Dual Tank Benchrest Rifle

[subscriber]

Wow, Bob, that is a lot of mechanism in a small space.  The potential for such small structures breaking makes me wonder when it is most likely to fail; if ever.  If a low strike force valve became self opening because something broke, I hope that occurs after a deliberate shot - when the chamber would always be empty.  Not after loading the next projectile, to go off when you are not yet fully lined up on target, or berm.

I am not so much concerned with a bench rifle, as a field rifle where it would be common practice to walk about with the chamber loaded, and either the striker decocked, or on safe.  Certainly, safe gun handling should have the muzzle never pointing at something valuable that does not need to be shot.  But a stray shot still has to go somewhere. 

I am not criticizing the design, but such small parts with a few hundred pound of force on them could be vulnerable to breaking eventually.  It is not the holding force that concerns me as much as slamming onto the seat repeatedly.  I think your holding force reduction should reduce the fatigue of the parts significantly, which sounds like a good thing.

The small valve stem with a vent drilled down its length seems potentially fragile, but probably less so, because it does not require as heavy a hammer blow as a conventional valve.  Am I right that you are making the valve stem from unhardened drill rod?  If so, it probably has good fatigue resistance, at the expense of potentially peening at the hammer strike face, or being bent.  Although bending would require it being struck slightly off axis, and I don't think there is enough clearance between stem and guide to allow that.

What you are doing is like making a functional piston engine with a tiny bore diameter.  Impressive!

[Rob M]

Thing is , all balanced valves have this level of complexity and theyre becoming quite common commerically. A lot of AEA models have them , fx uses balanced valves, so does daystate.Even your everyday action pistol with a spool valve is technically a balanced valve requiring very little hammer to produce the shot. Are they as reliable as conventional hammer valves ? NO .. not from what Ive seen. But Bobs not selling these guns commercially , so thats not an issue.

[subscriber]

As I understand it, balanced valve advantages are low cocking force and reduced vibration, starting from before the projectile starts moving until it leaves the barrel.   I can see these being of significant value for high FPE slug shooters.

Any other advantages, such as more consistent projectile velocity or POI (other than from reduced vibration)?

I presume that making the valves more robust will make them heavier,  sluggish in opening or closing, require more hammer strike energy, and increase system vibration.    Unless materials are optimized for properties and just the right amount of material is used in the right location. 

One possible improvement, if this was a commercial valve would be to harden the stem, then give it a spring temper.  Something stiff and very strong, but with very good fatigue properties.  Don't know how to improve the PEEK valve seat.  A narrow sealing area is going to take a beating eventually.  A bit like a racing engine:  You accept a shorter life between rebuilds for increased system performance.

[sb327]

Hey Bob,

If you’re concerned about the peek wanting to collapse (or yield), you might consider glass filled stock. I use glass filled pei for mine and haven’t had issues.

Subscriber,

The concern over the stem is not necessarily warranted due to the fact the hammer strike is SIGNIFICANTLY reduced with a bv. Also, the closing force is no different than a normal poppet valve so the concern with the seat being beat up is no different than any other valve.

Dave

[subscriber]

Thanks, Dave.  A lighter hammer strike would mean the valve stem does not work as hard.

It is not only closing force with the valve closed, but closing speed and valve mass that predicts closing impact.  That said, if any given valve configuration lasts long enough, then what does it matter if there might be a better material?

[Rob M]

Thanks, Dave.  A lighter hammer strike would mean the valve stem does not work as hard.

It is not only closing force with the valve closed, but closing speed and valve mass that predicts closing impact.  That said, if any given valve configuration lasts long enough, then what does it matter if there might be a better material?

harder materials are easier to unseat.. soft materials like nylon take more hammer pressure to get off the valve seat

[rsterne]

sub, I understand your concerns.... During development over the past few years, balanced valves had indeed had failures.... My first was the "spider" at the front end of my first one failing....



In the search for minimum restriction at he front of the valve I made 4 kidney-bean shaped holes, and after 100 shots or so, they broke.... The next version had eight 5/32" holes, and each web was twice the depth, so over 4 times as strong.... The area of the holes is still over twice that of the valve throat, and no more failures.... Marco has had several "cages/spiders" break in his 20mm, but his valve is designed to blow open quite violently, as he doesn't care about efficiency, just maximum power.... With such a design, the poppet collides with the front portion of the balance cylinder, and can put thousands of lbf. on the spider, particularly if there is no O-ring buffer between them.... I have always used such a buffer, because if it only collapses 0.025" it reduces the peak force by a factor of 10 or more....

