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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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Metal fatigue, springs
Replaced the door hinge assemblies in SWMBO's oven (purchased oven used). Made big difference. No longer need to push chair against oven door.
Questions about springs. These are 8" long, 1/2 inch diameter, 1.25 inches elongation when in use. My understanding from my 'strength of materials' classes 50+ years ago is that metal does not deform until you reach the elastic limit. I would guess that most springs in normal use NEVER reach that elastic limit of metal. This would include automotive valve springs, screen door springs etc. So how, when and why does a spring actually weaken. I need an education, all responses welcome. Ivan Vegvary |
#2
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Metal fatigue, springs
On Sat, 11 May 2013 15:02:17 -0700 (PDT), Ivan Vegvary
wrote: Replaced the door hinge assemblies in SWMBO's oven (purchased oven used). Made big difference. No longer need to push chair against oven door. Questions about springs. These are 8" long, 1/2 inch diameter, 1.25 inches elongation when in use. My understanding from my 'strength of materials' classes 50+ years ago is that metal does not deform until you reach the elastic limit. I would guess that most springs in normal use NEVER reach that elastic limit of metal. This would include automotive valve springs, screen door springs etc. So how, when and why does a spring actually weaken. I need an education, all responses welcome. Ivan Vegvary In the case of an oven door, it usually weakens from corrosion that's accelerated by heat. But your more pertinent question has to do with the phenomenon of fatigue, as metallurgists use the term. It's tricky and it's interesting, as an insight into the differences among metals. Fatigue failure is the result of repeated cyclical loading, often through thousands or even millions of cycles, at or even BELOW the elastic limit of the metal. That's the nastiest kind of fatigue, because you have to know the load and you have to understand the material. Aluminum, for example, has just a fraction of the fatigue strength of steel. Thus, in aircraft, it's something they watch out for all the time. Wherever a part is cyclically loaded to a high percentage of its yield strength, as with a spring, fatigue is an issue, and a frequent cause of failure. Wikipedia has a a pretty decent explanation. Yes, it really is that complicated: http://en.wikipedia.org/wiki/Metal_fatigue Part failure that occurs because of cyclic loading *above* the elastic limit actually is quite different. -- Ed Huntress |
#3
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Metal fatigue, springs
On Sat, 11 May 2013 15:02:17 -0700 (PDT), Ivan Vegvary
wrote: Replaced the door hinge assemblies in SWMBO's oven (purchased oven used). Made big difference. No longer need to push chair against oven door. Questions about springs. These are 8" long, 1/2 inch diameter, 1.25 inches elongation when in use. My understanding from my 'strength of materials' classes 50+ years ago is that metal does not deform until you reach the elastic limit. I would guess that most springs in normal use NEVER reach that elastic limit of metal. This would include automotive valve springs, screen door springs etc. So how, when and why does a spring actually weaken. I need an education, all responses welcome. Ivan Vegvary In an oven application, due to heat. Or poor materials. |
#4
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Metal fatigue, springs
Ivan Vegvary wrote in
: Replaced the door hinge assemblies in SWMBO's oven (purchased oven used). Made big difference. No longer need to push chair against oven door. Questions about springs. These are 8" long, 1/2 inch diameter, 1.25 inches elongation when in use. My understanding from my 'strength of materials' classes 50+ years ago is that metal does not deform until you reach the elastic limit. I would guess that most springs in normal use NEVER reach that elastic limit of metal. This would include automotive valve springs, screen door springs etc. So how, when and why does a spring actually weaken. I need an education, all responses welcome. As others have stated, it's probably due to the environment rather than the design of the spring per se. There's always lots of discussion in firearms circle about whether magazine & hammer springs fatigue. IF they are properly designed & manufactured, they shouldn't. That said, I have an ar-15 based target rifle with a highly regarded (and expensive) after-market trigger. The hammer spring will weaken over the course of ~ 9 months if it is stored cocked. At that point, it will no longer reliably fire military ammo, which tends to have harder primers. If you compare it with a new spring, it definitely deformed a bit under the constant tension. The manufacturer of the trigger makes their own springs, and I suspect they do not fully understand the finer points of spring fatigue. Doug White |
#5
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Metal fatigue, springs
Ed, thanks for your reply and the wonderful wiki link.
Ivan |
#6
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Metal fatigue, springs
On Sun, 12 May 2013 13:38:50 GMT, Doug White
wrote: Ivan Vegvary wrote in : Replaced the door hinge assemblies in SWMBO's oven (purchased oven used). Made big difference. No longer need to push chair against oven door. Questions about springs. These are 8" long, 1/2 inch diameter, 1.25 inches elongation when in use. My understanding from my 'strength of materials' classes 50+ years ago is that metal does not deform until you reach the elastic limit. I would guess that most springs in normal use NEVER reach that elastic limit of metal. This would include automotive valve springs, screen door springs etc. So how, when and why does a spring actually weaken. I need an education, all responses welcome. As others have stated, it's probably due to the environment rather than the design of the spring per se. There's always lots of discussion in firearms circle about whether magazine & hammer springs fatigue. IF they are properly designed & manufactured, they shouldn't. That said, I have an ar-15 based target rifle with a highly regarded (and expensive) after-market trigger. The hammer spring will weaken over the course of ~ 9 months if it is stored cocked. At that point, it will no longer reliably fire military ammo, which tends to have harder primers. If you compare it with a new spring, it definitely deformed a bit under the constant tension. The manufacturer of the trigger makes their own springs, and I suspect they do not fully understand the finer points of spring fatigue. Doug White There are a couple of factors that lead to sress-relaxation in springs kept under load. Creep is one of them. I forget the other. In any case, it isn't fatigue, as metallurgists and other engineers apply the term. -- Ed Huntress |
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