vibration analysis
TRANSCRIPT
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SIGNATURE ANALYSISSIGNATURE ANALYSISSIGNATURE ANALYSISSIGNATURE ANALYSIS
Which frequencies exist and what are the relationships to the fundamental exciting frequencies.
What are the amplitudes of each peak How do the peaks relate to each other If there are significant peaks, what are their source
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COUPLE UNBALANCECOUPLE UNBALANCECOUPLE UNBALANCECOUPLE UNBALANCE
1800 out of phase on the same shaft 1X RPM always present and normally dominates Amplitude varies with square of increasing speed Can cause high axial as well as radial amplitudes Balancing requires Correction in two planes at 180o
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OVERHUNG ROTOR UNBALANCEOVERHUNG ROTOR UNBALANCEOVERHUNG ROTOR UNBALANCEOVERHUNG ROTOR UNBALANCE
1X RPM present in radial and axial directions Axial readings tend to be in-phase but radial readings might be
unsteady Overhung rotors often have both force and couple unbalance each of
which may require correction
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Diagnosing UnbalanceDiagnosing Unbalance Diagnosing UnbalanceDiagnosing Unbalance
Vibration frequency equals rotor speed.
Vibration predominantly RADIAL in direction.
Stable vibration phase measurement.
Vibration increases as square of speed.
Vibration phase shifts in direct proportion to measurement direction.
900
900
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ECCENTRIC ROTORECCENTRIC ROTORECCENTRIC ROTORECCENTRIC ROTOR
Largest vibration at 1X RPM in the direction of the centerline of the rotors
Comparative phase readings differ by 00 or 1800
Attempts to balance will cause a decrease in amplitude in one direction but an increase may occur in the other direction
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ANGULAR MISALIGNMENTANGULAR MISALIGNMENT ANGULAR MISALIGNMENTANGULAR MISALIGNMENT
Characterized by high axial vibration 1800 phase change across the coupling Typically high 1 and 2 times axial vibration Not unusual for 1, 2 or 3X RPM to dominate Symptoms could indicate coupling problems
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PARALLEL MISALIGNMENTPARALLEL MISALIGNMENT PARALLEL MISALIGNMENTPARALLEL MISALIGNMENT
High radial vibration 1800 out of phase Severe conditions give higher harmonics 2X RPM often larger than 1X RPM Similar symptoms to angular misalignment Coupling design can influence spectrum shape and amplitude
RadialRadial
1x1x 2x2x4x4x
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BENT SHAFTBENT SHAFTBENT SHAFTBENT SHAFT
Bent shaft problems cause high axial vibration 1X RPM dominant if bend is near shaft center 2X RPM dominant if bend is near shaft ends Phase difference in the axial direction will tend towards 1800
difference
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MISALIGNED BEARINGMISALIGNED BEARINGMISALIGNED BEARINGMISALIGNED BEARING
Vibration symptoms similar to angular misalignment Attempts to realign coupling or balance the rotor will not alleviate the
problem. Will cause a twisting motion with approximately 1800 phase shift side to
side or top to bottom
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OTHER SOURCES OF HIGH AXIAL OTHER SOURCES OF HIGH AXIAL VIBRATIONVIBRATION
OTHER SOURCES OF HIGH AXIAL OTHER SOURCES OF HIGH AXIAL VIBRATIONVIBRATION
a. Bent Shafts
b. Shafts in Resonant Whirl
c. Bearings Cocked on the Shaft
d. Resonance of Some Component in the Axial Direction
e. Worn Thrust Bearings
f. Worn Helical or Bevel Gears
g. A Sleeve Bearing Motor Hunting for its Magnetic Center
h. Couple Component of a Dynamic Unbalance
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MECHANICAL LOOSENESS (A)MECHANICAL LOOSENESS (A)MECHANICAL LOOSENESS (A)MECHANICAL LOOSENESS (A)
Caused by structural looseness of machine feet Distortion of the base will cause “soft foot” problems Phase analysis will reveal aprox 1800 phase shift in the vertical
direction between the baseplate components of the machine
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MECHANICAL LOOSENESS (B)MECHANICAL LOOSENESS (B)MECHANICAL LOOSENESS (B)MECHANICAL LOOSENESS (B)
Caused by loose pillowblock bolts Can cause 0.5, 1, 2 and 3X RPM Sometimes caused by cracked frame structure or bearing block
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SLEEVE BEARINGSLEEVE BEARINGWEAR / CLEARANCE PROBLEMSWEAR / CLEARANCE PROBLEMS
SLEEVE BEARINGSLEEVE BEARINGWEAR / CLEARANCE PROBLEMSWEAR / CLEARANCE PROBLEMS
Later stages of sleeve bearing wear will give a large family of harmonics of running speed
A minor unbalance or misalignment will cause high amplitudes when excessive bearing clearances are present
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COMPONENT FREQUENCIES OF A SQUARE COMPONENT FREQUENCIES OF A SQUARE WAVE FORM.WAVE FORM.
