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I/C: KALLURI VINAYAK
Springs
• A mechanical spring is an elastic member
(generally metal) whose primary function is to
deflect under load and then to recover its original
shape and position when the load is released.
• Used for efficient storage and release of energy• Used for efficient storage and release of energy
• Strength and flexibility are two essential
requirements of spring design.
Spring Types
1. Helical springs (Tension / Compression)
2. Torsion spring
3. Leaf springs
4. Spiral spring
5. Belleville Springs
Leaf spring
5. Belleville Springs
Helical springsTorsion spring
Spiral spring
Belleville Spring
Stresses in Helical Springs
( )2323max
4842/16
2/
d
F
d
FD
d
F
d
FD
A
F
J
Tr
FDT
ππππτ +=+=+=
=
inside inside
Spring Index and shear stress correction factor
d
DC =
If we define the spring index to be as follows:
Then the foregone expression for maximum shear stress
can be expressed as:
323max
848
d
FDK
d
F
d
FDs πππ
τ =+=323 ddd πππ
Where Ks is called as the “shear stress correction factor” and
serves to correct the shear stress estimated from the torsion
alone for the direct shear. Here Ks is
C
CK s
2
12 +=
For the standard springs, C ranges between 6 and 12.
Curvature effect in fatigue loading
• Only in fatigue loading, the curvature of the wire
introduces more shear stress than estimated above
• Hence that expression for maximum shear stress
needs correction.
• Many factors have been suggested for correction.
Prominent are Wahl factor (K ) and BergstrasserProminent are Wahl factor (Kw) and Bergstrasser
factor (KB).
• These must replace Ks when incorporated.
Curvature effect in fatigue loading: Wahl factor and
Bergstrasser factor
24615.014 +− CC
Static Loading, only effect of direct shear:
C
CK s
2
12 +=
Fatigue Loading, effect of both direct shear and curvature:
Wahl factor (Kw) or Bergstrasser factor (KB) is used
34
24615.0
44
14
−+
=+−−
=C
CKor
CC
CK Bw
Fatigue Loading, effect of only curvature:
( )( )( )1234
242
+−+
==CC
CC
K
KK
s
Bc
Deflection and Stiffness:
aa
a
Gd
FDN
Gd
NFD
F
Uy
dAdJDNlFDT
Gd
DNF
Gd
NDF
AG
lF
GJ
lTU
24
3
24
2
2
4
3222
48
: theoremsecond so'Castiglian
.4/;32/;;2/
24
22
+=∂∂
=
====
+=+=
πππQ
a
aa
ND
Gd
y
Fk
Gd
NFD
CGd
NFDy
GdGdFy
3
4
4
3
24
3
24
8
8
2
11
8
D/d,Cindex spring gIntroducin
==
≅
+=
=
+=∂
=
Manufacturing processing at the ends and effect on total
coils
Compression Springs
Compression Springs
Formulas for the Dimensional Characteristics of Compression-Springs
Table 10–1
If interested:
For a thorough discussion and development of these relations, refer
Cyril Samonov, “Computer-Aided Design of Helical Compression Springs,
” ASME paper No. 80-DET-69, 1980.
SPRING MATERIALS
• Music wire, Oil-tempered wire, Hard drawn wire, Chrome-
vanadium wire and Chrome-silicon wire
mutd
AS = strength, tensileMinimum
Use Table 10-4 for “A” and “m”.
Table 10–4
SPRING MATERIALS
Table 10–5
Mechanical Properties of Some Spring Wires
SPRING MATERIALS
Unless otherwise specified, use MSS criterion for static design for springs
because the primary loading nature is shear.
