Idea for a compound slingshot crossbow...thing.

Discussion in 'Slingshot Crossbows' started by CaseyB, Feb 18, 2015.

  1. CaseyB

    CaseyB New Member

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    Honestly I forgot what my original inspiration for my idea was. I know part of it was the Slingshot crossbow made by Joerg. Anyway looking at that speargun rubber I also wondered if some form of mechanical advantage could be used to make that kind of poundage practical. The thread here https://www.theslingshotforum.com/f18/speed-testing-different-pulley-systems-34695/ seems to confirm that pulleys can be used to be effective.

    Central to my idea is a pair of special pulleys. This pulley is actually two pulleys mounted together along a shared axis. The large pulley has the bow string wound around it and attached to it. The small pulley has a string attached to it that leads to some very heavy rubber like speargun rubber. Since the two pulleys are conjoined they turn together. The difference in size between the pulleys would create a mechanical advantage ratio depending on how much bigger the large pulley is compared to the small one. Though I think 2 to 1 is high as you can go without it being too big. its kind of hard to me to describe my idea in words so I am posting a picture so you can better see what I'm talking about. Also Idk if such a pulley already exists or would actually work. If any of you know better please let me know.
     

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  2. JoergS

    JoergS Administrator

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    The problem with such devices is that there would be a powerful upwards vector. The string would not go straight, but swing upwards in an ark. I experimented with such devices employing wheels and gave up, the shots are unpredictable.

    Also keep in mind that you have to use very very high draw weights in order to make this effective. Friction and additional mass eat up a lot of the energy. If you want to exceed the speed and energy of a straight rubber band set, you need to use at least five times the draw weight, ideally even more.
     

  3. Gabe1983

    Gabe1983 Member

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    I had similar notions for a slingshot crossbow when I first started getting interested in the idea awhile back, but after much reading and seeing what others have done, I have been forced to conclude that Joerg is most likely right; a compound system, while looking cool, does not seem to offer many advantages in a rubber-based weapon and, as Joerg pointed out, could actually result in less power and speed, due to frictional losses and other causes. It seems to be far more effective to just use straight rubber (and you can get away with a lower draw weight). About the only thing a compound system would bring to the table, is that it would make the weapon easier to draw, but I believe there are better ways to accomplish this in a rubber-based system than pulleys.
     
  4. kindlebear

    kindlebear Slinger of Shots

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    Despite the loss of power, the vector could be controlled by guiding the string between an upper and lower rail.
    Like Jörg does with his pump action devices. (Oreo/Hexnut shooter etc.)

    I do agree on the negative aspects of the concept, but it will look badass anyway. So...why not build one Just4Fun and the possibility of learning during the process?
     
  5. JoergS

    JoergS Administrator

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    The hex nut shooter and the oreo shooter are both pulling the string perfectly straight. There is no up- or downwards vector. The slightest tendency of moving up or down will result in both friction and also malfunctioning as the string WILL jump the ball.

    You may be OK for very low draw weights, but as soon as things get serious, there is NO room for deviation.
     
  6. kindlebear

    kindlebear Slinger of Shots

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    Perhaps mount the pulleys, so they're a bit lower than the barrel. Afaik crossbowss are angled down a bit to prevent a hopping string. This could work here too.

    You're the experienced builder of rubber based destruction, Jörg. But allow us younglings to make our own mistakes.:D
     
  7. konni

    konni New Member

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    Real compound crossbow

    I was wondering how to increase efficiency of a pulley based sling crossbow. Basic ideas:

    * let the pulley travel forward, so that momentum ads up
    * minimize pulley travel path while maximizing advantage factor (that is 3:1 with 4 pulleys)
    * draw cycle with peak velocity at the end of shooting phase
    * simple, compact and light weight

    I came up with the following design (see attachment).

    What do you think?
     