COMPONENT FREQUENCIES OF A SQUARE COMPONENT FREQUENCIES OF A SQUARE WAVE FORM.WAVE FORM.
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COMPONENT FREQUENCIES OF A SQUARECOMPONENT FREQUENCIES OF A SQUARE WAVE FORM. WAVE FORM.
COMPONENT FREQUENCIES OF A SQUARECOMPONENT FREQUENCIES OF A SQUARE WAVE FORM. WAVE FORM.
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MECHANICAL LOOSENESS (C)MECHANICAL LOOSENESS (C)MECHANICAL LOOSENESS (C)MECHANICAL LOOSENESS (C)
Phase is often unstable Will have many harmonics Can be caused by a loose bearing liner, excessive bearing clearance
or a loose impeller on a shaft
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ROTOR RUBROTOR RUBROTOR RUBROTOR RUB
Similar spectrum to mechanical looseness Usually generates a series of frequencies which may excite natural
frequencies Subharmonic frequencies may be present Rub may be partial or through a complete revolution.
Truncated waveform
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RESONANCERESONANCERESONANCERESONANCE
Resonance occurs when the Forcing Frequency coincides with a Natural Frequency
1800 phase change occurs when shaft speed passes through resonance
High amplitudes of vibration will be present when a system is in resonance
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BELT PROBLEMS (D)BELT PROBLEMS (D)BELT PROBLEMS (D)BELT PROBLEMS (D)
High amplitudes can be present if the belt natural frequency coincides with driver or driven RPM
Belt natural frequency can be changed by altering the belt tension
High amplitudes can be present if the belt natural frequency coincides with driver or driven RPM
Belt natural frequency can be changed by altering the belt tension
BELT RESONANCEBELT RESONANCE
RADIAL
1X RPM
BELT RESONANCE
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BELT PROBLEMS (A)BELT PROBLEMS (A)BELT PROBLEMS (A)BELT PROBLEMS (A)
Often 2X RPM is dominant Amplitudes are normally unsteady, sometimes pulsing with either driver or
driven RPM Wear or misalignment in timing belt drives will give high amplitudes at the
timing belt frequency Belt frequencies are below the RPM of either the driver or the driven
Often 2X RPM is dominant Amplitudes are normally unsteady, sometimes pulsing with either driver or
driven RPM Wear or misalignment in timing belt drives will give high amplitudes at the
timing belt frequency Belt frequencies are below the RPM of either the driver or the driven
WORN, LOOSE OR MISMATCHED BELTSWORN, LOOSE OR MISMATCHED BELTS
BELT FREQUENCYHARMONICS
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BELT PROBLEMS (C)BELT PROBLEMS (C)BELT PROBLEMS (C)BELT PROBLEMS (C)
Eccentric or unbalanced pulleys will give a high 1X RPM of the pulley
The amplitude will be highest in line with the belts Beware of trying to balance eccentric pulleys
Eccentric or unbalanced pulleys will give a high 1X RPM of the pulley
The amplitude will be highest in line with the belts Beware of trying to balance eccentric pulleys
RADIAL1X RPM OFECCENTRICPULLEY
ECCENTRIC PULLEYSECCENTRIC PULLEYS
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BELT PROBLEMS (B)BELT PROBLEMS (B)BELT PROBLEMS (B)BELT PROBLEMS (B)
Pulley misalignment will produce high axial vibration at 1X RPM Often the highest amplitude on the motor will be at the fan RPM
Pulley misalignment will produce high axial vibration at 1X RPM Often the highest amplitude on the motor will be at the fan RPM