Table 10–6
Maximum Allowable Torsional Stresses for Helical Compression Springs
in Static Applications
Set removal or presetting• Is a process used in the manufacture of compression
springs to induce useful residual stresses
• The spring is made to a longer free length than required and
then is compressed beyond the elastic limit by 30% of the
length
• When the spring tries spring back, the plastic strain induced
opposes the same resulting in residual stress being set up
that are opposite in direction to the working stresses
• Hence the springs behave stronger in service
• Set removal must NOT be used for springs used in fatigue
loading
Critical Frequency of Helical Springs: Surging
• Spring surge or surging of springs is the problemand it is similar to the wave propagating in water
• If one end of springs is held stationary and otherend is disturbed, the springs vibrates violently
• Failure resulting from the resonance inherent insurging is found to be purely due to torsionalsurging is found to be purely due to torsionalshear and occurs at 45o to the wire axis.
• The governing equation for spring surging is thewave equation:
2
2
22
2
t
u
kgl
W
y
u
∂∂
=
∂∂
Here, u is the displacement, k is the spring stiffness, l is the length of the
spring, g is the acceleration due to gravity, W is the weight of the active part
of the spring
Contd.
The solution to this differential equation give the natural frequency of
vibration:
W
kgmπω =
W
kgmf
2=
m = harmonic number
222 γππ DNdd ( )( )
weightSpecific
44
222
=
===
γ
γπγπ
πγ a
a
DNdDN
dALW
The fundamental frequency, for m=1, should be from 15 to 20 times the
forcing frequency to avoid the resonance and hence in turn the surging of
the spring
Redesign to effect this normally involves increasing k or decreasing the W
STABILITY:• A compression spring is stable if it does not buckle under the
load
−−=
2/1
2
'
2'
10 11eff
cr
CCLy
λ
D
Leff
0αλ =
( )GE
EC
−=
2
'
1
( )EG
GEC
+−
=2
2 2'
2
π
( ) 2/1
02
'
2
2
21
+−
<⇒>EG
GEDL
C
eff απ
λ
For absolute stability and buckling not to occur,
Slenderness ratio Elastic constants
End -condition constants (α) for helical compression springs
Table 10–2, page 522
Helical Compression Spring Design for Static Service
• A helical coil spring force-deflection characteristic
• is ideally linear.
• For very small deflections, and near closure,
nonlinear behavior begins as the number of active
turns diminishes as coils begin to touch.
• The spring’s operating point to the central 75• The spring’s operating point to the central 75
percent of the curve between no load, F = 0, and
closure, F = Fs .
Fs = (1 + ξ )Fmax
Helical Compression Spring Design for Static Service
• In addition to the relationships and material
properties for springs, the recommended design
conditions are:
Spring index range : 6 ≤ C ≤ 12
No of active turns range : 3 ≤ Na ≤ 15No of active turns range : 3 ≤ Na ≤ 15
Robust linearity :ξ ≥ 0.15
Factor of safety at closure : ns ≥ 1.2
4 cost) material (relative- fom merit, of figure The
22 DNd tγπ=
Helical coil compression spring design for static loading.
From Table A-28; 1051
• A music wire helical compression spring is needed to
support an 89 N load after being compressed 50.8 mm.
Because of assembly considerations the solid height
cannot exceed 25.4 mm and the free length cannot be
more than 101.6 mm. Design the spring.
• Springs are almost always subject to fatigue loading.
• Automotive engine valves are supported by compression
springs that are subjected to millions of cycles of
operation without failure.
• Shot peening is used to improve the fatigue strength of
Design for Fatigue Load:
• Shot peening is used to improve the fatigue strength of
dynamically loaded springs. Shot peening can increase
the torsional fatigue strength by 20 percent or more.
• Springs are designed for infinite life based on
Zimmerli’s data.
Zimmerli’s Data: Shot Peening
• A cold working process used to produce a compressive
residual stress layer and modify mechanical properties of
metals
• Entails impacting a surface with shot (round metallic,
glass or ceramic particles of 1/64 inch diameter) withglass or ceramic particles of 1/64 inch diameter) with
force sufficient to create plastic deformation
Zimmerli’s Data:
• The best data on the torsional endurance limits of spring steels are
those reported by Zimmerli and discovered the surprising fact that
size, material, and tensile strength have no effect on the endurance
limits (infinite life only) of spring steels in sizes under 10 mm.
