18 COMPUTER AIDED ENGINEERING DESIGN
Again, an intermediate value is preferred, rather than an extreme value. From these two assumptions,
the mean coil diameter D is determined.
Step 3: From Eq. (1.15), calculate the maximum shear stress at the larger force
τ
max πmax
= 38
KFD
w d
(1.18)Step 4: Find the ultimate tensile strength (Sut) of the wire material from Eq. (1.16), selecting constants
A and b from Table 1.2. Usually, the torsional ultimate strength (Sus) and torsional yield strength (Sys)
are given by
Sus = 0.67Sut,Sys = 0.60Sut (1.19)Step 5: Find the safety factor (Ns) against yielding
NS
sys
=
τmax
(1.20)For static loading, the factor of safety should be between 1 and 2.
Step 6: If the safety factor appears to be less than 1 (or less than a desired value), perform the design
iteration by choosing another wire diameter from Table 1.1(b) and repeating the above steps, till an
acceptable value of Ns is achieved.
Step 7: Determine the spring rate
kFF
=
max – min
δ
(1.21)Use Eqs. (1.14) and (1.21) to calculate the active number of coils
N
Gd
Dk
a =
84
3 (1.22)A rounding off is done to the nearest^1
4
of the coil. For example, if Na = 8.6, it is rounded off to8.5 and if it is 8.09, it is rounded off to 8.0. This will increase the stiffness slightly.
Table 1.1(a) Common Spring Wire MaterialsASTM # Material SAE # Description
A227 Cold-drawn wire 1066 Least expensive general-purpose spring wire. Suitable for static
loading but not good for fatigue or impact. Temperature range
0 °C to 120°C.
A228 Music wire 1085 Toughest, most widely used material for small coil springs.
Highest tensile and fatigue strength of all spring wire. Temperature
range 0°C to 120°C.
A229 Oil-tempered wire 1065 General-purpose spring steel. Less expensive and available in
larger sizes than music wire. Suitable for static loading but not
good for fatigue or impact. Temperature range 0°C to 180°C.
A230 Oil-tempered wire 1070 Valve-spring quality-suitable for fatigue loading.