1X DRIVEROR DRIVEN
BELT / PULLEY MISALIGNMENTBELT / PULLEY MISALIGNMENT
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HYDRAULIC AND HYDRAULIC AND AERODYNAMIC FORCESAERODYNAMIC FORCES
HYDRAULIC AND HYDRAULIC AND AERODYNAMIC FORCESAERODYNAMIC FORCES
If gap between vanes and casing is not equal, Blade Pass Frequency may have high amplitude
High BPF may be present if impeller wear ring seizes on shaft Eccentric rotor can cause amplitude at BPF to be excessive
If gap between vanes and casing is not equal, Blade Pass Frequency may have high amplitude
High BPF may be present if impeller wear ring seizes on shaft Eccentric rotor can cause amplitude at BPF to be excessive
BPF = BLADE PASS FREQUENCY
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HYDRAULIC AND HYDRAULIC AND AERODYNAMIC FORCESAERODYNAMIC FORCES
HYDRAULIC AND HYDRAULIC AND AERODYNAMIC FORCESAERODYNAMIC FORCES
Flow turbulence often occurs in blowers due to variations in pressure or velocity of air in ducts
Random low frequency vibration will be generated, possibly in the 50 - 2000 CPM range
Flow turbulence often occurs in blowers due to variations in pressure or velocity of air in ducts
Random low frequency vibration will be generated, possibly in the 50 - 2000 CPM range
FLOW TURBULENCEFLOW TURBULENCE
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HYDRAULIC AND AERODYNAMIC HYDRAULIC AND AERODYNAMIC FORCES FORCES
HYDRAULIC AND AERODYNAMIC HYDRAULIC AND AERODYNAMIC FORCES FORCES
Cavitation will generate random, high frequency broadband energy superimposed with BPF harmonics
Normally indicates inadequate suction pressure Erosion of impeller vanes and pump casings may occur if left unchecked Sounds like gravel passing through pump
Cavitation will generate random, high frequency broadband energy superimposed with BPF harmonics
Normally indicates inadequate suction pressure Erosion of impeller vanes and pump casings may occur if left unchecked Sounds like gravel passing through pump
CAVITATIONCAVITATION
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BEAT VIBRATIONBEAT VIBRATIONBEAT VIBRATIONBEAT VIBRATION
A beat is the result of two closely spaced frequencies going into and out of phase
The wideband spectrum will show one peak pulsating up and down The difference between the peaks is the beat frequency which itself will be
present in the wideband spectrum
A beat is the result of two closely spaced frequencies going into and out of phase
The wideband spectrum will show one peak pulsating up and down The difference between the peaks is the beat frequency which itself will be
present in the wideband spectrum
WIDEBAND SPECTRUM
ZOOMSPECTRUM
F1 F2
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ELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMS
Stator problems generate high amplitudes at 2FL (2X line frequency )
Stator eccentricity produces uneven stationary air gap, vibration is very directional
Soft foot can produce an eccentric stator
STATOR ECCENTRICITYSTATOR ECCENTRICITYSHORTED LAMINATIONSSHORTED LAMINATIONSAND LOOSE IRONAND LOOSE IRON
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• Electrical line frequency.(Electrical line frequency.(FLFL) = ) = 50Hz = 3000 cpm.50Hz = 3000 cpm. 60HZ = 3600 cpm60HZ = 3600 cpm• No of poles.No of poles. ((PP) )
• Rotor Bar Pass Frequency (Rotor Bar Pass Frequency (FbFb) = ) = No of rotor bars x No of rotor bars x Rotor rpm. Rotor rpm.
