Swept volume.

Discussion in 'Piston & Spring' started by Tinks, Oct 24, 2018.

  1. Tinks

    Tinks Another tricky target at Lincs.....!!

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    A quick question for the tuners out there, what seems to be the minimum swept volume to achieve 11fpe in .177
    I say 11fpe, as I want a bit of room to set power easily at 10.4-10.7fpe.
    This question arose from my 23mm conversion peaking at 710fps with JSB 8.4gn and 730fps with Superdomes.
    If you need more information regarding the exact build specification please feel free to ask, thank you in advance.
     
  2. RobF

    RobF Administrator Staff Member

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    We've had 21's at 12 ft-lb... think we were about 30cc.
     
  3. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Hello Rob, thanks for that. Need to keep looking at mine then, as it currently has 34cc from 82mm stroke, I can push the stroke out to 88mm max but didn't want to as it means altering the laminated stock to allow the extra room on the cocking lever movement. More tinkering to be done it seems.
     
  4. Nick G

    Nick G Active Member

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    34cc will make the power easily, if your power will not alter by increasing the spring pre load , or you get into dieseling , the chances are your tube isn't sealing, either the sleeve , end cap or the piston seal itself .
     
  5. RobF

    RobF Administrator Staff Member

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    or it could be very tight...
     
  6. Nick G

    Nick G Active Member

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    Agreed , or the guide tight in the spring, the latch rod tight in the guide , or the piston not true to the rod , in the reduced tunes every little thing can add up and ruin the fun,:(
     
  7. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Hello Nick, thanks for the contribution, I am pretty sure the comp tube is not leaking, it was sealed at the TP (this is modified and has threaded inserts for quick TP size changes, so was plugged with an M5 screw and nylon washer), piston inserted and pushed down as far as my 11 stone frame could and it returned to the same position. Next test was to clamp this in a sash cramp and go for a smoke, after 5 mins the piston returned to the same position (maybe 1 mm lower).
    I had a very bad barrel lead in when I checked this, lots of visible light around a thumbed in pellet and could only get rid of this by lapping a 45 degree chamfer on the breach end of the barrel. This in turn gave me concerns of the void created at the barrel breach, which prompted the threaded TP insert machined with a 45 male to remove the void. This actually works really well and I can almost do away with the TP seal as I only lose 10 fps with it removed, will post a picture if you are interested.
    I have tried springs from 5N/mm up to 9N/mm each with dedicaated guides and upto 700fps they all perform as expected from the Prospero emulator, once the calculted peak pressure gets above 85 bar that's where things stop for me. This does sort of indicate a piston seal issue which I cannot rule out as I have not created a tune with o rings that go above 70 bar in a dynamic environment, my 2 other guns running o rings do not exceed a calculated peak pressure of 75 bar. Think I might try a different o ring section which can afford a little more crush than the 1.78mm BS018 I have now.
    So far this has been quite a journey, home made latch rod, profiled and hardened, piston head fitted to rod and turned from the rod to keep it true, pressed on delrin head holding the o ring and acting as a front guide, long top hat (40mm) sized to act as rear guide and support the spring, bronze washers to trim piston weight from 130g-180g, only to find it's a super sweet 9.5 ft/lb rifle, which made me think I was strangling it with too litlle swept volume.
    Thanks again for the pointers, I will update as soon as I get to the bottom of it, I'm sure you are correct about the leak, I just haven't found it yet.
    All the best, Richard.
     
  8. Paul_T

    Paul_T Member

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    Hello Richard

    Just out of interest how are you calculating the peak cylinder pressure?

    I'm still trying to get my head around this reduced diameter thing.

    Cheers
    Paul
     
  9. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Hello Paul, as above, the Prospero spreadsheet does it all for me. Enter your known values, spring wire diameter, spring OD, number of active coils, etc. along with the gun data, piston diameter, stroke, piston weight, calibre, barrel length, etc..... and see what the results look like compared to known gun. Once you have this you can fine tune the calculated output to match by varying the values on the top right of the spreadsheet.
    I have attached 2 images (cannot upload the xls file on here), the first shows where I can get to accurtely with increase in spring preload upto a spring power stroke of 99mm which equals 11mm preload. The second shows where I want to be, this requires a spring power stroke of 105mm or 23mm preload, however the calculated max pressure is now in excess of 100bar which I believe is causing my o ring to pass.
     

