1. Machinery Fault Diagnosis
A basic guide to understanding vibration analysis for machinery diagnosis.
1

2. Unbalance Vibration Monitoring
ω
m Unbalance is the condition when the geometric centerline of a rotation axis doesn’t
coincide with the mass centerline.
Funbalance = m d ω2
1X
Radial
MP MP
A pure unbalance will generate a signal at the rotation speed and predominantly in
the radial direction.
3

3. Static Unbalance Vibration Monitoring
Static unbalance is caused by an unbalance mass out of
the gravity centerline.
S
m
U
The static unbalance is seen when the machine is not in
operation, the rotor will turn so the unbalance mass is
at the lowest position.
The static unbalance produces a vibration signal at 1X,
radial predominant, and in phase signals at both ends
of the rotor.
4

4. Pure Couple Unbalance Vibration Monitoring
U = -U
-m
Pure couple unbalance is caused by two identical
S unbalance masses located at 180° in the transverse area of
the shaft.
Pure couple unbalance may be statically balanced.
m When rotating pure couple unbalance produces a
vibration signal at 1X, radial predominant and in
opposite phase signals in both ends of the shaft.
U b
5

5. Dynamic Unbalance Vibration Monitoring
U
Dynamic unbalance is static and couple unbalance at the
-m same time.
S
m
U
In practice, dynamic unbalance is the most common
form of unbalance found.
When rotating the dynamic unbalance produces a
vibration signal at 1X, radial predominant and the phase
will depend on the mass distribution along the axis.
6

6. Documentation of balancing Vibration Monitoring
Frequency spectra before/after balancing Balancing diagram
and balancing diagram.
.. after balancing
before ..
7

7. Overhung Rotors Vibration Monitoring
A special case of dynamic unbalance can be found in
overhung rotors.
The unbalance creates a bending moment on the shaft.
Dynamic un balance in overhung rotors causes high 1X
levels in radial and axial direction due to bending of the
shaft. The axial bearing signals in phase may confirm
this unbalance.
1X
Radial
Axial
8

8. Unbalance location Vibration Monitoring
The relative levels of 1X vibration
are dependant upon the location of
the unbalance mass.
9

9. Misalignment Vibration Monitoring
Misalignment is the condition when the geometric centerline of two
coupled shafts are not co-linear along the rotation axis of both shafts
at operating condition.
1X 2X
MP MP
Axial
A 1X and 2X vibration signal predominant in the axial direction is generally the
indicator of a misalignment between two coupled shafts.
10

10. Angular Misalignment Vibration Monitoring
Angular misalignment is seen when the shaft centerlines
coincide at one point a long the projected axis of both
shafts.
The spectrum shows high axial vibration at 1X plus
some 2X and 3X with 180° phase difference across the
coupling in the axial direction.
These signals may be also visible in the radial direction
at a lower amplitude and in phase.
1X 2X 3X
Axial
11

11. Parallel Misalignment Vibration Monitoring
Parallel misalignment is produced when the centerlines
are parallel but offset .
The spectrum shows high radial vibration at 2X and a
lower 1X with 180° phase difference across the
coupling in the radial direction.
These signals may be also visible in the axial direction
in a lower amplitude and 180° phase difference across
the coupling in the axial direction.
1X 2X 3X
Radial
12

12. Misalignment Diagnosis Tips Vibration Monitoring
In practice, alignment measurements will show a
combination of parallel and angular misalignment.
Diagnosis may show both a 2X and an increased 1X signal
in the axial and radial readings.
The misalignment symptoms vary depending on the
machine and the misalignment conditions.
The misalignment assumptions can be often distinguished
from unbalance by:
• Different speeds testing
• Uncoupled motor testing
Temperature effects caused by thermal growth should also
be taken into account when assuming misalignment is the
cause of increased vibration.
13

13. Alignment Tolerance Table Vibration Monitoring
The suggested alignment tolerances shown above are general values based upon experience and should not be exceeded.
They are to be used only if existing in-house standards or the manufacturer of the machine or coupling prescribe no other values.
14

14. Shaft Bending Vibration Monitoring
A shaft bending is produced either by an axial asymmetry
of the shaft or by external forces on the shaft producing
the deformation.
A bent shaft causes axial opposed forces on the bearings
identified in the vibration spectrum as 1X in the axial
vibration.
2X and radial readings can also be visible.
1X 2X
Axial
15

