Pme

[Misery]

ok at a game on saturday notice i was getting a bit of pme on full auto short stroking the best way to deal with this or am i thinking of something else?

[Iceni]

PME is usually fatal. I don't believe there is such a thing as a little bit of PME. Piston bounce (no padding and a piston that physically bounces off the cylinder head is sorted by adding some padding) and Double shooting (High speed motor on slower high ratio gears [the gears spin faster 18:1+] allows the bevel and spur to act like flywheels dragging a second shot out of the gear train energy - Sorted with correct gearing and motor choice) are not PME.

Once any PME has happened then there's going to be damage. There isn't space in a gearbox for damage not to happen, crushed piston, stripped teeth or collapsed bearings as well as many other things can and will happen.

The reason there is no such thing as a little PME is as soon as the gears cycle faster than piston you run out of space. The rack on the piston is moving forwards and can be imagined to be a solid bar, The teeth on the sector are out of sync and will not pick up at that very specific point in the cycle so they also can be thought of as a solid bar. If those 2 bars touch then it's game over for at least some parts. The only way to make more space is for either the piston, rack or gear train to move out of the way of each other.

There is also no single answer or solution that will solve the problem. You can reduce the teeth on the sector, allowing the gears a delay on pickup or speed up the piston. That isn't going to solve an attempt to push insane RPS out of an AEG. If you want RPS go dual sector with a slower gear train and bigger (faster) spring, or get an HPA engine that doesn't have the same mechanical limits.


So you need to look at what you have and access it, Post the parts up so we can judge them. Just be warned there are limits for an AEG and unless you are willing to spend a lot of time 25ish RPS should be considered the safe limit even after modifications.

13:1, HT motor, 2tooth SS, 7.4v lipo will get close to 25 rps without many issues. If you are wanting to push past that then the forum clams up a little, as anyone that has pushed past that and made it work did a hell of a lot of work and put some serious investment into the gearbox. And they probably have absolutely no faith in the durability or the gearbox.

 

Edited May 5, 2021 by Iceni

[Skara]

8 hours ago, Iceni said:

13:1, HT motor, 2tooth SS, 7.4v lipo will get close to 25 rps without many issues. If you are wanting to push past that then the forum clams up a little, as anyone that has pushed past that and made it work did a hell of a lot of work and put some serious investment into the gearbox. And they probably have absolutely no faith in the durability or the gearbox.

Not necessarily expensive, it's just a matter of knowing what you're doing

 

In my case I found the cause of PME in the spring(s) being too short, essentially what happened was the piston couldn't be moved fully forward because the backpressure created by the bb was stronger than the pressure applied to the piston by the spring, so it would reach a balance point about 1mm away from the cylinder head.

I solved the issue on both my M4s by removing one and two teeth respectively off the sector (pickup side) and slapping some longer springs in (SHS M90s). Both guns running on SHS HT motors, SHS 13:1 gears and 101% air seal, about 20/22 RPS on 7.4 batteries, over 30 on 11.1s..

 

There are other causes too, like the piston not being free to move in its guides, a bad AOE job, keeping all 15 teeth on the piston for some reason and running extreme ROFs at low energy output (1/1.1J with ROFs well over 35 RPS)..

 

 

Once you made sure that the piston doesn't snag on anything and the air seal is good, the easiest way to prevent PME is to knock off one/two teeth off the sector (be careful to not undervolume the gun, make sure the piston head can still retract past the port on the cylinder) and replace the spring with a slightly stiffer one OR install a thrust bearing on either the piston head or spring guide.

[Adolf Hamster]

9 hours ago, Iceni said:

13:1, HT motor, 2tooth SS, 7.4v lipo will get close to 25 rps without many issues. If you are wanting to push past that then the forum clams up a little, as anyone that has pushed past that and made it work did a hell of a lot of work and put some serious investment into the gearbox. And they probably have absolutely no faith in the durability or the gearbox.

 

it's as much a case of why expend the effort as technical possibility.

 

generally the hard limit is a function of the energy limit you have to deal with, so when folks see yanks with their 40rps builds they forget that being able to run 1.5+j in a "cqb" gun is part of what enables that to be practical.

 

not with a gearbox but as hpa i did tinker with the bleeding edge of fire rate up into the 50's when even the mighty cyma midcap struggles to keep up and frankly it didn't provide any benefit that outweighed being "that cunt with the buzz gun"

[ImTriggerHappy]

Pre engagement on any high ROF aeg is almost impossible to 100% prevent ever occurring.