• Unpeened springs were tested from a minimum torsional stress of
138 MPa to a maximum of 620 MPa and peened springs in the range
138 MPa to 930 MPa . The corresponding endurance strength
components for infinite life were found to becomponents for infinite life were found to be
utsyututsu
smsa
smsa
SSSorSS
MPaSMPaS
Peened
MPaSMPaS
Unpeened
557.035.067.0
534398
:
379241
:
≤≤=
==
==
Design for Fatigue Loading Based on Zimmerli’s
Data
criterion. failure fatigue aapply then and
,,,,,,,, minmax susyemama SorSSFFfindgivenFF ττ
minmaxminmax
22
FFFand
FFF ma
+=
−=
33
88
22
d
DFKand
d
DFK
FandF
mWm
aWa
ma
πτ
πτ ==
==
Zimmerli’s Data (Gerber criteria)
su
sm
sase
su
sm
se
sa
lineloadgivenforcordinatetionInter
S
S-
SS
S
S
S
S
=⇒=
+
2
2
sec
1
1
limit endurance thefindThen Ssa and Ssm are from
Zimmerli’s data.
Ssu= 0.67Sut
Factor of Safety,
a
saf
Sn
τ=
m
a
sm
sa
ut
se
se
susa
F
F
S
Sr
rS
S
S
SrS
==
++−=
222 2
112
Refer Table 6-7 ; page 307
Zimmerli’s Data (Goodman criteria)
su
sm
sase
su
sm
se
sa
lineloadgivenforcordinatetionInter
S
S-
SS
S
S
S
S
=⇒=
+
sec
1
1
limit endurance thefindThen Ssa and Ssm are from
Zimmerli’s data.
Ssu= 0.67Sut
Factor of Safety,
a
saf
Sn
τ=
m
a
sm
sa
sesu
susesa
F
F
S
Sr
SrS
SrSS
lineloadgivenforcordinatetionInter
==
+=
sec
Refer Table 6-6 ; page 307
Zimmerli’s Data (ASME- Elliptic criteria)
sy
sm
sase
sy
sm
se
sa
lineloadgivenforcordinatetionInter
S
S-
SS
S
S
S
S
=⇒=
+
2
22
sec
1
1
limit endurance thefindThen Ssa and Ssm are from
Zimmerli’s data.
Ssu= is to calculated
from Table 10-5
page 526
Factor of Safety,
a
saf
Sn
τ=
m
a
sm
sa
syse
syse
sa
F
F
S
Sr
SrS
SSrS
lineloadgivenforcordinatetionInter
==
+=
222
222
sec
Refer Table 6-8 ; page 308
Tension/ Extension springs: end preparation
Combined axial tension and
bending stress at A
Only
torsion at
B
Side
views
Tension /Extension springs:
Improved design
views
Analysis of stresses in tension springs
( )
( )
( )( ) d
rC
CC
CCK
K
dd
DKF
A
A
AA
11
11
1
2
1
23
2,
14
14
bygiven curvature,for factor correction strss Bending
416
moment bending tension axial combined todueA at stress tensilemaximum The
=−−−
=
=
+=ππ
σ
( )
( )
( ) ( )( ) d
rC
C
CK
K
d
FDK
B
B
BB
22
2
2
3
2,
44
14
bygiven curvature,for factor correction strss Torsional
8
bygiven is Bat stressshear torsionalmaximum The
=−−
=
=
=π
τ
Extension springF
A
B
( )
( )( ) d
rC
CC
CCK
dd
DKF
A
AA
11
11
1
2
1
23
2,
14
14
416
=−−−
=
+=ππ
σ
( )d
FDK BB 3
8=
πτ
Stress is to be computed at three locations
C
( ) ( )( ) d
rC
C
CK
d
B2
2
2
2 2,
44
14=
−−
=
π
34
24
83
−+
=
=
C
CK
d
FDK
B
BC πτ
INITIAL TENSION IN CLOSE-WOUND TENSION SPRINGS
• When extension springs are made with coils in contact with
one another, they are said to be close-wound.
