2. Analysis
A Logical Process
What is the problem ?
What are the symptoms ?
What machine information do I have ?
What data should I take ?
Where should I take data ?
3. Eight Step Analysis Procedure
1 - Define the Problem
2 - Determine the Machine History
3 - Determine the Machine Details
4 - Visual Inspection
5 - Data Collection
6 - Frequency Confirmation
7 - Vibration Direction / Phase
8 - Probing Studies
4. Step 1 - Define the Problem
Bearing, seal, or other frequent component
failures
Structural failures
Inability to manufacture a quality product
(machine tools)
Physical annoyance / noisy
Vibration is excessive
Does not meet performance standards
(API, ISO, AGMA, etc.)
5. Step 2 - Determine Machine History
When did the problem start?
It’s always been rough
The vibration has gradually increased over a period of
time
The vibration increased abruptly
Have any changes been made?
Repairs or parts replaced?
Modifications?
Machine speed?
Load or product?
Is the vibration temp. related?
6. Step 3 - Determine Machine Details
Machine RPM(s)
Type of bearings
For rolling element
bearings
number of rolling
elements (balls), ball
and pitch diameter
or, Mfgr and model
For fluid film bearings -
bearing configuration
Number of fan blades
Number of impeller vanes
Number of gear teeth
Coupling type
Machine critical speeds
Background sources
Baseline data
8. Step 4 - Visual Inspection
Loose, worn, or broken parts
Rotor wear or deposit build-up
Leaking seals
Cracks in the base, foundation,broken
welds
“Slow motion” study of rotor, belts, pulleys,
coupling, etc., with strobe light
9. Step 5 - Data Collection
Obtain tri-axial amplitude vs. frequency plots at
each bearing of the machine train
Unless the analyst is very comfortable with mixed
scales, use same amplitude scale (range) for all plots
For machine tools, analyze with:
The machine shut down to evaluate background
sources
The machine idling (no machining) to identify drive
related problems
Under machine conditions to identify vibration from the
actual cutting, drilling or grinding operation (chatter)
10. Step 6 - Frequency Confirmation
Determine if vibration frequencies are
EXACTLY related to machine RPM(s)
using:
Strobe light
Time-Synchronous averaging
High resolution FFT
Fmax 0-12,000 CPM
#FFT Lines - 3200 (minimum)
11. Step 6 - Frequency Confirmation
Fmax needs to be high enough to catch all
defect frequencies in machine
Resolution needs to be sharp enough to
separate closely spaced frequencies
Some machines may require more than
one set of FFT’s in order to capture all
problem frequencies with sufficient
resolution for analysis
12. Identifying the Problem Based on
Frequency
Many vibration
problems generate
frequencies related to
the rotating speed or
harmonics
Look for bearing and
electrical vibration
13. Step 7 - Vibration Direction / Phase
Determine if the vibration is directional or
non-directional by:
Comparing the horizontal and vertical phase
readings
Taking radial vibration amplitude readings
every 30° around each bearing
14. FFT’s Should Be Compared by
Direction: H,V,A
How do radial (horizontal and vertical)
readings compare to each other
How do radial readings compare with axial
measurements
16. Comparing Radial And Axial
Measurements
Only a few machine defects can cause high
axial readings
These include Misalignment, Bent Shafts, or
Unbalance of Overhung Rotors
General Rule:
Any time the axial amplitude exceeds 50% of
the highest radial, these defects should be
strongly considered
26. Rotor Unbalance Characteristics
Can Only Cause 1X RPM Vibration
WARNING: 1X not ALWAYS from unbalance
Amplitude proportional to distance between
center of mass and center of rotation
Uniform rotating force, changing in direction
and evenly applied radially
since mobility or “dynamic sensitivity” may
differ between H and V axes, amplitudes may
also differ
6-12
27. Rotor Unbalance Characteristics