    Attached Files:

  8. kohlqez

    kohlqez Accident-Prone

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    You don't want vertical pulleys because they don't pull straight like Jörg said maybe if you could fit a rail on to keep it from leaping over your projectile it might work. But it's probably easier to just do normal horizontal pulleys. But if you do horizontal make sure that the pulleys sit slightly below the projectile and not even with or you'll have problems with the string leaping the projectile.

    I wonder how diagonally aligned pulleys would work? Not too much of an angle of course but it's something to think about

    I don't think you want the pulleys moving, the way I imagine it it would be like adding a weight tho a slingshot pouch. The forward momentum might be good but you still have the backlash
     
    Last edited: Jun 16, 2015
  9. konni

    konni New Member

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    To get a speed advantage from pulleys, they need to move, there's no other way. The question is: forward, backwards, or sidewards. Current compound bow designs usually have the pulley or cams traveling sidewards to minimize shock/recoil. Forward movement should be, in theory, more efficient, of course. This sling crosbow study lets the pulleys move backwards, still obtains reasonable speeds.

    You'll lose some energy to accelerate the pulleys, yes, but a) they can be super light (I am thinking of u624zz, U groove micro bearings, 4 x 13 x 7mm , 4g each, see pic) and b) they move only 1/3 of the pouch's distance, and speed. Also the pulley momentum could, in principle, be transformed back into projectile energy, that is if a peak in speed is reached at end of draw cycle. This usually requires some cam or lever.

    [​IMG]
     
    Last edited: Jun 16, 2015
    Markus Oswald likes this.
  10. bigdh2000

    bigdh2000 Administrator

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    Jörg proved long ago that pulleys add no benefit. In fact, the benefit they provide is easily lost in the added points of friction they create, no matter what quality of pulley is used.
     
  11. konni

    konni New Member

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    That depends. Nobody really knows, until someone manages to do it :)

    I think Jörg never tried lightweight pulleys, and mechanical enhanced draw cycle (lever, cam) which ramps up at the end of draw cycle to gain back the energy in the moving parts. Have a look at the Mantisse Elite crossbow (video), for example (it is driven by air spring, but in principle could be driven by rubber as well). Similarly, there are the inswinger crossbows, see this one, for example.
     
    Last edited: Jun 16, 2015
  12. bigdh2000

    bigdh2000 Administrator

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    Feel free to proceed with your studies, but Jörg has had this same argument so many times he stopped responding concerning the matter.
     
  13. JoergS

    JoergS Administrator

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    Pulleys do work if done right. But they require a massive amount of draw weight in order to be more effective than straight rubber. Friction and additional mass take a serious toll.

    Of course you must use string to run through the rollers, never the rubber itself. The chafing (even if highly polished) will otherwise lead to very early rubber tearing. If you use string, it has to be heavy duty because of the high draw weights. This means more weight.

    If you want to shoot a serious steel ball (say, 15mm) significantly faster than with a normal slingshot in full butterfly, you will probably have to use between 350 and 400 lb of a draw weight. This means that even though you can use block and tackle based systems, you will need a winch to cock the weapon. We are talking an unwieldy, heavy, clumsy construction. But it can be done, for whatever reason.

    Handheld devices, you are limited to tiny ammo if you want an advantage over a slingshot. Look at the STS raptor, a full on compound bow that uses rubber instead of conventional limbs. You have to shoot 8mm ammo to get an improvement over a Y shaped slingshot... and that thing is a masterpiece in engineering, using high tech lightweight materials.
     
  14. deraNdy

    deraNdy Active Member

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    Joerg has allready said everything.
    And dont forget that a spring or even an air spring dont loose power when they are cocked. Rubber cools down and loose his power. When you load your crossbow and shoot it you will have more power as when you load it walk 5min around and than shoot it.
    But feel free to build what ever is in your mind. Have fun with building and failing or successfull building and shooting...
     
  15. konni

    konni New Member

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    Yeah, good points. A slingshot at full butterfly draw is hard to beat in terms of speed. It rules in cocking simplicity and compactness anyways. However a crossbow has some advantages as well: it can shoot arrows, can accommodate a sighting system, can be more accurate, and you can wait for a target to show up in a cocked position, and also it might be easier to shoot at moving targets. Personally I probably won't need these features anyway, but, after all, it might be just plain fun to shoot something different or just for the sake of curiosity to experiment something new.