• Spring manufacturers prefer some initial tension in close-
wound springs in order to hold the free length more
accurately.
( ) ( ) ( )
E
GNNcoilsofnumberActive
dNCdNdDL
kyFF
ba
bbo
i
+=
+−=++−=
+=
,
1212 :length Free
No of body coils
INITIAL TENSION IN CLOSE-WOUND TENSION SPRINGS:
• The initial tension in an extension spring is created in the
winding process by twisting the wire as it is wound onto
the mandrel.
• When the spring is completed and removed from the
mandrel, the initial tension is locked in because the spring
cannot get any shorter.
( )MPa
C
e Ci
−−±=
5.6
349.6
231
is, stress torsionalduncorrecte of range Preferred
105.0τ
PROBLEM
• A hard-drawn steel wire extension spring has a
wire diameter of 0.9 mm, an outside coil
diameter of 6 mm, hook radii of r1=2.55 mm
and r2= 2.1 mm, and an initial tension of 5 N.
The number of body turns is 12.17. From the
given information:given information:
(a) Determine the physical parameters of the spring
(b) Check the initial preload stress conditions
(c) Find the factors of safety under a static 24 N
load.
Solution:
( ) ( )
( )( )mmN
Gdk
turnsE
GNN
CCK
dDC
mmdODD
ba
B
/885.39.01079
57.1210198
107917.12
254.134/24
67.59.0/1.5/
1.59.06
434
3
3
=×
==
=××
+=+=
=−+=
===
=−=−=
( )( )( ) ( )
( ) ( ) ( ) ( )( )
mmyLL
mmk
FFy
mmdNdDL
mmNND
Gdk
o
i
bo
a
14.2589.425.20
89.4885.3
524
25.209.0117.129.01.5212
/885.357.121.58
9.01079
8
11
11
33
=+=+=
=−
=−
=
=++−=++−=
=×
==
( ) ( )( )( )
( ) [ ]
( ) range. preferred in thenot isIt .1.1027.248.126
5.1517.248.1265.6
367.549.6
231
07.899.0
1.5588
is stress initial duncorrecte The
min
67.5*105.0max
33
MPa
MPae
MPad
DF
i
i
i
uncorri
=−=
=+=
−−+=
===
τ
τ
ππτ
( )( )( )
15.5369.0
1.5248254.1
8
33
11 ===
ππτ B MPa
d
DFK
( )
( )
( )
526.115.536
55.818S
55.818181945.0S45.0S
18199.0
1783
d
AS
9.0
1
sy
utsy
190.0mut
331
===
===
===
τ
ππ
s
B
n
MPa
MPa
d
Contd.
( )( )
( )( )
( ) ( ) ( )( ) ( )( )
( ) 25.1364
25.1364181975.0S75.0S
,10219.0
4
9.0
1.51615.124
15.1167.567.5*4
167.567.54
14
14
67.5d
D
d
2rC
isA at bendinghook end in thesituation The
uty
231
2
11
1
2
1
11
===
=
+=
=−−−
=−−−
=
====
A
A
S
MPa
MPa
CC
CCK
C
ππσ
( )( )
( ) ( )( )
( ) ( ) ( )( )
( ) 595.1513
55.818,513
9.0
1.52420.18
2.1466.44
166.44
44
14
66.49.0
9.01.5
d
2r
is Bat hook -endin situation The
336.11021
25.1364
31
2
2
22
1
====
=−−
=−−
=
=−
=−
==
===
ByB
B
A
y
Ay
nMPa
C
CK
d
dDC
Sn
πτ
σ
�Close wound like helical coil
extension spring
�Negligible initial tension
�The ends connect a force at a
distance from coil axis to apply a
torque
�Wound with a pitch that just
separates the body coils to avoid
TORSION SPRINGS:
separates the body coils to avoid
intercoil friction.