A dominant unbalance shows 90° phase
difference between H and V
within usual 30° tolerance
Horizontal phase difference between
inboard and outboard bearings should
match vertical phase difference
Radial vibration much greater than axial
vibration
Overhung rotors are the exception
6-12
28. Rotor Unbalance Characteristics
Repeatable phase in all directions
except for minor beats and “chasing” phases
when balance has been largely corrected
May be amplified by resonance
May be the force behind the evidence of
looseness
6-12
29. Static Unbalance
The central principal axis is displaced parallel to the shaft axis
The unbalance is even at both ends, and is in the same direction
Couple Unbalance
The central principal axis intersects the shaft axis at the rotor
center of gravity
The unbalance is even at both ends, but in the opposite direction
Quasi-Static Unbalance
A combination of static and couple unbalance where central
principal axis intersects the shaft axis, but NOT at the rotor center
of gravity
Dynamic Unbalance
A random combination of static and couple unbalance
The centerline of unbalance does not intersect the centerline of
the shaft
Types Of Unbalance
6-11
31. Static (Force) Unbalance
SHAFT AXIS
BALANCE AXIS
Vibration amplitude and phase readings measured at the
two supporting bearings will be nearly identical
This unbalance can be solved with a single weight placed
in the center of the rotor, or with two identical weights
placed at the same location on each end
6-14
32. Vibration amplitudes will be reasonably the
same, but phase readings at the bearings will
differ by 180°
This unbalance can be solved with identical
weights placed at each end of the rotor 180°
apart
Coupled Unbalance
SHAFT AXIS
BALANCE AXIS
6-15
33. Quasi-Static Unbalance
SHAFT AXIS
BALANCE AXIS
A Static (or Coupled) Unbalance where vibration
amplitudes at one support are considerably
higher than the other and phase readings are the
same (or differ by 180°)
34. MOST COMMON TYPE OF UNBALANCE CONDITION
Accounts for any condition that does not meet the
requirements for static, couple or quasi-static unbalance.
Note that the balance axis does not pass through the shaft
centerline
Dynamic Unbalance
SHAFT AXIS
BALANCE AXIS
6-15
35. Cross Effect
A heavy spot of unbalance located at one
end of a rotor will not only create unbalance
vibration at that end, but will also create
some unbalance vibration at the opposite
end of the rotor as well
36. Overhung Rotors
Axial often greater than
radial
due to couple effect
Inboard axial phase
should equal outboard
axial phase
Usually corrected by:
Static first:
Brg 2, Plane A
Then coupled:
Brg 1, Equal weights in
Planes A and B, 180° apart
6-16
37. Residual Unbalance
Rotor Sensitivity =
Trial Wt. Size X Mounting Radius
Trial Wt. Effect (amplitude)
Trial weight effect is measured through the
vector length in the vector diagram
calculation during balancing
Residual Unbalance =
Rotor Sensitivity X Final Balance Amplitude
6-21
42. A Tolerable Residual
Unbalance for Modern
General Purpose Machines...
To satisfy MIL-STD and API, we should
meet ISO 1940 G 2.5
6-20
43. ISO 1940 G(rade) 6.3
Means 6.3 mm/sec Peak
Corresponds to 0.25 in/sec Peak
NOT AN ACCEPTABLE STANDARD for
General Purpose Machines in 2000 and
beyond
G 2.5 corresponds to 0.1in/sec Peak
This balance specification should be readily
achievable (for most machines) using modern
equipment and balancing techniques
6-20
45. Characterized by MAXIMUM 1X RPM amplitude
along an axis passing through shaft centerlines
A 90° change in transducer orientation yields 0° or
180° phase change
Eccentric Rotor
6-28
46. Eccentric Behavior
Gears have variation in mating force along
centerline of gears
Sheaves have vibration along centerline of
driver and driven pulley
Eccentric Motor Armature causes moving
unbalanced magnetic forces
2X Line Freq. And Pole Pass Freq.
