Inswinger patent from 1960

Discussion in 'Slingshot Crossbows' started by Gabe1983, Jun 30, 2015.

  1. Gabe1983

    Gabe1983 Member

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    Outswinger patent from 1960

    While browsing various crossbow patents on the Google Patent Search (as I often do; never know when you might find a new idea or two there), I ran across this 1960 patent for an inswinger-type (edit: it's actually an outswinger) crossbow:

    https://www.google.com/patents/US30...a=X&ei=wqiOVb2XFNDvoASl84KYBQ&ved=0CEQQ6AEwBQ

    One feature I really like about it is the built-in cocking lever (really more of a tension-release handle), which allows you to temporarily relieve tension on the bowstring so that it can easily be drawn back and engaged with the trigger mechanism.

    This weapon, as described in the patent, is powered by a metal spring, but I think it could easily be adapted to use Theraband or even speargun rubbers instead. I know it would be necessarily to have a fairly high draw weight, in order to compensate for the weight of the lever arms (plus another 30% to make up for hysteresis) and still have a respectable arrow speed, but I'm not sure how much.

    Anyone got any ideas on how to calculate this?


    [​IMG][​IMG]
     
    Last edited: Jul 9, 2015
  2. RalphG

    RalphG Member

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    Uhm, thats not an inswinger...
    In an inswinger the arms are pointing "forwards" -
    [​IMG]

    as for calculation you can try to use the simulation spreadsheet from torsionsfire- its for "regular" and "inswinger" (further down, page8) You`ll have to activate macros in excel after downloading it .

    It is made for torsion springs which have a constant spring force so you will need to calculate the force constant for the rubber somehow. rubber does not have a constant draw-force curve like steel springs so it wont be 100% exact but should give you an idea of what to expect.



    https://drive.google.com/file/d/0B2LGrb5XN8fKYUxpNENYcFZFQkk/view
     
    Markus Oswald likes this.

  3. Gabe1983

    Gabe1983 Member

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    Ah, okay. Thanks for the correction, RalphG. What would this be, then, an "outswinger?"
     
  4. JoergS

    JoergS Administrator

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    Outswinger it is.
     
  5. Gabe1983

    Gabe1983 Member

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    Hmmm...after a bit of digging, it seems what this thing is, is actually a slightly less-bulky version of the old Roman manuballistia (essentially a torsion-powered crossbow, similar in some respects to the ancient Greek gastrophetes...see video).

    [ame="https://www.youtube.com/watch?v=ipik__ief4w"]https://www.youtube.com/watch?v=ipik__ief4w [/ame]

    I wonder if it would be possible to use a scheme like this to break the "speed barrier" for rubber weapons? At its heart, when you break it down to basic components, this thing is essentially a pair of Class 1 levers linked by a bowstring on its outer tips.

    Now, according to various engineering texts I've been reading, if the input side of a Class 1 lever is shorter than the output side, the output side will exert less force on the load, but, assuming that it has sufficient input power, it will move the load a proportionately greater distance in the same amount of time it takes the input side to travel from one end of its arc to the other. In other words, it has a distance mechanical advantage and therefore a speed mechanical advantage.
    MA<sub>D</sub> = D<sub>L</sub>/D<sub>E</sub>
    where

    • MA<sub>D</sub> is the distance mechanical advantage
    • D<sub>L</sub> is the distance the load moves or the output distance
    • D<sub>E</sub> is the distance the effort moves or the input distance
    MA<sub>S</sub> = S<sub>L</sub>/S<sub>E</sub>
    where

    • MA<sub>S</sub> is the speed mechanical advantage
    • S<sub>L</sub> is the speed of the load
    • S<sub>E</sub> is the speed of the effort
    Since distance equals speed times time, or d = st, there is a distinct relationship between speed mechanical advantage and distance mechanical advantage.
    MA<sub>d</sub> = d<sub>L</sub>/d<sub>E</sub> = s<sub>L</sub>t/s<sub>E</sub>t = s<sub>L</sub>/s<sub>E</sub> = MA<sub>s</sub>
    Thus, Speed MA = Distance MA = 1/Force MA.