• Synchronous speed (Synchronous speed (NsNs)) = = 2xFL2xFL PP• Slip frequency ( Slip frequency ( FFS S )= )= Synchronous speed - Rotor rpm.Synchronous speed - Rotor rpm.
• Pole pass frequency (Pole pass frequency (FFPP )= )= Slip Frequency x No of Poles. Slip Frequency x No of Poles.
• Electrical line frequency.(Electrical line frequency.(FLFL) = ) = 50Hz = 3000 cpm.50Hz = 3000 cpm. 60HZ = 3600 cpm60HZ = 3600 cpm• No of poles.No of poles. ((PP) )
• Rotor Bar Pass Frequency (Rotor Bar Pass Frequency (FbFb) = ) = No of rotor bars x No of rotor bars x Rotor rpm. Rotor rpm.
• Synchronous speed (Synchronous speed (NsNs)) = = 2xFL2xFL PP• Slip frequency ( Slip frequency ( FFS S )= )= Synchronous speed - Rotor rpm.Synchronous speed - Rotor rpm.
• Pole pass frequency (Pole pass frequency (FFPP )= )= Slip Frequency x No of Poles. Slip Frequency x No of Poles.
FREQUENCIES PRODUCED BY ELECTRICAL FREQUENCIES PRODUCED BY ELECTRICAL MOTORS.MOTORS.
FREQUENCIES PRODUCED BY ELECTRICAL FREQUENCIES PRODUCED BY ELECTRICAL MOTORS.MOTORS.
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ELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMS
Loose stator coils in synchronous motors generate high amplitude at Coil Pass Frequency
The coil pass frequency will be surrounded by 1X RPM sidebands
SYNCHRONOUS MOTORSYNCHRONOUS MOTOR(Loose Stator Coils)(Loose Stator Coils)
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ELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMS
Phasing problems can cause excessive vibration at 2FL with 1/3 FL sidebands
Levels at 2FL can exceed 25 mm/sec if left uncorrected Particular problem if the defective connector is only occasionally
making contact
POWER SUPPLYPOWER SUPPLYPHASE PROBLEMSPHASE PROBLEMS(Loose Connector)(Loose Connector)
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ELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMS
Eccentric rotors produce a rotating variable air gap, this induces pulsating vibration
Often requires zoom spectrum to separate 2FL and running speed harmonic Common values of FP range from 20 - 120 CPM
ECCENTRIC ROTORECCENTRIC ROTOR((Variable Air GapVariable Air Gap))
ECCENTRIC ROTORECCENTRIC ROTOR((Variable Air GapVariable Air Gap))
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ELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMS
DC motor problems can be detected by the higher than normal amplitudes at SCR firing rate
These problems include broken field windings Fuse and control card problems can cause high amplitude peaks at
frequencies of 1X to 5X Line Frequency
DC MOTOR PROBLEMSDC MOTOR PROBLEMS
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ELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMSELECTRICAL PROBLEMS
1X, 2X, 3X, RPM with pole pass frequency sidebands indicates rotor bar problems. 2X line frequency sidebands on rotor bar pass frequency (RBPF) indicates loose rotor
bars. Often high levels at 2X & 3X rotor bar pass frequency and only low level at 1X rotor
bar pass frequency.
