using LiPo w/ sub 400 fps

[sabinn]

I didn't want to hijack the thread I read this in so I'm reposting here. Paraphrased for clarity.

Originally Posted by Kos-Mos

a 11.1v LiPo is too powerful and fast for a sub 400 fps setup. It will cause the gears to hit the piston before it is reset forward. That is a guarantee you will destroy gears (at first, the piston body will flex to allow the gears to reset, but the tooth will break off over time)

In your case, it would probably if/when you get a jam or stuck BB in the barrel/hop-up. The piston could not move forward fast enough because of the added air pressure.

I use a 11.1 LiPo in my CA P90 Proline shooting 340fps. Metal gears, full metal rack on piston. Is it a ticking time bomb? Would I be way better off going 9.6 NiMH?

[Maethori]

A 7.4v LiPo would work very well for that gun, I would think. An 11.1v LiPo is unnecessary for almost every gun out there.

[lurkingknight]

there's a few things to be concerned about when putting that much current and voltage into an AEG... one is shimming.. this prevents gears from twisting off axis when put under extreme torque forces when the motor starts up. Shimming also ensures you have good meshing of the gears so you don't grind them together in those extreme torque conditions.

secondly, angle of engagement... the extreme torque of the motor starting up slams the first sector tooth into the pickup tooth repeatedly. At full rest, the piston in the p90 sits so the pickup tooth is grabbed by a tiny bit of the first sector tooth. It's like chiseling away at the same spot over and over again. AoE correction moves the piston out so the first sector tooth hits flush with the pickup tooth every time and distributes the impact force evenly on the face of contact. This prolongs piston and gears.

thirdly, the p90 trigger contacts are pretty weak on the semi side. They generally arc and burn even when using a 9.6v nimh, but it's not as hot or pronounced as going to higher current 11.1 lipos. This is a good reason to get a msofet, which prevents trigger arcing.

These are generally the 3 concerns when going to high voltage/discharge batteries.

Kos-Mos is describing pre-engagement, the piston assembly will not reach full forward position before the gearset starts a second cycle, this is due to a weak spring (<350fps) This generally plows the sector gear teeth into the middle of the piston rack and is what kills pistons, or since you have a metal rack, it may kill your sector gear, or if not shimmed properly, jams the gears and the softest metal bends first in the path of least resistance... probably an axle or bearing, if not the gear teeth.

There are things you can do to lighten the piston assembly to make its return speed faster... you can swisscheese the piston... drilling out as much excess material as possible to save weight. You can pull out the bearings on the piston side and put a set on the spring guide side. But it's generally accepted that a sub 350fps full stroke setup has ROF issues that will lead to something blowing up.

The more accepted way to get <350fps and greater RoF (I'll just pull a number out here... let's say 20+) is to use a higher rated spring, let's say a 130 spring that would normally shoot 450 fps, and you file teeth off the sector gear to short stroke the piston cycle. A fraction of the distance pulled = a fraction of the return speed from full stroke.

This won't effect RoF as the gears physically don't change ratios (the gearing that determines RoF is on the spur gear itself and the spur side of the sector gear), you're just not pulling the piston back as far. You will actually probably lose a bit of RoF since you're using a stiffer spring.

On my p90, I use 11.1v lipos at a 20C-25C discharge for really fast trigger response, but I also use a computerized mosfet to slow the RoF down to reasonable levels. The unit I use allows me to shoot every semi shot or the first shot of a burst at 100% motor speed, then immediately after the first cycle, drops the voltage to lower my speed to 50% of maximum. Which means I get a semi trigger that will shoot as fast as I can pull the trigger. Some guys at nightfall thought I was shooting full auto when I was not. My typical game speed I run the gun at is 15rps full auto, but the trigger response is that if I can pull it fast enough, I can probably shoot up to that speed, if not faster, only limited by the mechanical portion of the trigger (length of pull and return speed of the trigger shuttle).

100% motor speed induces a white laserbeam of BB death, which in 99% of the case, is way too much fire. Unless someone isn't calling hits.

