Avenger / Leviathan project 60 FPE in .22

[subscriber]

I wonder if the double load shot dislodged (partly extruded) an o-ring in your transfer port.  Or blew a breech seal O-ring; accounting for the reduced muzzle velocity due to a partial leak.  Such a leak may be fixed by taking apart and reassembly; unless the O-ring was damaged in process.

I was born in the Transvaal.  Grew up mostly in the Western Cape.  Have been in the USA over 26 years.

[subscriber]

I could not get the valve to seat. In the end, I took a very light spring off of an electric motor bush that happened to fit perfectly

Perhaps this is also obvious:  You should cock the hammer when you fill the air tank from empty.  Else the hammer may rest on the valve stem (in some PCPs), and prevent the valve from seating.  So, it will leak continuously.

The fact that yours fills with a light spring may have solved your problem, and make no difference, should you remove the spring.  Providing you can get the valve to seat to start filling.  Gravity may then have something to do with it.  Position the air rifle such that gravity closes the valve...

Ek weet; ek praat te veel

[Lathaniel]

Have you been back to sunny SA since you left? Place had changed hectic over the last few years, but still an awesome place to live. I was born in natal, spent most of my childhood years in the Eastern transvaal, and a few of my adult in the southern Cape, now back in natal.
Ja, the blown o rings was my first suspicion, so I stripped, oiled and reassembled, but I figured while it's apart I may aswell do the valve stem. On the lighter spring, strangely, I found if u put a slightly stiffer spring in( from a chainsaw throttle) I actually increased mv. I'm going to order some acetal tomorrow and make a new poppet, and that will allow me to open the valve throat a bit more. I'm hoping that gets me where I want to be. I must be honest, I'm having a blast working on this pcp, first time ever doing that.

[subscriber]

Well, I don't own an Avenger, so it took me a while to realize that you are seeking and getting .25 caliber power from your .22.  In other words, you might be able to teach tuners about tuning the .22, more than they can teach you

Your valve spring experience is interesting.  I wonder if without a spring it is not slamming full open so hard that it bounces shut prematurely.  Whereas, with a soft spring, there is perhaps less bounce.  Strange things can happen.  For example (speculation), the spring is not so much controlling the valve as adding friction by applying a slight side load; should the spring not be perfectly coaxial with the spring.

Anyway, I try to have a working theory; and never dismiss results.  Even if I don't understand something because it seems to violate theory, rather than the result being deemed wrong, the theory is incomplete.

If I don't own an Avenger, why am I posting in this thread?  Because I wanted to try and help a fellow from SA.  And, I do try to be helpful in general, even if those biting their tongues might not thinks so.

What went through my head was that because the FX Impact M3 is reaching 100 FPE in .22, 60 FPE with the shorter Avenger barrel does not seem unreasonable.  Of course, the devil is in the details.

Ek het elke drie jaar SA besoek.  Laste keer was 2019.   My 90 jaar oud ouers  woon nog daar, en ek praat elke week met hulle.  As dit nie was dat die plek nou meer kaoties is nie, het ek miskien in SA kom aftree.   

[sb327]

I always enjoy reading your posts Subscriber. Whether you have an Avenger or not, the dynamics of getting/keeping air to the tail of the projectile are generally quite similar among conventional valved guns.

Dave

[subscriber]

Thanks Dave,

I am glad to see that a successful hot-rodder such as yourself is looking at this thread. 

[Lathaniel]

I really appreciate your help, i have been flying blind. Trying to theorise how to do things and why things don't work as well as one thinks they should. I agree with the valve spring, my thinking is that the spring absorbs some of thw force with the back pressure, resulting in less of an impact on the back,  and resultant bounce. Now thanks to your links, I am going to make a shorter poppet, which might alleviate that issue, but like you say  some things are contraintuituve, which I'm slowly starting to learn.
As u weer in suid afrika is, most u aseblief vir my n boodskap stour, dark kan ons by mekaar uit kom, en nog n bietjie leer. Wear by u ours.

[buddyb]

Have you had any issues with your regulator with the mods?

