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    Spring Failure Analysis

In our latest edition we offer some insight into spring failure analysis with
Spring Expert, Mark Hayes B.Sc., C.Eng., MIMMM.

Advanex EGST When our engineers receive a report on a failed spring the first question they should ask is "Does the report take into account all details of the raw material, spring manufacture and its use?". Of course, there are other questions like "Did we make it?", or "How did it get like that?", but the first question is fundamental. Whoever made it, and the circumstances causing its appearance to become altered are important, but the first need is to understand all details of the failure, and not to be side-tracked by headline features of the report.

Failure Report

A good failure analysis report will have looked at all the circumstances that lead to its failure to function, and will not have been written with a view to protecting the interests of the author, be that a material supplier, spring manufacturer, or the end user.

Assuming the spring service conditions are fully understood, the dimensions are correct, and the material is the one specified, the seven main criteria that the report should reveal are: Advanex EGST

A) Does the raw material conform to its supply certificate (if available) or specification?
B) Are the spring dimensions consistent with the drawing, albeit noting that some dimensions may have changed prior to recognition that the spring was not functioning correctly?
C) Are there marks on the spring from spring manufacture or use that could be important?
D) Is the appearance consistent with having been manufactured with all the required processes?
E) If the spring has fractured, then is the fracture shape consistent with the normal use, and is the fracture origin at the usual position of maximum stress? How many fractures were there?
F) What was the mechanism of failure?
G) Is the spring design capable of withstanding the service conditions?

Diagnosing The Cause Of Failure

It is all too easy to draw incomplete and inappropriate conclusions from the results shown in the report. Each member of the supply chain will, understandably, be anxious that they were not responsible for causing the failure, but our engineers know that an objective view needs to be taken.

The report might record that the hardness of the raw material was not exactly that expected so the raw material was the root cause of failure. This would only be an appropriate diagnosis if wrong hardness could only have arisen from the raw material and could not have arisen in springmaking or in service, and the wrong hardness could be directly attributable to the failure mechanism.

Advanex EGST The report might record evidence of marks on the spring surface from spring manufacture, so maybe this was the root cause. This would be an appropriate diagnosis only if the fracture initiated directly from or adjacent to this mark, and the mark was severe enough to be a stress raiser. Making springs without tool marks is impossible, but tool marks should be rounded and well blended in profile, and not sharp.

The report might observe evidence of slight corrosion from service. Again, this could be the root cause of failure, but only if the spring material is very susceptible to deterioration of performance due to active corrosion. Springs made from drawn carbon steel often go very rusty indeed and continue to function correctly.

Root Cause

Spring expert Mark Hayes has written spring failure reports for more than 30 years, and estimates that the root cause of failure is attributable to:
  • Raw material faults in about 15% of investigations.
  • Spring manufacturing faults occur about 20% of the time.
  • Spring design faults in half of all cases, and it is nearly always the end user who takes responsibility for the design.
  • Another 5% of investigations are inconclusive, but that leaves a few percent in which the service conditions were not foreseen, and some springs are abused.

This article provides a logical approach to the interpretation of spring failure reports. It also makes clear that an objective view of all details and not just the headlines is required to arrive at a correct diagnosis of the root cause of failure. Once the root cause is known, then remedial action becomes feasible and this should significantly reduce the chance of future failures.
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