Fault%20Analysis%20With%20Vibration[1]

Fault Analysis

Lecture Note

The main advantage of making vibration measurements on rotating machinery, is the possibility to detect faults, before they make the machine break down, and thereby reduce economical losses, such as damaged equipment and production loss. To this the constant percentage band width spectrum has shown to be the most efficient.

When a fault is detected, vibration analysis can be used to diagnose the fault.

Making diagnosis using vibration analysis requires skill and experience.

Additional measurements of FFT spectra and phase measurements isoften required.

In the following some simple rules for the most common machine faults are drawn up giving the fault type and a characteristic vibration measurements.

The spectra in the examples are all made as drawings, in order to emphasize the typical feature of each fault.

Fault Analysis

UnbalanceMisalignmentPulleys faultsBent shaftShaft crackMechanical loosenessJournal bearing faultsRolling element bearing faultsRotor rubElectrical motor problemsPumps ProblemsGear faults

Unbalance is the most common fault associated with rotating shaft. Unbalance vibration is mainly radial. On overhung rotor axial components may be present as well.

High 1X is often believed to be unbalance, however it can be misalignment, bent rotor or cracked shaft, and further investigation of what may cause the defect is often necessary.

Often Static Unbalance and Dynamic Unbalance are seen together. The phase difference across the shaft therefore may vary.

Sources of ImbalanceThe following machine problems are among the conditions that will

create imbalance:Uneven dirt accumulation on fan rotorsLack of homogeneity in cast parts, such as bubbles, blow-holes, porous sectionsRotor eccentricityRoller deflection, especially in paper machinesMachining errorsUneven mass distribution in electric motor rotor bars or windings Uneven erosion and corrosion of pump impellersMissing balance weightsBowed Shaft

Unbalance

Static Unbalance•Equal phase oneach bearing•Mainly radial vibration

Dynamic Unbalance• Phase changes 180 °across bearing• Mainly radial vibration

Overhung Rotor Unbalance• Both Radial and horizontal vibration• Often both Static and Dynamic unbalance are seen together

Typical Unbalance Spectrum

Typical Unbalance Spectrum

Please Note:Strong

unbalance cause

harmonics

RPMRadial

Misalignment is traditionally associated with a 2nd harmonic component, which according to some sources is due to to 2 times the stress reversal during one rotation. More probably the harmonic occurs due to distortion of the ideal sinusoidal vibration signal.

It is quite common that misalignment occurs on the 1st harmonic only in the spectrum. An investigation of the phase relationship across the rotor and across the coupling should therefore always be carried out for distinguishing misalignment from unbalance.

A misaligned rotor tend to wear in. That is after a while the bearing will get deformed after the misalignment. In the spectrum this is seen as the 2nd order component will decrease and the third order will increase as wear develops.

Causes of MisalignmentMisalignment is typically caused by the following conditions:

Inaccurate assembly of components, such as motors, pumps, etc.Relative position of components shifting after assemblyDistortion due to forces exerted by pipingDistortion of flexible supports due to torqueTemperature induced growth of machine structureCoupling face not perpendicular to the shaft axisSoft foot, where the machine shifts when hold down bolts are torqued.

Misalignment

B. Angular misalignment

A. Parallel misalignment

mm/s

1X 2X 3X

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1X 2X 3X

mm/s

10

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Axial Vibration approx.. 0 ° phase shifted1X , 2X or 3 X highest

Radial Vibration approx. 180 ° phase shifted2X often highest peak

Please Note:Misalignment may be appears on 1X component only

Please Note:Misalignment may be appears on 1X component only

A Bent Shaft to many extents is looking like a misalignment in the spectrum.

A phase measurement for axial vibration across the shaft will distinguish between misalignment and bent shaft as the bent shaft will produce a 180 Degrees Phase shift.