    As an example (assuming I understand this correctly):

    If the arms of your ballista each have an input side length (from inside tip to center-of-pivot) of 3-inches, and an output side length (outside tip to center-of-pivot) of 9-inches, it would have a 3:1 distance / speed advantage. This can be shown mathematically.

    The arms in the patent seem to travel in a 25-degree arc (based on protractor measurements of the patent drawings) from their cocked to their uncocked positions (personally, I would use a wider angle; actual Roman outswingers used 70-degrees, for example). So, the length of travel for each side of the lever can be calculated by (2π x D<sub>E</sub>) x ( deg travel / 360).

    The length of travel for the input side would be given by,

    (2π x 3) x (25 / 360) = 1.3 inches

    The length of travel for the ouput side would be given by,

    (2π x 9) x (25 / 360) = 3.9 inches

    3.9 / 1.3 = 3

    In other words, in the amount of time it takes the input end of the lever (where the bandset would be attached) to move 1.3 inches in a downward arc, the output end of the lever would move 3.9 inches in an upward arc.

    Conversely, the amount of power would be divided by a proportional amount. So, let's say each arm has its own bandset of 140 lbs initial draw weight, for a combined initial draw of 280 lbs. Subtracting 30% for hysteresis, this drops to about 196 lbs (or 98 lbs per side). The amount of force exerted on the bowstring (and whatever projectile is attached to it) will then be about 65 lbs (32.5 lbs per arm), which is comparable to many compound hunting bows.


    So, in other words, it's trading-off power for speed (but then, engineering is all about trade-offs, right?). I've noticed there seems to be a tendency for people (mostly neophytes) to want to build massively over-powered slingshots with extremely heavy draw weights; under normal circumstances, using rubber directly, this is overkill and doesn't offer many advantages. With a ballista-like design, such overkill may finally be justified.

    Please keep in mind, the calculations shown here do not take into account the weight of the arms. How much this may or may not affect such a weapon's performance can only be revealed through testing. However, my thinking is that in determining how much of a draw weight (and hence, how much rubber) to use on weapon of this type, you would need to account for the weight of the arm as part of the payload weight.

    Thoughts? Comments? Things I may have overlooked?
     
    Last edited: Jul 9, 2015
  6. tomfreds66

    tomfreds66 Member

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    This is a out swinger design. A inswinger will be at least 30% more efficient.
    There is Finnish chap whose name I can't remember that was building a inswinger Manu ballista, check out his blog for some really great information and ideas.
     
  7. Gabe1983

    Gabe1983 Member

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    Yes, I know it's an outswinger. I also realize the inswingers are more efficient. Anyway, my point in the previous post was trying to visualize how a weapon like the one in the patent drawing might work.

    Got a link to the Finnish chap's blog? I'd like to see it.
     
  8. tomfreds66

    tomfreds66 Member

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    Got a link to the Finnish chap's blog? I'd like to see it.[/QUOTE]

    Greek and Roman artillery wiki

    Samuli Seppanen
     
  9. seppman

    seppman Folding-Ladder Expert

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    Seppanen? Must be one of my finnish cousins... :)
     
  10. deraNdy

    deraNdy Active Member

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    Oh. I think i allready build such a crossbow...in Legotechnik. Not with such drawweights of course but you can find the levers and the ballista idea in it. And it was able to pierce a 3 mm thick mdf board. But i dont know if that was be more effektive than to use only rubber....but it was fun to build Powerful Lego Crossbow/Ballista with bullpupdesign: http://youtu.be/KSyVceOLUcQ
     
  11. Gabe1983

    Gabe1983 Member

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    I like it! That's very impressive performance for something built entirely out of Lego parts. I would prefer something a bit stronger, like multiplex plywood, myself. Especially if I was going to use higher draw weights.

    Honestly, it may not be more effective than direct rubber, especially at lower draw weights. It may only come into its own with a very heavy draw, or perhaps not at all. But, finding that out for sure would be all the more reason to build and test such a weapon. And it would look darn cool, too!
     
  12. Gabe1983

    Gabe1983 Member

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    Here's a repeating pistol crossbow that uses almost the same mechanism as the patent drawing (this one incorporates some kind of compound pulley system, though). Very impressive! I think this thing has some potential.

    https://youtu.be/y6A11ZyA5NE
     
    Last edited: Jul 9, 2015