High Speed Gears Stripping Pistons

[Bar1975]

hey guys..

I just recently upgraded my Mp5K (Version 3 GB) to a set of 13:1 High Speed Gears.

I have just stripped the last two pistons I've placed in this thing though. The stock piston that came with the gun had the pick up tooth completely crack off along with shredding of several teeth and a similar stock piston (from a different gun) just did the same thing without the pick up tooth cracking off though. (starting to shred teeth however)

At first i was running a high speed motor with the new gears so i figured that was the cause.. replaced it with the stock high torque motor. The HS Motor stripped the piston in 5 seconds flat (cracked pick up tooth) .. the high torque motor took several hundred rounds before starting to strip.

I had both the pistons properly set up for AOE (Using the Sorbo pad method) and two of the teeth removed. That part is all fine. It's not AOE.

I guess this is a standard case of Pre-Engagement and/or crappy pistons for high speed gear set? Is it possible that this is bad shimming? (I've seen the results of bad shimming and it's usually damage to the gears.. not the pistons so much) In both cases i've checked for proper meshing of the sector gear teeth with the piston teeth.. they meshed together fine. (Shimming was done to the bevel gear.. little to no vertical play in any gears.. i think it's shimmed ok)

Here is what I'm thinking will be my next move. Please advise..

1. New Polycarb piston with a few metal teeth (modify ultra high speed maybe?) Really don't want to spend a lot on a new piston. Maybe SHS with 7 metal teeth or something? (<$20)

2. Short stroke the piston and sector gear by a tooth? How many teeth do i remove? Would i really need to short stroke here?

Question: How many teeth should be on the piston and sector gear respectively for a higher speed set up? (we're only talking between 25fps and 30fps so not really all that high) Right now there are 15 piston teeth (one metal) and 16 metal sector gear teeth.. everything seems to mesh fine so not sure why I"m getting pre-engagement?

Running an M110 Spring and a 9.6V battery with Deans.

[Styrak]

What pistons were you using before (the 2 broken ones)?

[Bar1975]

standard stock pistons.. nothing special

[Styrak]

Why don't you just try a decent aftermarket piston then?
Could it be you're assembling the mechbox wrong? If you leave the gears engaged with the piston when putting it back together it will strip for sure.

Also you reposted this here, why? http://www.airsoftcanada.com/showthr...77#post1742177

[ThunderCactus]

I ran 1500rpm at 25rps and 380fps on my 249, with triple torque gears, never had a problem with my prometheus piston. Only removed the second tooth on it.
Have you done a dry run of the cycle? Open your mechbox, remove your spring (but not the piston) and just rotate the sector gear and see if anything obvious is wrong between the sector and piston

FYI DO NOT USE POLYCARBONATE PISTONS
polycarbonate is inherently too brittle to be used as a piston material. And you definitely don't need more than 3 metal teeth under 420fps
Try using a nylon fibre (marui) or POM piston.
I've always had luck with prometheus style pistons, but they tend to only perform reliably when matched with prometheus gears.

[Bar1975]

I've tested the sector and piston meshing without the spring in place. All the teeth match up and the sector teeth seem to engage the piston teeth properly.

I guess the issue is that in the second piston set up (with the high torque motor) i didn't swiss cheese the piston. Maybe if i do that it will help.

I'll get a new piston (nylon fiber).. swiss cheese it... and maybe remove one metal tooth from piston and sector..

is it ok to just remove 1 tooth from both piston and sector? or does a pre-engagement fix always mean 3 teeth removed?

[wind_comm]

short stroking to fix preengagment can be as many (or as few) as it takes.
generally though, it's used in combination with a heavier spring to gain back the lost fps.

lighten the piston and see where it gets you. if you still need to short stroke, ditch the m110 for an m130 or so and have at it.

[MaciekA]

Hehe, buying a new piston won't magically get rid of the PE guys ...

I like the M130 idea, which is what my MP5K runs. Somewhat necessary to overcome the loss of volume due to sorbo pad and the ultra short barrel, but also helps with preventing PE. The MP5Ks seem to come with M90 springs and the like and that's unlikely to push the piston back in time.

Now would be a good time for a neo motor if you don't have one yet, pulling an M130 at 13:1 will present some challenges.

[ThunderCactus]

You can run stock parts at 25rps no issue.
Shouldn't need to swiss cheese the piston or anything. I'm running 25rps with a system supercore piston, and that's a HEAVY reinforced piston.
Have you considered using a lighter spring as well?

