This document provides an overview of spectrum analysis techniques for analyzing machinery vibration problems. It discusses how spectrum analysis can be used to identify the location and cause of problems by examining the frequency and amplitude of vibration. The document outlines a two-step process of collecting information about machine components and operating speeds, and then analyzing the spectrum to identify issues like unbalance, misalignment, bent shafts, looseness, and bearing faults. It provides examples of typical vibration patterns and indicators for common machinery issues.

2. Spectrum Analysis
2006-12-07 ©SKF Slide 2 [Code]
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Basic Motions

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Module objective(s)
To introduce the concepts of spectrum analysis, and to provide
a general overview of how analysts recognize and analyze
typical machinery problems.

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FFT spectrum analysis
frequency
low
freq.
high freq.
time
frequency
amplitude
FFT spectra provide
information to help determine
the location of the problem,
the cause of the problem and,
with trending, how long until
the problem becomes critical

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Spectrum analysis - two step approach
• First, collect
useful
information, then
• Analyze
Asset Information Page – Front and Back

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Step 1 - collect useful information
Identify all components of the machine that could cause vibration
What are the possibilities?
• The number of fan blades or impellers, gear teeth, coupling type,
belts, sheave diameters, etc.
• Type of bearings at each bearing location and their defect
frequencies
• Speeds of other machines running in the same vicinity
• Orientation, vertical or horizontal, overhung or center-hung

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Identify components that cause vibration
1800 RPM
gearbox
gearmesh frequencies
2400 RPM
5 blade fan
motor

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Step 1 - collect useful information…
Obtain all historical machinery data
• Are previously recorded values, spectra, logs, control room data, or
overall trend plots available?
• Was a baseline recorded?
• Talk with operators. Often this provides insight into the original
problem (coupling changed, flow increased, fan cleaned, etc.).
• Check maintenance records and talk to the mechanics that service
the machine.

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Step 1 - collect useful information…
Identify the machine’s running speed
Knowing the machine's running speed is critical when analyzing an FFT
spectrum. There are several ways of determining running speed.
• Read the speed from instrumentation.
• Look for peaks in the spectrum at 1800 or 3600 RPM if the machine
is a induction motor. (1500 and 3000 RPM for 50 Hz countries.)
• The running speed peak in a vibration spectrum is often the first
significant peak reading from left to right. Look for this peak and for
peaks the harmonics of running speed.
• Use a tachometer.

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Step 1 - collect useful information
Identify what type of measurement produced the FFT spectrum
• Was it a displacement, velocity, acceleration, enveloped acceleration,
etc. measurement that produced the spectrum?
• Where was the probe positioned?
• Are plots scaled consistently for easy comparison?

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Step 2 - analyze
General Analysis Techniques
• Use a coin or the probe tip to get a hand-feel of the vibration on the
machine, the base, and any piping, ductwork, or other attachments.
• Acquire tri-axial (H, V, A) vibration data at each bearing.
• For analysis purposes, acquire relative phase measurements. This
improves the likelihood of a successful diagnosis.
• Evaluate overall vibration readings across the entire machine train.
• Where does the vibration appear to originate (highest amplitude)?
• What probe directions have the highest amplitude? (significant)
• Do the dominant frequencies have a relationship to rotating components?

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Spectrum analysis - identify frequency ranges…
1X
2X 3X
4X
frequency
amplitude
< 1X
10X
low frequency rotational events and harmonics
(i.e. unbalance, misalignment, looseness, etc.)
high frequency events
(i.e. bearing or gearmesh)

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Spectrum analysis - identify frequency ranges
• Identify any harmonics of running speed (1X, 2X, 3X, etc.).
• Identify bearing fault frequencies.
• Identify fan blade frequencies, if applicable.
• Identify number of gear teeth, if applicable.
• Identify pump impeller frequencies, if applicable.
• Identify adjacent machinery vibration, if applicable.
• With electric motors, identify peaks at line frequencies. Determine if
they are electrical or mechanical.

