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Thread: I'm really starting to think that these 21/22mm bores with their longer strokes...

  1. #76
    Snooper601 is offline I likes to polish my trophy
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    Quote Originally Posted by T 20 View Post
    It could all be down to piston breaking John

    Cardew showed the increase in cylinder pressure to have very little effect on the friction created by an O ring seal but did increase the friction created by a parachute seal.

    So maybe it's down to the faster temperature rise due to piston speed ?




    All the best Mick
    I agree, though there may also be an additional factor from the o-ring allowing more lube into the swept area due to the lack of scraper effects that parachute seals have?

    Cheers

    John
    Snooper601 Suspect a simple fault, or a simple engineer He who dies with the most toys wins!
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  2. #77
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    Or, as a parachute seal only scrapes on it's forward stroke - it pushes more lube forward into the compression area?

  3. #78
    Snooper601 is offline I likes to polish my trophy
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    Quote Originally Posted by Rickenbacker View Post
    Or, as a parachute seal only scrapes on it's forward stroke - it pushes more lube forward into the compression area?
    Complicated isn't it.

    Cheers

    John
    Snooper601 Suspect a simple fault, or a simple engineer He who dies with the most toys wins!
    QHAC Official lubricant development engineer.

  4. #79
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    Quote Originally Posted by T 20 View Post
    It could all be down to piston breaking John

    Cardew showed the increase in cylinder pressure to have very little effect on the friction created by an O ring seal but did increase the friction created by a parachute seal.

    So maybe it's down to the faster temperature rise due to piston speed ?




    All the best Mick
    I think it probably is down to the seals friction. To get the same power with a O ring you will need more pressure as the parachute seals grip the walls a bit at higher pressures, holding the piston forward for longer and ensuring the pellet accellerates faster. More pressure to get the same velocity = more dieseling. The O rings can also be tuned higher as they have less lost volume.

    Remember those 90s tunes where the cylinder bore was roughened to improve power? I think that this was pushing up the friction at the end of the forward stroke.

    If you look at the accellerometer recoil traces on a parachute seal you can see a little step about 1ms after the peak in forward recoil/surge. This step is where the friction force from the seal changes direction with the piston movement so is twice the magnitude of the recoil due to this force. The frictional force will change with pressure. Using force = mass * accelleration you can work out the size of the force. An O ring seal should have a much smaller step.

    BB

  5. #80
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    I dunno, I go on holiday for a week, and everyone gets excited about the small bore guns being hold sensitive, Is it time for me to say tell you "told you so" JB .
    I don't have access to fancy test equipment, but I do test consistantly every time , and not taking anything away from the other shooters or their shooting disciplines, but I am looking for an edge in Ft so my big scopes at high mag ,and unwieldy stocks, when shooting off the knee do show up any improvements or otherwise in the tunes that I try, and I am like a dog with a bone if I cant get some thing to work.
    My 25mm pistons are a lot lighter than the norm I run them at 175 gram, the reason I can do this is I think is the o ring, the cycle is not as soft as some , but that's not in my criteria, I don't call it "hold sensitivity" I call it sight picture movement, the less movement I get , in the sight picture , the more I hit , and whats more the easier it is to do so , simple as that.

    I have found that the 21mm and 23 mm tunes I have shoot better in the longer barrelled guns , I am not sure if this is because of the longer time the pellet has in the barrel, or because of the up front weight, so next up is a weight to add to the end of the Hunter carbine , then I have a plan for a tx bull pup, to move the weight back and try to reduce the barrel movement further.

    I have run o rings in 26mm 25 mm 23mm and 21mm, at 25mm and above detonation is not a problem at any power up to 12 ft lb, its only when you get down to 23mm that it becomes a problem, and not just a bit of smoke , one shot it makes the power then all the lube gone and nothing. I think the pressure in the cylinder must provide conditions that are just too close for comfort for combustion, ok in a diesel engine but not an air rifle,
    All good fun looking for the ultimate in air rifle design, but I am starting to to think we may be closer than we realise.

  6. #81
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    Quote Originally Posted by NickG View Post
    All good fun looking for the ultimate in air rifle design, but I am starting to to think we may be closer than we realise.
    I think so too, 23.75mm
    Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.

  7. #82
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    NickG......Haha! See, you snooze, you lose.....fancy going on holiday!!

    Hope you had a lovely time.


    JB....23.75....based on a model calculation? Or just what you fancy trying empirically or tube size availability?
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  8. #83
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    Quote Originally Posted by TonyL View Post
    NickG......Haha! See, you snooze, you lose.....fancy going on holiday!!

    Hope you had a lovely time.


    JB....23.75....based on a model calculation? Or just what you fancy trying empirically or tube size availability?
    Its a calculated guess, surely ? but isn't that the way most things are pushed forward, dream it up , try it out, then if it works develop it further, until you hit a brick wall then back to the drawing board

  9. #84
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    Nick knows me too well
    Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.

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