    Attached Files:

  10. Paul_T

    Paul_T Member

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    Cool thanks for that.
    I didn't know that spreadsheet existed.
     
  11. Nick G

    Nick G Active Member

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    Make sure you don,t have excessive crush on your o ring , and leave room in the width for expansion, my 21mm ( full piston )falls under its own weight with no top hat or spring inside,with a skirtless design if you have too much clearance on the piston head the piston pitching can make it bind .Also I tried delrin for o ring piston head in the past and they work for while then disintegrate .
     
  12. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Thanks Nick, mine is somewhat tighter than that, it needs a slight push with the tophat, spring and guide. Currently set up to give 0.4mm crush using 1.78mm CS BS018 in a groove 19.95 deep and 2.0mm wide, piston head clearance on front and rear guide is 0.10-0.12mm. So my next job is to make a new piston head going back to the original 82mm stroke and go as close as possible on tolerance and also reduce friction (no problemo_O).....a job for next week, as off hill walking this weekend.
    That's interesting about delrin piston head, I have tried a 23mm polyurethane u-cup seal (it's that fuzzy green one behind the comp tube) but it was so tight it had to be resized and I was not happy with the finish, so it sits on my desk until I can decide what to do with it. Beginning to wish I had gone for 22mm so I could have used the HW45 seal straight tfrom the start, all part of the learning curve.

    Many thanks again Nick.

    Here's the picture of the comp tube and barrel breach. IMAG0598.jpg IMAG0600.jpg
     
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  13. Nick G

    Nick G Active Member

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    That's interesting, I set my trans port so it contacts the end of the barrel, and seats the pellet flush with the end of the barrel, the skirt then seats in the "small" chamfer in the barrel, I find this mod is worth quite a bit of power . Your set up looks good however I would be worried that if the skirt has entered the rifling it may not be sealing.
    I have had 97 comp tube end plugs leak, its easy to sort put some loctite in the end of the tube and use the piston to create a vacuum to suck it into the threads and leave for a day, then remove the vacuum and allow to set for a couple of days , comp tube needs to be de-greased first.
     
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  14. hmangphilly

    hmangphilly The Doctor

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    Why the need to go super close on piston to bore tolerance ?

    Hw 45 seals nothing special
     
  15. Artfull-Bodger

    Artfull-Bodger Member

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    I take it your skimming the end of the Compression tube to set this Nick?
     
  16. Nick G

    Nick G Active Member

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    I do this on the tx by making a new trans port slightly longer, same principal in the 97 but a bit more difficult to achieve .
     
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  17. Tinks

    Tinks Another tricky target at Lincs.....!!

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    I was just taking all advise on board and trying to ensure the piston can't pitch and bind, which I guess with the offset TP is a possibility.
    As for the HW45 seal, my thinking was a 22mm seal off the shelf should be ok right from the start and easier to source than 23mm?
     
    Last edited: Oct 29, 2018
  18. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Hi Nick, this was a bit of a work around really since the breach lead in was so bad. I didn't intend that 45 chamfer to be as deep as it ended up but just couldn't get a light tight seal until it was that deep, then cut a 17 degree lead and lapped that with polishing paste. The threaded TP made it easier to experiment with different diameters and remove the void at the breach, as a bonus it seats the pellet consistently as the sliding breach closes.
    Will double check the comp tube for leaks and probably make a new full skirt piston, have this crazy idea to use carbon fibre tube for the skirt on a rotary piston.
    Hope to have something working before the weekend........fingers crossed.
    Thanks again.
     
  19. Nick G

    Nick G Active Member

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    I like crazy, carbon fibre very light and strong , how do you intend attaching it ?
     
  20. Tinks

    Tinks Another tricky target at Lincs.....!!

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    My initial thought would be epoxy resin, since it is used in the manufacture of the carbon fibre tube it should be compatible and hopefully strong enough to grip a steel piston body. Carbon fibre can be highly polished with the right stuff (see F1 and MotoGP bodywork). My plan was to have a bare tube attached to the piston head, no rear guide or bearing, to keep weight and load off the skirt of the rotary piston and see how it goes.
     

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