Modeling PCP Valve Lift and Dwell

[rsterne]

and closing, after the seat touches but while the Delrin is recompressing.... so five parts to the lift cycle.... The first, middle, and last are pretty well defined, it's the "just open" and "nearly closed" parts we really have no idea how long they last.... ie how much lift before they become insignificant compared with the stem closing force....

If you notice, I made a note above that the figures in my chart for FPE are twice as large as they should be (I forgot the 1/2 in E=1/2 MV^2).... Maybe if we just use the numbers in the chart as is, the extra 1/2 could represent the transition forces.... Maybe, sorta?....

Bob

[Bill G]

Good catch fellas.  I did not think about the valve stem having force imparted.  I'm making corrections now.  I'm currently at the chrony and have been making moves on the adjustments.  I just realized that I never took the o-rings out from the staley tune.


Thanks
Bill

[Bruce]

subscribing!

Bruce

[Bill G]

Made the changes to the calculator and considered the aspect of three parts to the motion that, I agree, we really don't know whats happening relative to time.   Also went out and shot some over the chrony while fiddling with the adjustments.  It looks and sounds really good but I'm still not conclusive.  I got interrupted by my policeman neighbor.  No troubles, we live on 5 acre track of land, and he just wanted to joint in the shooting. said it took him a min to figure out that is what he was hearing.  He thought I was shooting a suppressed fire arm and he wanted some of that.  After seeing what was up he got all "dark sided" on me and burned up about 50 pellets and drank 4 of my beers.  He never realized that pellet guns were so powerful and accurate.  Next time, I'm at his place.

So I back out the adjustments to have min tension and 2 turns in on stroke(shorter).  I plug it all into the laptop and it predicts 730fps as velocity.  I shoot 3 shots and get 717, 723, 719.  Mind you, I'm tethered to a 2900psi scuba tank.  I say wow!  so, I add 2 turns of increased spring tension.  Laptop says 847fps...... Gun says 834, 835, 833.  Now I'm getting all tickely on the inside.  What to do now, I ask myself?  I then lengthen the stroke only by 1 turn.  The laptop says...891 the gun says.....895,895,895.  Whaaaaat!!!  (I'm rounding the decimals like it matters).  I'm in disbelief, so I shorten the stroke by 1/2 turn.  Laptop 881 and Gun 885ish.  Now I'm even deeper disbelief and I run out of sunlight, chrony won't work.  %^&$#@&!  I'll resume in the morning since I have the day off to get over the jet lag.  I did manage to get 4 shots at 2700 using +4 on the spring and shortened stroke by 2 turns from max travel.  The result was avg 954fps with Jsb.  Thats quite away from coil bind and the additional travel would get me even more. That means I'm gonna need bigger pellets.  Barracudas at 950, you bet.  I'm running a .156 Porting and the throat, minus valve stem, have an area that is slightly larger than the area of the port.  valve cap that holding the valve spring has been moded to allow more air in to free flow as well. 

I'll take up again in the morning
Bill           

[bstaley]

and closing, after the seat touches but while the Delrin is recompressing.... so five parts to the lift cycle.... The first, middle, and last are pretty well defined, it's the "just open" and "nearly closed" parts we really have no idea how long they last.... ie how much lift before they become insignificant compared with the stem closing force....

If you notice, I made a note above that the figures in my chart for FPE are twice as large as they should be (I forgot the 1/2 in E=1/2 MV^2).... Maybe if we just use the numbers in the chart as is, the extra 1/2 could represent the transition forces.... Maybe, sorta?....

Bob

You have the ability to measure lift and calculate hammer energy and momentum, so cycle through various hammer energies from full valve lock to valve fully open at a given pressure, so you can plot energy vs. lift throughout the various 'stages' and at different PSI levels.  This will give you an idea of the relative forces being exerted in each stage.  Time should be easy to calculate from there.

[rsterne]

Good idea.... When I have the time I'll see if I can do that.... Even getting the data on one gun at one pressure would help.... doing it tethered to a regulated tank will give a constant pressure.... Could be the fall before I get the time needed though....

