Springs are used in all forms of transport – road, rail, air,
sea, snow and space (to deploy the solar panels) – and it is
often quoted that 40-50 percent of all springs are used in these
industries. Springs are also used in all white goods (washing
machines, cookers, etc.), toys, computers, televisions, telecommunications,
audio equipment, and mechanical, electrical
and electronic equipment. They are used in the mechanical
and electrical fi ttings of buildings, and sometimes even in
building foundations – as in the concert hall in Manchester,
England, which is mounted on springs to isolate the inside of
the hall from noise and vibrations on the outside. Factories
for assembly, food production, power generation and mining
rely upon them.
It seems extremely likely that every manufacturing industry
utilizes springs in most, if not all, of its products – a fact
that has been admirably demonstrated by other articles in this
edition. The title “Springs Everywhere” prompted IST to think
of the many environments in which springs are used and, in
particular, the fact that some spring designs are used in many
One of the services provided by IST is failure analysis
(at least one broken spring is received from somewhere in
the world every working day), but the results of the many
examinations that are carried out have to be kept confi dential.
Nonetheless, it is inevitable that generalized conclusions will
be drawn from these investigations. One of those conclusions
is that a common design fault that leads to spring failure is
inadequate consideration of the operating environment. Indeed,
it is often heard that a spring works without problem in most
sites around the world, but gives problems in only one or two.
It is nearly always the case that the working environment will be the cause of the problem, and that there will be some corrosion
on the failing springs but none at all on the springs that
are operating satisfactorily.
An example of the working environment causing a spring
failure problem has been quoted so often in the IST Spring
Failure and Prevention training course that it seems reasonable
now to put the results in print, albeit anonymously. A stainless
steel compression spring made from 4.4 mm diameter
wire was working within ink in a printing machine. After a
few months of use, the springs would fall into three to six
pieces. The problem was happening only at one plant despite
the spring’s use in many plants. Visual examination of the
fracture showed immediately that the spring had failed due to
fatigue, as pictured in Figure 1. The fracture is rather
green in color, but that is due to the green ink.
IST’s conclusion was that failure was due to corrosion
fatigue, which astonished the springmaker who saw no rust
and knew that the spring didn’t fail when fatigue tested. Further
investigation, though, revealed that the ink, suspended
in alcohol, was becoming acidic in use. It was only then that
the spring failed. The ink should have been neutral. When
the pH came under control, the spring lasted forever and the
springmaker received no further orders.
Failure analysis can have unwelcome results for spring
manufacturers, but this is not the main point I want to make in
this Cautionary Tale. The most important point is that control
of the environment is vital for satisfactory spring performance. The secondary point is that simultaneous corrosion and fatigue
can cause failure where either fatigue or corrosion alone would
not. In stainless steel, it might not be possible to see any red
rust at all; however with corrosion fatigue, you often see several
fractures in one spring. In contrast, most ordinary fatigue
failures are at one position only.
A similar story can be told about suspension springs for
cars. These springs are more susceptible to failure in some
countries than others. The most frequent failure mechanism
is stress corrosion cracking or corrosion fatigue. In countries
where the weather is hot and dry, the paint protection may be
sandblasted off, but the springs last for the life of the vehicle.
In hot and humid countries, the paint fi nish remains intact and
the springs last very well. In countries where the weather is
cool and wet, and where salt is used on the roads in winter, the
risk of failure occurs once the paint fi nish has been penetrated.
This is why OEM specifi ers require especially thick paint today
that will last > 720 hours in salt spray testing. However, once
the spring starts rusting the risk is always there, especially if
the strength of the steel is high. Tests at IST have shown that
suspension springs last three times as long before they fail by
a stress corrosion mechanism if the spring steel hardness is
reduced by 50Hv.
Mark Hayes is the Senior Metallurgist
at the Institute of Spring Technology
(IST) in Sheffi eld, England. Hayes manages
IST’s European Research Projects,
the spring failure analysis service, and
all metallurgical aspects of advice and
training courses given by the Institute.
Readers are encouraged to contact him
with comments about this column, and
with subjects that they would like to
be addressed in future installments, by phone at (011) 44
114 252 7984 (direct dial), fax at (011) 44 114 2527997 or
e-mail at firstname.lastname@example.org.