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71535 B © Copyright SPM Instrument AB 1997
What Is Vibration?
It is the response of a
system to an internal or
external force which causes
the system to oscillate.
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71535 B © Copyright SPM Instrument AB 1997
What Is Vibration Analysis?
Vibration analysis
is a non-destructive
technique which
helps early
detection of machine
problems by
measuring vibration.
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71535 B © Copyright SPM Instrument AB 1997
Detection By Vibration Analysis
1. Unbalance(Static, Couple, Quasi-Static),
2. Misalignment(Angular, Parallel, Combination)
3. Eccentric Rotor, Bent Shaft
4. Mechanical Looseness, Structural Weakness, Soft Foot
5. Resonance, Beat Vibration
6. Mechanical Rubbing
7. Problems Of Belt Driven Machines
8. Journal Bearing Defects
9. Antifriction Bearing Defects
(Inner race, Outer race, Cage, Rolling Elements)
10.Problems of Hydrodynamic & Aerodynamic Machines
(Blade Or Vane, Flow turbulence, Cavitation)
11.Gear Problems (Tooth wear, Tooth load, Gear eccentricity,
Backlash, Gear misalignment, Cracked Or Broken Tooth)
12.Electrical Problems of AC & DC Motor ( Variable Air Gap,
Rotor Bar Defect, Problems of SCRs)
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71535 B © Copyright SPM Instrument AB 1997
Basic Theory Of Vibration
Simple Spring Mass System
Upper
Limit
Neutral
Position
Lower
Position
Displacement
Max Acc
Mim Vel
Max Acc
Mim Vel
Max Vel
Mim Acc
It follows sine curve.
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71535 B © Copyright SPM Instrument AB 1997
Frequency & Amplitude
Frequency:
How many times oscillation is occurring
for a given time period?
Units: CPS(Hz), CPM
Amplitude:
It is the magnitude of vibration signal.
Units: Micron, MM/Sec, M/Sec2
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71535 B © Copyright SPM Instrument AB 1997
Physical Significance Of
Vibration Characteristics
Frequency - What is vibrating?
Source of the vibration.
Amplitude - How much is it vibrating?
Size (severity) of the problem.
Phase Angle - How is it vibrating?
Cause of the vibration.
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71535 B © Copyright SPM Instrument AB 1997
60 RPM
= 1 Rev / s
= 1 Hz
Frequency Measurement
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71535 B © Copyright SPM Instrument AB 1997
Amplitude Measurement
1. Displacement :
Total distance traveled by the mass.
Unit : Microns
2. Velocity :
Rate of change of displacement. It is the
measure of the speed at which the mass is
vibrating during its oscillation.
Unit : MM/Sec, Inch/sec
3. Acceleration :
It is the rate of change of velocity. The
greater the rate of change of velocity the
greater the forces (P=mf) on the machines.
Unit : M/Sec2, Inch/sec2
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71535 B © Copyright SPM Instrument AB 1997
B
C
A
a
b
c
t
t
t
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71535 B © Copyright SPM Instrument AB 1997
t
+
a
d
v
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71535 B © Copyright SPM Instrument AB 1997
Physical Significance Of Vibration
Amplitude
Displacement : Stress Indicator
Velocity : Fatigue Indicator
Acceleration : Force Indicator
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71535 B © Copyright SPM Instrument AB 1997
VIBRATION
SENSITIVITY DISPLACEMENT
VELOCITY
ACCELERATION
FREQUENCY
CPM
60 600 6000 60000 600 000
10
1
.1
.01
.001
When To Use Disp., Vel. & Acc.?
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71535 B © Copyright SPM Instrument AB 1997
What Is The Advantage Of Using
Velocity?
• Flat frequency range compared to
displacement & acceleration.
• Almost all machines generate fault
frequency between 600CPM to 60KCPM
• Velocity indicates fatigue.
• Velocity is the best indicator of
vibration severity.
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71535 B © Copyright SPM Instrument AB 1997
Scales Of Amplitude
Peak
Peak
to
Peak
RMS
Av.
Peak - a
Peak to Peak - 2a
RMS - 0.707 a
Average - 0.637 a
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71535 B © Copyright SPM Instrument AB 1997
Vibration Transducers
Produces electrical signal of vibratory motion
Proximity Probe - Displacement
Velocity Probe - Velocity
Accelerometer - Acceleration
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71535 B © Copyright SPM Instrument AB 1997
Proximity Probe
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71535 B © Copyright SPM Instrument AB 1997
• Permanently installed on large machines
• Measures relative displacement between the bearing
housing and the shaft.
• Called Eddy Current Probe
• Frequency range 0 to 60,000 CPM
• Used 900 apart to display shaft orbit
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71535 B © Copyright SPM Instrument AB 1997
Velocity Probe
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71535 B © Copyright SPM Instrument AB 1997
• Oldest of all.
• Produces signal proportional to velocity.
• Self generating and needs no conditioning electronics.
• It is heavy, complex and expensive.
• Frequency response from 600CPM to 60,000CPM
• Temperature sensitive
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71535 B © Copyright SPM Instrument AB 1997
Accelerometer
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71535 B © Copyright SPM Instrument AB 1997
• Produces signal proportional to
acceleration of seismic mass.
• Extremely linear amplitude sense.
• Large Frequency range
• Smaller in size
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71535 B © Copyright SPM Instrument AB 1997
Phase
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71535 B © Copyright SPM Instrument AB 1997
What Is Phase?
Phase is a measure of relative time difference between two sine waves.
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71535 B © Copyright SPM Instrument AB 1997
Importance Of Phase
• Phase is a relative measurement.
• Provides information how one part of a machine is vibrating
compared to other.
• Confirmatory tool for problems like-
1. Unbalance(Static, Couple, Quasi-Static).
2. Misalignment(Angular, Parallel, Combination).
3. Eccentric Rotor, Bent Shaft.
4. Mechanical Looseness, Structural Weakness, Soft Foot.
5. Resonance.
6. Cocked bearing.
• No correlation with Bearing defects, Gear defects,
Electrical motor defect.
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71535 B © Copyright SPM Instrument AB 1997
The Phase Angle is the angle (in degrees) the
shaft travels from the start of data collection to
when the sensor experiences maximum positive
force.
For example, the phase angle is 90° if the sensor
experiences its maximum positive force at 90°
after data collection was initiated by the
tachometer.
How Phase Angle Is Measured?
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71535 B © Copyright SPM Instrument AB 1997
Step - 1
The tachometer senses the notch in the shaft and triggers data collection.
At this point force equals zero.
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71535 B © Copyright SPM Instrument AB 1997
Step - 2
The high spot (heavy spot) rotates 90° to the sensor position.
At this point the unbalance force produces the highest positive
reading from the sensor.
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71535 B © Copyright SPM Instrument AB 1997
Step - 3
The high spot rotates 90 additional degrees,
the force experience by the sensor is again zero.
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71535 B © Copyright SPM Instrument AB 1997
Step - 4
The high spot rotates 90 additional degrees opposite the sensor position.
At this point the unbalance force produces the highest negative reading
from the sensor.
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71535 B © Copyright SPM Instrument AB 1997
Step - 5
The high spot rotates 90 additional degrees to complete its 360°
revolution, the force experienced by the sensor is again zero.
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71535 B © Copyright SPM Instrument AB 1997
Phase Angle
During the 3600 shaft rotation, the sensor
experiences it’s maximum positive force
when the shaft’s heavy spot is 900 from
it’s initial position when data collection was
initiated by the tachometer.
This measurement’s phase angel = 90°
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FFT
Spectrum
Analysis
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What Is FFT?
• Known as Fast Fourier Transformation.
• Developed by J.B. Fourier in 1874.
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71535 B © Copyright SPM Instrument AB 1997
FFT mathematically converts overall complex
signal into individual amplitudes at component
frequencies
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71535 B © Copyright SPM Instrument AB 1997
• Different machinery problems cause
vibration at different frequencies.