The only gearbox I have ever seen that it is guaranteed is a custom one with lots of electronics including a home sensor on the piston and uses inline bearings rather than the traditional piston guide rails. Everything  on it is one of a kind and that makes it impossible to maintain without access to the ability to manufacture the parts required.

 

The cost of building it was so ridiculous that it could never really be a realistic thing for the airsoft community. Better and easier to just go hpa.

 

Works well and is a very distinctive sounding gun though.

[Guest DrAlexanderTobacco]

Just to hop in here - What is PME, and how should I recognise it during a game? Just so I can keep an eye out. Cheers!

[Skara]

premature engagement (of the piston).

Essentially the sector gear manages to pick up the piston when (the piston) is still travelling forward, you'll hear an awful grinding noise all of a sudden followed (if you don't stop in time) by a very low powered shot.

 

Happens in full auto only

[ImTriggerHappy]

21 minutes ago, DrAlexanderTobacco said:

Just to hop in here - What is PME, and how should I recognise it during a game? Just so I can keep an eye out. Cheers!

 

I wouldn't worry about looking for it during a game as  mild pre-engagment will slowly chew at your pick up gears and if running a full plastic pinion you probably won't know until your gun fails. If severe or using metal teeth it normally kills the gun very fast and makes a horrible noise as it does.

 

If your guns stock then pre engagement is not generally a massive issue if you use the correct battery.

[Misery]

17 hours ago, Iceni said:

PME is usually fatal. I don't believe there is such a thing as a little bit of PME. Piston bounce (no padding and a piston that physically bounces off the cylinder head is sorted by adding some padding) and Double shooting (High speed motor on slower high ratio gears [the gears spin faster 18:1+] allows the bevel and spur to act like flywheels dragging a second shot out of the gear train energy - Sorted with correct gearing and motor choice) are not PME.

Once any PME has happened then there's going to be damage. There isn't space in a gearbox for damage not to happen, crushed piston, stripped teeth or collapsed bearings as well as many other things can and will happen.

The reason there is no such thing as a little PME is as soon as the gears cycle faster than piston you run out of space. The rack on the piston is moving forwards and can be imagined to be a solid bar, The teeth on the sector are out of sync and will not pick up at that very specific point in the cycle so they also can be thought of as a solid bar. If those 2 bars touch then it's game over for at least some parts. The only way to make more space is for either the piston, rack or gear train to move out of the way of each other.

There is also no single answer or solution that will solve the problem. You can reduce the teeth on the sector, allowing the gears a delay on pickup or speed up the piston. That isn't going to solve an attempt to push insane RPS out of an AEG. If you want RPS go dual sector with a slower gear train and bigger (faster) spring, or get an HPA engine that doesn't have the same mechanical limits.


So you need to look at what you have and access it, Post the parts up so we can judge them. Just be warned there are limits for an AEG and unless you are willing to spend a lot of time 25ish RPS should be considered the safe limit even after modifications.

13:1, HT motor, 2tooth SS, 7.4v lipo will get close to 25 rps without many issues. If you are wanting to push past that then the forum clams up a little, as anyone that has pushed past that and made it work did a hell of a lot of work and put some serious investment into the gearbox. And they probably have absolutely no faith in the durability or the gearbox.

 

wasnt actualy looking to go high rps bud they were just parts i got in at the time to replace what had broken from standard i just kinda went to far there was no effort made to make a high rof build originaly set out to just make it more reliable after it failed XD

had the gun apart on sunday to check and it was all fine nothing broken or worn,will take it apart again tomoz and get pics for u bud

 

tbh ill probs leave as is haha as i dont use full auto in game prefer semi was just when was out testing it on saturday it happened but will strip down again photograph and then rebuild it

 

Edited May 5, 2021 by Misery

[Misery]

completley forgot about this ended up doing another skirmish last week and eventualy got round to this today

i dont think its pme as such i think its more the timing for the tappet plate as i get about 10-15 rounds then it happens clear the rif and off we go again.so i reckon i need to get a delay chip if i want to run the 13-1 gears in that set up i have reverted back to stock gears for now and seems to be spot on mag dumped full mag no issues no weird sounds etc

 

these are the 13-1 gears no excessive wear i can see forgot to get pic of piston