Need for speed. Acceleration systems

Discussion in 'New project ideas' started by Adrinalino, Jul 17, 2013.

  1. Adrinalino

    Adrinalino New Member

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    Hi there, My name is Adrian and I am new here. I would like your advice and views on the following challenge.
    I would like to make a slingshot rifle that shoots small steel balls as fast as possible. One of the problems with rubber bands is that even fired without ammo, they still have a finite speed of about (I'm guessing here) 70 m/s. Tubes are even slower. One of the reasons for the finite speed is, in my view, material hysteresis and the fact that the rubber has to accelerate it self and the pouch too. The last fact I think is the main reason for tapered bands to shoot faster.
    An airgun shoots small bullets at about 200 m/s. It would be nice to reach that speed with a rubber powered gun.
    What we need is a mechanical construction that accelerates the contraction of the rubber.
    For this I came up with 2 ideas.

    1) The rubber pulls a string that makes a small wheel spin that is connected on the same axis to a bigger wheel that in turn pulls a string leading to the pouch.

    [​IMG]

    2) The rubber pulls a pulley that doubles the speed of the cord that it leads. This can be done twice so that the speed gets quadrupled.

    [​IMG]

    Both systems will dissipate energy and thus pulling force but I think that it is possible to compensate for that by simply adding more rubber.

    Please tell me what you think about these systems and I would also like to know what other methods you can think of.

    Cheers,

    Adrian.
     
  2. studer1972

    studer1972 scooter trash

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    folded or stacked theraband tan will be about as fast a contraction as you can get from rubber. It's the thinnest, so it has the highest surface area to interior rubber ratio around. Thereband is about as fast a contraction as you can get, although .030 latex is supposed to be a little faster than theraband gold. Tan is thinnest yet. From what I've read, too many folds or stacked strips tend to get tangled and result in more RTS (return to sender) shots. However, mounting them on a rifle style fram and shooting down a track would probably negate those issues.

    Build a sturdy frame and try as many different rubber propulsion methods as you can. Rubber powered mayhem is still more art than science. :(

    PS: Welcome to the forum!
     

  3. Yamroll2

    Yamroll2 New Member

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    I've had some very similar ideas. I'll have to draw them up and post them when I get home....
     
  4. Will

    Will Thread Hijacker

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    The second design would be quite easy to test using cheap pulleys and rubber tubes. But I don't see it increasing acceleration, only making an easier pull. Similar to using a series of pulleys to hoist a heavy object. As for the first, that is a good idea, and it would certainly provide increased power at what in this case could be called the flywheel. But I'm not sure how it would work with rubber. Some force still has to stretch the rubber away to power the flywheel to turn.
    I don't mean to discourage you, nor am I a mechanical engineer. Both are well-known forms of mechanical advantage.
    Welcome to the forum too, hope you don't think I was trying to smash your ideas.
     
  5. Moin moin,

    I had times try ....
    I think the remnants of it are with me still in the shed.
    Result, for me:
    Not recommended!
    1 It is surprisingly long sammer than expected. Especially when you have to move it on wheels! Each friction costs speed!
    2 Accuracy, too often, they spin too inaccurate.

    Conclusion: Dear good latex get so little mass as possible, accelerating, and have fun ......


    Hatte ich mal Versucht....
    Ich glaube die Reste davon liegen bei mir noch im Schuppen.
    Ergebniss, bei mir:
    Nicht zu empfehlen!
    1. Es ist erstaunlicherweise langsammer, als gedacht. Insbesondere, wenn man es über Rollen bewegen muss! Jede Reibung kostet Geschwindigkeit!
    2. Genauigkeit, leider zu oft, schleudert sie zu ungenau.

    Fazit: Lieber gutes Latex besorgen, so wenig Masse wie möglich Beschleunigen, und Spass haben......


    http://www.flickr.com/photos/[email protected]/

    ;-) Thorsten
     
    Last edited: Jul 17, 2013
  6. knifefightermike

    knifefightermike New Member

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    I would start with a single cam action. Maybe striker fired.

    The problem with rubber is that the total energy is so low AND the spring constant is so uneven: the spring constant varies lengthwise due to manufacturing tolerances on the thickness and due to the cut but also thermally and with time, AND the ultimate velocity is limited; that's why for a given set of bands, there is some mass of shot that maximizes either the speed or the energy but not both at the same time. You really can't make rubber do much work.

