Vacuum Crossbow - No limbs, no rubber.

Discussion in 'Crazy contraptions' started by BoyntonStu, Jul 21, 2014.

  1. BoyntonStu

    BoyntonStu New Member

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    Clever idea. Looks very fast.

    [ame]http://www.youtube.com/watch?v=YZUEC1ayOeY&index=8&list=UUVgRM2XsJJdB0sIjGn8_Mbg[/ame]
     
  2. kohlqez

    kohlqez Accident-Prone

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    That's pretty impressive, fairly tight grouping, obviously powerful, and makes a cool noise, I want one!
    Are you going to try to make one Stu? And how in the world does it work?
    I saw a break action barrel presumably pulling a piston to create a vacuum, was the piston somehow connected to the bow string?
    It's a very interesting piece that's one step away from no longer being a crossbow, thanks for sharing I hope you decide to do something with it.
     

  3. BoyntonStu

    BoyntonStu New Member

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    Here is my take on it:


    The vacuum chamber appears to be about 3" in diameter.

    That is about 7 square inches in area.

    Atmospheric pressure is about 14.7 lb/square inch.


    7 x 14.7 = 102.9 lb of initial force.

    Notice the 'crack' sound which appears to indicate the power.

    The concept is easier than an air rifle because it is easier to create the chamber of vacuum than to pump it to 100 psi.

    I hope that this helps.
     
  4. jlasud

    jlasud New Member

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    I've seen this crossbow,in another video, where it didn't look as powerful. Damn 2mm mild steel sheet is more than what the most,medieval crossbows could do,and they were 1000lb draw weights also,that couldn't do this. Because they had like 6" of acceleration length :D compared to 25" or how much this one have.

    Nice thing about this design,is that you can have a very long draw length. And I bet,you could have close to no "arms" to stick out,as they just need to hold the rollers. I think they just made some arms to look more like a crossbow.. I think this one should be much less affected by outside temperature than rubber or steel. Although if you go high up in the mountains, it'll be less powerful,while most powerful at sea level.
    I've been tempted to build something like this before, now it tickled my fantasy..
     
  5. BoyntonStu

    BoyntonStu New Member

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    Altitude has some effect, besides, what are you going to shoot at 2 miles high?


    [​IMG]
    [​IMG]
     
    Last edited: Jul 21, 2014
  6. jlasud

    jlasud New Member

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    Last edited: Jul 21, 2014
  7. BoyntonStu

    BoyntonStu New Member

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    Thanks, good info. I wish that they had posted the arrow speed.
     
    Last edited: Jul 21, 2014
  8. jlasud

    jlasud New Member

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    Got a silly idea,that might even work. i like simple and effective launchers. Less stuff to go wrong is always good.
    So this would use air pressure,but instead of pumping,using a spring powered piston,it would just need a tube, and a flechless arrow,and a release mechanism. The arrow itself would be the piston,so it would have to be stiff to resist very high compression forces, and not to bend, although the tube itself would keep it from doing that, and the pressure would get higher as more of the arrow is inside the tube already, less arrow sticking out,less chance to bend. Because I was unable to figure out a way to keep a small piston in place from outside the cilynder, without air pressure to escape,I thought to block the arrow itself. Here's a picture to clear all this nonsense.. It's totally not to scale, it's just to show everything in a child's drawing style :p
    Loading would be like the Spetsnaz ballistic knife. Put a piece plank or something on the ground ,and use your whole body weight to push the piston(arrow) inside the cilynder, until the retention mechanism clicks in,and holds the arrow until the trigger is pulled.
     

    Attached Files:

  9. jlasud

    jlasud New Member

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    P.S. I have one of those car back door? opener pistons. If you compress it and let it go,it kicks like it possibly could break ones jaw. And those, aren't even very long, or the max. that one could push. And diameter wise, much smaller than the vacuum crossbows cilynder. Also it should be much more efficient,because it has much less friction, not having 6 pulleys, cords. The arrow could be made so it has the least contact area with the cilynders inside.
     
  10. BeMahoney

    BeMahoney Builder of things

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    Bullsh!t removed

    This is awesome!

    5 cents about pressure (metric..)

    1 [bar] is 1kg per square centimeter.

    Under water, the "column" of water pressing on one square
    centimeter at a depth of 10 meters is ten times one hundred
    times one cubic centimeter; which is one liter of volume - or
    one kilogram..

    One cubic meter of air weighs 40 grams, if I´m right.
    (I´m not: it´s 1293grams!!!!!!!!)

    Normal atmospheric pressure is 1 bar (1kg/cm²)

    - a column of air, 1000 x 100 (cm) x 1cm³ of air
    (which is 1/10th of a cubic meter) should weigh

    100.000 times 1,293kg/1.000.000 which is 0,1293kg- or 129 grams

    (which simply is 1293 grams divided by ten..)