• Overall vibration coming out from machine
is combination of many vibration signals
form various machine parts and it’s
structure.
• FFT is a method of viewing the signal with
respect to frequency.
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71535 B © Copyright SPM Instrument AB 1997
CPM
t
CPM
t
Pure sinus
2 sine waves
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71535 B © Copyright SPM Instrument AB 1997
1
1.17033e-008
spec
j
400
0 j
x
i
i
0.900984
1.06721e-008
spec
j
400
0 j
trace 1
1.41421
-1.41421
x
i
100
0 i
3.75895
0.00679134
spec
j
400
0 j
trace 1
14.1221
-14.7102
x
i
200
0 i
CPM
Pure sinus
3 sine waves
Complex motion
CPM
CPM
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71535 B © Copyright SPM Instrument AB 1997
Spectrum
Analysis
Techniques
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71535 B © Copyright SPM Instrument AB 1997
Step - 1
Collect useful information
• History of machine.
• Control room data - speed, feed,
temperature, pressure etc.
• Name plate details - bearing no,
no of gear teeth etc.
• Design operating parameters,
critical speed.
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71535 B © Copyright SPM Instrument AB 1997
Step - 2 : Identify the type of
measurement procedure:
1. Identify measurement type-Disp, Vel, Acc.
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71535 B © Copyright SPM Instrument AB 1997
Conversion Of Spectrums
Displacement - Microns
Displacement - Microns
Speed - 2889 RPM
Velocity - mm/s
Acceleration - m/s2
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71535 B © Copyright SPM Instrument AB 1997
2. Measurement direction - Hori, Vert, Axial.
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71535 B © Copyright SPM Instrument AB 1997
Step - 3
Analyze:
1. Evaluate overall vibration reading of the entire
machine.
(a) Identify 1x RPM peak.
(b) Locate highest amplitude.
(c) What is the direction of the highest amplitude?
(d) What is the frequency of the highest amplitude?
2. See the values of Shock pulse, HFD etc.
3. See the trend - in case of sudden increase the
problem severity increases.
4. Analyze the frequency for possible defects.
5. Analyze the phase readings for confirmation if
necessary.
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71535 B © Copyright SPM Instrument AB 1997
Manual
Vibration
Analysis
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71535 B © Copyright SPM Instrument AB 1997
Detection By Vibration Analysis
1. Unbalance(Static, Couple, Quasi-Static),
2. Misalignment(Angular, Parallel, Combination)
3. Eccentric Rotor, Bent Shaft
4. Mechanical Looseness, Structural Weakness, Soft Foot
5. Resonance, Beat Vibration
6. Mechanical Rubbing
7. Problems Of Belt Driven Machines
8. Journal Bearing Defects
9. Antifriction Bearing Defects
(Inner race, Outer race, Cage, Rolling Elements)
10.Hydrodynamic & Aerodynamic Forces
(Blade Or Vane, Flow turbulence, Cavitation)
11.Gear Problems (Tooth wear, Tooth load, Gear eccentricity,
Backlash, Gear misalignment, Cracked Or Broken Tooth)
12.Electrical Problems of AC & DC Motor ( Variable Air Gap,
Rotor Bar Defect, Problems of SCRs)
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71535 B © Copyright SPM Instrument AB 1997
Unbalance
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71535 B © Copyright SPM Instrument AB 1997
Causes Of Unbalance
• Uneven distribution of mass of rotor.
• Dirt accumulation on fan rotors.
• Rotor eccentricity
• Roller deflection, especially in paper machines
• Machining errors
• Uneven erosion and corrosion of pump impellers
• Missing balance weights
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71535 B © Copyright SPM Instrument AB 1997
Types Of Unbalance
• Static Unbalance
• Couple Unbalance
• Overhang Rotor Unbalance
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71535 B © Copyright SPM Instrument AB 1997
Static Unbalance
Detection:
• Highest horizontal vibration
• Amplitude increases as square of speed.
• Dominant frequency at 1x rpm
• Horizontal to vertical phase difference 900 on
the same bearing housing
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71535 B © Copyright SPM Instrument AB 1997
Correction
Can be corrected by one balance weight
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71535 B © Copyright SPM Instrument AB 1997
2. Vibration Phase Analysis:
Angular - 1800 phase shift in the axial direction
across the coupling.
Offset - 1800 phase shift in the radial direction
across the coupling. 00 to 1800 phase shift
occur as the sensor moves from horizontal to
the vertical direction of the same machine.
Skew - 1800 phase shift in the axial or radial
direction across the coupling.
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71535 B © Copyright SPM Instrument AB 1997
Couple Unbalance
Detection:
• High horizontal & some times axial vibration
• Dominant frequency at 1x RPM
• 1800 phase difference between both bearings
horizontal as well as vertical direction.
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71535 B © Copyright SPM Instrument AB 1997
Correction
It requires two plane balancing
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71535 B © Copyright SPM Instrument AB 1997
Overhung Rotor Unbalance
Detection:
• High horizontal & axial vibration
• Dominant frequency at 1x RPM
• Axial readings will be in phase but radial
phase readings might be unsteady.
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71535 B © Copyright SPM Instrument AB 1997
Correction
Overhung rotors might be having both
static and couple unbalance and each of
which requires correction.
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Misalignment
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71535 B © Copyright SPM Instrument AB 1997
Causes Of Misalignment
•Thermal expansion - Most machines align cold.
•Machine vibrations.
•Forces transmitted to the machine by pipe or
support structure.
•Soft foot.
•Direct coupled machined are not properly aligned.
•Poor workmanship.
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71535 B © Copyright SPM Instrument AB 1997
Types Of Misalignment
1. Off set
2. Angular
3. Skew - Combination of
offset & angular
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Diagnosis Of
Misalignment
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Angular - Axial vibration at 1X RPM
Offset - Radial vibration at 2X or 3X RPM
Harmonics (3X-10X) generates as severity increases.
If the 2X amplitude more than 50% of 1X then
coupling damage starts.
If the 2X amplitude more than 150% of 1X then
machine should be stopped for correction.
1.Vibration Spectrum Analysis:
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71535 B © Copyright SPM Instrument AB 1997
Misaligned Or Cocked Bearing
1. Predominant 1X & 2X in the axial direction.
2. 1800 Phase shift top to bettom and/or side to
side of the axial direction of the same bearing
housing.
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71535 B © Copyright SPM Instrument AB 1997
Bent Shaft
Diagnosis:
1. High 1X axial and high 1X radial if bent is near the center.
2. High 2X axial and high 2X radial if bent is near the coupling.
3. The phase of the 1X component 1800 at opposite ends of rotor.
4. Phase difference of 1800 between left & right hand side and
also upper & lower sides of the same bearing housing in the
axial direction.
5. Amplitude of 1X & 2X rpm will be steady.
6. High bearing temperature.
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71535 B © Copyright SPM Instrument AB 1997
Eccentric Rotor
• When center of rotation is offset from geometric center.
• Dominant frequency 1X.
• Horizontal to Vertical phase difference either 00 or 1800
at the same bearing housing (Both indicate straight-line
motion).
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71535 B © Copyright SPM Instrument AB 1997
Resonance
• Occurs when forcing frequency coincides
with system natural frequency.
• Causes amplitude amplification which
results catastrophic failure.
• Requires changing of natural frequency
or change of operating speed.
• Bode plot is used to identify resonance.
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71535 B © Copyright SPM Instrument AB 1997
Beat Vibration
• It is the result of two closely spaced frequencies
going into and out of synchronization with one
another.
• The spectrum will show one peak pulsating up
and down.
• Low frequency vibration 5 to 100 CPM.
• Beat frequencies are not normally a problem
when the differences exceed 150 to 200 CPM.
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71535 B © Copyright SPM Instrument AB 1997
Mechanical
Looseness
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71535 B © Copyright SPM Instrument AB 1997
Causes
• It alone can not create vibration but in
the influence of Unbalance,
Misalignment, Bearing problems it
amplify the amplitude.