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Spectrum analysis - verify and determine
severity
Verify
•look for harmonics of the fault frequencies
Determine Severity
•compare vibration amplitude with ISO standards
•compare the current amplitude to past readings (best method)
•compare the amplitude to other readings obtained by similar
machines running under the same conditions

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Spectral pattern recognition
1X
2X 3X
4X
frequency
amplitude
< 1X
10X
low frequency rotational events and harmonics
(i.e. unbalance, misalignment, looseness, etc.)
high frequency events
(i.e. bearing or gearmesh)

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Harmonics…
A harmonic is a frequency that is an exact multiple of a non-sinusoidal
(impact type) fundamental frequency

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Harmonics
• The fault fundamental frequency may be produced from
a variety of sources
• shaft speed, a pump’s vane pass frequency, bearing fault frequencies, or
gear meshing frequencies
• With rolling element bearing faults, harmonics of the
fault’s fundamental frequency typically appear first
• fault frequency harmonics are the earliest indicators that a bearing
problem exists

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Sidebands…
Sidebands are produced when one frequency modulates another or when
one amplitude modulates another.

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Sidebands
• The dominant peak appears at the primary vibration frequency
• referred to as the center frequency
• The frequency that is modulating the center frequency appears as
two peaks equally spaced around the center frequency (sidebands)
• As a problem worsens, the sideband amplitudes increase and additional sidebands
appear
• The spacing between the center frequency and the sidebands identifies the
originating problem

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Waterfall plot
A waterfall plot trends spectrum changes.

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Phase & Uses

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Analyzing “Typical” Machinery
Problems

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Unbalance Principle

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Unbalance
static / mass
couple

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Unbalance – Single Plane…

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Unbalance – Two Plane…

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Unbalance – Over Hung type…An Exception…

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Unbalance - typical spectrum (pure unbalance)

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Unbalance - typical spectrum (Overhung Rotor)

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Unbalance - typical spectrum (pure unbalance)
Low Multiples of 1X
High 1X

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Unbalance - typical indications
• (Overall vibration) - high energy, low frequency, radial vibration
• (Spectrum) - higher than normal amplitude at 1X running speed frequency
• (Spectrum) - With the high 1X, there are little or no amplitude increases at
running speed harmonic frequencies (2X, 3X, 4X, etc.)
• (Spectrum) - 1X amplitude increases proportionally to speed increases
• (Phase) - Sensor shows 90° phase shift between horizontal and vertical
positions on the same bearing
• (Phase) - For predominantly static unbalance, there is usually no phase shift
across the machine or coupling in the same measurement position.

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Unbalance - summary
• If the radial measurement's 1X amplitude is high, and all harmonics (except
vane passing) are less than 30% of the 1X and do not increase with speed, there
may be unbalance.
• If the majority of vibration is in the radial plane, and the 1X amplitude is
medium to high in amplitude, and phase readings from vertical and horizontal
measurements on the same bearing differ by 90°, there may be unbalance.
• If there is a non-synchronous peak corresponding to the 1X running speed of a
nearby machine, there may be unbalance in the other machine.
• If vibration increases in both the axial and radial measurement planes, the
machine has an overhung mass, and the axial phase measurements across the
machine or coupling are in phase, there may be unbalance.

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Misalignment

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Misalignment

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Misalignment
Angular misalignment
Parallel misalignment

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Misalignment - cause and effects
Angular misalignment Parallel/offset misalignment

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Misalignment

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Misalignment… Aligned Machine

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Misalignment… Angular

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Misalignment… Offset

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Misalignment… Combination

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Misalignment - typical spectrum
high 1X and/or 2X, 3X

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Misalignment - typical indications
• (Overall vibration) - high energy, low frequency, radial and axial vibration
• (Overall vibration) - unbalance generally produces high motion in the horizontal
plane due to differences in bearing support stiffness. If vertical amplitudes are
higher than the horizontal readings at the same bearing, misalignment is
suspected.
• (Spectrum) - higher than normal 1X and harmonic amplitudes occur in both
axial and radial positions. If 2X is less than 30% of 1X, suspect unbalance, >30%
and <50%, misalignment probable, >50% and less than 100%, strong
misalignment,
• (Spectrum) – possible vibration at number of coupling elements times RPM.
• (Phase measurements) – in the radial readings on each machine, look for
combinations of 0° and180° degree phase relationships in the vertical and
horizontal planes.