15. Rotating Looseness Vibration Monitoring
Rotating looseness is caused by an excessive clearance between the rotor and the bearing
Rolling element bearing:
Rotation
frequency 1X
and harmonics
Radial
Journal bearing:
Rotation frequency
1X Harmonics and
sub Harmonics.
Radial
16

16. Structural Looseness Vibration Monitoring
Structural looseness occurs when the machine is not correctly supported by, or well
fastened to its base.
MP
• Poor mounting
• Poor or cracked base
MP
MP MP • Poor base support
• Warped base
Condition Monitoring
1X Structural looseness may increase vibration
amplitudes in any measurement direction.
Radial
Increases in any vibration amplitudes may
indicate structural looseness.
Measurements should be made on the bolts,
feet and bases in order to see a change in the
amplitude and phase. A change in amplitude
and 180° phase difference will confirm this
problem.
17

17. Resonance Vibration Monitoring
Resonance is a condition caused when a forcing frequency coincides (or is close) to the
natural frequency of the machine’s structure. The result will be a high vibration.
1st form of natural 2nd form of natural flexure 3rd form of natural flexure
flexure
v v v
tx tx
t tx t t
t
Shaft 1st, 2nd and 3rd critical speeds cause a
resonance state when operation is near these
critical speeds. f1st nat, flexuref2nd nat, flexuref3rd nat, flexure f
no harmonic relationship
18

18. Resonance Vibration Monitoring
• Resonance can be confused with other common problems in machinery.
• Resonance requires some additional testing to be diagnosed.
1 2
Resonance
Step-up
rev/min
Grad
Phase jump
MP MP
by 180°
rev/min
2
1 φ1= 240° 2 φ2= 60...80°
1
1. 1.
O. O.
Amplitude at rotation frequency fn by residual Strong increase in amplitude of the rotation
unbalance on rigid rotor. frequency fn at the point of resonance, step-up
dependent on the excitation (unbalanced condition)
and damping.
19

19. Resonance Diagnosing Tests Vibration Monitoring
Run Up or Coast Down Test:
• Performed when the machine is
turned on or turned off.
• Series of spectra at different RPM.
• Vibration signals tracking may
reveal a resonance.
The use of tachometer is optional
and the data collector must support
this kind of tests.
20

20. Resonance Diagnosing Tests Vibration Monitoring
Bump Test:
3
s 1 2
Excitation - force pulse Response - component vibration
F/a 3
D t t
ouble beat
5 ms 1
2 Decaying function
Shock component, natural vibration, vertical Frequency response, vertical Frequency response, horizontal
2
Natural frequency, nd mod.
vertical
1st mod.
Natural frequency,
horizontal
t
21

21. Journal Bearings Vibration Monitoring
Journal bearings provides a very low friction surface to support and guide a rotor through a cylinder
that surrounds the shaft and is filled with a lubricant preventing metal to metal contact.
High vibration damping due to the oil film:
• High frequencies signals may not be transmitted.
• Displacement sensor and continuous monitoring
recommended
Clearance problems (rotating mechanical looseness).
0,3-0,5X 1X
Oil whirl
Radial
• Oil-film stability problems.
• May cause 0.3-0.5X component in the spectrum.
22

22. Rolling Element Bearings Vibration Monitoring
1. Wear:
Wear
• Lifetime exceeded
• Bearing overload
• Incorrect assembly
• Manufacturing error
• Insufficient lubrication
Wear
The vibration spectrum has a higher
noise level and bearing characteristic
frequencies can be identified.
Increased level of shock pulses.
23

23. Rolling Element Bearings Vibration Monitoring
2. Race Damage:
4
3
Roller bearing geometry and damage frequencies: 1 - Outer race damage
2
2 - Inner race damage
3 - Rolling element damage
Angle of contact 4 - Cage damage
Dw D Arc diameter 1
d Rolling element diameter
Example of rollover frequencies:
Dpw Z Number of rolling elements
Ball bearing SKF 6211
n Shaft RPM
RPM, n = 2998 rev/min
Z n d Rollover frequencies
Ball pass frequency, outer race: BPFO = ( 1- cos ) Dimensions
D
260
Z d BPFO = n / 60 4.0781 = 203.77 Hz
Ball pass frequency, inner race: BPFI = ( 1+ cos ) d =77.50 mm
n D
260 d 2 D =14.29 mm BPFI = n / 60 5.9220 = 295.90 Hz
Ball spin frequency: BSF = D n ( 1- cos )
D
d60 2.fw = n / 60 5.2390 = 261,77 Hz
) Z
n d = 10
Fundamental train frequency: TFT = ( 1- cos
2 60 D
= 0 fK = n / 60 0.4079 = 20.38 Hz
24

