helical spring design - static load
DESCRIPTION
Helical Spring Design - Static LoadTRANSCRIPT
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Design of Helical Compression Springs Under Static And Fatigue
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Group Members :
Chandra Prakash
Ajeet Kumar Meena
Pavan Kumar
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Types of SpringsHelical:
Compression
Extension Torsion
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Some Review
F
y
k
linear springs: k=F/y
nonlinear springs:dy
dFk
Parallel
ktotal=k1+k2+k3
Series
321
1111
kkkktotal
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Helical Compression Springs
d diameter of wireD mean coil diameterLf free lengthp pitchNt Total coils
may also need:Do and Di
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End ConditionsPlain
Square
Plain Ground
Square Ground
Na=Active Coils
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F
F
F
F
Stresses in Helical SpringsSpring Index C=D/d
C
CKwhere
d
FDK ss 2
12,
83max
FF
T
T
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Curvature Stress
under static loading, local yielding eliminates stress concentration, so use Ks
under dynamic loading, failure happens below Sy: use Ks for mean, Kw for alternating
Inner part of spring is a stress concentration(see Chapter 4)
Kw includes both the direct shear factor and the stress concentration factor
CC
CKwhere
d
FDK ww
615.0
44
14,
83max
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Spring Deflection
Gd
NFDy a
4
38
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Spring Rate
Gd
NFDy a
4
38
aND
Gdk
3
4
8
k=F/y
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Helical Springs Compression
NomenclatureStressDeflection and Spring ConstantStatic DesignFatigue Design
Extension Torsion
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Static Spring Design Inherently iterative
Some values must be set to calculate stresses, deflections, etc.
Truly Designthere is not one “correct” answermust synthesize (a little bit) in addition to
analyze
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Material Properties
Sut =ultimate tensile strengthSut=Adb
Sys =torsional yield strengtha function of Sut and set
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What are You Designing?
Given
F, yk, y
Find
kF
Such that:Safety factor is > 1Spring will not buckleSpring will fit in hole, over pin, within vertical space
d, C, D*, Lf*, Na
*, clash allowance ()**, material**
design variables
+
* - often can calculate from given** - often given/defined
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Static Spring Flow Chart
STRESSES
Ns=Sys/
for shut spring if possibleif not, for max working load
if GIVEN F,y, then find k; If GIVEN k, y, then find F
D, Ks, Kw
material strengths
DEFLECTION
Lf, yshut, Fshut
Three things to know:• effect of d• shortcut to finding d• how to check buckling
ITERATE?
d, C
material
Na,
CHECK
buckling, Nshut, Di, Do
Nshut=Sys/shut
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Static Design: Wire Diameter
Three things to know:• effect of d• shortcut to finding d• how to check buckling
3max8
d
FDKs
Gd
NFDy a
4
38
*maintain units (in. or mm) for A, b
use Table 13-2 to select standard d near calculated d
)2(115.08
b
m
initialworksAK
FFCNd
Based on Ns=Ssy/ and above equation for :
Km=Sys/Sut
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Buckling
D
L
pendsonBucklingDe
f
Three things to know:• effect of d• shortcut to finding d• how to check buckling
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Helical Springs Compression
NomenclatureStressDeflection and Spring ConstantStatic DesignFatigue Design
Extension Torsion
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Modified Goodman for Springs Sfw, Sew are for torsional strengths, so von
Mises not used
0.5 Sfw
0.5 Sfw
a
m
Repea
ted
SfsC
B
Sus
A
fwus
usfwfs SS
SSS
5.05.0
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Fatigue Safety Factor
0.5 Sfw
0.5 Sfw
a
m
Sfs
Suslo
ad li
ne
Sa
a
i m
mload
mgooda
afs
SN
a,load = a,good at intersection
ausimfs
iusfsfs SS
SSN
…on page 828
Fi=Fmin
Fa=(Fmax-Fmin)/2
Fm=(Fmax+Fmin)/2
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What are you Designing?
Given
Fmax,Fmin, yk, y
Find
kF
Such that:Fatigue Safety Factor is > 1Shut Static Safety Factor is > 1Spring will not buckleSpring is well below natural frequencySpring will fit in hole, over pin, within vertical space
d, C, D*, Lf*, Na
*, clash allowance ()**, material**
design variables
+
* - often can calculate from Given** - often given/defined
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Fatigue Spring Design Strategy
d, C
if GIVEN F,y, then find k; If GIVEN k, y, then find F
Nshut=Sys/shut
CHECKbuckling, frequency,Nshut, Di, Do
D, Ks, Kw
material strengths
DEFLECTION
Lf, yshut, Fshut
Two things to know:• shortcut to finding d• how to check frequency
ITERATE?
material
Na,
STRESSES
ausimfs
iusfsfs SS
SSN
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Fatigue Design:Wire Diameteras before, you can iterate to find d, or you can use an equation
derived from relationships that we already know:
)2(1
min 134.11
67.0
8b
awfw
b
sfs
fsms
fsFK
S
AdFK
N
NFK
A
CNd
use Table 13-2 to select standard d near calculated d
Two things to know:• shortcut to finding d• how to check frequency
**see Example 13-4A on MathCad CD*maintain units (in. or mm) for A, b
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Natural Frequency: Surge
Two things to know:• shortcut to finding d• how to check frequency
Surge == longitudinal resonance
for fixed/fixed end conditions:
an W
kgf
2
1 (Hz)
…see pages 814-815 for more
ideally, fn will be at least 13x more than fforcing… it should definitely be multiple times bigger
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Consider the Following:
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