Best way to stop a rubber with sledge attached.

Discussion in 'General Slingshot Discussions' started by EyAlter, Nov 14, 2012.

  1. EyAlter

    EyAlter New Member

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    Hi, <br>I am planing on building a Slingshot Bow where a sledge in a tube is used to accelerate a 12mm steel ball. What is the best way to stop the sledge at the end of the shooting phase? I just want the ball to fly not the sledge too. I think otherwise the sledge will just destroy the front of the crossbow. <br><br>My plan so far is to have a magazine on top, so a pump gun action will reload a steel ball and cock the crossbow at the same time.
     
  2. JoergS

    JoergS Administrator

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  3. EyAlter

    EyAlter New Member

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    Wow, a reply from Joerg in person. I feel honoured. Do you recon i could wrap the rubber around the shaft vertically? Or will that be still to much of an impact? Would also be cool if you had pictures on how you made the crossbow that reloads automatically.
     
  4. JoergS

    JoergS Administrator

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    Automatically? I don't have one. I do have a pump gun repeater. You can see the mechanism in the bnisu part of this video (3:59 onwards):<br><br><embed pluginspage="http://www.macromedia.com/go/getflashplayer" src="http://www.youtube.com/v/PwbKnzan2Bo" width="425" height="350" type="application/x-shockwave-flash" wmode="transparent" quality="high" scale="exactfit"></embed><br><br>Best is to drill a hole through the sledge and push a steel rod through that hole. Attach the rubber bands to the ends of the rod, and cut a slot through the barrel. <br><br>Then wrap the rod ends with lots of rubber, and also use rubber cushioning for catching the sledge.
     
  5. EyAlter

    EyAlter New Member

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    You have 2 very nice Muschis. <img src="http://illiweb.com/fa/i/smiles/icon_razz.gif" alt="Razz" longdesc="9"><br><br>I am worried that the string will wedge itself between the ball and the tube, so I try to make a plunger instead.
     
  6. JoergS

    JoergS Administrator

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    You are right, as soon as the bands are getting too strong the string will jump the ball. Especially if you plan on using 12 mm steel balls (which are too small in my opinion). <br><br>I solved the problem by using a 6mm steel rod, slightly bent into a U-shape. <br><br>The steel rod can't jump over the ball. <br><br>I then attached (screwed on) two wooden blocks at the end of the slots. The shape resembles the shape of the steel rod so it won't bend on impact.<br><br>The rod is heavily cushioned by rubber.
     
  7. EyAlter

    EyAlter New Member

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    Why would you say 12mm is to small?<br>Ill make some CAD drawings soon and upload my design here.
     
  8. JoergS

    JoergS Administrator

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    12mm balls are too light for serious impact. <br><br>For a fast repeating action, you can't use more than 80 cm of a draw length. <br><br>And you can't exceed 18 kg of a draw weight. Better keep it at 12 kg - otherwise the pumping is awkward. <br><br>At this kind of draw weight and length, the 12 mm balls won't have much impact. You will get maybe 60 m/s. This means 12,6 J of energy and 0,42 N momentum, <br><br>Shooting 20 mm balls will deliver a much better destruction effect. The same bands will do 45 m/s, means, 32,4 J of energy and 1,44 N momentum.<br><br>Also smaller ammo requires very precise mechanisms. Larger balls are much more forgiving in that regard.
     
  9. EyAlter

    EyAlter New Member

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    So should I go for 20mm and 40cm draw length? Wanted to keep it compact but maybe I use a pully system if 40cm is to short.
     
  10. JoergS

    JoergS Administrator

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    40 cm is really short. Don't use pulleys, the friction will make the whole thing even less efficient. <br><br>You can (and should) use pretensed rubber, simply by making the slots shorter. But the barrel length (distance from fork to lock) should be at least 80 cm. You may keep the slots at maybe 48 cm. Then the rubber would be pretensed by factor 2 (16 cm relaxed band length, prestretched to 32 cm, and fully cocked stretched to 80 cm). <br><br>Again, this is only because you want a repeater. For a winch or stirrup equipped weapon, non of this applies and you could use strong bands and pulleys.