[XZIVR]

kind of an overly generalized statement to make about sub 400 fps.. there are many other factors. My indoor cqb death machine uses a 2200mah 11.1v 20c Lipo and fires 38 rounds per second at 280 fps. Probably 5000+ round through it now with no issues...

[lurkingknight]

other factors, I've got an open mind, what have you experienced as other factors?

[sabinn]

@lurkingknight

That's a great write up. Well done. Even someone with my bad gun doc skills understood that.

So the simplest way (not the cheapest) to improve the life of my P90 is 1) to install a mosfet and 2) pull back my piston to engage flush with the gear. Once tuned properly I can keep my 11.1 and still lower stress/fatigue on the internals. Do I get that right?

[lurkingknight]

at a minimum, it should stave off most issues. You can get a simple mosfet for anywhere between 15-50$, you don't need a fancy computerized unit.

Look up AoE correction on youtube, there's quite a number of visual guides to show how it's done. You'll need a v6 sorbo pad, which you can get from airsoftstore.ca. But you will have to measure how thick you'll need. A high quality superglue will stick it permanently to your cylinderhead.

It's never a guarantee that these things will stop a failure, but they'll definitely help prolong the life of your components.

[sabinn]

good info. thanks

[ThunderCactus]

That statement is completely false.
In extreme high speed setups (like 1500rpm+) you WANT the gun to be under 350fps.
My 249 at it's top speed does 380fps at 1400rpm, and it's done that for 100k rounds, on internals that have already shot 200k rounds from 350-380fps at 900-1200rpm
High quality upgrade parts can EASILY handle 1400rpm at 380fps with out any special modification

In short, the higher spring power in the gun, the more potential energy is built up in the gears when cycling, so when the piston is released, if you have really high potential energy, the sector gear will actually come around FASTER than it would if you were using a lower fps setup, increasing your chance of stripping the piston.

And Que Macie with more details;

[lurkingknight]

* as always, I reserve the right to say that I can possibly be wrong and that any advice I give is only the sum of my own research and limited experience.

Though we'll wait for macieK's intervention. :P

[coach]

Originally Posted by sabinn

I didn't want to hijack the thread I read this in so I'm reposting here. Paraphrased for clarity.


I use a 11.1 LiPo in my CA P90 Proline shooting 340fps. Metal gears, full metal rack on piston. Is it a ticking time bomb? Would I be way better off going 9.6 NiMH?

I'm using an 11.1 12c lipo in a second hand stock KA Galil that has been running that same lipo for a long time. Chronied in at a consistent 315fps

No major issues, but just Friday started developing an intermittent burst in semi so the trigger contacts likely need a bit of cleaning.

[MaciekA]

The above posts are super solid and I agree completely.

Kos-Mos' post is not a completely true statement... HOWEVER, his heart is in the right place and he is describing a problem that inexperienced AEG owners and/or doctors will experience from time to time. Pre-engagement is a total bummer when it happens.

The problem has been solved and re-solved by large numbers of techs out there, and it's also true that often it doesn't even need solving in the first place in some setups. Extremely high rate of fire issues come down to a few core issues.

Piston return time: Can we get the piston to return back to the front of the cylinder before the sector comes back again? I've seen data recently that indicated that the return time on a piston in a given setup was around 6 to 8 millseconds. Ignoring the position of the gears for a moment, that gives enough piston-return-time headroom for an astonishing 166 rounds per second. Even a conservative estimate of 12ms is still going to net you well over 80 piston returns per second.

I personally believe some number of pre-engagement cases are not due to the piston being too heavy, but experiencing too much friction on its way back to the front of the cylinder. I regularly run into pistons which are unable to move freely in the gearbox. When you guys are testing your mechboxes before final assembly, one of those tests should be to put the completely assembled piston into the gearbox without the spring or spring guide or gears and just see if it can drop up and down the path by using only gravity. Some of you will be surprised by the results. Something to think about...

Angle of engagement: can we ensure that the strike surface between the sector gear and the piston's first tooth is a 2D rectangle instead of a 1D line? It's also my opinion that a large number of people who destroy pistons and/or gears on high RoF setups are actually just not correcting for AoE and misdiagnosing or "underdiagnosing" (i.e. guessing) their pathology. Again, the incredibly fast return times of pistons in low-friction conditions would support this theory.

