Basic Vibration Analysis
Basic Vibration Analysis
Basic Vibration Analysis
Displacement
Upper Limit
Time Waveform
Heavy Spot Amplitude
+ 0
Time
Rotation 1 revolution 3600 RPM 60 Hz 1 Order = = = 3600 cycles per minute 60 cycles per second One times turning speed
Amplitude:
It is the magnitude of vibration signal. Units: Micron, MM/Sec, gs or M/Sec2
Amplitude
- How much is it vibrating? Size (severity) of the problem. is it vibrating? Cause of the vibration.
Frequency Measurement
60 RPM
= 1 Rev / s
= 1 Hz
Amplitude Measurement
1. Displacement :
The distance a structure moves or vibrates from its reference or rest position. Unit : Microns(p-p), mils(p-p)
2. Velocity
Rate of change of displacement. It is the measure of the speed at which the mass is vibrating during its oscillation. Unit : MM/Sec, Inch/sec (RMS or Peak)
3. Acceleration :
It is the rate of change of velocity. The greater the rate of change of velocity the greater the forces (F=ma) on the machines. Unit : M/Sec2, Inch/sec2 , gs (RMS)
A
B C
a
t
+ a
v t
Acceleration :
DISPLACEMENT
10
1 VELOCITY
.1
.01
.001
ACCELERATION
60
600
6000
60000
Scales Of Amplitude
Peak
RMS
Peak to Peak
Av.
a 2a 0.707 a 0.637 a
Vibration Transducers
Produces electrical signal of vibratory motion
Proximity Probe
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
Velocity Probe
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
Accelerometer
Produces signal proportional to acceleration of seismic mass. Extremely linear amplitude sense. Large Frequency range Smaller in size
Time Waveform
Heavy Spot Amplitude
+ 0
Time
Rotation 1 revolution 3600 RPM 60 Hz 1 Order = = = 3600 cycles per minute 60 cycles per second One times turning speed
Time Waveform
+
Amplitude
Time
1000 RPM
4 blades 4 X 1000 RPM = = =
1 revolution
Vibration occurs 4 times per revolution Vibration occurs at 4000 cycles per minute 4000 CPM
Time Waveform
+
Amplitude
Time
12 tooth gear
1000 RPM
1 revolution
12 teeth are meshing every revolution of the gear 12 x 1000 RPM = vibration occurs at 12,000 cycles per minute = 12,000 cpm = 200 Hz
Time Waveform
+ 0 -
Tim e
Time
+ 0
+
0 -
Time
Time Waveform
+ 0
Time
Time Waveform
Signal Acquisition
Transducer Amplitude Time Waveform
Overall Energy
FFT
Spectrum Amplitude Frequency
Frequency Domain
FFT - Fast Fourier Transform Separates individual frequencies Detects how much vibration at each frequency
Amplitude Amplitude
T i m e
T i m e
y c n e u q e r F
Amplitude
+ 0 -
Frequency Domain
Time
1x
Frequency
+
0 4x
Time Frequency
+ 0 -
Time
12x
Frequency
Phase
What Is Phase?
Phase is a measure of relative time difference between two sine waves.
Importance Of Phase
Phase is a relative measurement. Provides information how one part of a machine is vibrating compared to other. Confirmatory tool for problems like1. Unbalance 2. Misalignment 3. Eccentric Rotor, Bent Shaft. 4. Mechanical Looseness, Structural Weakness, Soft Foot. 5. Resonance. 6. Cocked bearing. No correlation with Bearing defects, Gear defects, Electrical motor defect.
Unbalance
What is unbalance?
The force created by a rotating body when its center of mass is offset from its center of Center of Mass = Center of Rotation rotation
Heavy Spot
Center of Shaft
Causes of unbalance
Improper assembly Material Buildup Wear Broken or missing parts
Characteristics of unbalance
High radial peaks at 1X shaft RPM Low axial vibration at 1X shaft RPM Low harmonics of shaft RPM 1X RPM sinusoidal pattern in the time waveform Can cause other faults to appear, especially looseness
Unbalance
Imbalance typically appears at the turning speed of the machine
Imbalance
Unbalance
Radial vibration at 1X shaft RPM is much more significant than in the axial direction