HPT and MPI testing. Why?

[shadowspirit]

People don't understand CS spring are pointless in a firearm. CS springs are no better than music wire, unless the environment is above 350° F. They are great for valve springs in engines because the internal temperature can be that high and since they live in a oil bath corrosion is not an issue.

Music wire and CS are better than stainless steel for life, but the benefits of corrosion resistance far outweigh that.

Aren't CS springs much better for life(number of compression/decompression cycles) than stainless and music wire? As for corrosion resistance, just spray/wipe some oil or other lube/protectant on it once in a while.

[shadowspirit]

Why forcing a spring to go solid is bad:

The first reason is impact. In cyclic compression, the coils gain speed in the direction of compression, even if the force compressing the spring stops short of the solid length, the momentum of the coils can carry them further back until they bottom out. This can physically distort the coils depending on the speed. The other reason is the maximum twisting stress allowable. When a spring goes solid, its stress has usually gone past the fatigue limit*; depending on the spring design this can happen well above solid. If the stress exceeds the yield limit, the spring will take a permanent set.

If space is limited, flat wire springs work extremely well, however, you can still exceed maximum stress levels if you aren't careful. The only major problem with flat wire springs, aside from their expense, is you cannot get really stiff springs without high OD/ID ratios, or very thick sections (square profiles). Flat wire spring are generally less stressed. So, to the question I would say that flat wire springs offer many benefits at the sole disadvantage of cost.


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* Steels have a fatigue limit, if the stress can be held below this value, the item can endure an infinite number of loading cycles. If the stress is above the fatigue limit, but still below the yield strength, cyclic loading will be high-cycle fatigue, with the number of cycles in the 10^6 range. If the stress is above the yield, but still below the ultimate limit, it will be subject to low-cycle fatigue with the number of cycle in the 10 to 10^3 range.

Is loading a 30 round AR mag to 28 rounds instead of 30 and example of the above explanation? Will loading 28 rounds give the spring substantially longer life?

[shadowspirit]

The other place than CS is better than if the wire diameter is over 0.080", interestingly, the diameter of the action spring is 0.072", right at the crossover point.

If you make three action springs, one from 17-7 stainless, one from music wire, and one from chrome-silicon, here is what you get: (note, my spring calculator won't even bother to design a CS spring smaller than 0.080", so all have a wire diameter of 0.080")

OD: 0.995"
Wire Dia - .080"
Free length - 10.70"
Total coils - 38
Active coils - 36
Ends - closed

Material . . . . . . . . . . . . . . . . . . Music Wire . . . . . . . . . . . . Chrome-Silicon . . . . . . . . . . . 17-7 Stainless
Rate . . . . . . . . . . . . . . . . . . . . . 2.134 lb/in . . . . . . . . . . . . . . 2.134 lb/in . . . . . . . . . . . . . . . 2.038lb/in
Maximum Load . . . . . . . . . . . . 16.174 lb(f) . . . . . . . . . . . . . .16.174 lb(f) . . . . . . . . . . . . . . 15.447 lb(f)
Max Shear Stress . . . . . . . . . 133,480 psi . . . . . . . . . . . . . 133,950 psi . . . . . . . . . . . . . . 120,790 psi

If all we were worried about were the "springy qualities" of these, there is no clear winner, none of these are appreciably "better" than the others. However, if you throw in corrosion resistance, they rank (1) Stainless, (2) Music Wire, and a distance third (3) Chrome-Silicon. Yes, the stainless has a lower maximum shear allowable, but as soon as you get a corrosion pit in the others, that number will drop like a rock.

How much more compression cycles can the CS springs last if corrosion is kept at bay? 2X? 3X? 4X?

[1168]

How much more compression cycles can the CS springs last if corrosion is kept at bay? 2X? 3X? 4X?

How often do you wear out a stainless or music wire spring? I actually wear these out more than most, but not particularly often.

“Just put oil on it” is silly, because the action spring is not a place where lubrication would be required if we could just stop tinkering. Saying that it’s the solution, is admission that there’s a problem, and I’m saying that problem doesn’t need to exist, because CS springs don’t solve any problem.

BTW, I used a CS spring yesterday in a T&E gun. The reward was failures to lock back with a Velos.

[Uncas47]

Is loading a 30 round AR mag to 28 rounds instead of 30 and example of the above explanation? Will loading 28 rounds give the spring substantially longer life?

It loads easier on a closed bolt. As far as lifespan, magazine springs are easily replaced along with any other spring in the gun.

[lysander]

Aren't CS springs much better for life(number of compression/decompression cycles) than stainless and music wire? [1] As for corrosion resistance, just spray/wipe some oil or other lube/protectant on it once in a while.[2]

1) No, they are not. Unless you are comparing them in a high temperature environment, chrome-silicon spring are no different than music wire, and only marginally better than 17-7 stainless*.

2) The driver here seems that everybody wants to to have the longest lasting spring. The stress on a spring is highest on the surface of the spring wire. So, any cracks, flaws or other damage to the surface of the wire will be the initiation point for fatigue cracking. Chrome-silicon spring are not very corrosion resistant, so any small corrosion spot will start a fatigue crack.

The longest lasting spring would be a stainless steel spring. In a test of 20 Carbines and 3/4 of a million rounds, not one action spring failed.

Now, if you want to play with spring rates, and pre-loads, and C-S is the only offering available, that's fine, they will work just as well as anything else and last a long time if you take care of them. They might even be better because you can get the spring rate you want, but don't try and convince everyone they are better springs because they are C-S. They aren't.

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* Careful, there are two popular types of stainless steel for springs, 17-7 and 302/304. The 300 series stainless steels work harden only and not nearly as good as 17-7 stainless.

[lysander]

How much more compression cycles can the CS springs last if corrosion is kept at bay? 2X? 3X? 4X?

ZERO.

In a room temperature environment (0° to 200° F), and spring wire diameter less than 0.080", there is no difference between chrome-silicon and music wire. 17-7 stainless does not have quite as high a yield strength as music wire, it is strong enough, and even gets you a little of that high temperature operational envelope back.

[1168]

* Careful, there are two popular types of stainless steel for springs, 17-7 and 302/304. The 300 series stainless steels work harden only and not nearly as good as 17-7 stainless.

do the 300 series ones have a tendency to look silvery and be inexpensive? I think I might have one in my “wtf is this” box.

[lysander]

Is loading a 30 round AR mag to 28 rounds instead of 30 and example of the above explanation? Will loading 28 rounds give the spring substantially longer life?

Not really. The difference in spring compression between 30 and 28 rounds is less than 3/8 inch, still far too close to solid for the spring's health.

[lysander]

I don't know is anybody makes action springs (or any AR springs) out of 302, or similar.

The action spring (and all the other SS springs in a AR) are supposed to have a passivated finish. To passivate SS you dip the part in a acid and etch off any free iron atoms on the surface, this make the parts more corrosion resistant. It also tends to make the surface look frosty.

Consumer 302 stainless stuff really does not get passivation treatment, as it is an expense. Your springs may be 302 or 17-7 and not passivated, which would make it shiny in appearence.