Introduction and basic concepts of vibration.

1. Introduction to vibration
R.Narasimha Swamy
Senior consultant
narasimhaswamy@yahoo.com

4. What is vibration?
• Vibration can be defined as simply the cyclic or
oscillating motion of a machine or machine
component from its position of rest.
• An important class of dynamics concerning the
linear and angular motions of bodies that respond
to the applied disturbances in the presence of
restoring forces.
• Examples are response of building structure to an
earthquake, unbalanced axle rotation, flow induced
vibrations of a car body, and the rattling of tree
leaves etc

5. Basic concepts
• Vibration sources are characterized by their
time and frequency domain properties.
• Categorized principally as:
– Periodic
• originate from the HVAC, DG, fans etc
• simplest form of periodic disturbance is harmonic.
• In the time domain, this is represented as a sinusoid
and in the frequency domain by a single line spectrum.
– Random disturbances
• originate from footsteps sound, conversation etc
• only statistical representations are possible.
• generally represented by its power spectrum.

6. What causes vibration? [1/2]
• Repeating forces
• Looseness
• Resonance

7. What causes vibration? [2/2]
• Change in direction with time, such as the force generated by a
rotating unbalance.
• Change in amplitude or intensity with time, such as the
unbalanced magnetic forces generated in a motor due to unequal air gap
between the armature and stator.
• Friction between rotating and stationary machine components like
between bow and the violin string to vibrate.
• Impacts, such as gear tooth contacts or the impacts generated by the
rolling elements of a bearing passing over flaws in the bearing raceways.
• Randomly generated turbulence force devices such as fans,
blowers, pumps, gas turbines or boilers.
• Improper installation without proper or improper mounts, isolators etc.
• Flanking transmission path in piping, building etc

8. Hazards of vibration
• Quality problems
• Severe machine damage
• High power consumption
• Machine unavailability due to breakdown
• Unnecessary maintenance
• Occupational hazards

9. Statistical model of vibration
• All mass-elastic systems have natural frequencies
– For linear system these frequencies are constant
• related only to the mass and stiffness distribution
– Non-linear effects require special treatment
• A few of the lower order frequencies are of interest
because the higher ones are more highly damped.
• For a frequency, a system vibrates in a particular
way, depicted by the relative amplitude and phase
at various locations - mode of vibration.

10. Statistical model of vibration Contd.,
 If there is no external force applied on the
system, the system will experience un-damped
free vibration. Motion of the system will be
established by an initial disturbance (i.e. initial
conditions).
 Furthermore, if there is no resistance or damping
in the system, the oscillatory motion will continue
forever with a constant amplitude. Such a system
is termed un-damped and is shown in the
following figure,

13. The exponential term
defines how quickly the
system “damps” down.
The larger the damping
ratio, the quicker it damps
to zero.
The cosine function is the
oscillating portion of the
solution, but the
frequency of the
oscillations is different
from the un-damped case.
Damped and un-damped natural frequencies

16. Characterization of vibration
• Displacement is represented as amplitude.
This important factor can also be
represented as velocity or acceleration to
make this factor qualified with reference to
time.
• Frequency
• FFT (spectrum)

17. Measurement types
• Vibration displacement: deviation of measured
point from rest position in mm or mil.
• Vibration velocity. Velocity with which measured
point moves about rest position in mm/s or ips.
• Vibration acceleration. Acceleration with which
measured point moves about rest position in m/s2
or “g” force.

26. Vibration measurement devices
• Transducers based on seismic mass displacement
measurement, capacitive, linear variable differential
transformer (LVDT), piezo-resistive, strain-gauges etc
• Frequency compensated electro-dynamic sensor,
specially designed for measuring low intensity, low
frequency acceleration.
• Servo accelerometers, working on force compensation
principle.
• Today, all these devices are neatly integrated in the
form of MEMS - a revolutionary semiconductor based
technology.

31. Impact sound

32. Impact insulation class (or IIC)
• IIC is an integer number rating of how well a building
floor attenuates impact sounds, such as footsteps.
• A larger number means more attenuation and better for
human comfort. It uses decibel scale.
• The IIC is derived from ASTM method E989, which in
turn uses a tapping machine specified in ASTM method
E492. This machine incorporates five steel-faced
hammers that strike the test floor and generate noise in
a room below.
• The IIC number is derived from sound attenuation
values tested at sixteen standard octave bands from
100 to 3150 Hz.

33. • IIC strength: Helps to rate structure-borne noise such
as footfall, a chair dragging on the floor, or other
realistic sounds in a single number.
• IIC weakness: Due to the nature of the testing
procedure, almost any assembly with carpet will meet
the IIC requirement. Meeting the IIC requirement does
not ensure the control of footfall noise. Conversely, if an
assembly does not meet the IIC requirement, it does not
necessarily mean that there will be a footfall noise
issue.
• The tapping machine frequently used for this test is not
designed to simulate any one type of impact, such as a
male or female footsteps, nor to simulate the weight of a
human walker. Thus the subjectively annoying creak or
boom generated by human footfalls on a limber floor
assembly may not be adequately evaluated by this
method (ASTM, E 1007, 5.2).

35. Structure borne sound
Sound travelling by means of structure vibration is usually
unnoticed but can become audible and a problem.

43. Thank you