1. Reliability Management Consultant
VIBRATION MONITORING
FUNDAMENTAL REALITIES
All machines vibrate – only dead machines don’t
Vibration is a symptom – to and fro motion
A body vibrates because it has mass and
stiffness & they are related.
2. Reliability Management Consultant
VIBRATION MONITORING
HARMFUL EFFECTS OF EXCESS VIBRATION
Increased Stresses On Risk of fatigue.
Machine Components
Increased Load On Bearings Reduced bearing life.
Higher Forces On Mountings Loosening of foundation
Damage to supporting
structure.
5. Reliability Management Consultant
Imagine
Imagine a bird perched on a thin twig of a tree.
And the bird goes up and down. In other words
it oscillates.How would you describe its motion
over a period of time?
Now imagine that two birds sit on the same
branch and the frequency of oscillation
increases to 20 times/min. The vibration
increases greatly and the birds fly off. What has
happened?
9. Reliability Management Consultant
Basic Theory Of Vibration
Simple Spring Mass System
Upper Limit
Neutral Position
Lower Position
Displacement
Max Acceleration, Min Velocity
Max Acceleration, Min VelocityMax Velocity, Min Acceleration
IT FOLLOWS SINE CURVE
11. Reliability Management Consultant
The significance
Displacement gives the strain energy
Velocity gives the kinetic energy
Acceleration gives the force
Spike energy/HFD/gE are special
ways to measure acceleration only.
13. Reliability Management Consultant
Amplitude Measurement
1. Displacement : Total distance traveled
by the mass. Unit : Microns, Mils
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
3. Acceleration : It is the rate of change of
velocity. The greater the rate of change of
velocity the greater the forces (P=mf) on the
Machines. Unit : M/Sec2, Inch/sec2
18. Reliability Management Consultant
General rule of direction
We measure H direction for machines
on the foundation.
We measure V direction for machines
on structures
We measure A direction when we
suspect misalignment
There are advantages, but there are
exceptions to this rule.
19. Reliability Management Consultant
Simplest Transducers
Finger – index finger –
displacement to even 10 microns
and acceleration or impacts FEEL
5 Rupee coin a go no-go test
Eye
22. Reliability Management Consultant
1. Produces signal proportional to
acceleration of seismic mass, can
get disp, and vel also.
2. Large Frequency range
3. Smaller in size
4. Can be used remote
Accelerometer
24. Reliability Management Consultant
Simple rules of Limits (Alarm)
For Velocity – 5 to 7 times of baseline
For Displacement – 4 to 5 times of Base
For Acceleration – 20 to 40 times Base
For Spike Energy – 30 to 50 times Base
25. Reliability Management Consultant
How do we set the limits?
Baseline = Y mm/s; G, D
Warning = 1.5 x Y mm/s; 10 G; 1.5 D
Alarm = 5 to 7 x Y mm/s;30-50G, 4/5D
Warning and Alarm Limits
26. Reliability Management Consultant
VIBRATION
Displacement:
Sensitive to low
frequencies.
Good guide for imbalance,
& other low frequency faults.
Velocity:
Has average sensitivity throughout the frequency range.
Reasonably sensitive to imbalance, misalignment, looseness and
rolling element bearing damage.
Acceleration:
Sensitive to high
frequencies.
Good for rolling element
bearing damage.
27. Reliability Management Consultant
Advantage Of Using Velocity?
Flat frequency range compared to
displacement & acceleration.
Almost all machines generate fault
frequency between 600CPM to 60KCPM
Velocity indicates fatigue.
Velocity is the best indicator of vibration
severity.
28. Reliability Management Consultant
Amplitude & Frequency
Amplitude:
It is the magnitude of Vibration signal.
Units: Micron, MM/Sec, M/Sec2
Frequency:
How many times oscillation is occurring
for a given time period?
Units: CPS(Hz), CPM
29. Reliability Management Consultant
Physical Significance Of Vibration
Characteristics
Amplitude - How much is it vibrating?
(severity) of the problem.
Frequency - What is vibrating?
Source of the vibration.
Phase Angle – Relation of vibration?
Cause of the performance.
31. Reliability Management Consultant
Detection By Vibration
Analysis
Unbalance
If the ratios of H:V:A = 5:4:1
And it is true for both bearings
on either sides, expect for
overhung rotors
32. Reliability Management Consultant
Detection By Vibration
Analysis
Misalignment
If the ratio of H:V:A = 4:2:3
Sometimes the axial may be
more than the horizontal. It
may be due to less stiff rotors.
33. Reliability Management Consultant
Detection By Vibration
Analysis
Mechanical Looseness, Structural
Weakness, Soft Foot
The ratio of H:V:A = 2:4:1 for foundation
The ratio of H:V:A = 2:1:4 for structures
The ratio of H:V:A = 2: 4: 3 for soft foot
35. Reliability Management Consultant
Detection By Vibration
Analysis
Electrical Problems
Vibrations are more to the motor side
than on the driven side or the
vibrations disappear suddenly on
switching off.
In some cases there is a distinct noise.
Or the needle starts to flicker when
near an electrical cable.
