chapter 1 evolution of a successful design

SPRINGS
6.34 MACHINE ELEMENTS THAT ABSORB AND STORE ENERGY
FIGURE 6.21 Location of maximum bending and
torsional stresses in twist loops. (Associated Spring,
Barnes Group Inc.)
6.5.4 Extension Spring Dimensioning
The dimensioning shown in Fig. 6.22 is generally accepted for extension springs.The
free length is the distance between the inside surfaces of the loops. The body length
is L B = d(N + 1). The loop opening, or gap, can be varied. The number of active coils
is equal to the number of coils in the body of the spring. However, with special ends
such as threaded plugs or swivel hooks, the number of active coils will be less than
the number of body coils.
6.5.5 Design Equations
The design equations are similar to those for compression springs with the exception
of initial tension and loop stresses. The rate is given by
Gd
k = = (6.24)
where PI is initial tension. Stress is given by
K
S =
w8PD
(6.25)
3 πd 4
P − PI
3 f 8D Na
FIGURE 6.22 Typical extension-spring dimensions. (Associated Spring,
Barnes Group Inc.)
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