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Misalignment - summary
• Misalignment produces higher than normal vibration at running
speed and possible harmonics of running speed out to approximately
5X running speed.
• Axial vibration at running speed is generally greater than 30% of the
lowest radial amplitude.
• Phase relationships across the coupling in the axial or radial planes
may indicate “pure” forms of misalignment but are ineffective when
dealing with complex misalignment or multiple sources of vibration.
• Misalignment generally produces combinations of zero and 180-
degree phase relationships in the vertical and horizontal planes.

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Bent Shaft

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Bent shaft
• Usually appears identical
to a misalignment
problem
• Phase measurement
comparisons along the
machine train are needed
to distinguish between
the two.

48. 2006-12-07 ©SKF Slide 48 [Code]
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Bent shaft - diagnosis
• Axial measurements
across machine
typically 180 degrees out
of phase, and
• Radial measurements
across machine
are typically in phase

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Bent Shaft

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Bent shaft - typical indications
• (Overall vibration and spectrum characteristics) Similar to
misalignment.
• (Phase measurements) In the axial position, a phase shift of 180°
exists across the shaft, and ...
• (Phase measurements) In a radial position, there is no phase shift
between bearings across the shaft.
• If both the above phase conditions are true, the problem is probably
a bent shaft.

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Bent shaft - summary
If the primary vibration plane is in the axial direction, and there is a
dominant 1X peak, and there is no coupling present, and there is a
180° phase difference in the axial direction across the shaft, and there
is no phase shift in the radial position across the machine, then there
may be a bent shaft.

52. 2006-12-07 ©SKF Slide 52 [Code]
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Mechanical looseness
• The machine has come
loose from its mounting
• A machine component
has come loose
• The bearing has
developed a fault, which
has worn down the
bearing elements, or the
bearing seat

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Looseness

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Structural looseness

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Mechanical looseness - diagnosis

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Mechanical looseness - indications
• (Overall vibration) - There is high energy radial vibration, especially
in the vertical measurement plane, and
• (Overall vibration) - There is normal (low) vibration in the axial plane
• (Spectrum) - There is an abnormally high running speed amplitude,
followed by higher than normal harmonics of running speed, and
perhaps 1/2 harmonics of running speed (from 2X - 10X). Harmonic
peaks may decrease in amplitude as they increase in frequency
(except at 2X, which, when measured in the vertical position, can be
higher in amplitude).

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Mechanical looseness - summary
• If there are a series of three or more synchronous or 1/2
synchronous multiples of running speed (range 2X to 10X), and their
magnitudes are greater than 20% of the 1X, then there may be
mechanical looseness.
• If the machine is rigidly connected (no coupling or belt),
and the radial 2X is high, then there may be mechanical looseness.

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Cocked bearing

59. 2006-12-07 ©SKF Slide 59 [Code]
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Cocked Bearings…

60. 2006-12-07 ©SKF Slide 60 [Code]
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Cocked Bearing

61. 2006-12-07 ©SKF Slide 61 [Code]
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Cocked bearing - summary
• Like misalignment, a cocked bearing usually generates considerable
axial vibration. However, phase measurements from the suspect
bearing’s axial position help differentiate the two.
• If the phase readings among the adjacent four sensor locations vary
considerably, a cocked bearing is indicated. Success in diagnosing
this condition is directly related to the stiffness of the bearing
support and the flexibility of the shaft.

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Soft foot…
• One of the most prevalent machine installation problems
• A condition where one or more feet of a machine are not sitting flat
and fully on the base
• When a soft foot is tightened down, it can cause a multitude of
problems from internal misalignment to motor air gap problems

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Soft Foot

64. 2006-12-07 ©SKF Slide 64 [Code]
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Soft foot
•A soft foot will typically produce a
marked increase or decrease in
amplitude when loosened
•Can often be corrected through
shimming while the machine is still
operating

65. 2006-12-07 ©SKF Slide 65 [Code]
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The need for signal processing
• Generally, low frequency vibration from unbalance or misalignment
masks the low amplitude bearing vibration signals and the bearing
faults are not detected
• Repetitious gearmesh and bearing faults typically emit low energy
vibration signals in the lower frequencies, especially in the early
states of failure. However, because these faults emit impact signals
that are highly pulse shaped and of very short duration, they
generate harmonics out to very high frequencies

66. 2006-12-07 ©SKF Slide 66 [Code]
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Typical Bearing Failure Rate
Detection by
Lubricant Analysis
Detection by Listen
and Feel
Detection by Vibration
Detection by
Acoustic Emissions