24. Rolling Element Bearings Vibration Monitoring
Outer race damage: Inner race damage:
(Ball passing frequency, outer range BPFO) (Ball passing frequency, inner range BPFI)
fn
BPFO 2 BPFO 3 BPFO 4 BPFO
Sidebands at intervals of 1X
BPFI 2 BPFI 3 BPFI 4 BPFI
Inner race damage frequency BPFI as well as
Outer race damage frequency BPFO as well as
numerous sidebands at intervals of 1X.
harmonics clearly visible
25

25. Rolling Element Bearings Vibration Monitoring
Rolling element damage: Cage damage:
(Ball spin frequency BSF) (Fundamental train frequency FTF)
Sidebands in intervals of FTF FTF and 2nd, 3rd, 4th harmonics
2 BSF 4 BSF 6 BSF 8 BSF
Cage rotation frequency FTF and harmonics
Rolling elements rollover frequency BSF with
visible
harmonics as well as sidebands in intervals of FTF
26

26. Rolling Element Bearings Vibration Monitoring
Lubrication Problems:
Major fluctuation in
• Race damage level of shock pulses
Lubricant contamination and damage
• Defective sealing frequencies
• Contaminated lubricant used
• Insufficient lubricant
Insufficient lubrication
• Underrating
Subsequent
small temperature
increase
Over-greasing • Maintenance error
Large temperature
• Defective grease
increase after
regulator lubrication
• Grease nipple blocked
27

27. Rolling Element Bearings Vibration Monitoring
Incorrect mounting.
Shock pulse
Air gap
Bearing rings out of round, deformed.
• Incorrect installation Damage
frequencies
• Wrong bearing storage envelope
• Shaft manufacturing error
• Bearing housing overtorqued.
Dirt
Bearing forces on floating bearing.
• Incorrect installation
• Wrong housing calculation
• Manufacturing error in bearing Fixed
housing Floating
Severe vibration
bearing bearing Bearing temperature
increases
Cocked bearing.
Axial 1X, 2X
and 3X.
• Incorrect installation
28

28. Blade and Vanes Vibration Monitoring
MP
A blade or vane generates a signal frequency called
blade pass frequency, fBP:
MP f BP =B n N Bn = # of blades or vanes
N = rotor speed in rpm
Identify and trend fBP.
An increase in it and/or its harmonics may be a
symptom of a problem like blade-diffuser or volute
air gap differences.
fBPF
Example characteristic frequency:
Radial
3 struts in the intake; x=3.
9 blades; Bn=9.
fBP x = N Bn x
Characteristic frequency = N 27
29

29. Aerodynamics and Hydraulic Forces Vibration Monitoring
MP MP
There are two basic moving fluid
problems diagnosed with vibration
analysis:
• Turbulence
• Cavitation
Cavitation: Turbulence:
1X fBPF Random
Random 1X
30

30. Belt Drive Faults Vibration Monitoring
MP MP
Belt transmission a common drive system in
industry consisting of:
• Driver Pulley
• Driven Pulley
MP MP • Belt
The dynamic relation is: Ø1 ω1 = Ø2 ω2
Belt frequency:
Ø
Ø1 ω 1
2
ω2 fB
3,1416 ω1 Ø1
l
l: belt length
31

31. Belt Drive Faults Vibration Monitoring
Belt Worn:
fn
The belt frequency fB and first two
Radial (or even three) harmonics are
1X,2X,3XfB visible in the spectrum.
2 fB generally dominates the spectrum
Pulley Misalignment:
1X of diver or driven pulley visible and
predominant in the axial reading.
Offset Angular Twisted
32