I ask because it seems  mine drops 200-400 psi on the regulator after each shot and if you shoot too quickly it affects accuracy and extreme spread.

[Lathaniel]

The reg seems OK, so far, after the mods i do use more air, im getting ~ 20 shots per fill now. I have noticed that shooting too quick, does have an effect, I have just slowed down and all is good so far.

[subscriber]

im getting ~ 20 shots per fill now.

What fill pressure are you using, Lathaniel?

[Lathaniel]

I'm going up to the full 310 bar pressure. Sometimes, slightly over or slightly under, hard to see those silly gauges

[sb327]

Not sure if it’s been discussed yet but running a smaller od poppet stem (you need to put in a bushing in the stem guide)
reduces the closing force as well as offering more flow area in the throat. This coupled with the heavier hammer will increase dwell time.

Dave

[Lathaniel]

What I did, was reduce the poppet valve stem to ~2mm, within 1cm from the poppet, the remaining 20mm is still 3mm od, would I need to look at a bushing still? I did that small piece, hoping not to affect the sealing in the valve guide, while increasing air flow in the valve throat. I will be honest, the valve guide is something that has bothered me, because as I see it, the stem moves relatively easily through it, meaning ( in my mind atleast) it dosent seal very well, and the hammer chamber is sealed not at all, how much air do you think we lose through the valve guide?

[subscriber]

I am not familiar with the parts, but if one looks at the potential leak path at the valve stem, it depends on the circumference, the radial clearance, and the overlap length between stem and guide.  The greater the overlap, the less leak, because the annular channel is in effect a very long narrow pipe, with a huge flow loss (pressure drop) across it.

A 3 mm diameter valve stem has a circumference of 9.43 mm.  If the radial gap is 0.02 mm, then the flow area is 0.1886 square millimeter.  Compared to 19.64 square millimeter for your 5 mm diameter TP flow area.  So, the leak is 1/104 of the TP area, assuming the radial clearance is correct.  If the radial clearance was 0.01 mm, then the valve stem leak area would be 1/208 of the TP area.

The missing piece of the equation is that flow area is not the only important consideration.  The "edge effect" of the valve stem leak is huge, where stagnant boundary layer air clings to both the surfaces of the valve stem and guide.    Compared to just the ID of the TP (down its length).   Here is where the length of the valve stem overlapping the guide helps reduce the leak.

The crude sketch below attempts to illustrate how much more efficient a large flow area is, compared to a narrow one; due to the stagnant boundary layer effect.  The read line is flow velocity of the air across the section.   The narrow gap acts as if it even narrower due to the boundary layer effect.  It has much less percentage effect on the TP.

That is not to suggest that one should ignore the valve stem leak.  I think the smaller the radial gap the better, up to the point that there is too much drag on the valve, and it does not open and close reliably.  I don't know if any PCP's use and O-ring seals on the valve stem, but somehow I get the impression that some leak at the stem while the valve is open, is considered normal.  It occurs only for a very short time.  Not long enough to matter very much.

The hammer chamber must be vented, or air compressing in front of the hammer will slow it down during "flight".

Perhaps someone with more specific knowledge and informed opinion will chime in.

[Lathaniel]

So if I understand correctly, the valve guide leak is negligible. And can be further negated by having a longer valve stem, which(up to a point) will also increase dwell time. This could also explain why having a stiffer valve spring produced more power than the flimsy one. Not so much preventing bounce, as maintaining a longer portion of the valve stem in the valve guide?

[subscriber]

It is not just the stem length, but the length where stem is shrouded by guide length.  So, the guide needs to have sufficient length too, for the leak to be minimized.  How long to approach zero leak?  Probably impractically long.

I would try to halve valve stem to guide clearance before trying to double the stem to guide overlap length.  I am at the disadvantage of not knowing what the packaging limits are.

A very long stem perhaps just opens the valve further, and takes longer to close, thus releasing more air.  An overly long stem my bend and ring when struck by the hammer; even if just elastically.

I think there may be more factors involved with the valve spring increasing power, over all else being identical, with the spring removed.  It was a small change.  The question is, does it still hold if you removed the spring?   I would like to understand this, but it seems less important than if it added more power.

