1 vibration basics0

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.

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.

WHAT IS VIBRATION ?
WHAT CAUSES VIBRATION ?
HOW DO WE MEASURE VIBRATION ?
HOW DO WE ANALYSE VIBRATION ?

WHAT IS VIBRATION ?
• IT IS MOTION OF MECHANICAL PARTS
BACK AND FORTH FROM ITS POSITION
OF REST.

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?

What causes Vibration ?
INDUCED FORCE
&
FREEDOM FOR MOVEMENT

Types of Forces ?
Inertia Forces
Acceleration & Deceleration
Looseness & Impacts

Degrees of Freedom?
Degrees of Freedom determines the
direction in which we measure vibration

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

Modes of Amplitudes
• DISPLACEMENT
• VELOCITY
• ACCELERATION
• SPIKE ENERGY

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.

Physical Significance Of Vibration
Amplitude
Displacement : Stress Indicator
Velocity : Fatigue Indicator
Acceleration : Force Indicator

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

UNITS OF MEASUREMENT
DISPLACEMENT - MICRONS
VELOCITY - MM / SEC.
ACCELERATION - MM / SEC2 , G

Scales Of Amplitude
Peak
PeaktoPeak
RMS
Av.
Peak - a
Peak to Peak - 2a
RMS - 0.707 a
Average - 0.637 a

AMPLITUDE DESCRIPTORS
• RMS - Displacement
Velocity
• PEAK - 1.414 RMS Displacement
Velocity
Acceleration
• PEAK-TO-PEAK - 2 * PEAK Displacement

Measurement direction -
Horizontal, Vertical, Axial.

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.

Simplest Transducers
 Finger – index finger –
displacement to even 10 microns
and acceleration or impacts FEEL
 5 Rupee coin a go no-go test
 Eye

Other Transducers
 Velocity
 Proximity Probes
 Accelerometers

Accelerometer

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

Fixing Transducers
 Come back to the same point
 9 inch probe
 Magnet
 Stud

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

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

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.

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.

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

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.

Analysis with overall?
What faults can we
measure and How?

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

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.

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

Analysis
Antifriction Bearing Defects
 The ratio of H:V:A = 5:3:1
 and the gSE or gE is also high

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.

1 vibration basics0

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