• It should be corrected first.
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71535 B © Copyright SPM Instrument AB 1997
Types
1. Structural frame/base looseness (1X)
2. Cracked structure/bearing pedestal (2X)
3. Rotating looseness - Loose bearing/improper
fit between component parts. (Multiple)
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71535 B © Copyright SPM Instrument AB 1997
1. Structural frame/base looseness (1X)
Caused by-
1. Structural looseness/weakness of machine feet,
baseplate & concrete base.
2. Deteriorated grouting.
3. Deterioration of frame or base
4. Soft foot.
5. Loose holding down bolts.
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71535 B © Copyright SPM Instrument AB 1997
Analysis
• Dominant freq 1X. Similar to
unbalance & misalignment.
• Horizontal to vertical phase diff 00 or
1800 at the same bearing housing.
• 1800 phase diff in the vertical direction
between two surfaces.
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2. Cracked structure/bearing pedestal (2X)
Caused by-
1. Crack in the structure or bearing pedestal.
2. Occasionally on some loose bearing housing bolts.
3. Loose bearing or improper component fit.
Analysis-
1. 2X RPM amplitude is > 150% of 1X RPM amplitude in
radial direction.
2. Amplitudes are somewhat erratic.
3. 2X RPM phase somewhat erratic.
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71535 B © Copyright SPM Instrument AB 1997
3. Rotating looseness
Caused by-
1. Loose rotor.
2. Bearing loose in the housing.
3. Bearing loose in the shaft.
4. Excessive bearing internal clearance.
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71535 B © Copyright SPM Instrument AB 1997
Analysis
1. Generates running speed harmonics up to 20X RPM.
2. Generates low amplitude frequencies of 1/2x (i.e. .5x, 1.5x, 2.5x…)
and 1/3 x also.
3. Presence of 1/2x will indicate more advanced looseness problems
like presence of rub. It indicate other problems like unbalance and
misalignment.
4. Phase measurement will be erratic.
5. Phase for loose rotor will vary from one measurement to next.
6. Bearing loose on the shaft will generate 1x RPM peak.
7. Bearing loose in the housing will generate 4x RPM peak.
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Journal
Bearing
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71535 B © Copyright SPM Instrument AB 1997
Cause: Excessive clearance and light radial loading.
This results in the oil film building up and forcing the
journal to migrate around in the bearing at less than
one-half RPM.
Oil whirl is a serious condition and needs to be corrected.
It can create metal to metal contact.
Oil Whirl
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71535 B © Copyright SPM Instrument AB 1997
A symptom peculiar for journal bearings is oil whirl.
It shows up as a vibration of approx. 0.42-0.48 X RPM.
Oil whirl can be diagnosed by increasing the load which
will decrease the subsyncronous vibration.
frequency
mm/s
rms
0.45X
2X
1X
severe oil whirl will cause looseness
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Journal Looseness
(Rotating Looseness)
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Rotor Rub
• Spectra similar to mechanical looseness.
• It may be partial or full annular rub.
• Often excites one or more resonances.
• It can excites high frequencies.
• Cascade diagram and shaft orbit are very
helpful in diagnosing rubs.
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71535 B © Copyright SPM Instrument AB 1997
Rolling
Element
Bearing
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Why Does Bearing Fail?
1. Improper lubrication
2. Contaminated lubrication
3. Heavier loading from unbalance,
misalignment, bent shaft etc.
4. Improper handling or installation.
5. Old age (Surface fatigue) .
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71535 B © Copyright SPM Instrument AB 1997
Bearing Failure Detection
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71535 B © Copyright SPM Instrument AB 1997
Bearing Failure Stages
• Earliest indications of bearing problems appear in
the ultrasonic frequency ranging from 1,200k to
3,600 kCPM.
• Spike Energy, HFD, and Shock Pulse evaluate
these frequencies.
Stage - 1
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71535 B © Copyright SPM Instrument AB 1997
Stage - 2
• Slight bearing defects begin to ring bearing component
natural frequencies, which predominantly occur in 30k
to 120K CPM range.
• Sideband frequencies appear above and below natural
frequency peak at the end of stage 2.
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71535 B © Copyright SPM Instrument AB 1997
Stage - 3
• Bearing defect frequencies and harmonics appear.
• More defect frequencies appear and nos of
sidebands grow, both around these and bearing
natural frequencies.
• When well-formed sidebands accompany bearing
defect frequency harmonics
• Bearing has to be replaced.
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71535 B © Copyright SPM Instrument AB 1997
Stage - 4
Towards the end-
• Amplitude of 1X and other running speed harmonics will increase.
• Discrete bearing defect and component natural frequencies
actually begin to disappear and are replaced by random, broad
band high frequency noise floor.
• Amplitude of both high frequency noise floor and spike energy
may decrease, but prior to failure, spike energy will usually grow
to excessive amplitudes.
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71535 B © Copyright SPM Instrument AB 1997
Rolling element bearings
FTF = Fundamental Train Frequency
BSF = Ball Spin Frequency
BPFO = Ball Pass Frequency Outer race
BPFI = Ball Pass Frequency Inner race
FTF .
.
1
2
1 .
d
D
cosF RPM
BSF .
.
D
.
2 d
1 .
d
D
cosF
2
RPM
BPFO .
N FTF
BPFI .
N (RPM - FTF)
RPM = Shaft rotation
d = Rolling element diameter
D = Pitch diameter
N = No. of rolling elements
= Contact angle
F
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71535 B © Copyright SPM Instrument AB 1997
Rolling element bearings
What do the bearing frequencies mean?
If:
FTF = 0.381 * RPM
BSF = 1.981 * RPM
BPFO = 3.047 * RPM
BPFI = 4.952 * RPM
The cage rotates 0.381 revolutions
The ball spins 1.981 revolutions
4.952 balls pass an inner race defect
3.047 balls pass an outer race defect
During one shaft revolution:
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Gear Box
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RPM1
RPM2
Z1
Z2
GMF (Gear Mesh Frequency)
Z1 .RPM1 = Z2 .RPM2
where Z = no. of teeth and RPM shaft speed
Gear box
Mating gears produce only one ‘Gear Mesh Frequency’
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Gear Box Defects
• Tooth wear
• Gear eccentricity & backlash
• Gear misalignment
• Cracked or broken tooth
• Hunting tooth
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frequency
velocity
time
velocity
Spur gear produce vibration in radial direction and
helical gear produce in radial & axial directions.
Sideband spacing reveals the defective gear or pinion.
All analysis should be done at maximum load.
Z.Rps
Z.Rps+2 Rps
Z.Rps+Rps
Z.Rps-2 Rps
Z.Rps-Rps
1/Rps
Gear box
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Normal Spectrum
•All peaks are of low amplitudes
•No natural frequency of gear excited
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Tooth Wear
•GMF may not change
•High amplitudes of side bands
•Presence of natural frequency of gear (fn)
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Gear Eccentricity & Backlash
•Excites GMF, fn & their side bands
•High amplitudes of side bands
•Sideband spacing reveals the defective
gear or pinion.
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Gear Misalignment
•Excites harmonics of GMF with side bands
•Amplitudes 2GMF or 3GMF higher than 1GMF
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Cracked / Broken Tooth
•High amplitudes of 1x RPM of gear or pinion
•Excites gear natural frequency.
•Time waveform indicates spikes at 1/RPM of broken or cracked tooth.
•Amplitudes of impact spikes in time waveform will be higher than
that of 1xRPM in FFT.
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Hunting Tooth
1. Faults on both gear & pinion occured
during manufacturing.
2. Causes high vibration at low
frequencies (less than 600CPM)
3. Gear set emits growling sound.
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Hunting Tooth Frequency
It is the rate at which a tooth in one gear mates with a particular
tooth in the other gear.
For well designed gearboxes HTF is low.