67. 2006-12-07 ©SKF Slide 67 [Code]
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Signal processing methods
overall vibration signal (including bearing fault signal)
bearing fault signal
•Enveloped Acceleration
•SEE (spectrum emitted energy)
•HFD (high frequency detection)

68. 2006-12-07 ©SKF Slide 68 [Code]
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Enveloped acceleration process
Band Pass Filter is specified
according to defect harmonic
frequencies of interest
low frequency events filtered out, high
frequency defect harmonics emphasized
due to low signal to noise ratio, these high
frequency harmonics are then summed and
folded back to the defect’s fundamental frequency
resulting enveloped signal,
measured in gE
defect fundamental
frequency
fundamental
frequency
harmonics

69. 2006-12-07 ©SKF Slide 69 [Code]
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Bearing fault concept…
Defect Force Response

70. 2006-12-07 ©SKF Slide 70 [Code]
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4.3x8.6x12.9x
17.2x
Filter
Smooth
4.3x4.3x
4.3x
104.3x
108.6x
112.9x
1x
4.3x
1x 4.3x8.6x12.9x
17.2x
4.3x4.3x4.3x
12.9x
17.2x
4.3x 4.3x4.3x
4.3x
104.3x
108.6x
112.9x
~10x
4.3x4.3x
4.3x
104.3x
108.6x
112.9x
Rectify
4.3x8.6x12.9x
17.2x 104.3x
108.6x
112.9x
Bearing fault concept

71. 2006-12-07 ©SKF Slide 71 [Code]
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The acceleration envelope process groups energy related to the bearing defect
and excludes all others.
Frequency - Hz
Filter #1 Filter #2 Filter #3 Filter #4
Rolls, Bearings Bearings Gears
FFT Spectrum
5 kHz - 40 kHz
500 Hz - 10 kHz
50 Hz - 1 kHz
5 Hz - 100 Hz
Envelope
Detector
Bandpass filter
Accelerometer
Felt
Selecting the enveloping filter…

72. 2006-12-07 ©SKF Slide 72 [Code]
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Selecting the enveloping filter
0 – 1,000 Hz
250 - 5,000 rpm
500 - 10,000 Hz
3
0 – 10,000 Hz
2,500 - … rpm
5,000 - 40,000 Hz
4
0 – 100 Hz
25 - 500 rpm
50 - 1,000 Hz
2
0 – 10 Hz
0 - 50 rpm
5 - 100 Hz
1
Analyzing
Range
Speed Range
Frequency
Band
Filter
Note: Detection is peak to peak

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Rubbing…

74. 2006-12-07 ©SKF Slide 74 [Code]
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Rubbing

75. 2006-12-07 ©SKF Slide 75 [Code]
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Oil Whirl

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Oil Whip

77. 2006-12-07 ©SKF Slide 77 [Code]
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Electrical Motor Cross Sectional View…

78. 2006-12-07 ©SKF Slide 78 [Code]
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Electrical Motor – Working Principle…

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AC Motors – Normal Vibration

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AC Motors – Broken / Cracked Rotor

81. 2006-12-07 ©SKF Slide 81 [Code]
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AC Motors – Eccentric Rotor

82. 2006-12-07 ©SKF Slide 82 [Code]
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AC Motors – Loose / Open Rotor

83. 2006-12-07 ©SKF Slide 83 [Code]
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AC Motors – Loose Stator Coils

84. 2006-12-07 ©SKF Slide 84 [Code]
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AC Motors – Phase Problems

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DC Motors

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DC Motors – Current Passing through bearings

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DC Motors – Faulty Comparator Card

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DC Motors – Faulty Carbon Firing

89. 2006-12-07 ©SKF Slide 89 [Code]
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Plain Bearing Clearances…

90. 2006-12-07 ©SKF Slide 90 [Code]
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Plain Bearing Problems…

91. 2006-12-07 ©SKF Slide 91 [Code]
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Gear Problems…

92. 2006-12-07 ©SKF Slide 92 [Code]
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Gear Meshing Frequency….