Bent Shaft

• Axial And Radial Vibration• 180 ° Phase shift in Axial Vibration• 0 ° Phase shift in radial vibration

1X 2X

mm/s

10

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Belt Drive Problem

Where D = Sheave DiameterL = Belt LengthRPM = Turn speed of sheave D

Where D = Sheave DiameterWhere D = Sheave DiameterL = Belt LengthL = Belt LengthRPM = Turn speed of sheave DRPM = Turn speed of sheave D

Mismatched, Worn, or Stretched BeltsMismatched, Worn, or Stretched BeltsMismatched, worn, or stretched belts, especially Vee belts, will generate vibration at the fundamental belt pass frequency and harmonics of it. Usually the second harmonic is dominant if there are two sheaves in the system. The Fundamental Belt Frequency FBF is given by the following formula. It is always sub-synchronous, meaning it is lower in frequency than 1X.

Fundamental Belt Frequency (FBF) = π*D*RPM/L

FBF

2x FBF

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Sheave misalignment will generate strong axial 1X components andaxial harmonics of the fundamental belt frequency.

Sheave Misalignment

ParallelAngular Both

Belt Drive Problem Cont.

Eccentric sheaves will generate strong 1X radial components, especially in the direction parallel to the belts. This condition is very common, and mimics imbalance. This can be checked by removing the belts and measuring again. 1X vibration of an eccentric sheave or a sheave with runout will usually also show up at the other sheave.

Eccentric Sheaves, Sheave Runout

• Center of rotation different from geometrical center

•Vertical an horizontal Phaseeither equal or 180 ° different

FanRPM

MotorRPM

10

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Belt Drive Problem Cont.

The Eccentric rotor will produce high vibration at the rotation speed. The Phase will be the same in both horizontal and vertical direction.If you try to balance an eccentric rotor, you may reduce the vibration readings in one direction, but the readings will increase in the other

Mechanical looseness produce a strongly distorted signal. The inter harmonics (½, 1/3 etc.) are attributable to the fact that the loose part bounces and thus does get excited every 2nd or 3rd revolution of the shaft.

Looseness between a machine and its foundation will increase the 1X vibration component in the direction if the least stiffness. This is usually the horizontal direction, but it depends on the physical layout of the machine. Low-order 1X harmonics are also commonly produced if the looseness is severe. It is often hard to tell imbalance from foundation looseness or flexibility, especially in vertical machines. If 1X tangential is much greater than 1X radial, looseness is suspected. If 1X tangential is lower than or equal to 1X radial, then imbalance is suspected. Foundation flexibility or looseness can be caused by loose bolts, corrosion, or cracking of mounting hardware.

mm/s

.5X 1X 1.5X 2X 3X

10

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Loose shaftOften seriesof sub harmoniccomponents½, 1/3, ... 1/n

mm/s

.5X 1X 1.5X 2X 3X

10

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Loose Foundation2X often highSub-harmonics

Looseness

The characteristics of Rotor Rub are very similar to mechanics looseness.

Rotor Rub

mm/

.5X 1X 1.5X 2X 3X

10

3.1

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• Symptoms same as Mechanical Looseness

• Subharmonics ½ ,1/3 etc.• Strong Harmonic pattern

Caused by truncation

Truncated Wave form

Shaft Cracks have been detected by continuously monitoring of 1st and 2nd harmonics, or by comparing run ups and coast down, where a cracked shaft will change the characteristic curve as it passes through the resonance. Shaft Cracks are often mistaken for the far more common misalignment.

Shaft Crack

Longitudinal Crack

Radial Crack

Shaft Cracks may be detectedby monitoring of

• Amplitude and Phase of 1X first and 2X and second harmonic of RPM.

• Monitoring of Coast downand Run - up characteristicswhen passing throughresonance

Shaft Cracks may be detectedby monitoring of

• Amplitude and Phase of 1X first and 2X and second harmonic of RPM.

• Monitoring of Coast downand Run - up characteristicswhen passing throughresonance

X/Y Position History

1X Run Up

Nyquist Bode

Oil Whirl Simplified explanation.In a journal bearing the shaft is “surfing” on an oil wave.Let us look at the speed profile of the oil film.At the boundary of the shaft the oil film has the same speed as the shaft.At bearing boundary the oil film is stationary.

