Uzi Extractor

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Generalzip

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I'm a structural engineer with a master's degree in civil engineering.

Let me think on this for a tad.

Here we have a spring (UZI extractor) that's left in its little hidey hole in the bolt for a decade or two.

If it exerts less sideways force on the 9mm case than it did originally, that would be stress relaxation.

If you take it out and it has a different shape than it originally did, it's less bent, that's creep.

Can you have stress relaxation in a spring without creep? What would be an example of that?

If you leave a gun magazine loaded for 20 years and you take the spring out, the spring will definitely be shorter than it was when it was put in the mag. With the creep accounting for the lower force exerted on the follower.
I mean if you want to measure dicks I have a degree in Course 3 (materials science and engineering) from MIT. Wasn't trying to attack you in any way. Just putting out the correct information.

Your example of the magazine spring is stress relaxation because a stored compressed spring does not undergo any change in displacement aka strain. The dimensions of the mag haven't changed over time. Now if you hung a 1kg weight on the same spring it would measure for purely an example 10" long. Come back in 20 years and it now measures 11". This is an example of creep because your strain has changed while under a constant stress load F=ma=(1kg)*g=9.81N

Again, it's very common for people to use the term creep to describe any change to a material because relaxation is not as common as creep in most industrial applications so don't feel bad. Regardless, fatigue is probably going to kill your moving parts before creep or relaxation. Weak coil springs can often be fixed just by just stretching them out a bit more. At least I've done this in the past on really old/used mags with no ill effects. The extractor is quite a bit different as its not really a true "spring" in the classical sense with F=kx.
 
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Dirk Hawthorne

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@Dirk Hawthorne I truly and honestly wish you the best.

I am capable of a superhuman feat: I can discuss engineering topics without getting touchy, and admit that I'm wrong when the occasion arises.

I'm referred to by the Guinness Book of World Records as "The Astonishing Miracle Man, An Amazingly Impossible Being."
 
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Dirk Hawthorne

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I mean if you want to measure dicks I have a degree in Course 3 (materials science and engineering) from MIT. Wasn't trying to attack you in any way. Just putting out the correct information.

Your example of the magazine spring is stress relaxation because a stored compressed spring does not undergo any change in displacement aka strain. The dimensions of the mag haven't changed over time. Now if you hung a 1kg weight on the same spring it would measure for purely an example 10" long. Come back in 20 years and it now measures 11". This is an example of creep because your strain has changed while under a constant stress load F=ma=(1kg)*g=9.81N

Again, it's very common for people to use the term creep to describe any change to a material because relaxation is not as common as creep in most industrial applications so don't feel bad. Regardless, fatigue is probably going to kill your moving parts before creep or relaxation. Weak coil springs can often be fixed just by just stretching them out a bit more. At least I've done this in the past on really old/used mags with no ill effects. The extractor is quite a bit different as its not really a true "spring" in the classical sense with F=kx.

I'm not measuring phalluses, I was just making it known that the subject is not completely alien to me. Nor did I think you were attacking me. Nor am I playing the hero of engineering.

Being proved wrong doesn't bother me. And I always give credit where it's due. After all, why not? Why be a baby about things?

Creep and strain relaxation are just variations on a theme, aren't they? A material has mechanical or heat energy stored in it and it uses the energy to restructure the lattices a little. In one case, the material is allowed to change dimensions, in the other case it isn't.

If you stick a load of ammo in a mag and leave it for 30 years, and you remove the spring from the mag, that spring is going to be shorter than the original length when was originally put in the mag. That is plain old creep. And that creep would account for the lower force, because the spring is shorter and it's deflecting less.

I actually had a Thompson mag from 1921 that was so creep-o-gized that the follower wouldn't even go up to the feed lips anymore, it stayed at halfway down the mag. If that's not creep, nothing is.

I don't see how the extractor in an UZI is not a spring in the classic sense. If it's not a spring, then what mechanical category is it? If it's not operating on F=kx, then what's powering it? The material in the extractor will not be aware that it's not a classic spring shape.