    If you can work in steel, you might make a rubber driven piston to drive an air powered pellet, like the single pump air rifles, and get your mechanical advantage pneumatically.
     
  7. JoergS

    JoergS Administrator

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    Unfortunately, Thorsten is right. Whatever you do to overcome the limit of rubber increases friction and the mass you have to move. The only input source is your muscle energy - not a lot, even if you are a strong man.

    The best implementation in that direction that I have seen is the Raptor rubber bow. It can shoot a .177 pellet at maybe 180 m/s. It clearly breaks the max. speed of straight rubber, but at a price - 80 lb draw. Very few people can master it. The Chinese that had designed the bow could not draw it back with maxed out rubber, so they sent it to me. After many failures, the second bow they sent worked.

    As soon as you go up in bullet size, you loose the speed advantage.

    Rubber is best shot straight, and it shines at heavy ammo. Forget using it for pellets. Air or gas is best for pellets. Rubber is best for big steel or lead balls.

    How fast is straight rubber? 80 m/s is achievable even without extremely tapered bands - everything faster than that leads to short bandlife and nasty handslaps. That isn't much if you shoot 0,5 gram pellets. But if you shoot 30 gram steel balls, then we are talking.

    I have tested mechanical setups a great deal, usually with frustrating results. The wheel idea is specifically dangerous. The string will lash out like a whip, and also the rotational motion leads to unpredictable hurling rather than straight shots.
     
  8. Adrinalino

    Adrinalino New Member

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    Thanks you guys for the replies. It feels especially good to get an answer from the Guru of slingshot shooting Joerg.
    Being heard headed I will give it a try any how and will upload some pictures of the experimental setting.

    Cheers,

    Adrian.
     
  9. JoergS

    JoergS Administrator

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    Good luck! The fact that this did not work for others doesn't mean it can't be done.
     
  10. Adrinalino

    Adrinalino New Member

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    This morning I built the concept. It does shoot actually pretty well. The first glass is already broken (I did not know there was a picture in that box :eek: ). It's at least good enough to continue with a real project.

    [​IMG]

    [​IMG]

    [​IMG]

    Cheers,

    Adrian
     
  11. FIAAO

    FIAAO Failureisalwaysanoption

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    Nice work, Adrian! Maybe you want to turn the pulleys 90 degrees, though?
     
  12. knifefightermike

    knifefightermike New Member

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    Interesting. Is that speargun tubing?
     
  13. studer1972

    studer1972 scooter trash

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    How does it shoot? (In both senses, please)
     
  14. Adrinalino

    Adrinalino New Member

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    This is just a quick setup to see if this works. I am thinking now of the real rifle.
    Cheers,
    Adrian.
     
  15. Adrinalino

    Adrinalino New Member

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    Experimental setup nr. 2

    I extended the pull and made the rubber part on the other side of the board.

    [​IMG]

    The rubber bands (thera tube black) are pre-tensioned. For the picture I left 1 tube unhooked so you can see the amount. In total I used 8 tubes.

    [​IMG]

    The strings are led over 2 rollers with ball bearings.

    [​IMG]

    Now the results :

    It shoots fine but no better than a single band slingshot. I don't have a crony to my disposal so in order to compare the speed I shot 15 mm marbles (that's 3/5 " for those who still live in the middle ages ;-) ) in several layers of cardboard and it penetrated about 6 layers. My slingshot rifle does the same with a single band.

    Conclusion : It's okay but not better, so for the next projects I'll stick to the proven concept. No pulleys, just rubber.

    Thanks for sharing your thoughts,

    Adrian.
     
  16. Brazilviking

    Brazilviking Thread Hijacker

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    Keep on going! People like you can evolve current concepts! It's not easy all the time, but you are in the right way!
     
  17. studer1972

    studer1972 scooter trash

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    Thanks for illustrating the mechanics. Looks good. From what I see, the pull and release are done manually, right?

    Edit: never mind, the release is done mechanically, sorry for not paying enough attention. :eek:
     
  18. Odins_Zornn

    Odins_Zornn Noob

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    If you decide to go with the first idea: taper the band at the wheel side for a consistent stretch. You will also need to use a smooth track to guide the projectile.
     
  19. MaddyMax

    MaddyMax New Member

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    it's always great to see people trying things to know if something works.

    personally i was too scared yet to experiment with "loose" pulley designs.

    keep on but take care! we don't want to see what happens if a cord rips apart or a screw get loose ...