Some bearing designs may develop instability at certain conditions of oil viscosity and bearing load. In such cases the oil film will pump around the shaft with about the average speed of the oil film speed profile. The speed of such pumping normally appears around 42 % - 47% of the shaft speed though instability has been reported in the range 30% to 70 % of shaft speed.

Clearance ProblemsIn a worn journal bearing harmonics up to 10 or 20 times the running speed may be seen.

Journal Bearings

wo= 0

wo ~ 0.3 - 0.5 ws

Oil Instability• normally 42 %- 47 % of

running speed• May appear from 0.3 -0.7X

in some occasions• Non Synchronous

Wear Clearance Problems

• Harmonic Seriesof Rotation Speed

wo= ws

mm/s

1X 2X 3X 4X 5X 6X 7X 8X 9X 10X...

10

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0.43X 1X 2X

10

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mm/s

Rolling element bearing faults normally start with small cracks or spalls, which produce very hard impacts by the passing of the balls.

As the bearing impacts are very short, they will contain energy in very high frequencies - the resonances of the force path will be excited and ring.

Detection of bearing wear is done by seeing increases of the resonances of the bearing and the machine structure in the 2kHz -14 kHz frequency range.

Using Envelope analysis the modulation of the high frequency can be analyzed. Envelope analysis provides thus an excellent tool for both detection and diagnosis of bearing signals.

If no modulation is present in the signal. There will be no peaks in the envelope spectra.

Rolling Element Bearings

Faults in Rolling Element Bearingsare Detected with CPB in the High frequency range

Envelope Spectra can be used both for Detection and Diagnosis of Rolling Element Bearing Faults

No Defects on Rolling Element Bearing

“Flat” Envelope Spectrum.

The rolling element bearing can be considered as a planetary gear with the inner ring as the sun weal and the balls as planets. Different defects will be repeated at frequencies which can be calculated with above formulas. The Ball Diameter and the Race diameters, as well as the contact angle beta is normally given by the manufacturer. The number of balls is given in newer literature from the bearing manufacturers. You may use the mounting diameters of the bearing for calculating the Pitch Diameter if the Outer and Inner Race diameter is not available.

It is general experience that these frequencies show up in a FFTspectrum at a very late stage of bearing wear.

With envelope analysis the bearing frequencies are seen at a very early stage of fault development however. The envelope analysis can beused for accurately predicting the breakdown of a bearing.

It should be noted that the balls will slip few percents in the bearings specially when lightly loaded.

Rolling Element Bearing Frequencies

D1 D2

PD D D=

+1 22

n = number of balls

f r = rotation frequency

The typical bearing fault start as a crack or spall in the outer race. Depending on bearing load a rolling element bearing can “survive”long time with an outer race fault.

An outer race spall will eventually develop to a wear. This can be seen in the envelope spectrum by the reduction of harmonics of the BPFO and an increase of the BPFO itself.

At as certain stage the balls off tracked by the outer race fault will cause a fault in the inner race. As the fault in the inner race is rotating into and out of the load zone, the fault frequency will be modulated with the rotation speed, and thus produce side bands with RPM spacing.

An inner race fault is often faster growing than an outer race fault.

In the end of a bearing fault, often faults and the balls are seen as well as inter modulation frequencies between the different fault types.

Typical Bearing Defects Development Envelope Analysis

3. Ball Defects • Requires Immediate action • Ball Spin FrequencyBSF with Harmonics.

• Often in combinations withabove with various inter-harmonics.

1. Outer Race Faults • Lead Time Month’s• Ball Pass Frequency Outer Race

( BPFO) and Harmonic

2. Inner Race Faults• Lead Time Days - Weeks• Ball Pass Frequency Inner Race(BPFI) With Side bands of Rotation speed

BPFO

RPM

BPFI

BSF

The earliest detection of bearing fault is done by placing the envelope filter on a resonance of the bearing. By doing so however, one miss the opportunity of classifying the defect depth, by the height of the peaks in the envelope spectrum.

