There is a good thread about modelling (conventional) PCP vavles, and how hammer weight, travel and spring force affect the FPE....https://www.gatewaytoairguns.org/GTA/index.php?topic=48892.That should keep you busy for a while, when you have questions, I'll do my best to answer them.... Bob
Been aware of this thread and seeing what folks have to say ? .... good links used that show how wildly variable the original question can be dealt with.If one wraps there mind around the basic dynamics in play ... that being the valves poppet is nothing more than a PLUG over a hole. On the sealed side of the plug sits Pressure. Higher the pressure or Greater the size / area of the plug harder it becomes to lift the plug and allow flow. Like a sink drain which uses a plug, the plumbing size below the plug controls how quickly along with volume that can pass threw. The size of the plug itself and area it has establishes POTENTIAL of flow that could pass by if or when the down stream area is equal to it, If less opening the plug further will gain you little and now you have farther to go in closing the plug to stop flow.In another abstract ...... With the poppet closed with pressure holding it shut ... THINK OF THIS LIKE A NAIL being driven into a board. lets just say nails already started and sitting they firmly resistant to doing anything. ( Plug in the hole )Your HAMMER is Lol .. a Hammer !! If you used a lighter hammer you would need to hit the nail VERY FAST & HARD to drive it further into the wood. If using a heavier hammer a much lighter strike on the nail could result in equal force applied to drive the nail inward an equal amount.The balance we strive for in creating an efficient PCP is that the force we deliver to the valves poppet ( Plug ) it that where we have good control on how far the poppet lifts ( Like driving the nail into the wood ) and based upon the resistance we have in doing so ( This the Area of poppet and pressure against it ) we find a balance in hammer weight and the spring energy that creates the motion sending it to hit the poppet stem.Enter the world of balanced / reduced force valves and your into the rabbits hole in near free fall !!While this may read like a child's lesson ... it makes those who read such threads hopefully grasp a basic level of whats going on in terms that hopefully make sense without all the technical verbiage.Scott
To distill that thread down as simply as possible....When the hammer first hits the valve stem, some of it's energy is lost decompressing the poppet material.... What is left over (the residual energy and momentum) governs the lift and dwell....The force to crack the poppet is the seat area times the pressure.... The energy lost in cracking the valve is that force, times the poppet compression distance, divided by 2.... Hence, harder materials (eg. PEEK), which compress less, are easier to crack open, leaving more energy and momentum to create the shot....Residual hammer energy creates valve lift, while residual momentum creates valve dwell.... Changing the hammer mass does not change the energy (lift), but it does change the momentum (dwell)….When you change the hammer mass, you therefore change the ratio of lift to dwell.... For any given hammer mass, increasing lift also increases dwell, and vice versa....Hammer energy is created by the valve spring.... how strong is it, what is the preload, and how far is it compressed during cocking....Once a poppet opens more than 1/4 of the seat diameter, the flow RATE does not increase, because of the "curtain effect" (throat area limits the flow)....The amount of air (energy) released by the valve is roughly the area under the lift to dwell curve, with the lift clipped by the curtain effect (if any).... Once the valve opens 1/4 of its diameter, the only way to increase power (FPE) is to increase dwell....There is a loose relationship between the hammer mass and the FPE it can release.... of particular importance in Big Bores.... However, if cocking force is not an issue, there are many combinations of hammer mass and spring energy that can produce the same FPE.... often with different efficiencies.... Balanced valves pretty much throw all of above out the window, because they are designed to be easy to open, and in fact many designs actually "blow open" to some degree, once cracked.... For this reason, they can be fussier to tune, and some cannot be tuned to create a bell-curve, they always have a declining velocity as pressure drops.... The less "balanced" the valve, generally the more it acts like a conventional valve, and the wider the tuning range possible....BobPS , Bill, if you figure out how to distill this down to an equation, please let us know, that would be most helpful....
I believe about 60 gr?.... It's not that difficult to calculate the hammer energy and momentum.... Figuring out what is left over after the valve cracks is much harder.... and quantifying the "minor" valve closing forces is next to impossible.... The "opening" force I talked about above.... The constant "closing" force is the stem are times the pressure (because the outer end of the stem is at atmospheric pressure)…. To both of those you have to add the hammer spring force.... The complications arise regarding how far the valve must open before the cracking force tapers off (pressure on both sides equalizes)…. and at what part of the shot cycle the flow past the poppet starts to build up a significant pressure difference which increases the closing force.... Then, just at the end of the shot cycle, as the poppet approaches the seat, the flow chokes, and there is a relatively sudden pressure differential across the poppet of about half the pressure (a condition of choked flow)…. Effectively, what this means is that the flow drops off just before the poppet seats, "clipping off" the last part of the dwell.... Very complicated to model.... Try cutting the hammer weight in half, and see what you have to do to compensate (increased travel or spring force)…. or if you had way too much hammer dwell in the first place, just enjoy the increased efficiency.... …. In my experience, small percentage changes in hammer mass do little....Bob
Regarding the Cothran valve, I wasn't trying to sell you on it. I spend more time talking people out of them then suggesting they buy a Cothran. They are very specialized valves for a very special purpose, and they perform that purpose better than most any other commercially available valve. However a lot of people buy them only to realize they aren't a good fit for their application.Making poppets.... Keep in mind I only have a cordless drill, a dremel and a radial arm saw with a drill chuck on one side..... Those are my "machine' tools, lol..I mount the peek to the stem first, then shape it using the stem as an arbor. the most critical step is the initial drilling of a centered, parallel hole in the peek. the closer to perfect the better, but it can be far from perfect, this just makes the next steps more challenging and take more time/effort. Use a significant interference fit where you basically hammer the stem into the peek and trust me, it will never move. for example, on my 2mm stems, i drill a 1/16 hole in the peek. You have to be careful and not bend the stem, if you do, time to start over.. for shaping and finishing the sealing surface, High speed is your friend. I run my dremel wide open. Rough shape with a file, then use sandpaper up to at least 800 grit to polish the sealing surface. I don't have a method to do anything other than flat, or actually slightly convex poppet faces. Once you get it where you think it's ready, paint the face of the poppet with a sharpie. insert the poppet into the valve and chuck the stem in a drill. run it at speed for 5 seconds or so, pulling pressure against the valve seat. Reverse direction, repeat a couple times. Remove poppet and inspect the face. If you have a nice ring evenly worn around it, you've done a good job and you are finished. If not, go back and repeat the shaping/polishing process. it's really not that bad. With my crude tools/skills I actually find peek to be more forgiving as it will polish like metal vs. fuzz up like when you get delrin too hot....If your valve seat is rough, a square of 500grit on the face of a poppet will smooth it right up....don't be afraid to mess up a poppet or two. it's part of the process.
Get used to making and tuning SSGs.... They are the key to eliminating hammer bounce....IMO, unless you are building a large caliber 22XX conversion, the MRod poppet is the way to go.... It will withstand up to 3000 psi on a 0.266" throat without extruding into the throat.... I only went to PEEK once I couldn't get enough hammer strike without excessive cocking effort.... A hint at what you are trying to achieve would help us give better advice....Bob
I generally make a longer rear cap, so that I can move the adjuster back, leaving more room for a longer, softer spring.... That is the key to making an SSG without making the gun harder to cock.... Of course if you are working with low power, eg. CO2, that isn't an issue....NoBob
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