Catastrophic Issue

[HaZarD SFD]

wow even the spring guide is cracked at the base! look near the screw.

[Nervikaire]

Same thing happened to me a while back... a Ver.2 tho: http://www.pbase.com/nervikaire/image/53025399

The reason of the break, in my case, was totally external... Slipped on a spot of ice, didn't have time to put the stock on my chest, so it was the first thing to hit the ground. The screw holding the stock cracked the gearbox. I didn't realize it at first so I got back up and started shooting again... Major failure hehe

[shadow_matter]

Illusion the piston head is not a stock Marui. Well to be fair, I have put over 35,000 rounds through the gun. So I guess it was inevitable. I'm just glad that it was the mechbox case and spring guide versus something really important and expensive. I ordered the parts from airsoftparts.ca. They are great guys to deal with, I told him the story and he shipped out the product express as soon as the post office opened this morning.

[vondnik]

Originally Posted by MadMax

Ouch. First busted V3 box that I've heard of now.

Where pics?

nope I was the first to bust one about 3 years ago.39k in a gen 3 is low, took about 300k to stress fracture mine.



ANd I've seen this kind of damage before, usualy what happens is one of the holding tabs of the spring guide breaks and the guide then gets smashed by the piston on the way back....

[Spa]

is that a aluminum piston? arnt those bad on the gears? and wouldnt you rather just have a poly 20$ one that can be easily and cheaply replaced after a season?

[vondnik]

Originally Posted by Spa

is that a aluminum piston? arnt those bad on the gears? and wouldnt you rather just have a poly 20$ one that can be easily and cheaply replaced after a season?

Ya it's an aluminium piston and piston head. Why would I want a 20$ poly that i have to replace 1-2 time a year when I got about 250k bbs in 3 years on this one and as far as I know it's still going strong ( so are the gears acually). 60% of my guns ran aluminium, steel or titanium pistons. If not they used the good old TM stock one ( witch IMO is better and cheaper that 90% of the aftermarkets ones)

[Dark-Angel]

wild guest like this, can it just be manufacturing defect. Seen how clean the break is, i see happen when i was working at mcdonald, a guy took the grill scraper that was in metal, he hit the frige not very hard with the scraper and it broke clean in half like in the pic. All so happened to my punching bag when some one kicked it and one of the hooks broke clean.

might be the reason why most essential metal structures and stress tested befor used

[ancorp]

Originally Posted by roughshadow

From all the working models of guns I can't see dryfiring being any more harmful then with ammo. It still cycles the same way.

However shooting a gun is slightly harmful to the gun. But it was meant to be shot so its only considered wear and tear.

Reputable gun pros if i am wrong please enlighten me on this.



As for the mechbox funny brake. It may have been just a bad mechbox.

Wrong. The piston slams harder and faster forwards due to no resistance in the barrel, due to the lack of ammo.

[The Saint]

Or it could be as vondnik said, the spring guide failed first and proceeded to kick the crap of the back of the gearbox. That sounds like a better explanation than the back of a version 3 simply giving out.

[MadMax]

I was thinking something along those lines, but if you really look at how a spring guide sits in a mechbox it's difficult to load the centre of the cross bar as it bears directly on the inside of a mechbox. The hole thru the bar reduces the cross section that so much that I doubt there's enough material to make much a difference in terms of strength in spreading load to the mechbox. The bar looks like it'd yield until the centre of the spring guide bears directly on the box, a kind of load the box is supposed to easily tolerate.

I suspect that there may have been a casting flaw which caused the rear to fail first. This would remove any support for the crossbar so it'd pop in short order. It's possible to tell where the crack ended. Look for two small sharp bent lips on the mechbox. Two small bits of yielded material can indicate a latest region of failure. Fatigue cracks start typically at a flaw like a void or microcrack and propagate until there isn't enough material to support the cyclic load. This region usually has a profusion of parallel ridges caused by incremental crack growth with each fatigue cycle.

At some point the crack grows far enough that the remaining material cannot support another fatigue cycle so you get a fast failure. This typically looks like a glassy surface which ends with two small sharp yielded points where the last material bent out and tore when the chunk falls off. This bent sharp bit is particularly useful for showing where a crack ended as it's often pretty obvious. It may be present on the larger fragment (mechbox) or smaller (bit that fell off).

A failure analysis (with a good magnifying glass or other optics) is necessary to really get to the cause of this failure. Without it it's difficult to discern cause from effect.

[Jayhad]

Originally Posted by MadMax

I was thinking something along those lines, but if you really look at how a spring guide sits in a mechbox it's difficult to load the centre of the cross bar as it bears directly on the inside of a mechbox. The hole thru the bar reduces the cross section that so much that I doubt there's enough material to make much a difference in terms of strength in spreading load to the mechbox. The bar looks like it'd yield until the centre of the spring guide bears directly on the box, a kind of load the box is supposed to easily tolerate.

I suspect that there may have been a casting flaw which caused the rear to fail first. This would remove any support for the crossbar so it'd pop in short order. It's possible to tell where the crack ended. Look for two small sharp bent lips on the mechbox. Two small bits of yielded material can indicate a latest region of failure. Fatigue cracks start typically at a flaw like a void or microcrack and propagate until there isn't enough material to support the cyclic load. This region usually has a profusion of parallel ridges caused by incremental crack growth with each fatigue cycle.

At some point the crack grows far enough that the remaining material cannot support another fatigue cycle so you get a fast failure. This typically looks like a glassy surface which ends with two small sharp yielded points where the last material bent out and tore when the chunk falls off. This bent sharp bit is particularly useful for showing where a crack ended as it's often pretty obvious. It may be present on the larger fragment (mechbox) or smaller (bit that fell off).

A failure analysis (with a good magnifying glass or other optics) is necessary to really get to the cause of this failure. Without it it's difficult to discern cause from effect.

SORRY to go slightly off topic, and looking like I am on MadMax's dick, but dude everytime you type you educate..... thank you

[Dark-Angel]

Originally Posted by Jayhad

SORRY to go slightly off topic, and looking like I am on MadMax's dick, but dude everytime you type you educate..... thank you

i second that. by any chance do you have any pics to describe what your saying, because what your saying doesnt make much sence since english isnt my primary language.

[ILLusion]

Here's the translation from MadMax-lese to simple English:

Gearbox originally made with a slight crack in the casting
Repeated stress cycles on the flaw caused the crack to increase in size
Finally, one last stress impact caused the crack to completely break free, causing the rear or the gearbox to break off.

[MadMax]

well, don't forget:

-after cracking, poorly supported spring guide would have broken easily
-do some fatigue surface analysis to determine if breaking mechbox is cause of spring guide failure or spring guide is cause of mechbox failure (differentiate cause-effect)
-without fatigue failure analysis, all we can do is pull ideas out of our ass

One thing comes to mind: If the spring guide failed well before the mechbox (SG failure is cause of mechbox failure), then there should be some significant evidence of wearing on inside of the fallen off chunk of mechbox. Very heavy cyclic scratching on the broken off bit would indicate that it was bearing hard against the damaged SG for many cycles before it broke off. If there is little evidence of cyclic wear, then the SG was not scrubbing the busted off piece before it broke off. Therefore, the SG could not have been loose before the piece broke free.

i.e.: if the when the piece falls off, there can be no significant force between the SG and the inside of the busted piece. Therefore the evidence or lack thereof of heavy scratching would indicate if heavy uneven bearing forces were bourne by the piece before it failed thereby indicating what failed first (SG or mechbox)