Finally, there are a number of ways we can lower velocity dramatically while still using a M120 or M130 spring, to ensure hyper-fast piston return time:

For example you can rob your cylinder of volume using sorbo and remove length from your barrel. Or do some fun dual sector gear stuff (edit: I meant to say short-stroke). In either case, these things require torque and power delivery, things I personally believe are the "pillars of the airsoft mechbox", but are still largely ignored. I'm still stunned when I meet people who have airsoftmechanics accounts and are running Tamiya (okay okay, you know who you are and I know you have a kickass excuse).

Unfortunately, those of us who know about these things are still fighting the "longer barrels are better" myth and general lack of awareness about neodymium motors and large LiPos, so there's a long way to go before these become viable suggestions and common knowledge. Until then, the PE boogeyman will reign...

[lurkingknight]

Originally Posted by ThunderCactus

That statement is completely false.
In extreme high speed setups (like 1500rpm+) you WANT the gun to be under 350fps.
My 249 at it's top speed does 380fps at 1400rpm, and it's done that for 100k rounds, on internals that have already shot 200k rounds from 350-380fps at 900-1200rpm
High quality upgrade parts can EASILY handle 1400rpm at 380fps with out any special modification

In short, the higher spring power in the gun, the more potential energy is built up in the gears when cycling, so when the piston is released, if you have really high potential energy, the sector gear will actually come around FASTER than it would if you were using a lower fps setup, increasing your chance of stripping the piston.

And Que Macie with more details;

I think I get what you're saying cactus, the potential energy of the spring returning the piston forward vs the potential energy of the gear train. Less energy from the 2 in opposite vectors to cause damage.

short stroking and AoE correction will eliminate any potential that the gear train even meets the piston on the return stroke and moves the greatest forces back to the strongest point of impact, which is the pickup tooth, the only place you want the sector gear to hit. Half stroking the piston halves the distance it travels, and with a stronger spring, makes up for the loss in air volume. The stronger spring ensures back to forward position before the gear train finishes a cycle.

I have a short stroked p90 gearbox on my bench right now it's got an shs 130 spring with 3 teeth off the sector gear, modified tappet plate and super swisscheesed piston. I leaned on the full auto trigger to see how well it worked and it shoots about 30-35rps just fine with a 11.1v 20c battery, no weird noises, no catastrophic failure. On top of that, this piston is only half metal teeth, so if something catastrophic were to happen, it probably would have already. This was a guy's primary indoor gun for the longest time and it's showing no signs of gears hitting the piston rack at all.


btw, I agree with your assessment about friction and piston return speed. While testing a mosfet for causes of an overheat, repeated, and successive fast firing of semi on a v6 and v3 gearbox, the cylinder noticeably rose in temperature, indicating heat build up from friction. I've also pulled out super hard O-rings from pistonheads, either ungreased or dried up, the o-ring was visibily oxidized and hardened from heat cycling.

[ThunderCactus]

That's an interesting point to bring up!
Only experience I've had with too much friction on the piston was using guarder mechbox shells (IE poor castings), and one guy was running his cylinder dry. Well the ring overheated and tore itself apart lol

[THe_Silencer]

Originally Posted by lurkingknight

thirdly, the p90 trigger contacts are pretty weak on the semi side. They generally arc and burn even when using a 9.6v nimh, but it's not as hot or pronounced as going to higher current 11.1 lipos. This is a good reason to get a msofet, which prevents trigger arcing.

All airsoft trigger contacts I've seen arc and eventually burn without a MOSFET, even when using only an 8.4v NiMH mini. This is only exacerbated when you run higher voltage batteries. This is even more over a reason to install a MOSFET, esp. if you are going to run 11.1v lipos. The statement about p90 trigger contacts being weak isn't necessarily true, it depends on the manufacturer. Classic Army in particular makes very durable trigger contacts in comparison to TM or any Chinese company. I've taken apart several year old non MOSFET CA aegs and the contacts still look almost good as new.