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Thread: Build advice needed

  1. #1
    Sharp Shooter

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    I stated in an earlier thread that I plan to build a single stroke pneumatic air rifle this year.


    This is just a rough preliminary draft
    Blue represents the trigger/ bolt housing. Green and red is the barrel and shroud. White is the valve and exhaust port. Yellow is the pump etc and pink is the pump tube.
    The idea is to push 200 cubic centimeters of air into the valve with a single push. That's a lot of force required and this has been done so the idea isn't new.
    A two arm lever mechanism that works on a double fulcrum was used and they state that they achieved almost 3000 psi in the valve with a single pump. My maths isn't good enough to calculate that - I just like the idea and am prepared to give it a go.

    Anyone here know the math behind this? There mechanism works, there rifle shoots. I just need to work out the volume area of the valve compared to the pump tube to get maximum results.

    And yes, I might just blow myself up
    Last edited by Timothy; 17-01-17 at 18:50.
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  2. #2
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    Hi Timothy, From the gas laws, P1xV1 = P2xV2 (if the temperature remains constant). In that equation,P1= Air pressure= 1 atm and V1 = 200 cm3(Given by you). And if Final pressure= P2 =3000 psi = 204 atm then you cansolve the equation for V2 (the volume of the valve). The volume of the valve should be 0.98039 cm3.

    Calculating the lever/pivot points/ becomes very complicated with a double pivot. Creating the most efficient double pivot lever- I wouldn't even know where to start !
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  3. #3
    Sharp Shooter

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    Quote Originally Posted by dnolanevans View Post
    Hi Timothy, From the gas laws, P1xV1 = P2xV2 (if the temperature remains constant). In that equation,P1= Air pressure= 1 atm and V1 = 200 cm3(Given by you). And if Final pressure= P2 =3000 psi = 204 atm then you cansolve the equation for V2 (the volume of the valve). The volume of the valve should be 0.98039 cm3.
    Calculating the lever/pivot points/ becomes very complicated with a double pivot. Creating the most efficient double pivot lever- I wouldn't even know where to start !
    Thanks, appreciate that. I kept on getting varying figures. Even using your above example I still get varying figures using online calculators for boyels gas law. Maths is not my strong point. Figuring out pivot points is. I do that every day LOL
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  4. #4
    Sharp Shooter

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    I finally figured it out. So I need a valve with a volume of 1 cm3 which would be a cylindrical tank of 1.3 cm high by 0.5cm radius so ID of 10mm and 13mm high/long. I have watched countless videos on valve modifications where everyone modds the valve to make more volume thinking that they will get more power and it doesn't really do that. Thinking more volume = more air = more oomph. But I stumbled on a vid and though he won't reveal the secret behind his valve he let loose that he was pushing 12.5 cubic inches of air into a space the thickness of a coin and that struck me as different so I did the math (well dnolanevans did) and it works. Note the volume calculation in relation to a average sized coin He states a result of almost 3000 psi per shot and with a average 695 FPS with 14 gr 5.5 pellets (15.5 foot pounds) and 593 FPS with a 25 gr jsp pellet (19.53 foot pounds). I will be experimenting with a crossman valve and a marauder valve. Will be modding the insides to decrease the volume to as close to 1 cm3 as possible and the pump tube will be 3.5 mm thick carbon tube that I will harden and then temper down to 45 RC - just in case something pops. Only putting this all up here if anyone is interested - will post as I get time to do the machining and build.


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  5. #5
    Sharp Shooter

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    And its started:
    Bam barrel in 177
    modified Crosman valve
    300mm x 27 mm gabriel shock with outer sleeve.
    Next to machine the parts, work out the mountings and cast the pump seal.
    Going as cheap as possible with this experiment to see if it works and what velocities I get before I start the 5.5
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  6. #6
    Sharp Shooter
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    Very interesting project... I'll be following this!
    Keep it coming!!
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  7. #7
    Sharp Shooter

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    Hi Timothy
    Very interesting project, I will be following this build. I love the mechanics of a single stroke pneumatic air rifle.
    Why don't you use hydraulic tube? You can buy it in many different diameters and wall thickness and specified burst pressures, it will save you a ton of work.
    Have you seen the plunger valve on the internet that is used in stead of a knock open valve? I like the simplicity of it, Jim Bowkett also uses this system in his SSP rifles.
    Anyway, good luck, I will be watching.
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  8. #8
    Sharp Shooter

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    Quote Originally Posted by sirshootalot View Post
    Very interesting project... I'll be following this!
    Keep it coming!!
    Thanks, will do.
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  9. #9
    Sharp Shooter

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    Quote Originally Posted by Buksie View Post
    Hi Timothy
    Very interesting project, I will be following this build. I love the mechanics of a single stroke pneumatic air rifle.
    Why don't you use hydraulic tube? You can buy it in many different diameters and wall thickness and specified burst pressures, it will save you a ton of work.
    Have you seen the plunger valve on the internet that is used in stead of a knock open valve? I like the simplicity of it, Jim Bowkett also uses this system in his SSP rifles.
    Anyway, good luck, I will be watching.
    Thanks. I plan too. But on this one I wanted to go as cheap as possible without sacrificing safety. So a car shock is more than adequate for the pump. The inner tube has a 1mm wall and is seamless and polished which cuts out a lot of work. The high pressure is not problematic in the pump tube. It's the valve. I will be machining a housing tube out of en45 for the valve and hammer so if anything does pop it's contained. High air pressure is not a joke. I recently saw a vid of a pcp failure where the valve shot right through the persons thigh! That's like a 50 cal bullet! Safety first. If I get 1800 - 2500 psi for the 177 with a single pump I will be happy and ready to spend some money doing the 5.5mm
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  10. #10
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    Hello Timothy,

    An interesting project, good luck with it.

    If I may make a suggestion, when making the valve housing,put a gas escape vent or 2 in there, pointing in a safe direction just in case something gives way in that Crosman valve, something similar to those used in bolt action powder burners.

    Cheers.
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  11. #11
    REP: Xhariep Jagters Colesberg

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    Will also be following your thread as I have a few spares myself but I'm thinking of a pcp pistol for my purposes...
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  12. #12
    Sharp Shooter

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    Quote Originally Posted by Juniper View Post
    Hello Timothy,
    An interesting project, good luck with it.
    If I may make a suggestion, when making the valve housing,put a gas escape vent or 2 in there, pointing in a safe direction just in case something gives way in that Crosman valve, something similar to those used in bolt action powder burners.
    Cheers.
    Thanks mate - I plan to Don't want to blow myself up.
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  13. #13
    Sharp Shooter

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    Started the barrel mountings - Below is the sample


    So in order to make sure that both pieces were aligned along the barrel I machined them from one piece.



    The finished mountings below and now to drill the holes and align them to the pump sleeve. I will put the pump sleeve in the lathe and lock the chuck. Then i will use a bit to scribe a line along the length of the pump sleeve and this will give me a perfect line to align the mountings to before I drill and tap the sleeve.

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  14. #14
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    Very interesting project! I will be following closely. Good luck with the build!
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  15. #15
    Sharp Shooter

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    Ok, having scrapped the original plan I learnt a few things and made some adjustments.


    Old relum stock, pump chamber, breech, hammer, and barrel mounted. That was the trickiest part - aligning it. Hopefully got it right.
    Still a lot of work to do and this project is possessing me. Need to remind myself to eat, sleep and pay attention to the family.
    At the moment she looks a little rough but after 8 yrs making knives I have learnt to focus on the mechanics and leave finishing till last - otherwise you never get finished.
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