    I see the problems you are mentioning. I think one can overcome those - partially.

    Lets have a closer look at those:

    1) Loss in resistance. This will be very low, because of bearings. And yes, string runs through the pulley, not the rubber (see drawing).
    2) Accelerated mass. This is also very low (a bearing weights 4g each. Rotational mass might be 2g roughly. So it will be for the two stationary pulleys and two moving pulleys: 2g x4 + 4g x2 plus maybe another 1g for the attachment. This weight travels at 1/3 the speed of the pouch). Strings: I plan to use Dyneema lines, ø2mm. They are specified at 410kg. And weight 1.8g per meter. So this might add up for 3g all together. I plan to shoot with the 142mm standard crossbow pistol arrows, 17g. Equals a 15mm or 16mm steel ball.
    3) Draw weight. That depends on the draw cycle efficiency. In theory, a pulley based system could be better than a direct driven system (if done right). In practice it is difficult however. Power stroke will be shorter on a crossbow compared to full butterfly, however a rubber based crossbow can have much longer stroke than a traditional (recurve) crossbow.
    I will use Theratube silver. It will be pre-stretched. So rubber resistance might be between 5kg to 15kg along the draw. Since there are two sides and pouch travels 3x the length, I am facing a max draw weight of 15kg x 2/3. Want to make probably around 50cm draw length. Some math on theoretical speed might be useful, that is, stored energy minus loss in accelerated system mass, etc...
     
    Last edited: Jun 17, 2015
  16. bigdh2000

    bigdh2000 Administrator

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    Oops, forgot to mention I was talking about reasonable handheld devices.
     
  17. bigdh2000

    bigdh2000 Administrator

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    Every weapon has a place. Crossbows are outstanding hunting tools but are almost useless as a defensive weapon. I define a defensive weapon as something that can be reused in under 3 seconds. True, a crossbow can be used as a bludgeoning device after the one shot, but reloading in under 3 seconds...very unlikely. They are fun though.

    As for shooting arrows, there are some exceptional slingbows out there that rival very powerful long and compound bows. In fact, many are even more efficient than bows and far more portable.
     
  18. konni

    konni New Member

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    As far as I know, those direct sling (cross) bows max out at around 150 to 200 fps. So the goal with a pulley based shooter would be to beat that. These tests have shown, that rubber + pulleys can do that, 414,68 fps were archieved (with a 12g arrow).
    My 3:1 pulley design should be some % more efficient due to different arrangement (around 30% faster in theory). Also with bearings it should have less friction, plus pulleys and strings are lighter.
    But I won't strive for any maximum since I mainly want it to be compact and light weight, so would be quite happy with lets say 250 fps.
     
    Last edited: Jun 17, 2015
  19. kohlqez

    kohlqez Accident-Prone

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    Honestly, I'd have trouble with reloading a slingshot in 3 seconds. And your definition of defensive, it seems to me, would be a case of multiple hostiles in relatively close quarters (ie zombie apocalypse, front lines of a battlefield, or full on raid.) my thinking is that (hopefully) such situations will be few. A more common situation that would require a defensive weapon would be a single intruder or even a siege, in either of those situations a deftly wielded crossbow would be effective for defense
     
  20. konni

    konni New Member

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    Which one for which purpose is another story. Lets focus here on how to improve performance by pulleys, that is, a true compound sling-X-bow.

    Currently the fastest shooting direct driven sling crossbow is Jörg's, shooting at 213 fps:
    Video

    Can't tell the draw length, but it is likely quite above 1m. So the pulleys serve two purposes: increase speed and reduce length.

    To keep it maneuverable (and cockable) the draw length should be around 50cm, the max draw should not exceed 150lb, and the overall length should not exceed 80cm (Bullpup).