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8/11/2019 Introduction to Designing Elastomeric Vibration Isolators.pptx
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8/11/2019 Introduction to Designing Elastomeric Vibration Isolators.pptx
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Introduction
Why elastomers?
Key design parameters
Loading
Configuration
Spring rates
Design considerations
Steps to designing a simple isolator
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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
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Elastomeric Isolators
Engineered properties can meet specificapplications
Modulus of elasticity
Internal dampening
Homogeneous nature allows for compactforms
Easily manufactured Molded
Formed in place
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Key Isolator Design Parameters
Configuration
Loading
Spring rate
Shear, Bulk, and Youngs modulus
Geometry
Ultimate strength
Internal dampening
Maximum displacement
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Simple Isolator Configurations
Planer Sandwich Form
Laminate
Cylindrical
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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
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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
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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
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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
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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
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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)
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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
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Compression Modulus vs. Shape
Factor [Gent]
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Laminate Isolator
Shear modulus is not effected byshape factor (if aspect ratio is
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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?
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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
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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)
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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 (
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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
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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)
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Questions?