When an UZI extractor gives up the ghost, I suppose that it could be either stress relaxation or creep. If the extractor was pulled out of the bolt and it was less bent than the original shape, then I would call it creep. If it's exactly the same shape as it was originally, then it's stress relaxation.
 
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Generalzip

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Again you’re close but it’s not creep. With an unloaded mag the length is X. With a loaded mag it is Y. Regardless of how long the spring sits in either scenario its length is ALWAYS X or Y. Aside obv from when being loaded or unloaded. So I understand you’re saying if you were to remove the spring and measure it then it would be shorter. That’s correct. BUT you’re not using a mag spring outside of the magazine. Because you applied a constant strain and the force of the spring reduced that is stress relaxation. It also results in some permanent (plastic) deformation you can measure once removed from the mag. Hope that helps. You can have plastic deformation from creep and/or relaxation. I think that’s what’s hanging you up.
 

Generalzip

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Specifically in the Uzi extractor it’s not a classic spring which follows F=kx. You are basically bending a rod. You’d have to used FAE to accurately analyze the stress throughout the part. Solidworks etc can do this. But it’s not using F=kx to solve that. I didn’t write solidworks code but I’d imagine it’s using many many iterations and integrations of the stress equation run over some proprietary model or Monte Carlo type simulation. A mechanical engineer here would know more about this than me I’m sure.

F=kx works because a spring is a simple repeating shape. An Uzi extractor is a complex geometry
 

CKxx

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Has anyone actually broken one? Mine has 15k rounds or so on it and it's going strong.
 

CKxx

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Specifically in the Uzi extractor it’s not a classic spring which follows F=kx. You are basically bending a rod. You’d have to used FAE to accurately analyze the stress throughout the part. Solidworks etc can do this. But it’s not using F=kx to solve that. I didn’t write solidworks code but I’d imagine it’s using many many iterations and integrations of the stress equation run over some proprietary model or Monte Carlo type simulation. A mechanical engineer here would know more about this than me I’m sure.

F=kx works because a spring is a simple repeating shape. An Uzi extractor is a complex geometry
Given the gun is from the 50s, you may be overthinking this.
 

Chef

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Maybe if we just stored our extractors out of the bolts, only installing them when we go to shoot them, it would prolong their life?
 

Dirk Hawthorne

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Given the gun is from the 50s, you may be overthinking this.

Of course we're overthinking it. That's what we do.
Again you’re close but it’s not creep. With an unloaded mag the length is X. With a loaded mag it is Y. Regardless of how long the spring sits in either scenario its length is ALWAYS X or Y. Aside obv from when being loaded or unloaded. So I understand you’re saying if you were to remove the spring and measure it then it would be shorter. That’s correct. BUT you’re not using a mag spring outside of the magazine. Because you applied a constant strain and the force of the spring reduced that is stress relaxation. It also results in some permanent (plastic) deformation you can measure once removed from the mag. Hope that helps. You can have plastic deformation from creep and/or relaxation. I think that’s what’s hanging you up.


Usually these things are more clear-cut: when you prestress a beam, the concrete beam shortens over time. That’s creep. At the same time, the force in the strands is reduced, independent of the concrete creep. That’s stress relaxation.

The dispute here is how the terms might be applied to a magazine spring, which is kind of a special case. Sort of.

1. As long as the spring is inside the mag and exerting force, you could rightly call the long-term reduction in force is an example of stress relaxation. Same strain, less force.

2. But the thing causing the reduced force is the fact that the spring is getting shorter. The spring used to be 12 inches long and now it’s 8 inches. That’s “creep.”

It’s hard for me to hold up a 1921 Thompson mag with the spring so shortened that the follower is halfway down the mag body and not call that an example of spring creep.

I can think of some decent counter arguments to what I wrote here, but I would tend to think of this magazine spring situation as creep, because the underlying reason for the reduction in load is the fact that the spring is getting shorter and deflecting less.
 

