Cautionary Tale: Springs Everywhere

by Mark Hayes

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 different locations.

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 m.hayes@ist.org.uk.

Contributed and first published by Springs Magazine (Summer 2004)

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