What Are Compression Springs? How
Compression Spring Works?
The four types of ends generally used
for compression springs are illustrated below. A spring with plain
ends has a non interrupted helicoid; the ends are the same as if a
long spring had been cut into sections.
A spring with plain ends that are
squared or closed is obtained by deforming the ends to a zero-degree
helix angle. Springs should always be both squared and ground for
important applications, because a better transfer of the load is
obtained.
Table 10–1 shows how the type of end
used affects the number of coils and the spring length. Note that the
digits 0, 1, 2, and 3 appearing in Table 10–1 are often used
without question. Some of these need closer scrutiny as they may not
be integers.
This depends on how a springmaker forms
the ends. Forys3 pointed out that squared and ground ends give a
solid length Ls of Ls = (Nt − a)d where a varies, with an average
of 0.75, so the entry dNt in Table 10–1 may be overstated.
The way to check these variations is to
take springs from a particular spring maker, close them solid, and
measure the solid height. Another way is to look at the spring and
count the wire diameters in the solid stack.
Set removal or presetting is a process
used in the manufacture of compression springs to induce useful
residual stresses. It is done by making the spring longer than needed
and then compressing it to its solid height. This operation sets the
spring to the required final free length and, since the torsional
yield strength has been exceeded, induces residual stresses opposite
in direction to those induced in service.
Springs to be preset should be designed
so that 10 to 30 percent of the initial free length is removed during
the operation. If the stress at the solid height is greater than 1.3
times the torsional yield strength, distortion may occur.
If this stress is much less than 1.1
times, it is difficult to control the resulting free length. Set
removal increases the strength of the spring and so is especially
useful when the spring is used for energy storage purposes. However,
set removal should not be used when springs are subject to fatigue.
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