Basic Vibration Analysis.ppt
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Basic Vibration Analysis
-I
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Basic Theory Of Vibration
Simple Spring Mass System
Upper
Limit
Neutral
Position
LowerPosition
Displacement
Max Acc
Mim Vel
Max Acc
Mim Vel
Max Vel
Mim Acc
It follows sine curve.
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1 revolution
Time
Amplitude
0
+
-
Time Waveform
3600 RPM = 3600 cycles per minute
60 Hz = 60 cycles per second
1 Order = One times turning speed
Heavy Spot
Rotation
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Frequency & Amplitude
Frequency:How many times oscillation is occurring
for a given time period?Units: Cycles Per Sec or Hertz, CPM
Amplitude:It is the magnitude of vibration signal.Units: Micron, MM/Sec, gs or M/Sec2
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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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60 RPM
= 1 Rev / s
= 1 Hz
Frequency Measurement
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Amplitude Measurement
1. Displacement :The distance a structure moves or vibratesfrom its reference or rest position.
Unit : Microns(p-p), mils(p-p)
2. Velocity :
Rate of change of displacement. It is themeasure of the speed at which the mass isvibrating during its oscillation.Unit : MM/Sec, Inch/sec (RMS or Peak)
3. Acceleration :
It is the rate of change of velocity. Thegreater the rate of change of velocity thegreater the forces (F=ma) on the machines.Unit : M/Sec2, Inch/sec2, gs (RMS)
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B
C
A
a
b
c
t
t
t
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t
+
a
d
v
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Physical Significance Of Vibration
Amplitude
Displacement : Stress Indicator
Velocity : Fatigue Indicator
Acceleration : Force Indicator
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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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What Is The Advantage Of Using
Velocity?
Flat frequency range compared to
displacement & acceleration.
Almost all machines generate faultfrequency between 600CPM to 60KCPM
Velocity indicates fatigue.
Velocity is the best indicator of
vibration severity.
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Vibration Transducers
Produces electrical signal of vibratory motion
Proximity Probe - Displacement
Velocity Probe - Velocity
Accelerometer - Acceleration
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Proximity Probe
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Permanently installed on large machines with sleeve
bearings.
Measures relative displacement between the bearing
housing(probe tip) and the rotating shaft.
Called Eddy Current Probe
Frequency range 0 to 60,000 CPM
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Velocity Probe
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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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Accelerometer
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Produces signal proportional to accelerationof seismic mass.
Extremely linear amplitude sense.
Large Frequency range
Smaller in size
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1 revolution
Time
A
mplitude
0
+
-
Time Waveform
3600 RPM = 3600 cycles per minute
60 Hz = 60 cycles per second
1 Order = One times turning speed
Heavy Spot
Rotation
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1000
RPM
1 revolution
Time
Amplitude
0
+
-
4 blades = Vibration occurs 4 times per revolution
4 X 1000 RPM = Vibration occurs at 4000 cycles per minute
= 4000 CPM
Time Waveform
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Time
0
+
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Time
Time
0
+
-
0
+
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Time Waveform
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Time Waveform
0
-
+
Time
Time Waveform contains all the differentfrequencies mixed together.
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Time Waveform
Example of a time waveform
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Signal AcquisitionTransducer
OverallEnergy
FFT
Waveform
Spectrum
Amplitud
e
Amplitude
Time
Frequency
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Frequency
1x
4x
12x
Time0
+
-
Time
Time
0
+
-
0
+
-
Frequency
Frequency
Frequency Domain
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Phase
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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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Diagnosing Machine Faults
Unbalance
Misalignment
General looseness or wear
Bearing defects
Gear defects
Belt defects
Electrical Faults Journal Bearing Faults
Resonance
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Causes of unbalance
Improper assembly
Material Buildup
Wear
Broken or missing parts
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Freq: 60.04 Hz
Order: 1.010
Spec: .390
Unbalance
Radial vibration at 1X
shaft RPM is much
more significant thanin the axial direction