Module 1: Overview of Vibration Controlsd Lecture 2...

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Module 1: Overview of Vibration Controlsd Lecture 2: Strategies of Vibration Control

The Lecture Contains:

Control of Vibration

Various Active and Passive Control Strategies

Reduction of excitation at the sourceIsolation of the SourceSystem RedesignRemedial Measures

Steps in Vibration Control

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Module 1: Overview of Vibration Control Lecture 2: Strategies of Vibration Control

Control of Vibration

Control of vibration or vibration suppression is possible using various passive and active methods

Passive action is independent of the resulting vibration – Open Loop System. Active method is dependent on the resulting vibration – Closed Loop System.

Various Active and Passive Control Strategies

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Reduction of excitation at the source

Examples:

Balancing of unbalanced inertia forces – rotors, enginesChanging the flow characteristics for flow induced vibrationsReducing friction, avoiding vortex shedding to reduce self-excitation,Reduce parameter variation for parametric excitation

Source provides the energy to maintain vibration. sources of vibration could be of several types:

Transient – for e.g. shock loading Forced excitation – Source (continuous) independent of Response Self-excited – Source generated by the Response for e.g. vortex induced vibration. Parametric excitation – System parameters (m,c or k ) change with respect to time.

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(ii) Isolation of the source

Modify the transmission path of vibration between source and the system to protect the system.

Example - Insertion of resilient elements – Springs, Dampers, Viscoelastic Materials, PneumaticSuspension etc. between the source and the system.

Very often vibration isolators are developed using a combination of springs and dampers. For example,viscoelastic materials are bonded to metal fasteners and used as anti-vibration mounts or isolators. Theconstruction of a typical bonded rubber spring for use under compressive loading is shown below.

A Typical Anti-vibration mount

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(iii) System modification

A large number of methods exist in this group including detuning, decoupling, using additive dampingtreatments ( constrained and unconstrained ), stiffeners and massive blocks (as foundation)

Consider the motion of the following single degree of freedom (SDOF) system:

(I) At low frequency the vibration is:

Stiffness controlled

(II) Near resonance, the vibration is

Damping Controlled

(III) Aat high frequency, the vibration is

Inertia Controlled

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Redesign of a Vibrating System

Redesign of a vibrating system involves modelling of materials - generally

Structural materials: metals and alloys Viscoelastic polymers: natural and synthetic rubbers (with additive)

For metals and alloys:

Stiffness is a function of elastic moduli ( E, G, K ) and the geometric dimensions depending on the typeof loading and deformation (bending, twisting etc.) Damping and Loss Factor are generally constant. Inertia depends on Density and Geometry.

MaterialsDensity(kg/m3)

EGPa

Loss factorO ( )

AI 2.7× 103 70 10-5

Brass 8.5 × 103 104 10-4

Steel 7.8 × 103 210 10-3

Cu 8.9 × 103 103 10-2

Cu-Mn(40 - 60) 7.2× 103 84 10-2

Concrete(dense) 2.3 × 103 27 10-2

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Viscoelastic Materials

Viscoelastic materials: butyl rubber, plasticized polyvinyl acetate, silicon rubber, polyurethane, thiokolRD etc.

Stiffness and Damping properties for viscoelastic materials are frequency and temperature dependentdue to transition from Glassy to Rubbery Phase.

Thiokol RD: The loss factor is 2 correspondingto a critical frequency of 7 Hz at

50C and around 800 Hz at 200C.

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Viscoelastic Materials

A qualitative plot of loss coefficient vs, Young's modulus for different classes of materials is shownhere for comparison.

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(iv) Use of Additive Layers

This involves addition of a secondary vibratory system to the original (primary) vibratory system which isunder excitation. Some secondary systems are vibration neutralizer, vibration absorber, tuned, self-tuned, impact absorbers. This strategy has been successfully used for suppressing vibration in verysmall to very large systems.

Examples: electric hair clippers, DC-9 aircraft, tractors, foot bridges, pipelines etc.

Viscoelastic materials are used as additive damping treatments: constrained and unconstrained layers

Extensional and shear deformation of the damping layer

Often spacers are designed to enhance extensional damping

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Steps in Vibration Control:

A. Identification and characterization of the source of vibration.

B. Specify the level to which the vibration should be reduced.

C. Select the method appropriate for realizing the vibration reduction level identified in step B.

D. Prepare an analytical design based on the method chosen in step C.

E. Realize in practice (i.e. hardware mechanization of) the analytical design constructed in step D.

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Step A: Identification and characterization of the source of vibration

Note: Often for a linear system, the analysis of the response helps in determining the nature of theexcitation. As shown here, the response can be analysed either in time domain or in frequency domain.

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Step B - Identify suitable response variable and decide on the accepted level ofvibration

Different design manuals/handbooks are available which corresponds to acceptable level of vibration forspecified field of applications. The table below is an excerpt of some of the frequently encounteredapplications and corresponding accepted level of vibration.

Source Total equivalentacceleration, m/s2

Hand tools Guideline 5 m/s2

Impact drill 10 - 110

Rock drill 5 - 13

Rail saw 3 - 6

Steel plate cutter 4 - 20

Chain saw 2 - 5

Grinder 1 - 3

Bench grinder 15

Bolt and nut wrench 5 - 15

Concrete vibrators 5 -20

Vehicles Guideline 1.15 m/s2

Excavator 1 - 5

Caterpillar with push plate 1 - 3

Motor sledge 2 - 5

Terrain vehicle 3 - 5

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Step C: Choice of a Method of Vibration Control

To control vibration effectively one can choose any of the five methods as discussed earlier or acombination of these methods.

Steps D and E will be discussed from Module 2 onwards. In the next lecture, we will discussmethod (iv) and (v) of vibration control.

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