Dirk Hawthorne

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They didn’t have engineers in the 50s? Wild they built that bomb the decade before.

Structural and mechanical engineers are experts (hopefully) in a field of study called "mechanics of solids." That's the main applied physics that we use to do our job.

That science was pretty sophisticated a long time ago. This conversation we're having could have been had 200 years ago.

The big modern advancements are in Materials Science, like concrete technology, welding, etc. And of course the use of computer methods to crunch big, complicated problems.
 

Dirk Hawthorne

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Specifically in the Uzi extractor it’s not a classic spring which follows F=kx. You are basically bending a rod. You’d have to used FAE to accurately analyze the stress throughout the part. Solidworks etc can do this. But it’s not using F=kx to solve that. I didn’t write solidworks code but I’d imagine it’s using many many iterations and integrations of the stress equation run over some proprietary model or Monte Carlo type simulation. A mechanical engineer here would know more about this than me I’m sure.

F=kx works because a spring is a simple repeating shape. An Uzi extractor is a complex geometry

I realize that the extractor force is not going to conform to f=kx. But it is a spring in all respects, is it not?
 

Dirk Hawthorne

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Maybe if we just stored our extractors out of the bolts, only installing them when we go to shoot them, it would prolong their life?

It would be a waste of your time. We're talking about decades here.

If you stuck a new extractor into your bolt, you wouldn't probably see any noteworthy reduction in function for 30 years.

Mechanisms are designed to account for the 10% or 15% reduction in spring force that occurs during their normal service life.

The recoil spring in my UZI is noticeably softer now than when it was installed, from being heated (and violently loaded and unloaded?), but it still works fine. And the spring assembly has been compressed since who knows when? Probably 1970s.

Same with magazines. You can leave them loaded for a LONG time before you deform the spring enough to make it not work well.

That being said, I did have an extractor go limp on me. I'll put a wrench on it and rebend it, and report back.
 

Generalzip

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Of course we're overthinking it. That's what we do.



Usually these things are more clear-cut: when you prestress a beam, the concrete beam shortens over time. That’s creep. At the same time, the force in the strands is reduced, independent of the concrete creep. That’s stress relaxation.

The dispute here is how the terms might be applied to a magazine spring, which is kind of a special case. Sort of.

1. As long as the spring is inside the mag and exerting force, you could rightly call the long-term reduction in force is an example of stress relaxation. Same strain, less force.

2. But the thing causing the reduced force is the fact that the spring is getting shorter. The spring used to be 12 inches long and now it’s 8 inches. That’s “creep.”

It’s hard for me to hold up a 1921 Thompson mag with the spring so shortened that the follower is halfway down the mag body and not call that an example of spring creep.

I can think of some decent counter arguments to what I wrote here, but I would tend to think of this magazine spring situation as creep, because the underlying reason for the reduction in load is the fact that the spring is getting shorter and deflecting less.
Concrete is an aggregate composite. The phenomena you are referring to with concrete has literally nothing to do at all with creep or relaxation of metals and polymers which are fully or partially crystalline structures. The mechanisms by which creep is formed is completely different and not relevant to the conversation we are having.

Apart from that I don’t really want to repeat myself. If you disagree that’s fine. Just wanted to share my thoughts for others that come across this post.

Bottom line, if the exterior wears out, replace it. Would be great if someone started making more.
 

Dirk Hawthorne

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Concrete is an aggregate composite. The phenomena you are referring to with concrete has literally nothing to do at all with creep or relaxation of metals and polymers which are fully or partially crystalline structures. The mechanisms by which creep is formed is completely different and not relevant to the conversation we are having.

Apart from that I don’t really want to repeat myself. If you disagree that’s fine. Just wanted to share my thoughts for others that come across this post.

Bottom line, if the exterior wears out, replace it. Would be great if someone started making more.

You know that round thing inside your car? It's called a "steering wheel." If you turn it, your car changes direction of travel. No kidding, I wouldn't lie to you.

Mods, fear not, we are done here.
 
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