Rubber as energy source for crossbows - pros and cons

Discussion in 'Slingshot Crossbows' started by JoergS, Sep 16, 2011.

  1. JoergS

    JoergS Administrator

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    I spent some time thinking about the pros and cons, and decided to publish my conclusions. I placed it on Ivo's great forum, the perfect spot for it.<br><br><a href="http://thearbalistguild.forumotion.com/t447-rubber-as-energy-source-pros-and-cons#3818" class="postlink" target="_blank" rel="nofollow">Read the post at the Arbalist Guild forum</a>
     
  2. Ryan Wigglesworth

    Ryan Wigglesworth Senior Member

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    I read that post, great stuff Jörg <img src="http://illiweb.com/fa/i/smiles/icon_smile.gif" alt="Smile" longdesc="2"> Your a benefit to the communities of all shooters , if not just for entertainment! <img src="http://illiweb.com/fa/i/smiles/icon_biggrin.png" alt="Very Happy" longdesc="1">
     

  3. John S

    John S New Member

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    Do you think they can improve TB gold? How many years till there is a TB platinum!?
     
  4. petersonOD

    petersonOD New Member

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    <table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>John S wrote:</b></span></td></tr>
    <tr><td class="quote">Do you think they can improve TB gold? How many years till there is a TB platinum!? </td></tr>
    </table>
    <span class="postbody"><br><br><br>Forget platinum, they need to make a "TB Joerg"</span>
     
  5. John S

    John S New Member

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    <table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>petersonOD wrote:</b></span></td></tr>
    <tr><td class="quote">
    <table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>John S wrote:</b></span></td></tr>
    <tr><td class="quote">Do you think they can improve TB gold? How many years till there is a TB platinum!? </td></tr>
    </table>
    <span class="postbody"><br><br><br>Forget platinum, they need to make a "TB Joerg"</span>
    </td></tr>
    </table>
    <span class="postbody"> YES!!! </span>
     
  6. McMilchreis

    McMilchreis New Member

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    I dont think that it would get any stronger. Its normal use is for training and thats pretty hard with the gold one.
     
  7. Simon J

    Simon J New Member

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    Crossbows have always been a fascination of mine, at school I was allowed to make one, it caused a bit of contention, and funnily enough it dissappeared when brought in for marking (how odd!!). Rubber based xbows have a great advantage, they work better the thinner they are, a wide prod sends the speed to the side rather than staright ahead. I have had a brief chat with Joerg about cams on youtube, and read the Arbalist Guild forum, but I'm not one for quitting. I'm going to find a way of using cams to accelerate the projectile in a compact weapon. I have some pretty cool ideas. Lets see if I have any success.
     
  8. JoergS

    JoergS Administrator

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    Simon, I hope you will find a way! I tested some with string instead of rubber, and the rubber as a power storage behind the cams. Friction took the let off advantage away, and the string leashed out like a bullwhip, dangerous.
     
  9. Simon J

    Simon J New Member

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    Can I assume you are using off centre cams? My design involves using two stationary rollers at the front (as in your dead play reducing designs) and a floating pulley (per side), it's axis is attached to two lengths of TB silver tube attaching it to the back (butt end). Therefore the the rubber from the leather (in the cocked postion) takes on an "S" shape. Currently I'm awaiting some alimunium tube to overcome a problem with the bearings being pulled out by the stretching rubber. Early indications are looking good. The system is under constant tension but I think a hinge near the middle of the stock with facilitate removing tension, act as a loading lever and make it compact for transport. I am sure I will hit several more problems that a more experienced slingshot artisan would foresee or be able to easily overcome. The weight and friction of the pulleys need to be at a minimum but TB silver should overcome a fair amount of resistance.
     
  10. Simon J

    Simon J New Member

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    Another future idea in the realm of day-dreamed solutions is to have the majority of the rubber and pulley mechanisms inside a box section which becomes a heated environment to push the speeds further. One stage at a time though.
     
  11. Simon J

    Simon J New Member

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    I've put together a bit of a contraption, the pulley system on TB silver gives an enormous amount of tension. It is sitting in the garage awaiting some fool hardy friend to load it with me. It requires two people at the moment as both ends of the rubber needs to be detatched during storage otherwise it would be under perminent tension. Once we have shot it a few times, if it proves to accerate a 20mm steel beyond 100m/s I will design something similar to Joerg's early x-bows with the pulleys sliding towards the butt to remove perminent tension, and allow easy cocking. I need to work out how to upload photos, a picture would speak a thousand words here I feel.
     
  12. yulzari

    yulzari New Member

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    Pulleys could allow a longer band for the same weapon length (ie the rubber is attached at the butt end, run forward over a pulley and returns to the release mechanism) or run the same but with a shorter fat rubber (eg speargun) with a cable running over the pulley. Overcentre and/or combination of pulleys and cable (ie block and tackle system) could potentially raise the speed at the pouch compared to the actual speed of the rubber.<br><br>However, rubber does have an inherent speed limit. Middleton suggests this is just over 80mps with a trivial weight bullet and about 60mps with a practical load. Pulleys introduce an unnecessary complication and friction and adding stronger rubber will not make it faster. <br><br>To increase power one has then to look at adding mass to the load. Firstly using lead instead of steel or glass. Secondly increasing the diameter of the ball. Whilst a boule shooting catapult rifle is powerful, it is not practical to carry in the field. What we need is to be able to increase the weight but not the diameter. This leads us to the concept of sectional density. <br><br>Essentially you make the ball heavier by keeping the existing diameter but increasing the length so now we are looking at launching a rod not a ball. Rods are not compatible with pouches and will tumble if launched from a pouch and quite likely hit the weapon, us or someone else which is not good. So we need a channel to keep it on line and a pusher powered by the rubber to push it up the channel (or tube). Now we can add as much rubber as we can cope with and balance it by using a heavier rod material and/or longer rod. However the rod will tumble due to aerodynamic forces once launched so we need a stabilizing device. This can simply be a lightweight wooden rod or plastic tube attached to the back long enough to allow the front heavy centre of gravity to keep the device stable at 60mps. <br><br>Fortunately our forefathers invented this long ago. The arrow or bolt. The fletchings are unnecessary for our weighty bolts (though a waisted rear like an air rifle pellet would allow for a shorter tail) and the 'catapult bolt' would fit in the aforementioned channel. Thus we can increase the power of our weapon without going down the route of an artillery crew served boule catapult. For those who enjoy playing at tank gun designers, a heavy hard steel headed catapult bolt (with the rubber to reach 60mps) should exceed the armour piercing capacity of most handguns (but ricochet wildly when it fails if you are daft enough to use metal as your target).<br><br>Other than reminding folk that this produces a device that will go through most backstops and kill on the other side so caution is necessary, constructive criticism is welcomed.