What caused this failure? (Piston stripped)

[Hectic]

Okay so I've been building "the ultimate dmr" lol and it's been nothing but a pain in the ass.
To the issue

The pick up tooth as you can see is nicely chopped and the fourth (third I guess as the first tooth is shaved from the factory) tooth was also stripped/broken) the other two teeth I shaved for clearance/aoe. Now what I'd like to know is what can cause such a failure. The gun seemed to run great (aside from some over spin from time to time) then it just quit. Here is the build.
Vfc 417 shell
Shs13:1 gears
Lonex a1 motor.
11.1 lipo.
Merf aab
All in all nothing super fancy. The only "odd ball" thing is its a high fps setup, should be sub 450fps but still high for the 30+rps I'm going for. So what I did was picked up the all metal lightweight shs piston (i hear they fit better in the vfc gearbox, I had to "slim out" the lonex piston) but before I stick it all together am I missing something? I have a sorbo and correct the aoe

[SuperCriollo]

Kinda looks like pre-engagement to me

[ThunderCactus]

Failed on the pickup side, so probably an aoe issue, or you shaved too many teeth down, so if its grqbbing right from 1st to 4th tooth, it doesnt pickup on the 4th until its letting go of the 1st, changing the aoe of that particular pickup.
You only ever need to shave off 2 teeth at the very most. 3 is way too far.

[Hectic]

Okay so the sorbo AND sorbo buddie is too thick (that's why I had to shave the third tooth me thinks)
Will remove, shave a seccond tooth off this shs piston and have at er.
I though at first looked like PL but then the perfect gear tooth shaped notch in the pick up tooth was making me think that that was the actual failure not so much the broken 4th tooth. So basically the sector is just holding on by the tip of the tooth and once the pointy bit is at the right angle it just shears the pic up tooth off like a blade.
So never more than 2 teeth thus always having 2 teeth or more share the load.

[lurkingknight]

what cylinder head are you using. and did you glue the sorbo to the rubber pad or did you remove the pad?

[pestobanana]

Issue is pre-engagement. Pre engagement stripped the 3rd and 4th tooth to the point that the sector gear could no longer engage them, and each gear would slip off the pick up tooth. Since the plastic pick up tooth was acting as a release tooth, it failed the way it did.

Your pre-engagement issue is most likely caused by misfeeding. You can't just slap those parts together and expect your gun to last.

[Hectic]

It is the stock vfc cylinder head, pom piston head (lonex or modify I think)
The pad was glued directly on and then the sorbo buddie. With it removed (the buddie) it looks as tho all will clear with half the 3rd (2nd after the pic up) shaved off
I didn't just slap them together lol. I took my time building it. (A lot of time actually this vfc gearbox is tight and my ocd made shimming it just right annoying as all hell lol)
And the third tooth was not "stripped" it was removed by me for AOE as it was ever so slightly engaging, I really only shaved half off of the 2nd and 3rd as that was all they needed to clear. However as was brought to my attention by shaving the 3rd tooth I caused my own problem. When the sector grabs the pic up and starts the piston reward it can only go so far before the other teeth have to start doing the work (sharing the load) right around the time the seccond tooth is fully seated the pic up tooth is barely hanging on by "the skin of its tooth" so to speak now you have a plastic/polymer pick up tooth with a metal tooth point pressing against its edge trying to hold a ~450fps spring thus shearing a nice gear tooth shaped notch in the pic up tooth and causing the failure. (Look closely at the pic up tooth) after holding the gear and piston together I can see exactly how it failed.
You are not incorrect it is pre engagement but it happened after the piston had alrdy failed. The pic up tooth was sheared off and the gears kept trying to pick up the piston until it finally tried to grab the 4th tooth and when the 4th to the failed the piston went forward and the sector gear was stopped about mid way through its cycle.

[lurkingknight]

sorbo buddy with the neoprene PLUS stock vfc cylinder head and pad is incorrect aoe.