93. 2006-12-07 ©SKF Slide 93 [Code]
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Gear Phase Related Problems…
• Gear Assembly Phase Frequency (GAPF) can result in Fractional Gear
Mesh Frequencies (if NA>1).
• It literally means (TG/NA) gear teeth will contact (TP/NA) pinion teeth
and will generate NA wear patterns, where NA in a given tooth
combination equals the product of prime factors common to the
number of teeth on the gear and pinion (NA = Assembly Phase Factor).
• GAPF (or harmonics) can show up right from the beginning if there
were manufacturing problems.
• Also, its sudden appearance in a periodic survey spectrum can
indicate damage if contaminate particles pass through the mesh,
resulting in damage to the teeth in mesh at the time of ingestion just
as they enter and leave meshing or that gears have been reoriented.

94. 2006-12-07 ©SKF Slide 94 [Code]
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Gears – Phase assembly problems

95. 2006-12-07 ©SKF Slide 95 [Code]
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Gear – Broken Teeth

96. 2006-12-07 ©SKF Slide 96 [Code]
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Gear - Eccentricity

97. 2006-12-07 ©SKF Slide 97 [Code]
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Gear – Excessive loads

98. 2006-12-07 ©SKF Slide 98 [Code]
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Gear – Hunting tooth

99. 2006-12-07 ©SKF Slide 99 [Code]
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Gear – Loose bearing fit

100. 2006-12-07 ©SKF Slide 100 [Code]
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Gear – Teeth Misalignment

101. 2006-12-07 ©SKF Slide 101 [Code]
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Belt Alignment – An practical Approach!
Is this the way!!

102. 2006-12-07 ©SKF Slide 102 [Code]
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Belt Related Problems - Basics

103. 2006-12-07 ©SKF Slide 103 [Code]
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Belt Related Problems…

104. 2006-12-07 ©SKF Slide 104 [Code]
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Eccentric Pulley…

105. 2006-12-07 ©SKF Slide 105 [Code]
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Belt – Eccentric Pulley

106. 2006-12-07 ©SKF Slide 106 [Code]
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Belt Angular Misalignment – in Pulley…

107. 2006-12-07 ©SKF Slide 107 [Code]
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Belt Vertical Misalignment – in Pulley…

108. 2006-12-07 ©SKF Slide 108 [Code]
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Belt offset Misalignment – in Pulley…

109. 2006-12-07 ©SKF Slide 109 [Code]
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Belt – Misalignment

110. 2006-12-07 ©SKF Slide 110 [Code]
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Belt – Resonance

111. 2006-12-07 ©SKF Slide 111 [Code]
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Proper Belt Alignment Technique…

112. 2006-12-07 ©SKF Slide 112 [Code]
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Cavitation

113. 2006-12-07 ©SKF Slide 113 [Code]
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Flow Turbulence

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Hydraulic / Aerodynamic forces

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Resonance…
•A condition where a structure vibrates at very large
amplitudes for a given force level
•Not a source of vibration, an amplifier of vibration
•Most resonance conditions are from a rotating or periodic
force produced inside the machine
•Occurs when a machine operates at a speed near one of
its natural frequencies

116. 2006-12-07 ©SKF Slide 116 [Code]
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Bump Test…

117. 2006-12-07 ©SKF Slide 117 [Code]
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Resonance Modes…

118. 2006-12-07 ©SKF Slide 118 [Code]
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Resonance Modes…

119. 2006-12-07 ©SKF Slide 119 [Code]
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Resonance Modes…

120. 2006-12-07 ©SKF Slide 120 [Code]
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Resonance Modes…

121. 2006-12-07 ©SKF Slide 121 [Code]
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Resonance…concepts
• Mass
• Stiffness
• Damping
Machine designers design
machinery components
so that the machine
operates at a speed 20-
30% higher/lower than
the resonant frequencies

122. 2006-12-07 ©SKF Slide 122 [Code]
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Correcting resonant conditions
• Bracing or Stiffening
• Changing the speed of the machine
• Damping treatments
• Reduce or eliminate the vibration source

123. 2006-12-07 ©SKF Slide 123 [Code]
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ISO 10816-1 (horizontal shaft)

124. 2006-12-07 ©SKF Slide 124 [Code]
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ISO 10816-1 (overhung – horizontal shaft )

125. 2006-12-07 ©SKF Slide 125 [Code]
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ISO 10816-1 (vertical shaft )

126. 2006-12-07 ©SKF Slide 126 [Code]
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2006-12-07 ©SKF Slide 126 [Code]
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Quick review
QUESTIONS ?