bearing storage
  • Shaft manufacturing error
  • Bearing housing overtorqued.
    Dirt
    Damage
    frequencies
    envelope
    Shock pulse
    Air gap
    Bearing forces on floating bearing.
  • Incorrect installation
  • Wrong housing calculation
  • Manufacturing error in bearing
    housing Severe vibration
    Bearing temperature
    increases
    Fixed
    bearing
    Floating
    bearing
    Cocked bearing.
  • Incorrect installation
    Axial 1X, 2X
    and 3X.
    28©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Blade and Vanes
    MP
    MP
    f
    BPF
    Radial
    29
    Example characteristic frequency:
    3 struts in the intake; x=3.
    9 blades; Bn=9.
    f
    BP x = N Bn x
    Characteristic frequency = N 27
    Identify and trend fBP.
    An increase in it and/or its harmonics may be a
    symptom of a problem like blade-diffuser or volute
    air gap differences.
    A blade or vane generates a signal frequency called
    blade pass frequency, fBP:
    fBP = Bn N Bn = # of blades or vanes
    N = rotor speed in rpm©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Aerodynamics and Hydraulic Forces
    There are two basic moving fluid
    problems diagnosed with vibration
    analysis:
  • Turbulence
  • Cavitation
    MP MP
    f
    1X BPF 1X
    Cavitation: Turbulence:
    Random Random
    30©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Belt Drive Faults
    MP MP
    MP MP
    Belt transmission a common drive system in
    industry consisting of:
  • Driver Pulley
  • Driven Pulley
  • Belt
    The dynamic relation is: Ø1 ω1 = Ø2 ω2
    Ø1
    Ø2
    ω1
    ω2
    Belt frequency:
    : belt length
    l
    fB 3,1416 1 1
    l
    31©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Belt Drive Faults
    Pulley Misalignment:
    The belt frequency fB and first two
    (or even three) harmonics are
    visible in the spectrum.
    Radial
    2 f
    B generally dominates the spectrum
    Belt Worn:
    1X of diver or driven pulley visible and
    predominant in the axial reading.
    f
    n
    1X,2X,3XfB
    Offset Angular Twisted
    32©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Belt Drive Faults
    Eccentric Pulleys:
    The geometric center doesn’t coincide with the rotating
    center of the pulley.
    High 1X of the eccentric pulley visible in the spectrum,
    predominant in the radial direction.
    Easy to confuse with unbalance, but:
  • Measurement phase in vertical an horizontal directions
    may 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 the
    driver or driven 1X, this frequency may be visible in the
    spectrum.
    Belt
    direction
    33©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Gear Faults
    Spur Gear:
    Planet Gear:
    Worm Gear:
    gear wheel
    gear wheel pair
    gear train
    Driving gear
    Driven gear
    Gear (wheel) Pinion
    Gear
    Worm gear
    Bevel gear
    Bevel gear
    Sun gear
    Carrier
    Planet gear
    Ring (cone)
    Bevel Gear:
    34©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Gear Faults
    Starting point of
    tooth meshing
    Pitch point
    Working flank
    Non working flank
    Right flank
    Pitch line
    Left flank End point of tooth
    meshing
    1
    2
    4 3
    5
    6
    85
    86
    88 87
    89
    Top land
    Tip edge
    Pitch surface
    Root mantel flank
    Flank line
    Flank profile
    Tooth
    Tooth
    space
    Root flank
    Gear Meshing:
    Gear meshing is the contact pattern of the pinion
    and wheel teeth when transmitting power.
    The red dotted line is the contact path where the
    meshing teeth will be in contact during the
    rotation.
    Gear mesh frequency fZ can be calculated:
    F
    z = z fn
    Where z is the number of teeth of the gear rotating
    at f
    n.
    35©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Gear Faults
    Incorrect tooth meshing
    Wear
    MP
    MP
    z2
    z1
    fn1
    fn2
    fz
    MP MP
    X
    Detail of X:
    fz
    fz
    2 f
    z 3 fz
    fz
    2 f
    z 3 fz 4 fz
    36©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Gear Faults
    Incorrect tooth shape
    MP
    MP
    fz
    Detail of X:
    X
    fz
    fz
    and
    harmonics
    Sidebands
    Tooth break-out
    MP MP
    z1 z2
    X fz
    Detail of X:
    fn1 fn2
    37©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Gear Faults
    Eccentricity, bent shafts
    MP MP
    X
    Detail of X:
    fz
    fz
    and
    harmonic
    sidebands
    “Ghost frequencies” or machine frequencies
    fz
    f
    M “Ghost frequency”
    Cutting tool
    Gearwheel being
    manufactured
    z
    M
    Worm drive part of the
    gear cutting machine
    38©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    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
    f
    L: line frequency
    n
    bar: number of rotor bars
    p: number of poles
  • Stator eccentricity
  • Eccentric rotor
  • Rotor problems
  • Loose connections
    39©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Electrical Motors
    Stator Eccentricity:
    Loose iron
    Shorted stator laminations
    Soft foot
    MP MP
    1X
    2 f
    L
    2X
    1X and 2X signals
    f
    L without sidebands
    Radial predominant
    High resolution should be used when
    analyzing two poles machines.
    Radial
    40©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Electrical Motors
    Eccentric Rotor:
    Rotor offset
    Misalignment
    Poor base
    MP MP
    1X
    2 f
    L
    2X
    Radial
    f
    p
    t [ms]
    T
    slippage
    f
    p, 1X, 2X and 2fL signals.
    1X and 2fL with sidebands at fP.
    Radial predominant.
    High resolution needed.
    Modulation of the vibration time signal with the slip
    frequency fslip
    T
    slip 2-5 s
    41©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Electrical Motors
    Rotor Problems:
    1X
    Radial
    f [Hz]
  1. Rotor thermal bow:
    Unbalanced rotor bar current
    Unbalance rotor conditions
    Observable after some operation time
  2. Broken or cracked rotor bars:
    1X
    Radial
    2X 3X 4X
    1X and harmonics with sidebands at fP
    High resolution spectrum needed
    Possible beating signal
    42©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
    Electrical Motors
    1X
    Radial
    f [Hz]
  3. Loose rotor bar:
    f
    bar and 2fbar with 2fL sidebands
    2f
    bar can be higher
    1X and 2X can appear
    f
    n
    Radial
    2f
    2f n L 2f
    L excessive signal with sidebands at 1/3 fL
    Electrical phase problem
    Correction must be done immediately
    2X f
    bar 2fbar
    Loose connections:
    43

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