�The wire in the torsion spring is
in bending
Free
End
Free
end
location
Back
angle
Angular
rotation,
proportio
-nal to Fl
TORSION SPRING
Fixed
End
location
angle
For all positions of the moving end θ + α =Σ = constant.
turnspartial;body turns
integer360
integer
==
+=+=
pb
pob
NN
NNβ
Bending Stress :
The bending stress can be expressed as
22 1414
''
CCKand
CCK
factorcorrectionstressisKwhere
I
McKσ
−+=
−−=
=
3
3
32,
32
)1(4
14
)1(4
14
d
FlKisequationbendingthe
d
c
IandFlMngSubstituti
unitythanlessalsoandKthanlessalwaysisK
CC
CCKand
CC
CCK
i
io
oi
πσ =
==
+−+
=−−−
=
Torsional stiffness:
( ) radiansEd
Ml
dE
Fl
EI
Fl
l
ye 44
22
3
64
64/33
:deflection End
ππθ ====
The end deflection is bending of a cantilever beam whereasThe end deflection is bending of a cantilever beam whereas
the coils undergo bending action under M = Fl requiring
application of Castigliano theorem.
Strain energy in bending, ∫= EI
dxMU
2
2
bbc
DNDN
c
Ed
MDN
Ed
FlDN
dI
EI
dxFl
EI
dxlF
FF
Ul
bb
44
4
0
2
0
22
6464
64/
2
==⇒
=
=
∂∂
=∂∂
= ∫∫
θ
π
θππ
The Force ‘F’ will deflect through a distance “lθ”
Torsional stiffness
( ) ( )
atebae
bb
t
eect
NEd
MDNNN
D
llN
Defining
D
llN
Ed
MD
Ed
Ml
Ed
Ml
Ed
MDN
EdEd
4
21
21
44
2
4
1
4
21
64;,
3
3
64
3
64
3
6464
(rad), deflectionangular totalThe
=+=+
=
++=++=
++=
θπ
πππθ
θθθθ
Stiffness expressions in torque/radian units:
bc
cDN
EdMk
64
4
==θ ( )21
4
64
3
ll
EdMk
e
e +==
πθ
Stiffness values in torque/turn values (i.e 2π rad /turn) :
at
sDN
EdMk
64
4
==θ
π24
' ×==EdM
k π24
' ×==EdM
k( )
ππ
23 4
' ×==EdM
k
Torsional stiffness
πθ
264'
' ×==at
sDN
EdMk π
θ2
64'
' ×==bc
cDN
EdMk
( )π
πθ
264
3
21
'
' ×+
==ll
EdMk
e
e
at
sDN
EdMk
8.10
4
'
' ==θ bc
cDN
EdMk
8.10
4
'
' ==θ ( )21
4
'
'
8.10
3
ll
EdMk
e
e +==
πθ
Tests show that the effect of friction between the coils is such that the
constant 10.2 (i.e 64/2π) should be increased to 10.8
Torsion spring supported on round bar or pin:
cb
b
N
DND
+= ndeformatioafter and before balance volumefrom,'
'θ
�When the load is applied to a torsion spring, the spring winds up, causing
a decrease in the inside diameter of the coil body.
�Ensure that the inside diameter of the coil never becomes equal to or less
than the diameter of the pin, in which case loss of spring function would
ensue
�The helix diameter of the coil D′ becomes
( ) ( )[ ]b'
cb
cb
NπDAθNπD'A
N
××=+××
+θ
( )pin
pinc
b
pin
cb
bpinpini
DdD
DdN
DdN
DNDdDDD
−−∆−
++∆=
−−+
=−−=−=∆
'
1
' '
θ
θ
The new inside diameter D′i = D′ − d makes the diametral clearance ∆
between the body coil and the pin of diameter Dp
Design of Torsion Springs for Strength:
•Static strength
mutd
AS =
Table 10–6
First column entries in Table 10–6 can be divided by 0.577 (from distortion-
energy theory) to give