Safety in spring design and within the spring manufacturing process can take many forms, and a
Cautionary Tale on this subject is by its nature a summary of points raised during the last 50 Cautionary Tales written for Springs. Each article contains advice about how to stay safe in the spring industry and how to avoid pitfalls that the Institute became aware of in the normal course of business.
As pressure grows from both spring end users and from material supply costs, most spring manufacturers have reduced costs through improved designs and weight reductions. Testing spring materials has increased, with the knowledge of fatigue life and relaxation characteristics now available. A number of spring design software packages exist and are continually being updated to provide the design engineer with an optimized design. These are vital to reducing the pre-production phase on any new design, but the nature of the beast is leading towards life cycle testing of the final design.
This is increasingly important so that the spring user can be confident the design will meet the safety critical application for which it is intended. It is worth mentioning here that material suppliers are equally aware of this aspect and continue to improve the material characteristics for these applications. Springs remain a vital part of many safety critical designs and their range of working environments continues to expand; from deep sea applications to outer space, within some highly corrosive atmospheres and with temperature sensitive applications.
Some IST staff members have been invited to visit manufacturing plants during our visits to countries where health and safety concerns are not top priorities, and safety inspections are virtually unknown. During one visit, we watched the coiling of springs on a coiler. When the wire ran out the next coil was brought and threaded up. The setter had not used the machine guard so far, and it looked as if it may never have been used. So the setter was surprised that IST insisted on its use. Reluctantly the setter did as he was asked, but didn’t immediately ask why.
The first two springs coiled okay and the setter again said the guard was not necessary. But IST continued to insist on its use, and rightly so, because the third spring broke and a piece of it hit the guard, which did its job – saving the operator and observers from serious injury. The setter then asked how we knew there was a risk, as he had inspected the tools on his coiler, which, surprisingly, were undamaged. He threaded up the wire and continued to use it without further mishap, and during a subsequent visit two years later, we noticed that he was still using the guard, but that none of his fellow setters did. Even though some equipment designs have been updated to include the latest guarding requirements, these requirements continue to be improved and refurbishment of older equipment is often preferred to new purchases.
Safety on the shop floor is one thing, but many springs are in safety critical applications which need to operate as designed. The safety relief valve is the most obvious one to use to demonstrate this aspect. Relief valves used to be simple heavy weights that would lift if the pressure inside a vessel exceeded the set value. English railway engineer John Ramsbottom observed that these valves could be tampered with by adding extra weights or locking them down to get a bit more performance on steam trains. Realizing the safety risk, in 1856 he invented the tamper-proof device shown in Figure 1. This remains an important market for springs today.
The moral of this Cautionary Tale: Stay safe while designing and manufacturing components that will keep others safe.
Mark Hayes is Technical Advisor to the Institute of Spring Technology (IST) in Sheffield, England. He is also the principal trainer for the spring training courses that the Institute offers globally. Readers are encouraged to contact IST with comments about this cautionary tale, and with subjects that they would like to be addressed in future tales, by email email@example.com.