[Hectic]

Well at the onset it looked good in respect to the pick up tooth to sector meshing but yes I see where shaving the extra tooth to compensate for the extra padding caused it to fail.
I have removed the sorbo buddie. Have to shave the seccond tooth a bit and then it will be good. Just packed it away before dinner gonna grab some more low res wire and do that while I have it apart.
I had never used the sorbo buddie before but because of the higher fps it was recommended to "protect the sorbo pad from wear" basically.
I'm sure in a setup like this (likely any 30rps+) something will fail or start to act up well before the sorbo pad gets worn out without the buddie so I don't see any issue with leaving it out.
I will say that shs piston does seen to fit quite nicely, it doesn't get hung up like the lonex one did before I shaved it down and (without weighing it) it say it's easily 20% lighter than the lightened lonex as well. Hopefully that will make the piston travel faster reducing pre engagement chances. Only thing that worries me is friction. I know it's not tight but I'd think the polymer pistons would be sort of slick but metal on metal should I use special lube? (i usually use madbull grease or super lube)

[pestobanana]

No. It failed due to pre-engagement. Pre-engagement damaged the third, fourth, and fifth teeth enough to be unable to engage in the distance the pickup tooth can be pulled. The pickup tooth would never fail this way from AoE. The slant you are seeing in the pick up tooth is from the gears skipping over it as they are unable to engage the other teeth. The pickup tooth would have been the last one to fail. If the pickup tooth failed first, you wouldn't be seeing any damage on the other teeth, and the slant would either be in the opposite direction or it would have broken off cleanly.

Pre-engagement is caused by feeding issues. You did not tune your setup properly to feed at your rate of fire and that is why it failed.

[Stealth]

+1 vote on pre engagement, but I think the failure was due to the piston not being to glide freely along the rails and getting stuck.

Did you ever check if the piston was able to move freely?

You do so by assembling the piston with the piston head, but remove the piston head o-ring. Reassemble the mechbox with only the compression parts. Screw both halves together with a few screws. Shake the mechbox up and down. Does the piston move? If not you need to sand down the guide rails.

[Hectic]

I did adjust the piston to glide freely, I watched a video that described the issue of the lonex piston being a little too wide, however the rails were not the issue. The lower part of the rear of the piston (beside the rack/pickup tooth area) as it would bind against the shell, after that the piston would slide freely with the screws in and all.
And again, the 3rd tooth did not break, it was shaven by me. The fourth tooth broke and or the pickup tooth broke in the same instance. To me the damage (and how the lil bits of piston looked and where the gears where jammed) says the pic up failed but not quite and the seccond tooth grabbed on and also the 4 piston tooth made contact with the sector and the whole thing locked up.
Most of the reason being the lack on the third tooth that I shaved off like an idiot trying to correct an incorrectable aoe.
Gonna grab the wier and such and hopefully get it put together without the sorbo buddie and with only 2 teeth max shaved off and that should do it. Looking at it now I'm not sure how I didn't see that was going to be a problem before hand. Nor can I imagine how it lasted for the 1000 rounds or so of testing I put it through (no jams or miss feeds after I sorted the hop up out) guess those lonex pistons are tough as nails lol.

I know it looks like PE. The last gun I broke was exactly that, all the teeth in the middle of the piston were stripped off. And looking at the pic I posted it does look that way due to the razor blade removeal of the teeth the just looked chewed up lol.
But putting a sector gear with the piston and looking at how the teeth mesh you can easily see that the third tooth (3rd after the pic up) on the piston will bear almost the entire load (assuming the 1st and 2nd are shaved). Just as the 4th tooth starts to mesh up the pick up tooth is off of the sector gear.

[pestobanana]

The pickup tooth was the last to fail, and only failed because each cycle was causing it to skip off 16 teeth like something on a grinder. The pickup tooth did not cause your failure.

If you don't want to listen, that's fine. You can go and waste your time and money on more pistons.

[freshblake]

I can't see how he would get pre engagement with shaving the 3rd and 4th teeth.

It looks like he did all that shaving to do exactly that - prevent PE. Unfortunately it created a lot of stress on the other teeth on the piston which is why it stripped and he was able to get 1k rounds.

If he had a crazy thick sorbo on the cylinder head his piston would be sitting too far back thus creating a poor angle of engagement opposite to the original angle he was trying to correct.

Assuming the first tooth is already removed you only need to brush that second tooth with a file and ensure the cylinder head has the proper thickness of spacer for a proper AoE.

[pestobanana]

Originally Posted by freshblake

I can't see how he would get pre engagement with shaving the 3rd and 4th teeth.

It looks like he did all that shaving to do exactly that - prevent PE. Unfortunately it created a lot of stress on the other teeth on the piston which is why it stripped and he was able to get 1k rounds.

If he had a crazy thick sorbo on the cylinder head his piston would be sitting too far back thus creating a poor angle of engagement opposite to the original angle he was trying to correct.

Assuming the first tooth is already removed you only need to brush that second tooth with a file and ensure the cylinder head has the proper thickness of spacer for a proper AoE.

Removal of those teeth is related to AoE and has absolutely nothing to do with pre-engagement.