Bob

[PakProtector]

Good idea.... When I have the time I'll see if I can do that.... Even getting the data on one gun at one pressure would help.... doing it tethered to a regulated tank will give a constant pressure.... Could be the fall before I get the time needed though....

Bob

Awwww, come on Bob, I am sure thare's stuff you can ignore so it can happen before that...
cheers,
Douglas

[rsterne]

Tell my wife to give me a reprieve from sitting behind the Motel registration desk.... *grin*....

Bob

[PakProtector]

Tell my wife to give me a reprieve from sitting behind the Motel registration desk.... *grin*....

Bob

I don't think it would have the desired effect Sir...but given the pix you have posted of the surrounding area, I'd love to come out for a stay. I could keep you company at the desk...
cheers,
Douglas

[bstaley]

Good idea.... When I have the time I'll see if I can do that.... Even getting the data on one gun at one pressure would help.... doing it tethered to a regulated tank will give a constant pressure.... Could be the fall before I get the time needed though....

Bob

Chrony readings to go along with the lift and energy numbers would be good too!

[rsterne]

I have been thinking about how the hammer weight may be proportional to the square root of the bullet weight, and this morning I woke up with the thought I should check Lloyd's spreadsheet on internal ballistics, and I find that in fact the dwell required to keep the valve closing at a specific point in the barrel (choose 33%, 50%, whatever) is proportional to the square root of the bullet weight if we keep the caliber fixed.... This is with no transfer port volume, no bore drag, not including the mass of the air, and using 100% efficiency.... This need for increased hammer strike as the bullet weight increases is consistent with reality.... I just never quantified it before.... I'm thinking my hammer weight proportional to the square root of the bullet weight is looking pretty good now.... On further investigation, if you leave the SD constant, the dwell is also constant, so this appears to work across caliber changes as well.... eg. double the bore, four times the bullet weight, the dwell stays the same.... However, we will need bigger ports, more lift and more hammer energy....

If the pellet weight increases fourfold, and we double the hammer weight, we increase the dwell by only 1.414 times.... We would also need enough spring and/or travel increase (also 1.414 times) to get the momentum (and the dwell) to the required value for the four times heavier pellet (ie double).... This leads to a variety of choices in the case of that 4 times heavier pellet.... which requires twice the dwell....

1. We can leave the spring and travel alone, and use a hammer 4 times heavier.... No change in energy.... If we examine this over large weight and caliber changes, it has no relationship to common practice....
2. We can leave the hammer weight and spring alone, and double the travel.... This doubles the velocity of the hammer (half from the increase in travel and half from the increase in spring force).... and therefore produces 4 times the energy....
3. We can leave the hammer weight and travel alone, and quadruple the average spring force (doubling the velocity).... This also produces 4 times the energy....
4. We can do any combination of the above which results in twice the momentum and dwell.... If we double the hammer weight, and double the average spring force, the energy (and hence the lift) also doubles.... This results in a lift to dwell curve of the same shape as we started with, twice as high and twice as wide, with 4 times the area.... That, incidently, is the same as the increase in pellet weight, a factor of 4....

When I originally suggested that we increase hammer weight by the square root of the pellet weight, I was trying to come up with a way to make an educated guess of how big to make the hammer that would work for any cailber and bullet weight.... As we know, there are any number of answers, many of which can be made to work.... Let's think for a minute of what happens if we "scale up" a bullet/pellet in all dimensions, by doubling it.... We have twice the caliber, 4 times the bore area, 8 times the weight, and twice the SD.... I would suggest that incresing the hammer weight 2.83 times, and increasing the travel and/or spring force to increase the energy by a similar amount would be a very workable solution.... We would end up with a lift to dwell curve of the same shape we started with, and with 8 times the area.... To me, this makes sense, and should allow good decisions to be made when building a larger caliber gun where we have no previous experience, as an example.... Incidently, the required hammer velocity ends up being the same if you change the weight by the square root of the pellet weight, and the area under the curve is in proportion to the pellet weight.... To get that hammer velocity using a heavier hammer, you need to use a stronger spring, increased travel, or both....

