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2
Objectives
Define Vibration
� What is it, where does it come from?
Describe measurement methods
Examine amplitude measurements
� Differences between the three
Choose proper transducer
� Right probe for the application
3
General DescriptionVibration : defined………..
– Movement of a body about a reference.• Response to some form of excitation•The excitation is generally referred to as a forcing function
4
Sources of VibrationImbalance
Misalignment
Belts
Gears
Bearings
� Rolling Element
� Sleeve
Looseness
Electrical Anomalies
Oil Instability
Resonance
5
X
Y
Y • Amplitude
X • Time
Legend
General Description
6
Vibration Measurement Parameters
Vibration breaks down into two separate areas called domains.
� The time domain displays a plot called a
waveform where the amplitude is displayed
over time, such as an oscilloscope monitors
an electrical signal.
� The frequency domain displays amplitude as
a function of how often something occurs in
one cycle.
7
Lets imagine that the
shaft is rotating
(The center of the shaft is really
what we are plotting, not the mass
near the edge. We just use the
mass for illustration)
Introduction to Vibration
8
Am
plit
ud
e
Locate it’s peak positive
position with a red mark.
Time
Introduction to Vibration
9
Am
plit
ud
e
Time
As the shaft rotates, time
also shifts, so we’ll locate
this new position forward
with respect to time.
Introduction to Vibration
10
Am
plit
ud
e
Time
Continuing this process…
The mass is now 180 degrees
from it’s original position.
We could call this peak negative.
Introduction to Vibration
11
Am
plit
ud
e
Time
As the mass returns to the“zero” reference point,notice that it is now 180degrees from the correspondinggreen marker.
Introduction to Vibration
12
Having completed 1 revolution,
the mass returns to peak positive
position. This is one cycle.
Am
plit
ud
e
Time
Introduction to Vibration
13
Am
plit
ud
e
Time
Connecting the reference locations with a curve...
Introduction to Vibration
14
Am
plit
ud
e
Time
Introduction to Vibration
15
Am
plit
ud
e
Time
Introduction to Vibration
16
Am
plit
ud
e
Time
Introduction to Vibration
17
Time
Am
plit
ud
eIntroduction to Vibration
Introduction to Vibration
Y
X
Am
pli
tude
Time
Observed in the time domain as the
amount of time to complete a cycle
The resulting waveform resembles
a sine wave.
Introduction to Vibration
Y
X
Am
pli
tude
Time
Every detected signal appears.
The time waveform becomes
very complicated, busy.
Introduction to Vibration
Each waveform signal is then
“converted” to it’s respective spectral peak.
Frequency
Am
pl it
ude
21
Waveform Generation
22
2 Domains : TIME and FREQUENCY
FFT
Measurement Parameters
23
Amplitude
3 ways to display
� All three show severity
� Some better than others
Displacement
� Distance a body moves
� Point “A” to point “B”
Velocity
� Measures the speed
� Time from point “A” to point “B”
� rate of displacement
Acceleration
� Total force acting
� Change of direction
24
Am
plit
ud
e
Frequency
Displacement
Displacement accentuates LOW frequencies,
and attenuates HIGH frequencies.
How the different units treat data...
Amplitude Relationships
25
Acceleration
Am
plit
ud
e
Frequency
Amplitude Relationships
Acceleration accentuates HIGH frequencies,
and attenuates LOW frequencies.
26
Amplitude RelationshipsA
mplit
ud
e
Frequency
Velocity
Velocity does neither. It treats all frequencies equally.
27
Amplitude Relationships
AccelerationDisplacement
Velocity
Am
plit
ud
e
Frequency
28
(D)isplacement Mils Peak-to-Peak
(V)elocity In/Sec Peak
(A)cceleration G’s RMS
The Figure below show the relationship of these three types of units.
D
VA
-
Am
plit
ud
e
Period is Time “T”
360°= One (1) Shaft Revolution
Peak to Peak
Average
R.M.S.
0 to Peak
Time
Amplitude
29
Phase: is the relationship between two events. You measure phase in degrees of rotation.
