vibration isolation

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Vibration Isolation System

BY

Naziya I. Ghanchi.

Roll No.-03M. E.- Structures

Saraswati college Of Engineering.

6 th January 2014.

1



What Is Vibration?

Vibration is the physical movement or oscillation of amechanical part about a reference position.

OR

Vibration are time dependent displacements of aparticle or a system of particles w.r.t an equilibriumposition.

The structure undergoes vibration when it is

disturbed from its static equilibrium position and toallow it to vibrate. This motion that vibrates thebuilding could be resulted from the wind orearthquake-induced force or due to any other reason.

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Why do we care about vibration?

Vibration control of civil structures is more recent ascompared to machines & aerospace vehicles.

Earthquakes and wind loads - main sources ofstructural vibrations.

Control vibrations by: changing rigidity, mass,damping, shape, or applying passive or active controlforces.

High strength may result in high acceleration levels,so increasing strength alone wont always work.

3



Why do we care about vibration?

In some instances, vibration can be the main causeof serious damages that can destroy astructure/machine or can lead to a human’s

discomfort.Because of these issues, there are always many waysto provide vibration isolation, that is to lesser theimpact of the vibration or to provide vibration control

so that damage is minimized.

4



What is isolation?

Isolation units are the basic elements of a vibrationisolation system which are intended to providethe decoupling effect to a structure or machine.

Effectiveness of an isolation is measured in terms ofminimum force or motion transmitted to supports orsurroundings

Materials used for isolation have elastic and damping

properties.Materials- cork, rubber, felt, metal springs etc.

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Why vibration isolation system?

Vibration isn’t always easy to predict, which is whyengineers must both design systems to eliminatevibrations and also use vibration isolation to control

the problem after a system is designed to the best ofthe engineer’s ability.

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What is Vibration isolation system?

Vibration isolation is the process of isolating anobject, such as a part of a structure or piece ofequipment, from the source of vibrations.

Vibration isolation concerns means to bring about areduction in a vibratory effect.

A vibration isolator in its most elementary form maybe considered as a resilient member connecting thesuperstructure/equipment and foundation.

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Function of Vibration isolation

The function of an isolator is to reduce themagnitude of motion transmitted from a vibratingfoundation to the superstructure/equipment.

or

To reduce the magnitude of force transmitted from

the equipment/superstructure to its foundation.

8



Function of Vibration isolation

The essential features of an isolator are resilientload-supporting means and energy dissipatingmeans.

In certain types of isolators, the functions of theload-supporting means and the energy-dissipatingmeans may be performed by a single element, e.g.,natural or synthetic rubber.

In other types of isolators, the resilient load-carryingmeans may lack sufficient energy-dissipatingcharacteristics, e.g., metal springs; then separateand distinct energy-dissipating means (dampers) are

provided. 9



Difference between vibrationdamping and isolation

Vibration damping is a termed used to reduce theamount of energy that’s produced by the system.

A vibration damper takes energy out of the system.When you increase the damping in a mechanism orstructure there will be a reduction in vibration andnoise and the dynamic stresses applied will be

reduced with a resulting benefit to the fatigue life – among many other benefits.

10



Difference between vibrationdamping and isolation

Vibration isolation is a termed used for preventingthe vibration from being communicated from onepart of a structure to another. Isolators will often

have some inherent damping (like rubber mountingfeet.)

A good vibration isolation system will lower thenatural frequency of a mechanical system below the

excitation frequency. This keeps the natural andexcitation frequency “out of sync” which in turnreduces the amount of vibration and potentialproblems.

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Classification of Control Methods

Passive isolation.

Active isolation.

Hybrid control.

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Active/Feedback control:External source of power drives actuators (i.e., providesinput voltage) . Voltages required are computed by controller using

certain algorithms with inputs from sensors.Sensors measure motion (strains, displ, vel, accl.) Actuators apply forces to structure, thereby adding ordissipating energy.Examples of sensors are acceleromters, strain gauges.Examples of actuators are tendons, solenoids,piezoelectric stacks, active mass dampers (AMD).Destabilization possible.External power may not be available during

earthquake.




Passive control:No external power required.Passive control device (TMD, Base Isolator) imparts

forces that are developed directly as a result of motionof structure (i.e., no actuator involved).Total energy (structure + passive device) cannotincrease, hence inherently stable.Relatively inexpensive.

Reliable during earthquakeNot as effective as active, hybrid, semi-active control.




Hybrid control:Uses active & passive devices. Advantages of both active and passive systems are

present and their limitations are reduced.Essentially an active control systemExamples: viscous damping with AMD, base isolationwith actuators, TMD+AMD).



Passive control

Passive vibration isolation refers to vibration isolationor mitigation of vibrations by passive techniques suchas rubber pads or mechanical springs.

Passive vibration isolation is a vast subject, sincethere are many types of passive vibration isolatorsused for many different applications.

A few of these applications are for isolation of civil

engineering structures from earthquakes like baseisolation.

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Passive control: Base isolation

Fig. 2: (a) Schematic of base isolated building, (b) Model, (c) Rubber bearing



Passive control: Base isolation

Base isolators for seismic isolation ofbuildings, bridges, etc.Structure mounted on a suitably flexible base such

that the high frequency component of ground motionis filtered out and the fundamental vibration period islengthened. This results in deformation in theisolation system only, thus keeping the structureabove almost rigid. However, if the earthquakeexcitation contains a major component of thisfundamental period, there will be large sidesway(albeit almost rigid) motions.Not suitable for tall slender buildings (subject to highwind loads). For these auxiliary dampers (viscous,viscoleastic) are deployed

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Passive control: Tuned Mass Damper

Fig. 3: (a) TMD schematic, (b) Response

2



Passive control: Tuned Mass Damper

TMD, usually having mass about 1% that ofstructure, fitted to top of building. It is tuned toreduce vibration for given frequency range. Absorber mass takes up vibratory energy, leaving themain mass (building) almost static.Not very useful for earthquake excitations whichoccur over wide frequency range.Main system properties (stiffness-k1, mass-m1)known, absorber system properties (stiffness-k2,mass-m2) to be designed such that absorberfrequency equals excitation frequency (w2=w).

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Active Control

Fig. 5: Schematic of an active control system



Active control

First full scale application of active control to abuilding was done on Kyobashi Seiwa building(Japan) in 1989. Two AMD’s were used. Primary one

weighs 4t and damps transverse motion. Secondaryone weighs 1t and damps torsional motion.

Can also use Magnetorheological fluid dampers(semi-active), active tendons, etc.



Active control with TMD

Fig. 8: AMD on Kyobashi Seiwa building



Active control with TMD

Fig. 7: Schematic of AMD applied to building



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Thank you :)

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vibration isolation

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