HTF = GMF . (lowest common prime factor) /(Z1.Z2)
Ex:
Z1 = 63, Z2 = 12
Z1 = 63 = 7.3.3
Z2 = 12 = 3.2.2
lowest common prime factor = 3
HTF = GMF. 3 / (63.12) = GMF / 252
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71535 B © Copyright SPM Instrument AB 1997
Belt Drive
d D
C
BF (Belt Frequency) = Rps. .D / L

BF is the rotational frequency of the belt. Worn or mismatched belts will
produce radial vibration at BF and its harmonics. 2.BF is often dominant
with 2 sheaves in the system. BF is a subsyncronous vibration (below 1X).
L C D d
D d
C
   

2 157
4
2
. ( )
( )
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71535 B © Copyright SPM Instrument AB 1997
force
Eccentric sheaves will generate strong 1X radial components.
A common condition.
Looks like unbalance, but shows at both sheaves.
Belt Drive Eccentric sheaves
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Belt Drive
Off-set Angular
Axial vibrations at 1X and BF with harmonics.
Sheave misalignment
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Fans
Fan Blade Pass Frequency = RPM x no. of fan blades
(FBPF)
Mainly radial vibrations
FBPF with 1X RPM sidebands.
Unbalanced horizontal or axial vibrations.
Inadequate blade clearance (at FBPF or RPM harmonics)
Uneven velocity distribution across fan inlet gives FBPF vibrations
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Vibration signature depends upon operating condition.
pressure, temperature, speed, cavitation….
Centrifugal pumps
at Vane Pass Frequency = RPM . no. of impeller vanes
and harmonics.
Gear pumps
at Gear Mesh Frequency and 1X sidebands
Screw pumps
at thread rate = RPM . number of threads and its
harmonics.
Pumps
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Electrical
Motors
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f
p
RPM sync 

120
Asyncronous AC motors
Running speed:
where p = number of poles
f = line frequency
If no. of poles = 2.
The motor will have a sync. RPM of 3000
where fs = slip frequency
AC Motors
fs = RPMsync -
RPM
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AC Motors
All electrical machines vibrates at 2 . line frequency.
The reasons are expanding and collapsing magnetic fields.
This vibration is normal (to a degree).
The amplitude is a measure of the quality of the construction.
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Coast Down Test
Vibration suddenly drops down after switching off the power
AC Motors
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AC Motors
Faults possible to detect with vibration analysis:
Rotor thermal bow
Air gap eccentricity
Loose rotor
Eccentric rotor
Loose windings
Necessary data:
No. of stator slots (SPF)
No. of rotor bars (RBPF)
Line frequency (LF)
Slip frequency (SF)
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In order to find 2 X slip sidebands around FL, a analyzer
with high resolution, and preferably zoom around 50 Hz
is needed.
current clamp
Frequency analysis of input current
AC Motors
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71535 B © Copyright SPM Instrument AB 1997
D.C. Motor
•Broken field winding
•Bad SCRs
•Loose connections
•Loose or blown fuses
•Shorted control cards
Faults possible to detect with vibration analysis:
112
71535 B © Copyright SPM Instrument AB 1997
Identification
• Broken field winding, Bad SCRs, Loose connections
Peaks at 6.LF
• Loose or blown fuses, Shorted control cards
Peaks at 1.LF to 5.LF
For 3 phase rectified D.C. motors
113
71535 B © Copyright SPM Instrument AB 1997
-suppresses vibrations not synchronous with the measured object.
3
0
One measurement
1X 2X 3X
3
0
1X 2X 3X
Average of 10 measurements
Sampling always starts at
’tacho signal’ from roll 1.
Synchronous time averaging
Roll 1
Roll 2
VIB
RPM
114
71535 B © Copyright SPM Instrument AB 1997
Enveloping
Detects low amplitude high frequency repetitive
bearing or gear mesh defects.
These defects emit repetitive low energy impact
signals that are highly pulse shaped and of very
short duration. They generate harmonics in the
very high frequency ranges.
These techniques filter out low frequency
vibration energies and therefore isolate &
emphasize specific high frequency signals.
115
71535 B © Copyright SPM Instrument AB 1997
ISO 2372
Class Class Class Class Class Class
Limits
1 Step
116
71535 B © Copyright SPM Instrument AB 1997
Case - 1 Pump Impeller Wear
Summary
This case documents a 100 hp monitored
over a period of 2 years (1500 rpm).
An increase of overall vibration levels and
spectrum showed high vane pass levels
of 3 x RPM.
117
71535 B © Copyright SPM Instrument AB 1997
The pump has – 3 vanes on the impeller
and an internal inspection indicated
impeller wears.
Impeller changed and both the overall
levels of vibration and the vane pass
frequency returned to normal,
acceptable levels.
118
71535 B © Copyright SPM Instrument AB 1997
Analysis
Overall Vibration Levels:
Overall vibration levels were on the
boarder line of being in an alarm state
for several months and remained stable,
it was decided to monitor closely. Then
suddenly overall vibration levels jumped
to 0.95 inch/sec ie 390 % change.
119
71535 B © Copyright SPM Instrument AB 1997
Vibration Spectrum :
The frequency spectrum collected from
the pump inboard bearing shows a
predominant peak at a frequency of
3000 rpm. The spectral plot showing
the historical growth in amplitude of
this predominant peak.
It is noted that the machine deterioration
appears more severe on the basis of
overall readings than on the basis of the
frequency spectrums.
120
71535 B © Copyright SPM Instrument AB 1997
One possible explanation for the
observation is that there was an
increase in vibration amplitudes at
frequencies beyond the range.
121
71535 B © Copyright SPM Instrument AB 1997
Theoretical Consideration:
Problems with blades and vanes are
usually characterized by high
vibration or near the harmonic
frequency corresponding to the
rotation speed X No. of blades. Blade
pass frequency usually exists in
normal operating machines, but
harmonics usually indicates problem.
122
71535 B © Copyright SPM Instrument AB 1997
Problem Diagnosis:
The data collected pointed towards the
pump impeller as the source of high
vibrations because of dominant peaks
at the blade pass frequency as well as
growing side band harmonics.
123
71535 B © Copyright SPM Instrument AB 1997
Results
Corrective Action:
The pump impeller was removed and
replaced.
Findings:
Overall vibration reading taken after
repair amplitudes significantly reduced.
124
71535 B © Copyright SPM Instrument AB 1997
Conclusions
The objective of this case was to
demonstrate the concept that the blade
pass frequency (No. of blades x shaft
speed) and surrounding harmonics
gives an indication of impeller
condition. This case over all vibration
increases trend stopped the machine
and changed impeller.
125
71535 B © Copyright SPM Instrument AB 1997
Vibration signature of the pump took on a
new shape without high amplitudes at
blade pass frequency and the overall
vibration levels returned to acceptable
levels.

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VIBRATION-ANALYSIS.ppt

  • 1. 1 71535 B © Copyright SPM Instrument AB 1997 What Is Vibration? It is the response of a system to an internal or external force which causes the system to oscillate.
  • 2. 2 71535 B © Copyright SPM Instrument AB 1997 What Is Vibration Analysis? Vibration analysis is a non-destructive technique which helps early detection of machine problems by measuring vibration.
  • 3. 3 71535 B © Copyright SPM Instrument AB 1997 Detection By Vibration Analysis 1. Unbalance(Static, Couple, Quasi-Static), 2. Misalignment(Angular, Parallel, Combination) 3. Eccentric Rotor, Bent Shaft 4. Mechanical Looseness, Structural Weakness, Soft Foot 5. Resonance, Beat Vibration 6. Mechanical Rubbing 7. Problems Of Belt Driven Machines 8. Journal Bearing Defects 9. Antifriction Bearing Defects (Inner race, Outer race, Cage, Rolling Elements) 10.Problems of Hydrodynamic & Aerodynamic Machines (Blade Or Vane, Flow turbulence, Cavitation) 11.Gear Problems (Tooth wear, Tooth load, Gear eccentricity, Backlash, Gear misalignment, Cracked Or Broken Tooth) 12.Electrical Problems of AC & DC Motor ( Variable Air Gap, Rotor Bar Defect, Problems of SCRs)
  • 4. 4 71535 B © Copyright SPM Instrument AB 1997 Basic Theory Of Vibration Simple Spring Mass System Upper Limit Neutral Position Lower Position Displacement Max Acc Mim Vel Max Acc Mim Vel Max Vel Mim Acc It follows sine curve.