Eccentric Stator will show a line of maximized
vibration
Eccentric bearings rarely cause a problem
because of precision manufacture
when reaching late stages of deterioration, a
bearing may exhibit eccentric behavior
Eccentric impellers may cause a hydraulic
unbalance between stationary and moving
vanes (high vibration at BPF and harmonics)
6-28
47. Eccentric Rotors
Balancing attempts may cause a drop of
amplitude in one radial direction with a
simultaneous increase in another
May cause significantly HIGHER vibration
in one radial direction than another
6-29
48. Bent or Kinked Shaft
Kinked - Bent at or near one bearing
Bent - Uniform bow between bearings
6-30
49. Bent Shaft
Common causes of a bent shaft
Manufacturing or machining errors
Mishandling during transportation
“Bow” due to thermal growth or uneven heating
due to rotor bar problems
Sag from sitting in one spot for extended time
Two types of bent shaft
Kinked Shaft - Bend is at or near one bearing
Bowed Shaft - Bend is uniform between two
bearings
6-30
50. Bent Shaft Symptoms
Predominant vibration is at 1 X RPM. This
can be accompanied by a 2 X RPM peak
Axial vibration amplitude normally exceeds
50% of highest radial amplitude
Radial vibration is usually uniform
Axial phase analysis must be used to verify
problem and type of bend
6-30
51. Identifying a Kinked Shaft
Produces a twisting motion of the bearing
Axial phase readings taken 90° apart will produce
a phase shift of 90°.
6-31
53. A Critical Speed LOOKS like a Bent
Shaft
A rotor operating close to or at a critical
speed may exhibit “bent shaft” behavior
Shaft deflection is possibly imitating bent
shaft behavior
6-31
54. Electric Motors
A growing vibration amplitude may indicate
shorted laminations…
the localized heating imposes a shaft bend
as the rotor heats up
may straighten or not, depending on whether
the bending exceeds elastic limits (reaches
permanent or plastic limits)
6-31
55. Misalignment
Still the most common source of vibration
(except in plants where PM has been successful
and laser alignment fully implemented)
6-32
56. Three Types of Misalignment
Combination (most common)
Angular
Parallel (or Offset)
6-32
57. Misalignment
MOST COMMON PROBLEM FOUND INITIALLY
Temperature changes can affect alignment
Machines aligned cold can go out when warm
Different expansion rates in the driver and driven bases can
cause misalignment with changes in ambient temperature
Bases or foundations can settle
Shims or bases can rust or corrode
Grouting can shrink or deteriorate
May increase energy demands
More likely to be visible as an increase in RPM when
aligned correctly (power to product, not problem)
6-32
58. Misalignment (Cont.)
Resulting forces are shared by driver and driven (not
localized)
Differences in mass and stiffness may cause different
amplitudes on the driver and the driven, but the frequencies
will be present on both
The level of misalignment severity is determined by
the machine’s ability to withstand the misalignment
If the coupling is stronger or more pliable (can withstand
more misalignment) than the bearings, the bearings can fail
with little or no damage to the coupling
A coupling’s ability to withstand more misalignment does not
reduce the higher forces resulting from this misalignment. It
merely transfers these higher forces to the bearings
6-32
59. Thermal Growth Rule of Thumb
On start-up, vibration should start high and
ease up
the pre-set “misalignment” eases as thermal
growth is achieved
the machine “grows” (or “shrinks” for cryogenic
applications) into alignment
Applicable to machines with significant thermal
growth
60. Symptoms Of Misalignment
Radial vibration is highly directional
1X RPM, 2 X RPM, and 3 X RPM can be present, IN
ANY COMBINATION, depending on the type and extent
of misalignment
Misalignment Symptom Direction
Angular 1X RPM Axial
Parallel 2 X RPM Radial (H & V)
Combination 1/2/3 X RPM Radial and Axial
Problems internal to the coupling usually generate a 3 X
RPM vibration.
6-33
61. Symptoms are a function of...
Coupled system’s ability to accommodate
misalignment
Offset
Speed
Torque
Corrosion, sludging and other factors
affecting coupling stiffness
6-33
62. Component Failure
Coupling versus bearing
the stiffer component wins out and survives
Gears
Belts
Sheaves
Blades
Etc...