Also one misses the opportunity of being able to analyze above defects in the envelope spectra.

For getting the best information about modulations of random noise produced by a rolling element bearing, it is recommended to place the envelope filter in the high frequency at a place where signal isavailable, but not amplified by resonances. (There should be maximum 10dB variation across the envelope filter range).

Bearing Mounting Defects Analyzed With Envelope Analysis

RPM

2*RPM

2*BPFO

Lubrication Defect

Rotor Misalignment Rotor Unbalance

Radial Tension of Bearing

Misalignment ofouter Race (Cocked)

Slip of Race inthe Mounting Seat

2*RPM

1*RPM

2*BPFO

Harmonicsof RPM

Increase ofBackgroundlevel

RPM

A motor with loose, broken or shortened rotor bars will produce modulation of the rotation speed with the slip frequency.

An efficient way of analyzing this fault is making zoom FFT around the motor rotation speed of the motor current.

The motor current can be analyzed using a current probe on one of the motor current supply lines.

If the side bands appear less than 45 dB below the RPM component, alert caution should be taken.

Side bands appearing less than 35 dB below the RPM component should be regarded as shut down criteria.

Please refer to the application note BO 0269 “ Vibration Diagnostics for Industrial Electric Motor Drives” for a detailed description of diagnostics of electrical motors.

Electrical Motor:Cracked Rotor Bars

Loose Rotor Bars may also cause Sidebands of Line frequency around Rotor barpassing frequency and 2*RBPF

Loose Rotor Bars may also cause Sidebands of Line frequency around Rotor barpassing frequency and 2*RBPF

StatorBars

StatorBars

RotorsBars

RotorsBars

Pole Pass Freq. = Slip Freq.* No. of PolesSlip Freq. = Synch Speed - RPMRotor Bar Freq. = No. of rotor Bars * RPM

Pole Pass Freq. = Slip Freq.* No. of PolesSlip Freq. = Synch Speed - RPMRotor Bar Freq. = No. of rotor Bars * RPM

Broken Rotor BarsCracked Rotor BarLoose Rotor BarShorted Rotor LaminationsPoor End Ring Joints

• Side bands of Slip Freqaround 1X, 2X 3X etc.< - 35 dB = Serious> - 45 dB = OK.

(1X- n*Slip Freq) 1X (1X+n*Slip Freq) ZoomSpectrumZoom

Spectrum

35 dB 45 dB

1X 2X RBPF

Lin freq.spacing

The electrical magnets of an electric motor are contracting twice for every period of the net frequency. Thus electrical faults are appearing at twice the net frequency.

The slip frequency is the difference between the rotation frequency of the rotor and the net frequency.

The pole pass frequency is the number of poles times the slip frequency.

An eccentric electric motor will produce side spaced with the pole pass frequency around twice the net frequency.

Zoom is required to analyze these faults.

Electrical Motor Problems

Stator Eccentricity Looseness of Stator SupportShored Stator Laminations

• 2nd Harmonicof line frequency

Pole Pass Freq. = Slip Freq.* No. of PolesSlip Freq. = Synch Speed - RPM

Pole Pass Freq. = Slip Freq.* No. of PolesSlip Freq. = Synch Speed - RPM

mm/s

1X Line 2x 2*Line freq.

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mm/s

1X Line 2X 2*Line freq.

Eccentric Rotor (Statical)• 2 * Line frequency and Sidebands of Pole Pass Freq.around 2 * line frequency

Loose stator coils in synchronous motors may generate high vibration at the coil passing frequency which is the number of stator coils times the RPM.

Modulation is often present and can be seen as side bands spaced with RPM.

DC motors are often controlled by Silicon Controlled Rectifiers ( SCR ).

At the SCR frequency which is usually 6 times the line frequency, increases will show problems with the SCR.

