Page 413 - Analysis and Design of Machine Elements

Springs
Figure 14.3d has a swivel hook, which tends to reduce stress concentration in hooks, 391
as the hook can turn freely. The cost of a spring is affected by end configurations.
Cylindrical helical torsion springs
A cylindrical helical torsion spring is used to exert rotational moment, or torque.
Torsion springs are wound in the same way as for compression and extension springs,
except that the ends are configured for transmitting torque conveniently [5]. As a
torque is applied, the spring deflects about its axis by rotation. Figure 14.4 presents
typical ends of cylindrical helical torsion springs. Torsion springs are used where
a torque needs to be applied, or rotational energy needs to be stored, such as in
clothespins to provide gripping force, in spring hinges to close doors, or in animal
traps and so on. Due to limited space, Figures 14.2–14.4 only present typical end
configurations of cylindrical helical springs. More end configurations can be found
in design handbooks [6, 7].

14.1.2.2 Belleville Springs
A Belleville spring is essentially a circular disc in a conical shape with a central hole, as
shown in Figure 14.5a. They are capable of obtaining a high spring rate within limited
axial space. Belleville springs are used either in series or in parallel. Stacking in series
in Figure 14.5b provides a larger deflection for a given load, while stacking in parallel
as shown in Figure 14.5c provides a higher load for a specified deflection [1]. Different
combinations of Belleville springs can be designed to obtain the desired load-deflection
relationship, that is, a characteristic curve, similar to Figure 14.5d. They have wide appli-
cations when space is limited and high loads with small deflections are required.

14.1.2.3 Spiral Springs
Spiral springs are wound from flat strips of metal with each turn wrapped tightly on its
inner neighbour in the form of a spiral, as shown in Figure 14.6a. The strip extends when
the free end is loaded with a torque. The load-deflection relationship of a spiral spring
is shown in Figure 14.6b. A spiral spring works in confined spaces, capable of storing
far more energy than a compression spring. They are used in clocks, watches and other
storage devices.

(a) (b) (c) (d)
Figure 14.4 Cylindrical helical torsion springs with various end configurations.

(a) (b) (c) (d)
Figure 14.5 Belleville spring and its characteristic curve.