ROTOR PROBLEMSROTOR PROBLEMS
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ROTOR BAR FREQUENCIES ROTOR BAR FREQUENCIES (SLOT NOISE)(SLOT NOISE)
ROTOR BAR FREQUENCIES ROTOR BAR FREQUENCIES (SLOT NOISE)(SLOT NOISE)
POLEPOLE
MINIMUMMINIMUM
POLEPOLE
MAXIMUMMAXIMUM
MAXMAX
MINMIN
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CALCULATION OF GEAR MESH CALCULATION OF GEAR MESH FREQUENCIESFREQUENCIES
CALCULATION OF GEAR MESH CALCULATION OF GEAR MESH FREQUENCIESFREQUENCIES
20 TEETH20 TEETH
51 TEETH51 TEETH
1700 RPM1700 RPM
31 TEETH31 TEETH
8959 RPM -- HOW MANY TEETH ON THIS GEAR?8959 RPM -- HOW MANY TEETH ON THIS GEAR?
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GEARSGEARSNORMAL SPECTRUMNORMAL SPECTRUM
GEARSGEARSNORMAL SPECTRUMNORMAL SPECTRUM
Normal spectrum shows 1X and 2X and gear mesh frequency GMF GMF commonly will have sidebands of running speed All peaks are of low amplitude and no natural frequencies are present
14 teeth
8 teeth GMF= 21k CPM
2625 rpm
1500 rpm
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GEARSGEARSTOOTH LOADTOOTH LOAD
GEARSGEARSTOOTH LOADTOOTH LOAD
Gear Mesh Frequencies are often sensitive to load High GMF amplitudes do not necessarily indicate a problem Each analysis should be performed with the system at maximum load
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GEARSGEARSTOOTH WEARTOOTH WEAR
GEARSGEARSTOOTH WEARTOOTH WEAR
Wear is indicated by excitation of natural frequencies along with sidebands of 1X RPM of the bad gear
Sidebands are a better wear indicator than the GMF GMF may not change in amplitude when wear occurs
14 teeth1500 rpm
8 teeth2625 rpm
GMF = 21k CPM
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GEARSGEARSGEAR ECCENTRICITY AND BACKLASHGEAR ECCENTRICITY AND BACKLASH
GEARSGEARSGEAR ECCENTRICITY AND BACKLASHGEAR ECCENTRICITY AND BACKLASH
Fairly high amplitude sidebands around GMF suggest eccentricity, backlash or non parallel shafts
The problem gear will modulate the sidebands Incorrect backlash normally excites gear natural frequency
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GEARSGEARSGEAR MISALIGNMENTGEAR MISALIGNMENT
GEARSGEARSGEAR MISALIGNMENTGEAR MISALIGNMENT
Gear misalignment almost always excites second order or higher harmonics with sidebands of running speed
Small amplitude at 1X GMF but higher levels at 2Xand 3X GMF
Important to set Fmax high enough to capture at least2X GMF
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GEARSGEARSCRACKED / BROKEN TOOTHCRACKED / BROKEN TOOTH
GEARSGEARSCRACKED / BROKEN TOOTHCRACKED / BROKEN TOOTH
A cracked or broken tooth will generate a high amplitude at 1X RPM of the gear
It will excite the gear natural frequency which will be sidebanded by the running speed fundamental
Best detected using the time waveform Time interval between impacts will be the reciprocal of the 1X RPM
TIME WAVEFORM
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D0
D1DB
Note : shaft turning outer race fixed
F = frequency in cpmN = number of balls
BPFI =
BPFO =
BSF =
FTF =
Nb
2
Pd
2Bd
12 (
(
Bd
Pd
COS
RPM(
(
1-
1 + COS X
Nb
2 ( 1 - Bd
Pd
COS
(
X RPM
((
1 -Bd
Pd( COS2 (
XRPM
Bd
PdX RPM
EfftekEfftekDiagnostic EngineersDiagnostic Engineers St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866
ROLLING ELEMENT BEARINGS ROLLING ELEMENT BEARINGS STAGE 1 FAILURE MODESTAGE 1 FAILURE MODE
ROLLING ELEMENT BEARINGS ROLLING ELEMENT BEARINGS STAGE 1 FAILURE MODESTAGE 1 FAILURE MODE