32. Belt Drive Faults Vibration Monitoring
Eccentric Pulleys:
The geometric center doesn’t coincide with the rotating
center of the pulley.
High 1X of the eccentric pulley visible in the spectrum,
predominant in the radial direction.
Belt
direction
Easy to confuse with unbalance, but:
• Measurement phase in vertical an horizontal directions
may be 0° or 180°.
• The vibration may be higher in the direction of the belts.
Belt Resonance:
If the belt natural frequency coincides with either the
driver or driven 1X, this frequency may be visible in the
spectrum.
33

33. Gear Faults Vibration Monitoring
Spur Gear:
Worm Gear:
gear wheel
Driving gear
Gear
Driven gear
gear wheel pair
gear train
Gear (wheel) Pinion Worm gear
Planet Gear:
Bevel Gear:
Ring (cone) Bevel gear
Sun gear
Bevel gear
Planet gear
Carrier
34

34. Gear Faults Vibration Monitoring
Gear Meshing:
Gear meshing is the contact pattern of the pinion
and wheel teeth when transmitting power.
Left flank End point of tooth
meshing
Working flank
85 The red dotted line is the contact path where the
89 meshing teeth will be in contact during the
86 rotation.
Right flank 88 4 87 3
5 2
6 1
Pitch point Non working flank Flank line Top land
Starting point of
tooth meshing Pitch line
Tooth
space Tip edge
Pitch surface
Tooth Root flank
Gear mesh frequency fZ can be calculated:
Fz = z fn Root mantel flank
Where z is the number of teeth of the gear rotating
at fn. Flank profile
35

35. Gear Faults Vibration Monitoring
Incorrect tooth meshing
MP
fz fz 2 fz 3 fz 4 fz
fn1
z2
z1
MP
fn2
Wear
fz 2 fz 3 fz
MP MP Detail of X:
X
fz
36

36. Gear Faults Vibration Monitoring
Incorrect tooth shape
fz
MP
fz Detail of X:
X
MP
Tooth break-out
MP MP fz and
X Detail of X: harmonics
z2 Sidebands
z1
fz
fn1 fn2
37

37. Gear Faults Vibration Monitoring
Eccentricity, bent shafts
fz and
MP MP Detail of X:
harmonic
X
sidebands
fz
“Ghost frequencies" or machine frequencies
fz f M “Ghost freque
Gearwheel being
manufactured
Cutting tool
Worm drive part of the
gear cutting machine
zM
38

38. Electrical Motors Vibration Monitoring
Electromagnetic forces vibrations:
Twice line frequency vibration: 2 fL
fL : line frequency
Bar meshing frequency: fbar = fn nbar
nbar: number of rotor bars
p: number of poles
Synchronous frequency: fsyn = 2 fL / p
Slip Frequency: fslip = fsyn - fn
Pole pass frequency: fp=p fslip
• Stator eccentricity
• Eccentric rotor
• Rotor problems
• Loose connections
39

39. Electrical Motors Vibration Monitoring
Stator Eccentricity:
Loose iron
Shorted stator laminations
Soft foot
MP MP
1X 2X
Radial 1X and 2X signals
2 fL fL without sidebands
Radial predominant
High resolution should be used
when analyzing two poles machines
40

40. Electrical Motors Vibration Monitoring
Eccentric Rotor:
Rotor offset
Misalignment
Poor base
MP MP
fp 1X 2X
Tslippage
R
adial
2 fL
t [ms]
fp, 1X, 2X and 2fL signals.
Modulation of the vibration time signal with the slip
1X and 2fL with sidebands at fP. frequency fslip
Radial predominant. Tslip 2-5 s
High resolution needed.
41

41. Electrical Motors Vibration Monitoring
Rotor Problems:
1. Rotor thermal bow:
1X
Radial
Unbalanced rotor bar current
Unbalance rotor conditions
Observable after some operation time
f [Hz]
2. Broken or cracked rotor bars:
1X 2X 3X 4X 1X and harmonics with sidebands at fP
Radial High resolution spectrum needed
Possible beating signal
42

42. Electrical Motors Vibration Monitoring
3. Loose rotor bar:
1X 2X fbar 2f bar
Radial
fbar and 2fbar with 2fL sidebands
2fbar can be higher
1X and 2X can appear
Loose connections: f [Hz]
fn 2f L
2fL excessive signal with sidebands at 1/3 fL
Radial Electrical phase problem
Correction must be done immediately
43