Note that this is not an argument for light or heavy hammers.... If you are pleased with the results you get in .22 cal with a hammer of X grams, and you want to build a .44 cal, shooting a bullet 8 times as heavy, then you could use a hammer of (square root of 8 ) = 2.83X grams, and once you get the travel and the spring force dialed in, you should be working with a gun operating in a similar manner.... All I am suggesting is that it gives us a workable "scaling factor" to use as a design tool.... Choose a constant that "works for you"....

Bob

[Motorhead]

Bob,
As always good overall conveyance of the point of view being shared here.
Which now gets me thinking that when My R&D was happening and came to the weight hammer that worked best it was a .22 caliber rifle shooting 18 grain pellets.  Having now built another somewhat similar rifle tho .177 caliber that likes 10.3 grain pellets, it has me pondering using the previous pellet weight to hammer weight ratio of 1.5 building yet another hammer to fit within this model figure ( 15-17 grams )

Easy enough too knock one out ... have to do so and share the findings 

[rsterne]

Here is a graphic representation of the above....



In this case, I looked at what was the required dwell for a pellet 4 times as heavy (same caliber).... The dwell needs to double to make the valve close in the same position (eg. 33% of barrel length or whatever).... If you use the square root of 4 for the increase in hammer weight, that means the weight doubles.... If you adjust the spring and travel to maintain the same hammer velocity with the new hammer, you get twice the energy (lift) and twice the momentum (dwell).... The curve is the same shape, and contains 4 times the area.... It is my belief that is the best place to start to get the gun to shoot well with the new heavier pellet....

An example would be going from 25 gr. in .25 cal to 100 gr..... That would suggest going from a 60 gr. Disco hammer to one of 120 gr.... If you increase the travel from 0.58" to 0.82" (and providing the spring can handle the extra travel without going coil bound).... that should maintain the same hammer velocity.... 0.82" / 0.58" = 1.414, which is the square root of two (the increase of the hammer weight).... This is just one way you could meet the energy and momentum requirements.... but I think it would be a good place to start.... I'm not suggesting you wouldn't need other changes, you would need larger porting to take advantage of the additional lift, and provide more flow for the heavier bullet.... but everything is kept in proportion by following this guideline....

Bob

[Bill G]

I think your on the money with the theory.  If you where searching for a single hammer to serve across all small calibers,  I think you may have reversed that argument.  I would seem to me that you would be better suited to discover and engineer a hammer specific to your desired caliber and energy output.  Also choose a spring that delivers the required performance and have the hammer throw long.  I've done more testing with my spread sheet and every adjustment that I make and get a prediction for velocity is within less than 1% from shot to shot.  Shortening stroke is the same as reducing spring tension.  The exception is when I get the spring compressed beyond 75% of its free length.  I think that the force curve turns upwardly very sharply above 75% compression.  Below 75% and it is almost 1:1.  The deviation could be the fact that the two adjustments have different thread pitches so evaluating the adjustments by way of turns, instead of distance/turn, could account for the difference. The larger the ports and better the valve breathes, the steeper the string gets.  This can be over taken by lowering the fill psi and still getting the desired velocity. If I'm willing to take a 3% deviation in velocity, I'm loosing 6shots(30% of the string) and gaining 25% in energy output.  I think what I need now is a lighter hammer to test the out come.  How much lighter, Who knows?  I'm gonna figure that I can get this thing to tell me though.  If I put a known spring at max compression and hypothetically reduce the mass of the hammer until the dwell is correct for the desired output, That should be the hammer mass required.  If I then employ that hammer and the result are as accurate as i have been getting, then computation matches empirical data and tuning a gun just got a whole lot quicker.  Yesterday when I was tinkering, I had a problem getting all the calculations to match up with empirical outputs and couldn't grasp why.  So I took a guess.  I removed the spring and tested the spring rate.  It was supposed to be a 10# spring from MSC.  When I set up my little rig and began adding weight, It turned out to be an actual 8lb 12oz spring rate.  Adjusted that input and it all came together.   I'm very excited about this thing.

Bill 

[rsterne]

When the hammer strike requirements are low.... for example in small calibers and using low pressures.... there are many more solutions that can work.... You can use lighter hammers, lighter springs, less travel, and still get enough energy and momentum to open the valve.... However, when you start pushing towards lots of FPE, if you try and use too light a hammer, or too short a travel, you can't even pull it back because the spring is too stiff.... You want to run a .50 cal or bigger on 4500 psi.... be prepared for a big heavy hammer with lots of travel.... and even then it will be hard to cock....

