05-fault analysis 4

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

UnbalanceMisalignmentEccentricityBent shaftShaft crackMechanical loosenessJournal bearing faultsRolling element bearing faultsRotor rubCavitationElectrical motor 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.

Unbalance

Static Unbalance•Equal phase oneach bearing

•Mainly radialvibration

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

RPMRadial

Typical Unbalance Spectrum

Typical Unbalance Spectrum

Please Note: Strong

unbalance cause

harmonics

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.

Misalignment

B. Angular misalignment

1X 2X 3X

mm/s

10

3.1

1

0.31

A. Parallel misalignment

1X 2X 3X

mm/s

10

3.1

1

0.31

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

Radial Vibration approx. 180 ° phase shifted2X often highest peak

Please Note:Misalignment often appears on 1X component only

Please Note:Misalignment often 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

3.1

1

0.31

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.

Eccentricity

• Center of rotation different from geometrical center

•Vertical an horizontal phaseeither equal or 180 ° different

FanRPM

MotorRPM

10

3.1

1

0.31

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.

mm/s

.5X 1X 1.5X 2X 3X

10

3.1

1

0.31

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

mm/s

.5X 1X 1.5X 2X 3X

10

3.1

1

0.31

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

1

0.31

• 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

mm/

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

10

3.1

1

0.31

wo= 0

wo ~ 0.3 - 0.5 ws0.43X 1X 2X

10

3.1

1

0.31

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

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 resonancesof 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 FFT spectrum 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 be used 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 Frequency

BSF with Harmonics.• Often in combinations with

above 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 theopportunity 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 is available, 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

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 MotorCracked 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.

10

3.1

1

0.31

10

3.1

1

0.31

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

The faster a fluid travels by an object the lower the pressure will be, this phenomenon is well known as Bernoullis 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 collapseof small bubbles that occurs duringlocal boiling at certain condition of the fluid (low dynamic pressure)The Collapses are short in timeand thus wide in Frequency.

• The resonances are exited throughoutthe spectrum• Specially high Frequencies are exited• In Envelope Spectra an increase of thebackground level with no distinct linesare seen.

CPB Spectrum Envelope Spectrum

05-fault analysis 4

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