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-   -   Does static tension wear out a spring? (http://www.physicsforums.com/showthread.php?t=661309)

codybias Dec28-12 05:30 PM

Does static tension wear out a spring?
 
Alright, here's the context of my question:

I have a pistol for home defense, and I keep a magazine loaded with ammunition in case I ever had to use it. Otherwise, I'd have to scuffle around and load the magazine before it could be used for its intended purpose, and that would sort of eliminate the advantage of having the pistol in the first place. I am concerned that the magazine spring, being under constant tension due to it being loaded, will have a tendency to "set" and lose elasticity. This would cause it to have insufficient pressure to push rounds into the chamber as the weapon is discharged, or initially racked.

I've done quite a bit of research on gun-owner forums regarding this issue. However, answers on these forums are commonsensical, and not substantiated by physics. In most instances, answers are one of three possibilities:

1. It is cycles of tension (loading and unloading) that cause the most damage to the spring (not constant tension). So don't worry about keeping the magazine fully loaded.

2. Constant tension will cause the spring to set. As such, you should cycle your magazines every few months.

3. Cycles of tension (loading and unloading) cause the most strain on a string [due to metal fatigue], but heavy tension also causes strain. As such, you should keep your magazine loaded, but not "fully" loaded (i.e. download a few rounds).

Right now, I have chosen answer "3" above as the best solution, but I'd like to know what your answer is.

Here's some additional information:

a. The spring is under some tension regardless of whether the magazine is loaded or unloaded. Specifically, when the magazine is disassembled for cleaning and maintenance, and the spring is taken out and allowed to expand to its full length, it is about 50% longer than it is in the magazine's unloaded state. It is compressed even further in the magazine's loaded state. (I understand that spring tension probably increases exponentially as spring length decreases linearly so that's why I elected to chose answer "3" as a compromise)

b. The exact chemical composition of the spring is probably proprietary (I couldn't find it anywhere), but most sources say it is some form of steel. Here's a link to the exact spring I use in my pistol's magazine ( http://glockstore.com/pgroup_descrip...e%2BSprings%2F ). I understand that the fundamentals of spring mechanics won't change with the material, but I figured if you had a better idea as to the chemical makeup of the spring you would probably be able to provide a more accurate answer.

So, all in all, does having the magazine spring constantly under tension wear it out? And what is the best way (in regards to longevity of the magazine and reliability of the weapon to fire) to keep the ammunition loaded?

A.T. Dec28-12 06:31 PM

Re: Does static tension wear out a spring?
 
Quote:

Quote by codybias (Post 4211249)
Right now, I have chosen answer "3" above as the best solution, but I'd like to know what your answer is.

Get a revolver.

AlephZero Dec28-12 07:04 PM

Re: Does static tension wear out a spring?
 
Creep and fatigue are well understood, and there is no particular problem designing something with a creep life of tens of years, or a fatigue life of millions of cycles, if that is what is required. Some materials have "infinite" life, provided the stress never exceeds a critical value.

If the spring is properly designed for what it has to do, it will last as long as the rest of the gun mechanism. If it isn't, it might not. There's not much more one can say about it, without getting into the specifics of a particular design.

Tayrantula Dec29-12 01:12 PM

Re: Does static tension wear out a spring?
 
There are a few things to consider:

The phenomenon known as creep, as mentioned above, only affects materials at or above ~0.4x their melting point, in absolute temperature (kelvin). This is unlikely to be an issue in regular service unless your springs are made of something absurd like lead, which actually creeps are room temperature. (I am assuming the temperature increases as a result of firing the weapon are small)

Stress-strain cycles, on the other hand, play a major role in spring wear. Ferrous material like iron and most steels exhibit an infinite lifetime under a particular amount of stress amplitude - not the absolute stress, which is generally far less - (the so-called "fatigue limit"). Less ductile materials like aluminum and titanium have a finite cycle life regardless of the stress amplitude; however, parts designed with these materials generally have lifetimes in the millions of cycles and fail by different modes long before the lifetime is reached.

So obviously, the life of the spring depends on proper design and materials choice. The spring steel that your gun would most likely use is a moderately-high carbon steel, with potentially nickel, silicon and manganese alloying agents in small quantities. Properly designed, it would last far longer than the other components of the gun that are regularly undergoing thermal stress, diffusion, and much larger fatigue cycles.

It's safe to say that storing your mag in a properly designed gun, will not wear out the spring prematurely. However, removing 1 or 2 rounds would increase the odds that you are maintaining the spring stress below the critical fatigue limit.

As a footnote, springs in regular circumstances follow Hooke's Law, which states that F=-kx (k being a materials, or "spring" constant and x being displacement). Thus, spring force is linear to displacement. I saw this forum post from an article on the main page, and as a 4th year materials engineer student I couldn't resist signing up to answer your question. Hope this helps!

codybias Dec29-12 06:54 PM

Re: Does static tension wear out a spring?
 
Thanks for the detailed response. I sincerely appreciate it!

codybias Dec29-12 06:56 PM

Re: Does static tension wear out a spring?
 
Quote:

Quote by A.T. (Post 4211282)
Get a revolver.

I thought about it. Reliability vs. Ammo Capacity. It's a long standing debate amongst gun owners.

russ_watters Dec29-12 07:46 PM

Re: Does static tension wear out a spring?
 
Welcome to PF, both of you! Great question and great answer!


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