63. Outboard End - Possible Symptom
The bearing next to the coupling may act
as a PIVOT
relative to machine / bearing stiffness
This may mean a stabilized INBOARD
bearing and severe vibration AWAY from
the coupling
6-32
64. Other Sources of High AXIAL Vibration
ANGULAR misalignment
Bent or kinked shaft
Misaligned bearings
Axial resonance
Worn thrust bearings
Worn helical or bevel gears
Motor hunting for magnetic center (sleeve bearings)
Couple unbalance or couple component of unbalance
Highly pronounced on overhung rotors
6-33
65. PHASE ANALYSIS IS
ESSENTIAL
To assess misalignment or other
abnormal or excessive 1X, 2X and 3X
RPM vibration behavior
6-33
66. Angular Misalignment
Produces predominant 1 X RPM peak in the axial
direction, with an elevated 2 X RPM possible
Marked by 180 phase shift across the coupling in the
axial direction
Axial forces cycle from max to min once per revolution
Radial forces remain relative uniform throughout revolution
6-35
67. Parallel Or Offset Misalignment
Produces a predominant 2 X RPM peak in the radial
spectrum
Marked by 180 phase shift across the coupling in the
radial direction.
Axial forces remain relative uniform throughout revolution
Radial forces cycle from max to min twice each revolution
6-36
68. Very Pronounced Parallel
Misalignment on a SINGLE AXIS
May cause behavior similar to other highly
directional vibration
might be mistaken for resonance, eccentricity
or the like…
Highly elliptical shaft orbit (flattened)
6-36
69. Misalignment Vs. Type C
Looseness
Excessive misalignment (whether
angular or parallel) may generate a
progressively HIGHER number of
running speed harmonics
6-36
70. Misaligned / Cocked Bearing
Generates
considerable 1 X RPM
and possibly 2 X RPM
Axial Twisting motion
NOT resolved by
balancing or aligning
6-37
72. Coupling Problems
Long or short spacer
noticeable radial 3 X RPM vibration
much HIGHER axial 3 X RPM vibration
Gear Type Lock-up
may cause thrust bearing failure
mostly 1X RPM with axial > radial
when misalignment is present
some mesh (# of grid X RPM) & harmonics
Christmas Tree (2 X to 6 X, decreasing by 25% for
each harmonic increment
6-38
73. Coupling Problems
Loose Coupling
sidebands around BPF or GMF
Text: “does not drive rotor at uniform speed
(RPM)”
when torque is applied continuously, looseness will
not be manifest unless severe misalignment is
present
Modulation mostly due to occasional shift of
coupling causing a pivoting effect resulting in
eccentric placement of the rotor within casing
6-38
77. Fn and Resonance
CONCEPTS
Natural Frequency
Critical Speed
Resonance
6-39
78. Fn and Resonance
Every machine element has natural
frequencies based on mass, shape and
stiffness
Resonance occurs when a forcing
frequency coincides with one of these
natural frequencies
6-39
79. VIBRATION DUE TO RESONANCE
Every object, including every element or part of a
machine, has “natural frequencies” at which “it likes to
vibrate”
Determined by machine mass and stiffness as well as moment
of inertia (shape) for the concerned axis
CAN NOT CAUSE VIBRATION , it merely serves
as a “mechanical amplifier” (10x-100x)
Resonance is a very common cause of excessive
vibration because:
Machines consist of many individual elements
Stiffness of each machine component differs in all directions,
meaning several natural frequencies within the range of the
generated forcing frequencies
6-39
80. Identifying Resonance
Vibration will be highly directional
Changing the exciting forcing frequency to
observe reaction
Change the mass or stiffness of the
suspected resonant machine component
Perform a bump test
modally tuned hammer test
6-39
81. FORCING FREQUENCIES
Almost ANY frequency present in the
spectrum can become a forcing frequency
Unbalance, misalignment, torque or blade
pass pulses, harmonics of each, etc.
6-39