If you use a heavy spring, there's too much inertia built up in a light gearset when the sector let's go of the piston and the gears shoot back around too quickly. So use of a lighter spring better regulates their speed.
I use a very heavy prometheus gearset, so they're much less prone to rapid acceleration from that built up inertia

[Stealth]

If you post up a picture of the piston, I'm sure we can all tell you pretty quickly what exactly caused the issue.

If the pickup tooth ONLY is gone, then it's AoE, if the rack looks like a train ran over it, then it's PE.

What battery are you running?

[lurkingknight]

what battery are you using, what exact piston(s) and what spring if not known then what fps is the gun shooting when you can get a shot out of it?

If it's not aoe as you say, I agree with maciek, if it's anything less than a 110 with that speed gearset, you're probably running into preengagement even with a slower motor. a sub 360 spring probably isn't strong enough to return the piston in time before the next cycle begins.

Check for warping of the piston guiderails in the gearbox itself as well.. put in a new piston with no spring and see how it moves. Check for trueness of the guides on the piston.

Properly setup, a m130 with 3 teeth shortstroked off the pickup side of the sector gear will net you 300~ fps. There shouldn't be a need to swiss cheese but it certainly doesn't make it worse.

[Bar1975]

The spring is a madbull M110. The motor is a stock High Torque motor that came with the CA Mp5k. The piston was the stock piston that came with the gun. (not an aftermarket)

This piston stripping only started happening after i upgraded to high speed gears.

The piston itself looks like all the pictures i've seen of Pre Engagement.. the pick up tooth was cracked off and several of the teeth before it were stripped. The second piston I added (another stock piston i had lying around) is showing signs of the same thing about to happen.

I don't want to stick a high end piston in this thing only to see it break like the previous two. I want to resolve the root cause of the issue which im pretty sure has to be per-engagement. I can say that the AOE is perfect.. (Sector is engaging at 12:00) I've got pretty good at setting up the AOE with sorbo pads. (i even added a small spacer on the piston head to bring it to exactly 12:00 sharp)

I never imagined i'd have to stick a M130 in this little gun though to be honest.. that seems pretty powerful for a small CQB gun like this? I get the loss of volume and what not.. but an M130 is what i need to return that piston fast enough to avoid pre-engagement? Seems excessive..

[Bar1975]

sorry.. to fully answer the question..

Battery: 9.6V with Deans
FPS: ~310-320fps with .2g
RPS: with 9.6V is ~25rps

[Azathoth]

Originally Posted by ThunderCactus

I ran 1500rpm at 25rps and 380fps on my 249, with triple torque gears, never had a problem with my prometheus piston. Only removed the second tooth on it.
Have you done a dry run of the cycle? Open your mechbox, remove your spring (but not the piston) and just rotate the sector gear and see if anything obvious is wrong between the sector and piston

FYI DO NOT USE POLYCARBONATE PISTONS
polycarbonate is inherently too brittle to be used as a piston material. And you definitely don't need more than 3 metal teeth under 420fps
Try using a nylon fibre (marui) or POM piston.
I've always had luck with prometheus style pistons, but they tend to only perform reliably when matched with prometheus gears.

Never had a problem with Prometheus pistons and Siegetek gearsets or Marui ones for that matter however i never really strayed to far into the ACM market.

Another option instead of sticking in a higher velocity spring is to use spacers on your spring guide.

Their are a lot of combinations of motor, spring, gear ratio, battery that you can use to get your desired RoF. For example you could have moved to a high speed motor and a much larger battery 11.1+ volt battery with 60+ Amps continuous discharge to achieve your intended RoF instead of putting in a new set of gears.

[wind_comm]

Originally Posted by Bar1975

I never imagined i'd have to stick a M130 in this little gun though to be honest.. that seems pretty powerful for a small CQB gun like this? I get the loss of volume and what not.. but an M130 is what i need to return that piston fast enough to avoid pre-engagement? Seems excessive..

not really. I'm full stroke on an m120 with a _heavily_ swiss'd shs 15t and even-heavier-than-prommy-siegetek-gears, and the slowest motor known to man (frankentorque) and still occasionally get some slight pre-engagement. (among other issues)

also breaking 25-30 rps without any effort, despite the slow motor and bushings.

the issue you're describing sound almost like the release tooth is collapsing (thus stripping the rest in a hurry). pictures would help.