    - so an altitude of 1km is reducing power by rd 13% I guess.

    This is what Wikipedia says:

    Troposphäre from sea level up to 7 km (Polar region) and 17 km (Tropen), begrenzt durch die Tropopause
    Stratosphäre bis zur Stratopause in 50 km Höhe
    Mesosphäre bis zur Mesopause in 80 bis 85 km Höhe

    starting from 1bar=1kg/cm² - which is the pressure of that air "lake"
    we live in, an atmosphere of linear density would have to be
    1000g/1293g= 773 meters..


    Bull**** Alert! EDIT:
    1m³ of air weighs 1,293 kg/m3; to stack that up a column 1x1cm of
    7,733km is needed.. - sorry, woke up today remembering the feel
    sth was wrong with these calculations..

    But in reality it´s this:

    [​IMG]

    reality is pretty interesting.. thanks for making me think about this!


    kind regards,

    Be
     
    Last edited: Jul 22, 2014
  11. BeMahoney

    BeMahoney Builder of things

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    If the pic seems strange:

    Warmth and Temperature are two completely different things..
    Warmth is infrared radiation; Temperature is telling how fast
    molecules are moving (vibrating).. Gases under pressure are
    less agile.. or colder..
     
  12. BoyntonStu

    BoyntonStu New Member

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    Here's how it goes together. The 'good' part start at about 5:00.

    Do you understand what happens when the trigger is pulled?

    [ame]http://www.youtube.com/watch?v=VcoiFQwV7u4[/ame]
     
  13. BoyntonStu

    BoyntonStu New Member

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    In the U.S. the unit for Heat is the BTU (British Thermal Unit)

    1 BTU of Heat will raise the Temperature of 1 pound of water 1* Fahrenheit.

    1 Calorie of Heat will raise the Temperature of 1 gram of water 1* Centigrade.


    (12,000 BTU/hr/day defines 1 Ton of ice melt cooling.)
     
  14. kohlqez

    kohlqez Accident-Prone

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    I GET IT! You totally lost me with the numbers and the math but that second video was all I needed I get it now.
    Seriously when you guys start with the numbers it's like you're speaking a different language at least I could understand the guy in the videos :D
     
  15. BoyntonStu

    BoyntonStu New Member

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    The great thing about Math is that it is a universal language.

    ( IMHO Learning Math is a worthwhile goal.)

    I could not understand a single word in the video except 'vacuum' but the video and the hand waving explained a lot.

    A vacuum is begging to be filled. The neat thing is that there is infinite air available for each shot. Unlike compressed air.

    The limitation on power is the diameter and length of the vacuum chamber.

    Brilliant!
     
  16. kohlqez

    kohlqez Accident-Prone

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    I know basic, intermediate, and a little bit of slightly advanced math but I try to avoid math whenever possible as it causes me headaches. Also I'm pretty sure I'm dyslexic especially where numbers are involved
     
  17. BeMahoney

    BeMahoney Builder of things

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    One cubic meter of air weighs 1,293 kg!

    SORRY! - I edited yesterdays post...

    It was full of BullSH!T! - all numbers wrong..

    But I corrected it (must say I tried to..)

    greets,

    Be

    (Those forty grams are the amount of water one cubic meter of air holds at
    40°C, 80% rel. Hum. - like in Florida at 4p.m.?)
     
    Last edited: Jul 22, 2014
  18. jlasud

    jlasud New Member

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    For me, the results speak for themselves. And i think this design can be improved. I would take away the arms,and 4 out of 6 pulleys,thus reducing a bit of it's size,weight, some friction, cost,and stuff to break. I would take away the plug with holes for the cylinder,which lets atmosperic pressure push the plug. I think that cap with holes,slows down the airflow into the cylinder.
    The rail could be lower placed on the cylinder,taking away a bit of weight,and size.
    Here's a shooting video at 50m. If someone has an audio/video editing program,it would be possible to calculate the average velocity of the arrow at 50m by looking at the time from the launching sound to impact sound. But to me it sounds about 1 sec to get to 50m and those are probably the 55 gram arrows,described on their page. That's impressive power.. [ame]https://www.youtube.com/watch?v=B5wtksl__Qg[/ame]
     
  19. Slinger

    Slinger New Member

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    I guess the pulleys are needed as an aid system for cocking the "bow" to increase the leverage. The end cap is required to keep the rope centered and prevent it from scraping the grease from the vacuum chamber, all the other holes are required too to let the piston slide back and forth as they let the air out of the chamber when the "bow" is being cocked, and into it once more after shooting to return the piston to it's 1st position along side the vacuum behind it creating the required pressure difference that operates the weapon
     
  20. jlasud

    jlasud New Member

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    You could center the rope with the pulleys.. The end cap is there is to stop stuff getting into the cylinder i think.