I have never used hardened stems, or hammers, in any of my builds.... My .457 Hyabusa had a 1/2 lb. hammer that took nearly 30 lbf. to cock, and it never showed signs of the collision between hammer and stem.... so I think that worry is overblown in PCPs.... Has anyone ever seen a stem peened, or a hammer dented?.... Yes, I have seen photos of thin stems bent, of course.... The hole in the stem is 3/64" (0.047"), and the stem is 1/8", so the reduction in compressive strength compared to a solid stem is only 14%.... The threaded portion is inside the poppet, so much of the forces are distributed.... Most poppets are simply pressed onto the stem, mine are threaded on....

Another point of failure I have experienced is having the O-ring on the stem of the balance chamber pull the end off the stem, and I'm not the only one.... I increased the distance between the O-ring and the end, which cured that, some have gone to metal stems, which is my choice on this design....

While it seems like there is a lot going on in a small space, I have done what I can to prevent failures.... I may still have missed something, and you are correct some failures in balanced valves can cause the gun to fire unexpectedly.... Those that have occurred to me have just resulted in the opposite, an inability to fire, requiring emptying the reservoir to disassemble....  Thanks for your concerns, they are very welcome, as it is always possible to miss something, and safety is always paramount....

Bob

[subscriber]

Thanks, Bob

[mackeral5]

Seeing that damage reminded me of this one....





This is from when I was being hard headed and insisted on running a peek seal on a modified Cothran valve.  Just the right (wrong) amount of hammer preload resulted a tank dumping machine gunning episode.  The Cothran sucked the valve return spring over the poppet, wrecking the valve seat in the process. For good measure, the muzzle blast proceeded to scatter my chronograph and light kit across the shop.....

In hindsight, I did not understand how to properly size and locate the vent.  This all happened by combining a vent that was way too big, too much balance chamber volume, super hard poppet material, and then some idiot trying to force the valve to dwell by leaning on hammer strike........

I recall being extremely frustrated for a week or two before finally giving up on the peek and going to a delrin seal.....  Lots of nitrogen and lead went to waste during that episode.

[rkr]

This kind of makes me wonder, with small calibers like .184 do we really benefit from balanced valves? Couldn't we just use 2mm valve stem and PEI poppet to get the long dwell with reasonable hammer strike while keeping it all simple and robust? After all we are now talking about small throat diameter valves and thus much reduced force keeping the valve closed.

[Rob M]

This kind of makes me wonder, with small calibers like .184 do we really benefit from balanced valves? Couldn't we just use 2mm valve stem and PEI poppet to get the long dwell with reasonable hammer strike while keeping it all simple and robust? After all we are now talking about small throat diameter valves and thus much reduced force keeping the valve closed.

good point..

[rsterne]

Quite possibly.... However, I am not scaling down these valves, they were designed for a .257 cal, and I am simply going to taper the transfer port from valve to barrel.... This is partly to hedge my bets because the longest (only) barrel I could get for the .183 cal was 26", and I want to run regulated, so the most pressure I can run (and what I ordered the regulator at) is 3600 psi.... Otherwise I won't get enough shots for a benchrest gun, even with having 1.4 liters of 300 bar air on board.... I figure I can always throttle the gun down, and it doesn't take ridiculous amounts of hammer strike to make a valve of this size work, it is based on the ones in my last two Sporters.... The .172 and 6mm are regulated at 2800 psi and the .224 and .257 are unregulated at 3800 psi....

Besides, I can always make a small-stemmed conventional valve if these don't work out.... 

Speaking of not working out, I had to scrap the second poppet above.... The vent wandered off-center to the point it was coming through the side of the stem and I had to scrap it.... Since the regulated gun will be running "only" 2200 psi, I decided to make a single piece PEEK poppet and lose the aluminum balance chamber.... The dimensions are the same, just all PEEK (and the steel stem).... The vent is drilled beside the stem, just though the PEEK.... WAY easier to make.... 

Bob

[rsterne]

Well, I got the valve bodies roughed out today....



You will notice that one of the poppets is now all PEEK, as one gun is only 2200 psi and others have reported no problems at those pressures with similar dimensions.... If you look closely on that one, you can see the concave seat, machined with an 5 deg. undercut so that it only seals around the outer edge.... Also, I didn't have to drill the stem for the balance chamber vent, there was enough room beside the stem, through the PEEK....

Next is to machine the cage/spider for the front to fit in that recess, retained by a snap ring.... All the port, retaining hole and O-ring machining have yet to come....