I think there is a flow limit reached after a certain valve lift is reached.  More lift just increases valve dwell.  Also, the further the valve opens, the harder it will slam shut.  The flow is then limited by the flow area through the valve throat, less stem area.  Some of the links I posted above almost certainly go into detail about this.

[Lathaniel]

Even coked, and using gravity to help assist with a seal, I never achieved a seal without a spring. I suspect that my little pump fills too slowly to create enough pressure on the rear of the poppet, to make a proper seal. Another person I know, removed the spring, and managed to get a seal, he was, however filling from a tank, so very quick pressure, to create enough force to seal? Or perhaps things don't align 100% in my assembly, so it needs that bit of pressure from the valve spring to close it properly. While waiting for that acetal, I am going to work my frankenstein poppet face and see if anything changes, with my new valve stem. Everything is done by hand, i have a drill and grinder, so things take some time and don't always align as I'd like them to off the bat, but perciverance usually sees me through.

[subscriber]

OK.  You can't fill without the valve stem spring.  It is weak and can't do any harm; while appearing to help with more power.  That may change after a few 100 shots to bed the valve better.

I am guessing that a valve with slightly larger head and stem diameters, using appropriate lift should boost your power a bit more.  As long as the valve is not too heavy for your hammer.  Certainly, the hammer will need to work harder to open a larger diameter valve against the same air pressure due to the larger area.

[Lathaniel]

So, I decided to take lunch today, and go home to play with my avenger. Good news I've got the poppet seating and the rifle filling now. I have attached a photo of the valve assembly, too. What you say makes sense, something I sort of thought of, but dismissed. When I originally opened the valve throat, the poppet got stuck in it. So I reversed the poppet, using the back as the new face. However, after drilling a new hole in the poppet, the pressure blew the valve stem out, so i put a screw in the original valve stem hole, which solved that problem, but did increase the weight of the poppet. That's why I've been trying to find delrin, is to make a new poppet. And get away from the extra weight and frankenstein look. I have 2 more questions if you'd be so kind, subscriber. 1) is peek much better than delrin? I've ordered the acetal, and it's cheap R27 a meter, where peek is R380 a meter, with acetal being so cheap, I don't mind wasting that money and ordering peek too, if it's worth it, what are your feelings?
2)When I receive the material, i was thinking of making the poppet with a conical shape on the back, easier to cut through the pressurised air, so means less hammer to open the valve, or am I barking up the wrong tree?

[subscriber]

Peek is almost 50% stronger than the stronger form of acetal.  Peek is almost like metal in its stability and machinability.  Note that Peek's impact strength is not as good as acetal's, so if you valve slams shut hard, it may suffer brittle failure, depending on the design.

https://www.mcmaster.com/peek/wear-and-chemical-resistant-peek-rods-and-discs/
Tensile Strength: 14,000-16,000 psi (Excellent)
Impact Strength: 0.6-0.95 ft.-lbs./in. (Poor)

Stronger acetal:
https://www.mcmaster.com/acetal/wear-resistant-easy-to-machine-delrin-acetal-resin-rods-and-discs/
Tensile Strength: 10,000-11,000 psi (Good)
Impact Strength: 1-1.5 ft.-lbs./in (Poor)

Weaker acetal (slippery):
https://www.mcmaster.com/acetal/slippery-wear-resistant-easy-to-machine-acetal-copolymer/
Tensile Strength:
Blue: 7,600 psi (Good)
Red: 5,900 psi (Good)


Both the peek and first acetal are about the same hardness, so a tough call.   The second acetal is much weaker.  So, it depends which one you can get.   I think plenty of PCP valve makers on this forum use Delrin / acetal because it is 1/3 of the price of peek.  Not sure which grade they use.  Probably what they can get...


Creating a shape on the back of a the valve to smooth air flow is a good idea, providing it does not choke the flow.  Think of car engine intake valve head shape.

Pressure differential across the valve, and front and rear areas for air pressure to act on affect force to open valve.  When the valve is open, the closing force follows similar principles, except the pressure on the TP side of the valve is high initially.  Then drops once the pellet accelerates down the barrel.