  • 5. 5 71535 B © Copyright SPM Instrument AB 1997 Frequency & Amplitude Frequency: How many times oscillation is occurring for a given time period? Units: CPS(Hz), CPM Amplitude: It is the magnitude of vibration signal. Units: Micron, MM/Sec, M/Sec2
  • 6. 6 71535 B © Copyright SPM Instrument AB 1997 Physical Significance Of Vibration Characteristics Frequency - What is vibrating? Source of the vibration. Amplitude - How much is it vibrating? Size (severity) of the problem. Phase Angle - How is it vibrating? Cause of the vibration.
  • 7. 7 71535 B © Copyright SPM Instrument AB 1997 60 RPM = 1 Rev / s = 1 Hz Frequency Measurement
  • 8. 8 71535 B © Copyright SPM Instrument AB 1997 Amplitude Measurement 1. Displacement : Total distance traveled by the mass. Unit : Microns 2. Velocity : Rate of change of displacement. It is the measure of the speed at which the mass is vibrating during its oscillation. Unit : MM/Sec, Inch/sec 3. Acceleration : It is the rate of change of velocity. The greater the rate of change of velocity the greater the forces (P=mf) on the machines. Unit : M/Sec2, Inch/sec2
  • 9. 9 71535 B © Copyright SPM Instrument AB 1997 B C A a b c t t t
  • 10. 10 71535 B © Copyright SPM Instrument AB 1997 t + a d v
  • 11. 11 71535 B © Copyright SPM Instrument AB 1997 Physical Significance Of Vibration Amplitude Displacement : Stress Indicator Velocity : Fatigue Indicator Acceleration : Force Indicator
  • 12. 12 71535 B © Copyright SPM Instrument AB 1997 VIBRATION SENSITIVITY DISPLACEMENT VELOCITY ACCELERATION FREQUENCY CPM 60 600 6000 60000 600 000 10 1 .1 .01 .001 When To Use Disp., Vel. & Acc.?
  • 13. 13 71535 B © Copyright SPM Instrument AB 1997 What Is The Advantage Of Using Velocity? • Flat frequency range compared to displacement & acceleration. • Almost all machines generate fault frequency between 600CPM to 60KCPM • Velocity indicates fatigue. • Velocity is the best indicator of vibration severity.
  • 14. 14 71535 B © Copyright SPM Instrument AB 1997 Scales Of Amplitude Peak Peak to Peak RMS Av. Peak - a Peak to Peak - 2a RMS - 0.707 a Average - 0.637 a
  • 15. 15 71535 B © Copyright SPM Instrument AB 1997 Vibration Transducers Produces electrical signal of vibratory motion Proximity Probe - Displacement Velocity Probe - Velocity Accelerometer - Acceleration
  • 16. 16 71535 B © Copyright SPM Instrument AB 1997 Proximity Probe
  • 17. 17 71535 B © Copyright SPM Instrument AB 1997 • Permanently installed on large machines • Measures relative displacement between the bearing housing and the shaft. • Called Eddy Current Probe • Frequency range 0 to 60,000 CPM • Used 900 apart to display shaft orbit
  • 18. 18 71535 B © Copyright SPM Instrument AB 1997 Velocity Probe
  • 19. 19 71535 B © Copyright SPM Instrument AB 1997 • Oldest of all. • Produces signal proportional to velocity. • Self generating and needs no conditioning electronics. • It is heavy, complex and expensive. • Frequency response from 600CPM to 60,000CPM • Temperature sensitive
  • 20. 20 71535 B © Copyright SPM Instrument AB 1997 Accelerometer
  • 21. 21 71535 B © Copyright SPM Instrument AB 1997 • Produces signal proportional to acceleration of seismic mass. • Extremely linear amplitude sense. • Large Frequency range • Smaller in size
  • 22. 22 71535 B © Copyright SPM Instrument AB 1997 Phase
  • 23. 23 71535 B © Copyright SPM Instrument AB 1997 What Is Phase? Phase is a measure of relative time difference between two sine waves.
  • 24. 24 71535 B © Copyright SPM Instrument AB 1997 Importance Of Phase • Phase is a relative measurement. • Provides information how one part of a machine is vibrating compared to other. • Confirmatory tool for problems like- 1. Unbalance(Static, Couple, Quasi-Static). 2. Misalignment(Angular, Parallel, Combination). 3. Eccentric Rotor, Bent Shaft. 4. Mechanical Looseness, Structural Weakness, Soft Foot. 5. Resonance. 6. Cocked bearing. • No correlation with Bearing defects, Gear defects, Electrical motor defect.
  • 25. 25 71535 B © Copyright SPM Instrument AB 1997 The Phase Angle is the angle (in degrees) the shaft travels from the start of data collection to when the sensor experiences maximum positive force. For example, the phase angle is 90° if the sensor experiences its maximum positive force at 90° after data collection was initiated by the tachometer. How Phase Angle Is Measured?
  • 26. 26 71535 B © Copyright SPM Instrument AB 1997 Step - 1 The tachometer senses the notch in the shaft and triggers data collection. At this point force equals zero.
  • 27. 27 71535 B © Copyright SPM Instrument AB 1997 Step - 2 The high spot (heavy spot) rotates 90° to the sensor position. At this point the unbalance force produces the highest positive reading from the sensor.
  • 28. 28 71535 B © Copyright SPM Instrument AB 1997 Step - 3 The high spot rotates 90 additional degrees, the force experience by the sensor is again zero.
  • 29. 29 71535 B © Copyright SPM Instrument AB 1997 Step - 4 The high spot rotates 90 additional degrees opposite the sensor position. At this point the unbalance force produces the highest negative reading from the sensor.
  • 30. 30 71535 B © Copyright SPM Instrument AB 1997 Step - 5 The high spot rotates 90 additional degrees to complete its 360° revolution, the force experienced by the sensor is again zero.
  • 31. 31 71535 B © Copyright SPM Instrument AB 1997 Phase Angle During the 3600 shaft rotation, the sensor experiences it’s maximum positive force when the shaft’s heavy spot is 900 from it’s initial position when data collection was initiated by the tachometer. This measurement’s phase angel = 90°
  • 32. 32 71535 B © Copyright SPM Instrument AB 1997 FFT Spectrum Analysis
  • 33. 33 71535 B © Copyright SPM Instrument AB 1997 What Is FFT? • Known as Fast Fourier Transformation. • Developed by J.B. Fourier in 1874.
  • 34. 34 71535 B © Copyright SPM Instrument AB 1997 FFT mathematically converts overall complex signal into individual amplitudes at component frequencies
  • 35. 35 71535 B © Copyright SPM Instrument AB 1997 • Different machinery problems cause vibration at different frequencies. • Overall vibration coming out from machine is combination of many vibration signals form various machine parts and it’s structure. • FFT is a method of viewing the signal with respect to frequency.
  • 36. 36 71535 B © Copyright SPM Instrument AB 1997 CPM t CPM t Pure sinus 2 sine waves
  • 37. 37 71535 B © Copyright SPM Instrument AB 1997 1 1.17033e-008 spec j 400 0 j x i i 0.900984 1.06721e-008 spec j 400 0 j trace 1 1.41421 -1.41421 x i 100 0 i 3.75895 0.00679134 spec j 400 0 j trace 1 14.1221 -14.7102 x i 200 0 i CPM Pure sinus 3 sine waves Complex motion CPM CPM
  • 38. 38 71535 B © Copyright SPM Instrument AB 1997 Spectrum Analysis Techniques
  • 39. 39 71535 B © Copyright SPM Instrument AB 1997 Step - 1 Collect useful information • History of machine. • Control room data - speed, feed, temperature, pressure etc. • Name plate details - bearing no, no of gear teeth etc. • Design operating parameters, critical speed.