Remember, at 950 fps, the FPE is twice the bullet weight.... Instead of calculating hammer weight as (for example) 14 times the square root of the bullet weight .... you could just as easily use 10 times the square root of the FPE....

100 grain bullet.... square root is 10.... hammer is 14 x 10 = 140 grams....

200 FPE.... square root is 14.14.... hammer is 10 x 14.14 = 141.4 grams....

I think that formula will well work for other weights and velocities as well.... In fact, it might be easier to understand than one relating to bullet weight.... It's possible that the constant should be varied for different fill pressures, I haven't even considered that.... or possibly that can be just looked after with springs and travel....

Bob

[Motorhead]

Stock Marauder hammer is 78 grams .... which by the formula being discussed would be ideal with a 52 grain pellet    Yikes !!

[PigButtons]

Hi guys, new to the forum and really enjoying it. I hope you don't mind if I enter into your thread with some observations. I've been enjoying this thread so much that it caused me to join the forum. So please understand that I'm not critiquing the content of the thread but just trying to add to the thread's progress in what I hope is a constructive manner.

1. The focus of the thread has drifted organically from the original post from shot string optimization to performance of observed behavior vs formula predictions as they relate to hammer weight and pellet weight. A logical and useful progression, but drift never the less.
2. The assumption that the volume of air doubles because the dwell doubles is somewhat suspect. If the pressure in the tank remains constant then the assumption is correct. But if the pressure drops 10% the air density also drops 10% (or close enough for this discussion ) hence the volume per given dwell time will also drop 10%. Couple this with the fact that the drop in pressure also represents a drop in energy to force the air out of the tank and volume is further decreased.
3. The force on the valve and the stem are only significant when lifting the valve off of its seat. Once open the pressure is practically equal on both sides of the valve and therefore insignificant when calculating any further resistance to opening or closing pressures. ( You many want to make the case for drag caused by the air flow over these parts but I would think that a small enough variable to ignore). The observed difference in valve opening height is probably due to the significant difference in energy expended getting the valve off of its seat when the pressure changes.
4. In most air guns the lift and dwell are not such well rounded graphs due to the mechanical limits on valve / hammer travel. Once the valve / hammer have bottomed out the remaining energy / momentum is transferred to the stop and drops to zero and immediately begins its recoil. Depending on the elasticity of the materials there may be significant rebound as well, not to be confused with the valve bounce mentioned in the thread.

[PakProtector]

Stock Marauder hammer is 78 grams .... which by the formula being discussed would be ideal with a 52 grain pellet    Yikes !!

Yeah...and I hope my next set of throat cutting throws out Bob's, 'keep the area the same from valve seat to barrel port' practice...'cause I want some more power, and the Maruader valves seem to be an exception to his usual work.

I think I will use the set screw striker to get a wee bit more travel, and take a grinder to the spring pocket to lighten slightly and attempt sticking in the 15# spring.
cheers,
Douglas

[rsterne]

Going further with the thoughts of using FPE instead of pellet weight to estimate the required hammer weight.... I made up the following graphs.... I used 5, 10, and 15 as the constants for the three curves.... The second graph is just an expanded portion of the first, showing more detail below 100 FPE....



By including the three curves I am trying to show that there is a large latitude of hammer weights that will work, some better than others, of course.... If you were to try and use a light - fast hammer in a 1000 FPE gun, you might need both hands to cock it.... If you follow along a curve what you are basically doing is using a constant hammer velocity.... and keeping with the same shape lift to dwell curve (ie relationship of energy to momentum).... Light - fast hammers have less momentum for a given energy (or more energy for a given momentum).... Heavy - slow hammers have more momentum for a given energy (or less energy for a given momentum).... It is, of course possible to draw such curves through any hammer / FPE combination which you know works well.... whether between the curves shown, or lighter or heavier....

Bob

[Motorhead]

Liking that ... a lot !!

If an accurate range ... sure going to make the task of fiddling a lot easier for those who wish to do so.