Earliest indications in the ultrasonic range These frequencies evaluated by Spike EnergyTM gSE, HFD(g)
and Shock Pulse Spike Energy may first appear at about 0.25 gSE for this first
stage
gSE
ZONE BZONE A ZONE C ZONE D
EfftekEfftekDiagnostic EngineersDiagnostic Engineers St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866
ROLLING ELEMENT BEARINGS ROLLING ELEMENT BEARINGS STAGE 2 FAILURE MODESTAGE 2 FAILURE MODE
ROLLING ELEMENT BEARINGS ROLLING ELEMENT BEARINGS STAGE 2 FAILURE MODESTAGE 2 FAILURE MODE
Slight defects begin to ring bearing component natural frequencies These frequencies occur in the range of 30k-120k CPM At the end of Stage 2, sideband frequencies appear above and below natural
frequency Spike Energy grows e.g. 0.25-0.50gSE
ZONE A ZONE B ZONE C ZONE D
gSE
EfftekEfftekDiagnostic EngineersDiagnostic Engineers St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866
ROLLING ELEMENT BEARINGS ROLLING ELEMENT BEARINGS STAGE 3 FAILURE MODESTAGE 3 FAILURE MODE
ROLLING ELEMENT BEARINGS ROLLING ELEMENT BEARINGS STAGE 3 FAILURE MODESTAGE 3 FAILURE MODE
Bearing defect frequencies and harmonics appear Many defect frequency harmonics appear with wear the number of sidebands grow Wear is now visible and may extend around the periphery of the bearing Spike Energy increases to between 0.5 -1.0 gSE
ZONE A ZONE B ZONE C ZONE D
gSE
EfftekEfftekDiagnostic EngineersDiagnostic Engineers St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866
ROLLING ELEMENT BEARINGS ROLLING ELEMENT BEARINGS STAGE 4 FAILURE MODESTAGE 4 FAILURE MODE
ROLLING ELEMENT BEARINGS ROLLING ELEMENT BEARINGS STAGE 4 FAILURE MODESTAGE 4 FAILURE MODE
Discreet bearing defect frequencies disappear and are replaced by random broad band vibration in the form of a noise floor
Towards the end, even the amplitude at 1 X RPM is effected High frequency noise floor amplitudes and Spike Energy may in fact decrease Just prior to failure gSE may rise to high levels
gSE
ZONE A ZONE B ZONE C
High just priorto failure
EfftekEfftekDiagnostic EngineersDiagnostic Engineers St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866
GEARSGEARSHUNTING TOOTHHUNTING TOOTH
GEARSGEARSHUNTING TOOTHHUNTING TOOTH
Vibration is at low frequency and due to this can often be missed Synonymous with a growling sound The effect occurs when the faulty pinion and gear teeth both enter
mesh at the same time Faults may be due to faulty manufacture or mishandling
fHt = (GMF)Na(TGEAR)(TPINION)
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OIL WHIP INSTABILITYOIL WHIP INSTABILITYOIL WHIP INSTABILITYOIL WHIP INSTABILITY
Oil whip may occur if a machine is operated at 2X the rotor critical frequency. When the rotor drives up to 2X critical, whirl is close to critical and excessive
vibration will stop the oil film from supporting the shaft. Whirl speed will lock onto rotor critical. If the speed is increased the whip
frequency will not increase.
oil whirl
oil whip
EfftekEfftekDiagnostic EngineersDiagnostic Engineers St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866St. James House, Temple Sowerby, Cumbria, CA10 1RS, United Kingdom. +44(0)1768362866
OIL WHIRL INSTABILITYOIL WHIRL INSTABILITYOIL WHIRL INSTABILITYOIL WHIRL INSTABILITY
Usually occurs at 42 - 48 % of running speed Vibration amplitudes are sometimes severe Whirl is inherently unstable, since it increases centrifugal forces
therefore increasing whirl forces