  • 40. 40 71535 B © Copyright SPM Instrument AB 1997 Step - 2 : Identify the type of measurement procedure: 1. Identify measurement type-Disp, Vel, Acc.
  • 41. 41 71535 B © Copyright SPM Instrument AB 1997 Conversion Of Spectrums Displacement - Microns Displacement - Microns Speed - 2889 RPM Velocity - mm/s Acceleration - m/s2
  • 42. 42 71535 B © Copyright SPM Instrument AB 1997 2. Measurement direction - Hori, Vert, Axial.
  • 43. 43 71535 B © Copyright SPM Instrument AB 1997
  • 44. 44 71535 B © Copyright SPM Instrument AB 1997 Step - 3 Analyze: 1. Evaluate overall vibration reading of the entire machine. (a) Identify 1x RPM peak. (b) Locate highest amplitude. (c) What is the direction of the highest amplitude? (d) What is the frequency of the highest amplitude? 2. See the values of Shock pulse, HFD etc. 3. See the trend - in case of sudden increase the problem severity increases. 4. Analyze the frequency for possible defects. 5. Analyze the phase readings for confirmation if necessary.
  • 45. 45 71535 B © Copyright SPM Instrument AB 1997 Manual Vibration Analysis
  • 46. 46 71535 B © Copyright SPM Instrument AB 1997 Detection By Vibration Analysis 1. Unbalance(Static, Couple, Quasi-Static), 2. Misalignment(Angular, Parallel, Combination) 3. Eccentric Rotor, Bent Shaft 4. Mechanical Looseness, Structural Weakness, Soft Foot 5. Resonance, Beat Vibration 6. Mechanical Rubbing 7. Problems Of Belt Driven Machines 8. Journal Bearing Defects 9. Antifriction Bearing Defects (Inner race, Outer race, Cage, Rolling Elements) 10.Hydrodynamic & Aerodynamic Forces (Blade Or Vane, Flow turbulence, Cavitation) 11.Gear Problems (Tooth wear, Tooth load, Gear eccentricity, Backlash, Gear misalignment, Cracked Or Broken Tooth) 12.Electrical Problems of AC & DC Motor ( Variable Air Gap, Rotor Bar Defect, Problems of SCRs)
  • 47. 47 71535 B © Copyright SPM Instrument AB 1997 Unbalance
  • 48. 48 71535 B © Copyright SPM Instrument AB 1997 Causes Of Unbalance • Uneven distribution of mass of rotor. • Dirt accumulation on fan rotors. • Rotor eccentricity • Roller deflection, especially in paper machines • Machining errors • Uneven erosion and corrosion of pump impellers • Missing balance weights
  • 49. 49 71535 B © Copyright SPM Instrument AB 1997 Types Of Unbalance • Static Unbalance • Couple Unbalance • Overhang Rotor Unbalance
  • 50. 50 71535 B © Copyright SPM Instrument AB 1997 Static Unbalance Detection: • Highest horizontal vibration • Amplitude increases as square of speed. • Dominant frequency at 1x rpm • Horizontal to vertical phase difference 900 on the same bearing housing
  • 51. 51 71535 B © Copyright SPM Instrument AB 1997 Correction Can be corrected by one balance weight
  • 52. 52 71535 B © Copyright SPM Instrument AB 1997 2. Vibration Phase Analysis: Angular - 1800 phase shift in the axial direction across the coupling. Offset - 1800 phase shift in the radial direction across the coupling. 00 to 1800 phase shift occur as the sensor moves from horizontal to the vertical direction of the same machine. Skew - 1800 phase shift in the axial or radial direction across the coupling.
  • 53. 53 71535 B © Copyright SPM Instrument AB 1997 Couple Unbalance Detection: • High horizontal & some times axial vibration • Dominant frequency at 1x RPM • 1800 phase difference between both bearings horizontal as well as vertical direction.
  • 54. 54 71535 B © Copyright SPM Instrument AB 1997 Correction It requires two plane balancing
  • 55. 55 71535 B © Copyright SPM Instrument AB 1997 Overhung Rotor Unbalance Detection: • High horizontal & axial vibration • Dominant frequency at 1x RPM • Axial readings will be in phase but radial phase readings might be unsteady.
  • 56. 56 71535 B © Copyright SPM Instrument AB 1997 Correction Overhung rotors might be having both static and couple unbalance and each of which requires correction.
  • 57. 57 71535 B © Copyright SPM Instrument AB 1997 Misalignment
  • 58. 58 71535 B © Copyright SPM Instrument AB 1997 Causes Of Misalignment •Thermal expansion - Most machines align cold. •Machine vibrations. •Forces transmitted to the machine by pipe or support structure. •Soft foot. •Direct coupled machined are not properly aligned. •Poor workmanship.
  • 59. 59 71535 B © Copyright SPM Instrument AB 1997 Types Of Misalignment 1. Off set 2. Angular 3. Skew - Combination of offset & angular
  • 60. 60 71535 B © Copyright SPM Instrument AB 1997 Diagnosis Of Misalignment
  • 61. 61 71535 B © Copyright SPM Instrument AB 1997 Angular - Axial vibration at 1X RPM Offset - Radial vibration at 2X or 3X RPM Harmonics (3X-10X) generates as severity increases. If the 2X amplitude more than 50% of 1X then coupling damage starts. If the 2X amplitude more than 150% of 1X then machine should be stopped for correction. 1.Vibration Spectrum Analysis:
  • 62. 62 71535 B © Copyright SPM Instrument AB 1997 Misaligned Or Cocked Bearing 1. Predominant 1X & 2X in the axial direction. 2. 1800 Phase shift top to bettom and/or side to side of the axial direction of the same bearing housing.
  • 63. 63 71535 B © Copyright SPM Instrument AB 1997 Bent Shaft Diagnosis: 1. High 1X axial and high 1X radial if bent is near the center. 2. High 2X axial and high 2X radial if bent is near the coupling. 3. The phase of the 1X component 1800 at opposite ends of rotor. 4. Phase difference of 1800 between left & right hand side and also upper & lower sides of the same bearing housing in the axial direction. 5. Amplitude of 1X & 2X rpm will be steady. 6. High bearing temperature.
  • 64. 64 71535 B © Copyright SPM Instrument AB 1997 Eccentric Rotor • When center of rotation is offset from geometric center. • Dominant frequency 1X. • Horizontal to Vertical phase difference either 00 or 1800 at the same bearing housing (Both indicate straight-line motion).
  • 65. 65 71535 B © Copyright SPM Instrument AB 1997 Resonance • Occurs when forcing frequency coincides with system natural frequency. • Causes amplitude amplification which results catastrophic failure. • Requires changing of natural frequency or change of operating speed. • Bode plot is used to identify resonance.
  • 66. 66 71535 B © Copyright SPM Instrument AB 1997 Beat Vibration • It is the result of two closely spaced frequencies going into and out of synchronization with one another. • The spectrum will show one peak pulsating up and down. • Low frequency vibration 5 to 100 CPM. • Beat frequencies are not normally a problem when the differences exceed 150 to 200 CPM.
  • 67. 67 71535 B © Copyright SPM Instrument AB 1997 Mechanical Looseness
  • 68. 68 71535 B © Copyright SPM Instrument AB 1997 Causes • It alone can not create vibration but in the influence of Unbalance, Misalignment, Bearing problems it amplify the amplitude. • It should be corrected first.
  • 69. 69 71535 B © Copyright SPM Instrument AB 1997 Types 1. Structural frame/base looseness (1X) 2. Cracked structure/bearing pedestal (2X) 3. Rotating looseness - Loose bearing/improper fit between component parts. (Multiple)
  • 70. 70 71535 B © Copyright SPM Instrument AB 1997 1. Structural frame/base looseness (1X) Caused by- 1. Structural looseness/weakness of machine feet, baseplate & concrete base. 2. Deteriorated grouting. 3. Deterioration of frame or base 4. Soft foot. 5. Loose holding down bolts.
