GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on November 28, 2013, 03:57:22 PM
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Forgive the duplicate post, this is also in my Hayabusa thread in the Machine Shop Gate.... but I felt it was important enough to get additonal traffic from MRod owners here in the PCP Gate.... When I pulled the .224/.257 valve out of the Hayabusa this morning to install the new .308/.357 valve I found that the MRod poppet was deformed slightly from the load.... Here is a photo after a few hundred shots....
(http://i378.photobucket.com/albums/oo221/rsterne/Hayabusa%20PCP/IMG_3853_zps6e21540f.jpg) (http://s378.photobucket.com/user/rsterne/media/Hayabusa%20PCP/IMG_3853_zps6e21540f.jpg.html)
The throat of the valve is 0.281", and you can see that the OD of the poppet is no longer a cylinder, it is belled out slightly on the end from the forces involved.... The original OD was 0.330" and it is now 0.350".... At 3000 psi, the force on the poppet was 256 lbs, assuming a 0.33" sealing diameter, and may well now be greater than that, but the fact remains that was too much for the poppet to withstand without distorting....
I know from past experience that a 1/4" throat on an MRod valve works fine, but pushing the throat ID to 9/32" is evidently overstressing the poppet material.... It may stay like this forever, not getting any worse.... or it might not.... Nice to know, anyway....
Bob
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Yea, such a thin sealing margin with a .281 throat is really putting quite the shear load on the O.D. of poppet head.
I too also have had no issue at .250 throat on OEM poppets ... but then again only subject them to @1900# pressure within regulated rifles.
Can you say ... PEEK ;D
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And that is why I am running PEEK poppets in my Marauder valve with the same size throat...:) Mine looked just like that. Now when I tried running .265 throat it seemed to tolerate that OK.
cheers,
Douglas
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Thank you. That explained the problems with my custom .22 pumper. 39.6cc x 10+ into ~1.8cc = too much for Delrin on a .284 throat. :o :'( I just finished a new valve back half with a .234 throat, .096 stem, and .218 transfer port. :) It now dumps 15 pumps reliably. 8) Chrony time in the A.M.
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So it looks like a 1/4" throat on an MRod poppet is fine, 9/32" is too big, and 17/64" (0.266") MAY be OK at 3000 psi.... Bigger than that, or more pressure, and you need to go PEEK....
Bob
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So it looks like a 1/4" throat on an MRod poppet is fine, 9/32" is too big, and 17/64" (0.266") MAY be OK at 3000 psi.... Bigger than that, or more pressure, and you need to go PEEK....
Bob
15/64" looks good so far, I'll know better in a week or so. ??? If not, Tap Plastics in Sacramento is not that far. 8)
I have a question. What is a reasonable efficiency range for a flat-top piston and valve?
Wrong button ... no edit done ;)
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Pump and Dump guns are not that great in terms of the FPE out compared to the total volume of air you compress, which is the way the efficiency is usually measured..... For example your pump (from above) is 39.6 cc x 10 = 396 cc = 24 CI for 10 pumps.... In a PCP, at 1.0 FPE/CI that should yield 24 FPE.... From 15 pumps, that would be 36 FPE.... and my guess is you ain't gonna get there.... My .22 cal Uber-Pumper, with 14.3 gr pellets, using a pump from a 1400 with a shortened stroke (33 cc) did 17.9 FPE at 10 pumps at 0.89 FPE/CI, but at 14 pumps it only reached 21.5 FPE at 0.76 FPE/CI and at 20 pumps (where it was starting to retain air) it did 25.4 FPE at 0.63 FPE/CI.... The highest efficiency I recorded with that gun was at 3 pumps where it did 7 FPE at 1.16 FPE/CI....
Now if you set up the gun for retained air (also called an ACP or Air Conserving Pumper) you can get better efficiency at higher power because you are basically shooting a PCP with an onboard pump, where the valve is closing before the pellet is half way down the barrel instead of blowing unused air out after Elvis has left the building....