Bob

[subscriber]

This thread is another treasure trove for those designing improved PCPs for commercial production.  Keep it up, Bob.

[rsterne]

Well, there have been a few of those.... The SSG, the use of plenums, reversed bottles, dual side-by-side bottles, simplified balanced valves to mention a few.... Not necessarily new ideas (there seldom are) but I did my best to promote these, and others.... I'm glad to see guys on the GTA find them useful, and flattered that many of the improvements in PCPs over the last decade and a half have spread into the commercial realm....

Bob

[PikeP]

This kind of makes me wonder, with small calibers like .184 do we really benefit from balanced valves? Couldn't we just use 2mm valve stem and PEI poppet to get the long dwell with reasonable hammer strike while keeping it all simple and robust? After all we are now talking about small throat diameter valves and thus much reduced force keeping the valve closed.

It's what I do and personally recommend (and have for the longest) for small calibers. This valve otoh probably benefits from being balanced, where as I would have to force an oversized poppet on my small .234" .25 cal throat just to have it make sense to balance. I currently run peek with .02" sealing margins.

The minimum clearance from piston the chamber will be 0.075", which is 1/4 of the valve throat diameter.... This means the valve can reach full flow without the poppet touching the piston.... That volume can be tripled if desired, to make the valve open easier....

Bob

I know .075" lift is where the valve reaches peek flow but wouldn't it need to lift significantly higher to produce max power ie: need more minimum distance between chamber and piston in your adjustment? I know it can be tripled but .075" seems really tiny if I am envisioning the space between between and chamber correctly, my guess is you would run somewhere in the middle?

[rsterne]

The valve will absolutely open more than 0.075" when going for maximum power.... The lift and dwell are inevitably intertwined, for a given pressure, valve and hammer mass.... You don't NEED the extra lift past the 1/4 diameter where curtain area equals throat area, but you may end up with that to get the dwell you require.... On the other hand, every time I have physically measured the valve lift when a PCP is tuned for the knee of the curve, I have never found it to be greater than 1/4 the throat diameter.... If it is greater than that, the gun is more than likely an air hog, and tuned way up on the plateau for the pressure you are using.... That's my experience, anyways.... Having said that, I expect the best tuning volume to be in the middle of the range I have allowed for, and the poppet to never hit the piston....

Bob

[mackeral5]

Bob, while it may be the less exotic of the two, I am looking forward to the .22 and seeing your results with the Monster Redesigned pellets.  I recently started shooting them out of a cleaned up Fortitude barrel, which i believe is around a 1:22 or so twist. It is sleeved up to 18mm with cf tubing, in a Brod.  While I have not tried them at slower speeds, they do not seem to mind running in the 990fps range.  They are not as difficult to get running fast as I anticipated---21.5" from barrel port to muzzle, around 2000psi reg.  Testing has shown many instances of 2-3 shots in basically the same ragged hole at 100 yards.  Unfortunately, more times than not 5+ shot groups open up to 1.5-2".  I hope some barrel/breech stability improvements (barrel band at tank block/Cobra reg) will help shrink those groups.  I have not tried head sizing/sorting/weighing...

[PikeP]

The valve will absolutely open more than 0.075" when going for maximum power.... The lift and dwell are inevitably intertwined, for a given pressure, valve and hammer mass.... You don't NEED the extra lift past the 1/4 diameter where curtain area equals throat area, but you may end up with that to get the dwell you require.... On the other hand, every time I have physically measured the valve lift when a PCP is tuned for the knee of the curve, I have never found it to be greater than 1/4 the throat diameter.... If it is greater than that, the gun is more than likely an air hog, and tuned way up on the plateau for the pressure you are using.... That's my experience, anyways.... Having said that, I expect the best tuning volume to be in the middle of the range I have allowed for, and the poppet to never hit the piston....

Bob

Makes sense, you need much less lift and dwell at 3600 psi than you would at 2400 psi for the same given power, and a lighter hammer needs more lift than a heavier one (even if marginal and often times negligible) due to the reduced momentum it contains fighting the valve closure.

Are you going to try out x-rings on this build or will you be staying with a traditional o-ring?

[rsterne]

The goal is about 80-90 FPE for the .183 ca (3600 psi), but only 50 FPE for the .22 cal at 2200 psi.... I'm going to start with the same hammer for both, with more spring on the higher pressure....

I have a couple of 1/4" X-rings for the balance chamber piston, and will try those.... Scott (Motorhead) has good success with them....

Bob