introduction to designing elastomeric vibration isolators.pptx

8/11/2019 Introduction to Designing Elastomeric Vibration Isolators.pptx

1/26


2/26

Introduction

Why elastomers?

Key design parameters

Loading

Configuration

Spring rates

Design considerations

Steps to designing a simple isolator


3/26

Elastomers

An eastomer is any elastic polymer

Silicone Rubber

Butyl Rubber

Fluorosilicone Rubber

Material selection dependent on application

Ulitimate Loading

Sensitivity to Environment

Internal Properties


4/26

Elastomeric Isolators

Engineered properties can meet specificapplications

Modulus of elasticity

Internal dampening

Homogeneous nature allows for compactforms

Easily manufactured Molded

Formed in place


5/26

Key Isolator Design Parameters

Configuration

Loading

Spring rate

Shear, Bulk, and Youngs modulus

Geometry

Ultimate strength

Internal dampening

Maximum displacement


6/26

Simple Isolator Configurations

Planer Sandwich Form

Laminate

Cylindrical


7/26

Spring Rates

Isolator spring rate sets system resonantfrequency

Ratio of resonant frequency to input frequency

plus dampening control amount of isolation For an elastomer, spring rate is determined by

Shape factor

Loading: shear, compression, tension

Material Properties: bulk, shear, and youngsmodulus


8/26

Transmissibility

0.1

1

10

0 0.5 1 1.5 2 2.5 3

Transmissibility

fr/f

Transmissibility vs. Frequency Ratio

= 0.01

= 0.05

= 0.1

= 0.5


9/26

Shape Factor

Ratio of load area to bulge area

Easy to calculate for simple shapes simply loaded

Planer sandwich forms are simple

Tube form bearings are more difficult, but can beapproximated as a planer form


10/26

Shear Spring Rate

Design isolator to attenuate in shear if possible

Dependent on load area, thickness, and shearmodulus

Shear modulus is linear up to 75%-100% strain

Shear modulus for large shape factors is alsoeffected by high compressive loads

When aspect ratio exceeds 0.25 a correctionfactor is added to account for bending


11/26

Compression Spring Rate

Designed properly, compression canprovide high stiffness

Depends on load area, effectivecompression modulus, and thickness

Effective compression modulus Linear up to 30% strain

Can be tricky to compute


12/26


13/26

Finding Modulus

Many elastomers are listed with only with Durometer Shore hardness

Ultimate strength (MPa or psi)

Contacting manufacturer may be useful

Perform tests Shear stress is 1/3 Youngs modulus as poissonsratio

approaches 0.5

Use Gents relation between Shore A hardness and Youngsmodulus (if you gotta have it now)


14/26


15/26

Computing Ec

Gent provides a reference graph

Hatheway found empirically that the

transition zone is (Ec

/E)(t/D)1.583=0.3660

Can calculate Ecor the simple case of a circular

load area of diameter D and thickness t

Find the break points

First break point: (t/D)1.583= 0.366(E/EB)

Second break point: (t/D)1.583= 0.366


16/26

Compression Modulus vs. Shape

Factor [Gent]


17/26


18/26

Laminate Isolator

Shear modulus is not effected byshape factor (if aspect ratio is


19/26


20/26

Design Considerations

What is being Isolated?

What are the inputs?

Are there static loads? What are the environmental conditions?

What is the allowable system response?

What is the service life?


21/26

Example Design Process for a Simple

Isolator

Single excitation frequency

Circular cross section, planar geometery

All other components infinitely rigid Low dampening

Attenuation provided in shear


22/26

Design Process

Specifications

Mass, input vibration, required attenuation, max

displacement

Use transmissibility to determine resonance

frequency and spring rate

Find isolator minimum area (A)

Total number of isolators and max allowable stress

Select modulus (G)


23/26

Design Process (cont)

Knowing area (A), modulus (G), and springrate (ks), calculate thickness (t)

Calculate radius, verify aspect ratio < 0.25 to avoid

bending effect Find static deflection

Is static plus dynamic deflection < max allowabledeflection?

Find static shear strain

Low strain reduces fatigue (


24/26

Conclusion

Careful selection of parameters necessary to usemethods presented. Can get complicated quick Low strains

Low loads

Try to stay clear of the transition zone betweenYoungs and the bulk modulus

For multiple input frequencies, need to considerif dampening () is necessary

May need to include considerations other thanjust isolation Stresses due to CTE mismatch


25/26

References

P. M. Sheridan, F. O. James, and T. S. Miller, Design ofcomponents, in Engineering with Rubber (A. N. Gent,ed.), pp. 209{Munich:Hanser, 1992)

A. E. Hatheway, Designing Elastomeric MirrorMountings,Proc. of SPIE Vol. 6665 (2007)

Daniel Vukobratovich and Suzanne M. VukobratovichIntroduction to Optomechanical design

A. N. Gent, On The Relation Between IndentationHardness and Youngs Modulus, IRI Trans. Vol. 34,pg.46-57 (1958)


26/26

Questions?

introduction to designing elastomeric vibration isolators.pptx

Documents