Bob
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Thank you for that. But I was actually asking about the pumping efficiency. :-[ Ie how much pressure might I be building in the valve? I guess I need to be clearer sometimes :-[ :-[
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The only accurate data I have is for my .25 cal Disco Carbine Pumper.... That gun built 1500 psi in 15 pumps, filling a 5.5cc valve from a swept volume of 54cc.... So, the amount of air compressed was 15 x 54 = 810 cc into a 5.5 cc space, which should result in a pressure of 810 / 5.5= 147 bar, which is 2135 psi.... Based on that, the pumping efficiency was 1500 / 2135 = 70%.... The efficiency drops off as the pressure increases.... At 10 pumps it built 1090 psi which is 77% efficency, and at 5 pumps the pressure was 600 psi which is 84% efficiency....
At 10 pumps you are trying to cram 396 cc of air into 1.8 cc, which means the pressure should be 220 bar (3200 psi).... At that pressure I'm betting your pumping efficiency is 50% or less.... Are you sure your valve is only 1.8 cc?.... BTW, I MELTED a Delrin F-T-P on my Uber-Pumper with the 1400 pump tube.... Your Delrin poppet has more to deal with than just pressure, the air temperature inside the valve will be softening it as well, even at much less than 3000 psi....
Bob
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The valve is nearly a direct copy of a 1377 valve, with only a little volume increase, and major head-space decrease with an extended check poppet. As for the heat, the piston and valve are made of steel, as i have plenty of that, and brass is expensive. :D The Delrin getting soft is one thing I will research. I just might be going down and getting some PEEK after all. :-\ As far as efficiency, 50% will give me a number to use to guesstimate pressure. ::)
Thanks again. Bart
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15/64" looks good so far, I'll know better in a week or so. ??? If not, Tap Plastics in Sacramento is not that far. 8)
NOT Tap plastics .... But Interstate Plastics on commerce circle over by the State fair off exposition ave.
They sell PEEK, Delrin, Nylon etc .....
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15/64" looks good so far, I'll know better in a week or so. ??? If not, Tap Plastics in Sacramento is not that far. 8)
NOT Tap plastics .... But Interstate Plastics on commerce circle over by the State fair off exposition ave.
They sell PEEK, Delrin, Nylon etc .....
:-[ :-[ :-[ :-[ Thanks, I was looking for fiberglassing supplies the other day and so I mixed myself up. :-[ :-[ :-[ ;D
Still not that far. :D
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The Material they are using is softer than Delrin but I don't know what it is they use. With Delrin in a concave face the outer edge does the sealing and gets the full force of the area flexibng the outer edge as you show. The amount of flaring at the edge depends on the angle of the concave and flexilbility of the material. In the US FT the amount the lock frees up to speed things is countered by the exhaust valves tendency to grow at the perimeter and drop velocity.
In a harder material like delrin you can get a good seal going at one pump and it can take the load of 14 pumps (or about 1500 PSI) without any noticeable set taking place. In a PCP where it sits at that level constantly the thing does yield and takes a permanent set. We pull them at about the one month point and trim the outer edge to restore velocity (if it has lost any) or just turn the hammer spring in to compensate.
Scaling up the valve volume of an airgun almost always involves scaling up the stem size and the head size. At the end of the day the porting is the only thing that needs to scale up. Everything else bigger just makes you need a bigger hammer. I'd bush the valve thru hole down to 2MM and try a US FT Delrin Valve to replace the Factory exhaust. Then it will beg for a lighter hammer and hammer spring.
Think hard and small. Big is in your way!
TimmyMac1
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Using a small stem is important to reduce the closing force of the valve, the area of the stem (and the pressure) is by far and away the largest component of that force, and it is that force that competes with the hammer momentum over the dwell of the valve.... If I had not already made my .308/.357 Hayabusa valve for a 5/32" Disco stem I would, now that I know more than I did a year ago, have made it to use a 1/8" stem.... Going down to 2mm on a gun that is intended to produce over 200 FPE would likely just cause the stem to force it's way though the head of the poppet, especially on Delrin.... I purposely DIDN'T change the stem size on the new valve for my Hayabusa because I wanted it to be a test of the PEEK material, which, BTW, I am totally sold on for large valves at high pressure.... In fact, if you are getting distortion on the Delrin poppets on your US FT, I would suggest you experiment with it, it is pretty amazing stuff....
While I agree that large ports are problematic if you don't need them, you simply can't force enough air through a 1/4" hole to produce 250 FPE using 3000 psi.... unless you are into 3 foot long barrels.... With the quest for huge amounts of FPE comes the necessity to have either big valves or mondo pressure (or both).... The throat of the valve is just one point of restriction in the system, and no amount of increasing ports downstream of it can overcome a throat that is too small for the job being asked of it.... In fact, hogging out the ports way bigger than the area of the throat will lose performance instead of gaining it, as the pressure will drop before reaching the pellet.... Every point in the porting should be balanced in area, not only to the other points, but also to the job (FPE) expected of it, for best efficiency.... For the absolute maximum in power, that diameter (or area) will be the size of the bore itself....