  • 71. 71 71535 B © Copyright SPM Instrument AB 1997 Analysis • Dominant freq 1X. Similar to unbalance & misalignment. • Horizontal to vertical phase diff 00 or 1800 at the same bearing housing. • 1800 phase diff in the vertical direction between two surfaces.
  • 72. 72 71535 B © Copyright SPM Instrument AB 1997 2. Cracked structure/bearing pedestal (2X) Caused by- 1. Crack in the structure or bearing pedestal. 2. Occasionally on some loose bearing housing bolts. 3. Loose bearing or improper component fit. Analysis- 1. 2X RPM amplitude is > 150% of 1X RPM amplitude in radial direction. 2. Amplitudes are somewhat erratic. 3. 2X RPM phase somewhat erratic.
  • 73. 73 71535 B © Copyright SPM Instrument AB 1997 3. Rotating looseness Caused by- 1. Loose rotor. 2. Bearing loose in the housing. 3. Bearing loose in the shaft. 4. Excessive bearing internal clearance.
  • 74. 74 71535 B © Copyright SPM Instrument AB 1997 Analysis 1. Generates running speed harmonics up to 20X RPM. 2. Generates low amplitude frequencies of 1/2x (i.e. .5x, 1.5x, 2.5x…) and 1/3 x also. 3. Presence of 1/2x will indicate more advanced looseness problems like presence of rub. It indicate other problems like unbalance and misalignment. 4. Phase measurement will be erratic. 5. Phase for loose rotor will vary from one measurement to next. 6. Bearing loose on the shaft will generate 1x RPM peak. 7. Bearing loose in the housing will generate 4x RPM peak.
  • 75. 75 71535 B © Copyright SPM Instrument AB 1997 Journal Bearing
  • 76. 76 71535 B © Copyright SPM Instrument AB 1997 Cause: Excessive clearance and light radial loading. This results in the oil film building up and forcing the journal to migrate around in the bearing at less than one-half RPM. Oil whirl is a serious condition and needs to be corrected. It can create metal to metal contact. Oil Whirl
  • 77. 77 71535 B © Copyright SPM Instrument AB 1997 A symptom peculiar for journal bearings is oil whirl. It shows up as a vibration of approx. 0.42-0.48 X RPM. Oil whirl can be diagnosed by increasing the load which will decrease the subsyncronous vibration. frequency mm/s rms 0.45X 2X 1X severe oil whirl will cause looseness
  • 78. 78 71535 B © Copyright SPM Instrument AB 1997 Journal Looseness (Rotating Looseness)
  • 79. 79 71535 B © Copyright SPM Instrument AB 1997 Rotor Rub • Spectra similar to mechanical looseness. • It may be partial or full annular rub. • Often excites one or more resonances. • It can excites high frequencies. • Cascade diagram and shaft orbit are very helpful in diagnosing rubs.
  • 80. 80 71535 B © Copyright SPM Instrument AB 1997 Rolling Element Bearing
  • 81. 81 71535 B © Copyright SPM Instrument AB 1997 Why Does Bearing Fail? 1. Improper lubrication 2. Contaminated lubrication 3. Heavier loading from unbalance, misalignment, bent shaft etc. 4. Improper handling or installation. 5. Old age (Surface fatigue) .
  • 82. 82 71535 B © Copyright SPM Instrument AB 1997 Bearing Failure Detection
  • 83. 83 71535 B © Copyright SPM Instrument AB 1997 Bearing Failure Stages • Earliest indications of bearing problems appear in the ultrasonic frequency ranging from 1,200k to 3,600 kCPM. • Spike Energy, HFD, and Shock Pulse evaluate these frequencies. Stage - 1
  • 84. 84 71535 B © Copyright SPM Instrument AB 1997 Stage - 2 • Slight bearing defects begin to ring bearing component natural frequencies, which predominantly occur in 30k to 120K CPM range. • Sideband frequencies appear above and below natural frequency peak at the end of stage 2.
  • 85. 85 71535 B © Copyright SPM Instrument AB 1997 Stage - 3 • Bearing defect frequencies and harmonics appear. • More defect frequencies appear and nos of sidebands grow, both around these and bearing natural frequencies. • When well-formed sidebands accompany bearing defect frequency harmonics • Bearing has to be replaced.
  • 86. 86 71535 B © Copyright SPM Instrument AB 1997 Stage - 4 Towards the end- • Amplitude of 1X and other running speed harmonics will increase. • Discrete bearing defect and component natural frequencies actually begin to disappear and are replaced by random, broad band high frequency noise floor. • Amplitude of both high frequency noise floor and spike energy may decrease, but prior to failure, spike energy will usually grow to excessive amplitudes.
  • 87. 87 71535 B © Copyright SPM Instrument AB 1997 Rolling element bearings FTF = Fundamental Train Frequency BSF = Ball Spin Frequency BPFO = Ball Pass Frequency Outer race BPFI = Ball Pass Frequency Inner race FTF . . 1 2 1 . d D cosF RPM BSF . . D . 2 d 1 . d D cosF 2 RPM BPFO . N FTF BPFI . N (RPM - FTF) RPM = Shaft rotation d = Rolling element diameter D = Pitch diameter N = No. of rolling elements = Contact angle F
  • 88. 88 71535 B © Copyright SPM Instrument AB 1997 Rolling element bearings What do the bearing frequencies mean? If: FTF = 0.381 * RPM BSF = 1.981 * RPM BPFO = 3.047 * RPM BPFI = 4.952 * RPM The cage rotates 0.381 revolutions The ball spins 1.981 revolutions 4.952 balls pass an inner race defect 3.047 balls pass an outer race defect During one shaft revolution:
  • 89. 89 71535 B © Copyright SPM Instrument AB 1997 Gear Box
  • 90. 90 71535 B © Copyright SPM Instrument AB 1997 RPM1 RPM2 Z1 Z2 GMF (Gear Mesh Frequency) Z1 .RPM1 = Z2 .RPM2 where Z = no. of teeth and RPM shaft speed Gear box Mating gears produce only one ‘Gear Mesh Frequency’
  • 91. 91 71535 B © Copyright SPM Instrument AB 1997 Gear Box Defects • Tooth wear • Gear eccentricity & backlash • Gear misalignment • Cracked or broken tooth • Hunting tooth
  • 92. 92 71535 B © Copyright SPM Instrument AB 1997 frequency velocity time velocity Spur gear produce vibration in radial direction and helical gear produce in radial & axial directions. Sideband spacing reveals the defective gear or pinion. All analysis should be done at maximum load. Z.Rps Z.Rps+2 Rps Z.Rps+Rps Z.Rps-2 Rps Z.Rps-Rps 1/Rps Gear box
  • 93. 93 71535 B © Copyright SPM Instrument AB 1997 Normal Spectrum •All peaks are of low amplitudes •No natural frequency of gear excited
  • 94. 94 71535 B © Copyright SPM Instrument AB 1997 Tooth Wear •GMF may not change •High amplitudes of side bands •Presence of natural frequency of gear (fn)
  • 95. 95 71535 B © Copyright SPM Instrument AB 1997 Gear Eccentricity & Backlash •Excites GMF, fn & their side bands •High amplitudes of side bands •Sideband spacing reveals the defective gear or pinion.
  • 96. 96 71535 B © Copyright SPM Instrument AB 1997 Gear Misalignment •Excites harmonics of GMF with side bands •Amplitudes 2GMF or 3GMF higher than 1GMF
  • 97. 97 71535 B © Copyright SPM Instrument AB 1997 Cracked / Broken Tooth •High amplitudes of 1x RPM of gear or pinion •Excites gear natural frequency. •Time waveform indicates spikes at 1/RPM of broken or cracked tooth. •Amplitudes of impact spikes in time waveform will be higher than that of 1xRPM in FFT.