Synchronous Motors, DC Motors

DC MotorsSilicon Controlled Rectifiers (SCR)

SCR firing frequency increase may show:• Bad SCR•Loose Connections•Broken Field Windings

Synchronous MotorsLoose Stator Coils

• RPM spaced Sidebandsaround Coil Pass Frequency.

1X 2X SFC Freq.= 6*Line freq. 2*SCR

1X 2X Coil Pass Freq.

1 RPMspacing

Spectrum Example on DC Motor

The faster a fluid travels by an object the lower the pressure will be, this phenomenon is well known as Bernoulli's law, and it is the reason that aero planes can fly and turbo machines are working.

The lower the pressure, the lower the boiling temperature of water.

In some instances the water of a pump may start boiling locally as a result of the local fluid speed will decrease local dynamic pressure and hence decreased the boiling point below the fluid temperature.

When the local pressure increases again the small bubbles formed in the boiling process collapses very rapidly. The rapid collapse causes shock pulses which may be strong enough to break apart fragments of metal on the location it occurs - cavitation wear.

The collapsing bubbles also induce shock waves which are transferred through the structure. Since the pulses are very short, they have a very high frequency content, and they will excite resonances throughout the spectrum range.

CavitationCavitation is caused by the collapse of small bubbles that occurs during local boiling at certain condition of the fluid (low dynamic pressure) The Collapses are short in time and thus wide in Frequency.

-The resonances are exited throughout the spectrum

-Specially high Frequencies are exited

- In Envelope Spectra an increase of the backgroundlevel with no distinct lines are seen.

Pumps Problems

Constant Percentage Bandwidth Spectrum Envelope Spectrum

Centrifugal PumpsThe following spectrum, containing broadband high-frequency noise,indicates cavitation in a centrifugalpump due to low inlet pressure.

Pumps Problems Cont.

Gear PumpsGear pumps are commonly used for pumping lube oil, and they almost always have a strong vibration component at the tooth mesh frequency, which is the number of teeth on the gear times the RPM. This component will be highly dependent on the output pressure of the pump. If the tooth mesh frequency changes significantly, such as the sudden appearance of harmonics or sidebands in the vibration spectrum, it could indicate a cracked or otherwise damaged tooth.

Typical Gear Pump Spectrum

Pumps Problems Cont.

Screw Pumps

The screw type pump can generate a multitude of frequency components in the vibration spectrum. Thread wear or damage will usually produce strong harmonics of the thread rate, which is the number of threads times the RPM.

Gear box spectra contains a range of frequencies of the different tooth mesh and its harmonics.

The tooth mesh frequency itself, is generally dependent of gearbox load, whereas gear box wear can be diagnosed as an increase of the 2nd and 3rd harmonic.

Side bands around the tooth mesh frequency and its harmonics, are quite common and contains information about gearbox faults.

As a general rule a distributed faults such as runout and gear misalignment will produce side bands which have high amplitudesclose around the fault, whereas distributed faults produce side bands which are spread more widely.

Cepstrum analysis produce an excellent tool for analyzing the energy in each side band family.

With signal enhancement analysis you may collect the vibration spectrum from a single shaft inside a gear box.

Use of Order analysis will eliminate smearing the many components together due to small speed variations.

Gear Boxes

Gear Boxes Produce Complex Spectra

Cepstrum Analysis and Time Domainaveraging greatly simplifies the job ofthe analysis in assessment of Gear Box faults.

Gear Box Spectrum• Faults produce Side Band families

around tooth mesh frequency (TMF)

and harmonics TMF.

Gear Box Spectrum• Faults produce Side Band families

around tooth mesh frequency (TMF)

and harmonics TMF.

Gear Box Cepstrum• The Energy of each Side Band

Family (fault) is easily assessed.

Gear Box Cepstrum• The Energy of each Side Band

Family (fault) is easily assessed.

TMF 2*TMF

Fault%20Analysis%20With%20Vibration[1]

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