Bob
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So why don't we see valve systems that are made of steel similar to automotive designs? This is what 3500psi did to my poppet
(http://i1284.photobucket.com/albums/a568/edgeer1/WP_000731_zps4739af1e.jpg) (http://s1284.photobucket.com/user/edgeer1/media/WP_000731_zps4739af1e.jpg.html)
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Steel to steel can work, provided it is lapped in.... The main difference as I see it is that automotive valves don't have to hold pressure for long periods of time, only very short time periods.... I don't know how good they would be holding 3000 psi for months at a time, which we expect of our PCPs....
Bob
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Steel poppet on a Ti seat might seal up well...Ti is a bit lower modulus than Steel and would perhaps settle in properly and seal up. It would be wicked low energy to kick off its seat and open up...:)
cheers,
Douglas
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Unfortunately, the energy required to crack the valve is a relatively small part of the hammer energy.... assuming the valve is working properly, not robbing huge chunks of energy like my Disco valve was.... Most of the hammer's energy and momentum go into creating lift and dwell respectively....
Bob
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So why don't we see valve systems that are made of steel similar to automotive designs? This is what 3500psi did to my poppet
(http://i1284.photobucket.com/albums/a568/edgeer1/WP_000731_zps4739af1e.jpg) (http://s1284.photobucket.com/user/edgeer1/media/WP_000731_zps4739af1e.jpg.html)
Hehe.... We do! It's just a matter of time.
http://www.gatewaytoairguns.org/GTA/index.php?topic=55930.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=55930.0)
If poppet valves had better flow characteristics than tulip valves I guarantee F1 would be doing it. But their not .
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Significantly different flow characteristics between an F1 intake valve operating at less than 1 bar and a PCP exhaust valve operating at 200 bar.... and also a significantly different mode of operation (direct mechanical vs knock-open).... The HPA in our PCPs acts more like water than air because of the density....
Sean's valve is beautiful, but it's also not steel to steel.... I think that for our purposes his valve design may be close to optimum, particularly if paired with a more streamlined head on the poppet (maybe a hemisphere?).... Don't forget that our rather clunky looking poppets have a spring that is very close to (or is the same as) the OD of the poppet, and if the airflow is on the outside of the spring instead of through it, they may not be as bad as you think for flow....
Bob
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I'm not entirely convinced the two are really all that different. Yes the PCP is operating at 200 bar but we have seen, I'm sure, the Ping pong ball blow a hole through the paddle like it was made of tissue paper. The energy released was as a result of vacuum. Which is also causing the flow on the intake side of an engine.
To me working fluids are working fluids. Flow is flow. We frequently use wet flow models to study air flow because we can more easily see and describe what we are actually seeing.
The whole science of flow and fluid dynamics is nowhere near in the bag yet. If it was we wouldn't be spending millions every year designing and testing new theories and making great strides in ship design and even wing design. Shocking we still don't know everything there is to know about a silly wing.
Ultimately what I'm trying to say is I think there are a lot more chapters to be written in the flow book.
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Fortunately.... Life would be VERY dull if we didn't have anything left to learn.... *grin*....
Bob
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Sure am intrigued by the Theoben type valves ( AKA: taper seat automotive head types )
Recently working with a HATSAN AT44 platform, again find myself working with this type seat/poppet design.
(http://i1320.photobucket.com/albums/u532/scott_schneider1/DSCF9099_zps857a4a17.jpg)
What I DO NOT understand at this point is why a valve seat/ poppet of this smaller design and area is seemingly able to flow the volume of what appears to be larger in a more conventional poppet as Bobs is sharing ??/
Yuppers .... learning every day :-\
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Easier to open, reaches full flow faster......once the pellet starts moving flow has diminishing returns over time.
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If you are stating that tapered valves are easier to open, that has not beem my experience, or that of many that mave tried them.... They tend to wedge into the seat.... This is obviously not as much problem in a car engine with steel valves and seats.... and Sean's PEEK with a very narrow seat will help that situation as well....
Bob