CB
360°
270°
Phase
30
Time
0
+
-
Tach Pulse
Peak of Vibration Signal
Phase Measured
between the two
360 degrees
Phase Measurement
31
A
B
You can also use phase to describe the relationship
between two events, as shown below. Disk A is 180º
out of phase with disk B.
Phase
32
Common Vibration TermsDisplacement� distance a body travels
� units of mils (pk. to pk.)
Velocity� rate at which displacement occurs
� units of inches per second in/sec (pk.) or mm/sec RMS
Acceleration� rate of change of velocity
� units of G’s (RMS)
Frequency� how often an event occurs per unit time
Phase� relationship between two events relative to a reference
33
1-24
� Signal processing– time waveform or spectrum
� Problem detection– examine both sets of data
– is there a problem?
� Diagnosis– what is the source ?– Root Cause
� Choosing the transducer– the right one for the best data
An effective program...
34
Amplitude and Frequency - Review
Amplitude
� Acceleration
� G’s
� Velocity
� In/sec
� Displacement
� mils
Frequency
� Cycles per minute - CPM
� Cycles per second - CPS / Hz
� Revolutions per minute -RPM
� Orders - Frequency/TS
RPM vs CPM
35
1-26
TransducersFunction
� Convert mechanical signal to electrical
Three basic types
� Displacement transducer
� Velocity transducer
� Accelerometer
36
shaft
non contact
displacement
transducer
bearing
Transducers
37
Displacement Xducers
38
Displacement Xducers
39
Displacement XducersAdvantages +
� Measures relative movement
� good for sleeve brg. machines
� very good for heavy machines
Disadvantages -
� permanently installed
� frequency response : DC to 1KHz
� power required
40
Connection
Case
Conductor Coil
Spring
Magnet
Damper
Velocity Xducer
41
Velocity XducerAdvantages +
� best signal to severity ratio
� good signal to noise ratio
� no power required
� single dif. / int. needed
� usually pretty hardy
Disadvantages -
� usually big
� heavy
� freq. 10 to 2Kz
� temperature sensitive
� comparatively expensive
� magnetic field sensitive
� orientation sensitive
� wear and temp. changes calibration
42
Settle Time
Bias Voltage10.5 Vdc
20 - 30 Vdc@
2-8mAC.C.
Supply Voltage
Amplifier
PreloadedRef. Mass
Base
Mica InsulatorConductive PlatePiezoelectric Crystal
Electrical Insulator
to signal analyzer
Accelerometers
43
AccelerometersAdvantages� broad frequency range
� small, light, rugged
� ICP needs no signal conditioning
� easy to mount
Disadvantages� poor as a hand held
� limited signal to noise ratio
� reads acceleration
� power required
� double integration needed
44
hand held
magnet glue wax stud
natural frequency of accelerometer
frequency
12345
5
4 3 2 1
Accelerometers
45
Overall Level Criteria
The overall level is a single number calculation of the unfiltered amplitude of a vibration waveform.
46
Summary of Overall Vibration Standards
An insurance agent responsible for insuring companies and their equipment established the Rathbone chart in 1939. So he could set an adequate premium, he had to know the running condition of the machinery. The agent based his chart on casing measurements made on heavy, slow-speed machines.
47
Summary of Overall Vibration Standards
48
ISO Vibration Chart
49
DuPont Severity Chart
50
Locations for Data Collection
51
Frequency Regimes
Synchronous
� NxRPM where N is an integer
Sub synchronous
� <1 x RPM
Non-synchronous
� F x RPM where F is >1x RPM but not integer
52
Causes – Sub-synchronous
Another component in the machine
Another machine
Belt drives
Hydraulic instability� Oil whirl, oil whip
Rubs� rotor, shaft, wheel
Cage� fundamental train - rolling element brgs.
53
Causes - Synchronous
Imbalance
Pitch line run-out
Misalignment
Bent shaft
Looseness
Blade / vane pass
Recips
Gears
Slot / Rotor Bar pass
54
Causes – Non-synchronous
Another machine
Belt multiples
Brgs.
Resonance
Electrical
Chains
Compressor surge
Detonation
Sliding surfaces
Lube pumps
Centrifugal clutches
U-joints