  • 98. 98 71535 B © Copyright SPM Instrument AB 1997 Hunting Tooth 1. Faults on both gear & pinion occured during manufacturing. 2. Causes high vibration at low frequencies (less than 600CPM) 3. Gear set emits growling sound.
  • 99. 99 71535 B © Copyright SPM Instrument AB 1997 Hunting Tooth Frequency It is the rate at which a tooth in one gear mates with a particular tooth in the other gear. For well designed gearboxes HTF is low. HTF = GMF . (lowest common prime factor) /(Z1.Z2) Ex: Z1 = 63, Z2 = 12 Z1 = 63 = 7.3.3 Z2 = 12 = 3.2.2 lowest common prime factor = 3 HTF = GMF. 3 / (63.12) = GMF / 252
  • 100. 100 71535 B © Copyright SPM Instrument AB 1997 Belt Drive d D C BF (Belt Frequency) = Rps. .D / L  BF is the rotational frequency of the belt. Worn or mismatched belts will produce radial vibration at BF and its harmonics. 2.BF is often dominant with 2 sheaves in the system. BF is a subsyncronous vibration (below 1X). L C D d D d C      2 157 4 2 . ( ) ( )
  • 101. 101 71535 B © Copyright SPM Instrument AB 1997 force Eccentric sheaves will generate strong 1X radial components. A common condition. Looks like unbalance, but shows at both sheaves. Belt Drive Eccentric sheaves
  • 102. 102 71535 B © Copyright SPM Instrument AB 1997 Belt Drive Off-set Angular Axial vibrations at 1X and BF with harmonics. Sheave misalignment
  • 103. 103 71535 B © Copyright SPM Instrument AB 1997 Fans Fan Blade Pass Frequency = RPM x no. of fan blades (FBPF) Mainly radial vibrations FBPF with 1X RPM sidebands. Unbalanced horizontal or axial vibrations. Inadequate blade clearance (at FBPF or RPM harmonics) Uneven velocity distribution across fan inlet gives FBPF vibrations
  • 104. 104 71535 B © Copyright SPM Instrument AB 1997 Vibration signature depends upon operating condition. pressure, temperature, speed, cavitation…. Centrifugal pumps at Vane Pass Frequency = RPM . no. of impeller vanes and harmonics. Gear pumps at Gear Mesh Frequency and 1X sidebands Screw pumps at thread rate = RPM . number of threads and its harmonics. Pumps
  • 105. 105 71535 B © Copyright SPM Instrument AB 1997 Electrical Motors
  • 106. 106 71535 B © Copyright SPM Instrument AB 1997 f p RPM sync   120 Asyncronous AC motors Running speed: where p = number of poles f = line frequency If no. of poles = 2. The motor will have a sync. RPM of 3000 where fs = slip frequency AC Motors fs = RPMsync - RPM
  • 107. 107 71535 B © Copyright SPM Instrument AB 1997 AC Motors All electrical machines vibrates at 2 . line frequency. The reasons are expanding and collapsing magnetic fields. This vibration is normal (to a degree). The amplitude is a measure of the quality of the construction.
  • 108. 108 71535 B © Copyright SPM Instrument AB 1997 Coast Down Test Vibration suddenly drops down after switching off the power AC Motors
  • 109. 109 71535 B © Copyright SPM Instrument AB 1997 AC Motors Faults possible to detect with vibration analysis: Rotor thermal bow Air gap eccentricity Loose rotor Eccentric rotor Loose windings Necessary data: No. of stator slots (SPF) No. of rotor bars (RBPF) Line frequency (LF) Slip frequency (SF)
  • 110. 110 71535 B © Copyright SPM Instrument AB 1997 In order to find 2 X slip sidebands around FL, a analyzer with high resolution, and preferably zoom around 50 Hz is needed. current clamp Frequency analysis of input current AC Motors
  • 111. 111 71535 B © Copyright SPM Instrument AB 1997 D.C. Motor •Broken field winding •Bad SCRs •Loose connections •Loose or blown fuses •Shorted control cards Faults possible to detect with vibration analysis:
  • 112. 112 71535 B © Copyright SPM Instrument AB 1997 Identification • Broken field winding, Bad SCRs, Loose connections Peaks at 6.LF • Loose or blown fuses, Shorted control cards Peaks at 1.LF to 5.LF For 3 phase rectified D.C. motors
  • 113. 113 71535 B © Copyright SPM Instrument AB 1997 -suppresses vibrations not synchronous with the measured object. 3 0 One measurement 1X 2X 3X 3 0 1X 2X 3X Average of 10 measurements Sampling always starts at ’tacho signal’ from roll 1. Synchronous time averaging Roll 1 Roll 2 VIB RPM
  • 114. 114 71535 B © Copyright SPM Instrument AB 1997 Enveloping Detects low amplitude high frequency repetitive bearing or gear mesh defects. These defects emit repetitive low energy impact signals that are highly pulse shaped and of very short duration. They generate harmonics in the very high frequency ranges. These techniques filter out low frequency vibration energies and therefore isolate & emphasize specific high frequency signals.
  • 115. 115 71535 B © Copyright SPM Instrument AB 1997 ISO 2372 Class Class Class Class Class Class Limits 1 Step
  • 116. 116 71535 B © Copyright SPM Instrument AB 1997 Case - 1 Pump Impeller Wear Summary This case documents a 100 hp monitored over a period of 2 years (1500 rpm). An increase of overall vibration levels and spectrum showed high vane pass levels of 3 x RPM.
  • 117. 117 71535 B © Copyright SPM Instrument AB 1997 The pump has – 3 vanes on the impeller and an internal inspection indicated impeller wears. Impeller changed and both the overall levels of vibration and the vane pass frequency returned to normal, acceptable levels.
  • 118. 118 71535 B © Copyright SPM Instrument AB 1997 Analysis Overall Vibration Levels: Overall vibration levels were on the boarder line of being in an alarm state for several months and remained stable, it was decided to monitor closely. Then suddenly overall vibration levels jumped to 0.95 inch/sec ie 390 % change.
  • 119. 119 71535 B © Copyright SPM Instrument AB 1997 Vibration Spectrum : The frequency spectrum collected from the pump inboard bearing shows a predominant peak at a frequency of 3000 rpm. The spectral plot showing the historical growth in amplitude of this predominant peak. It is noted that the machine deterioration appears more severe on the basis of overall readings than on the basis of the frequency spectrums.
  • 120. 120 71535 B © Copyright SPM Instrument AB 1997 One possible explanation for the observation is that there was an increase in vibration amplitudes at frequencies beyond the range.
  • 121. 121 71535 B © Copyright SPM Instrument AB 1997 Theoretical Consideration: Problems with blades and vanes are usually characterized by high vibration or near the harmonic frequency corresponding to the rotation speed X No. of blades. Blade pass frequency usually exists in normal operating machines, but harmonics usually indicates problem.
  • 122. 122 71535 B © Copyright SPM Instrument AB 1997 Problem Diagnosis: The data collected pointed towards the pump impeller as the source of high vibrations because of dominant peaks at the blade pass frequency as well as growing side band harmonics.
  • 123. 123 71535 B © Copyright SPM Instrument AB 1997 Results Corrective Action: The pump impeller was removed and replaced. Findings: Overall vibration reading taken after repair amplitudes significantly reduced.
  • 124. 124 71535 B © Copyright SPM Instrument AB 1997 Conclusions The objective of this case was to demonstrate the concept that the blade pass frequency (No. of blades x shaft speed) and surrounding harmonics gives an indication of impeller condition. This case over all vibration increases trend stopped the machine and changed impeller.
  • 125. 125 71535 B © Copyright SPM Instrument AB 1997 Vibration signature of the pump took on a new shape without high amplitudes at blade pass frequency and the overall vibration levels returned to acceptable levels.