Pulley system effectiveness

Discussion in 'General Slingshot Discussions' started by Chase, Feb 11, 2014.

  1. Chase

    Chase New Member

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    Hi guys!

    I've got a question for the ones experienced in creating complex slingshots.

    As we know, the butterfly style is much more effective than the ussual one.
    If the elongation is two times greater, the energy accumulated have to be 4 times greater in theory.
    But, we use different initial lengths of rubber for those styles, so that's not that easy. Also, not all of that energy is transfered to the projectile.

    And what about the pulley system?
    I'm planning to create something similar to Joerg's bullpup crosbow, the one which has the pulleys in front. But I want to attach the rubber at the rear end, let it go all the way to the front end to the pulleys and only then back again to the trigger mechanism and the projectile.

    So, I want to ask, how effective will it be, in your opinion?))
    I do know basic physics, that is why I'm asking for real life experience.

    the questions are:

    1) how much more energy can butterfly style draw give to the projectile, comparing to the ussual draw?
    how much lighter the projectiles have to be?

    2) If I use the pulley system where the rubber is drawn for 160cm in total, but only 80 of them are the way of projectiles acceleration (and the length of the device in total), "where" device's effectivenes is between 80cm draw (ussual style) and 160 draw(butterfly). I know about theoretical 3/4 of butterfly))))

    in your opinion, of course))
     
  2. ruthiexxxx

    ruthiexxxx ruthiexxxx

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    I had wondered about pulleys for more drawlength and was relieved by Joerg saying that friction losses detracted seriously from any advantage. This relieved me of the necessity of trying as my mechanical aptitude is not impressive.
    But there is huge advantage in greater draw length. if you do not like full butterfly (as I don't !) then have you considered starships? At half butterfly i get a 60" draw on one of mine. Hexnuts just smoke through steel cans for very little effort!
     

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  3. Chase

    Chase New Member

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    thank your for the answer))

    ah, that's bad! i'll try to reduce the friction as much, as it is only possible))
    Thanks for your advice, but unfortunately, i do not consider that type of slingshots, as my main goal is to make slingshot which is very compact and very powerful))

    Ideally, it should fit in my pocket)) and butterfly is very hard to aim for me.

    and what about aproximate numbers? can anyone say how much times faster the projectile is?))
     
  4. JoergS

    JoergS Administrator

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    I don't see how you can make a pulley equipped slingshot that gives you 80 cm of a pretension and STILL fits in your pocket. Maybe I should send you my Jason Vorhees pants :)

    You can gain a bit from the draw extension, very much based on the ammo weight and band set you are shooting. 5 to 15% more energy can be hoped for. The best effect is reached if you shoot bands about half the draw weight you can normally handle. You will find those much harder to draw on the pulley slingshot due to a) friction, and b) the pretension. Therefore you can forget about your heaviest band set, as won't be able to shoot it on the draw extended shooter.

    So if you compare the results you can achieve with your pulley monster and then with your strongest conventional setup, conventional outperforms the pulley every time.

    The pulley based shooters also "eat" band sets due to the chafing. Rubber is sooooo vulnerable in stretched condition. Even smoothly polished rollers will half the lifetime of your bands.

    I gave up on this concept some time ago for these reasons. I would only consider them for extremely small pistol based designs, and even then only for tubular rubber (which is a lot slower to begin with).

    If you don't want to shoot butterfly (which I recommend learning, it is the key to powerful slingshot shooting), then a starship is the way to go. I understand your argument regarding the bulk. But here is the challenge that I have for you: Design a starship that is TRULY folding.

    Nobody has really mastered that task, Saunders came close with the Wrist Rocket Pro (http://sausa.com/product.php?id=2&category=16), but that extension is still not good enough. Double it and we are talking.

    I'd do it, but can't, as starships are illegal here.
     
  5. ruthiexxxx

    ruthiexxxx ruthiexxxx

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    It is a great shame that starships or wrist bracing are illegal in Germany as I would LOVE to see what Joerg could do with this. The mind truly boggles in anticipation !
     
  6. Chase

    Chase New Member

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    Nobody does)) and that's why i believe my project is something unussual.

    Thank you for so detailed response!

    My goal is to achieve maximum speed for the projectile, even if it will considerably reduce it's mass. rifle's bullets aren't very heavy)))

    Joerg, could you give me any advice except buttefly and starship?
    I believe there must be some mechanical way to increase speed.
    Even in middle ages people managed to converse strong but slow movement to weaker but much more fast)))
    there must be a way to do something with rubber, which already gives us pretty high velocity.

    By the way, thank you for the forum and for your channel, as it is just awesome))
     
  7. uberfreak9

    uberfreak9 Lord Of Crows

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    When you have any pulley system the friction coefficient is what determines its efficiency. But the static and dynamic friction coefficient drops significantly between a soft and a hard metal, almost instantaneously in slingshot firing, which is why I use a 1/2-inch brass sleeve over a machine oiled 3/8-inch stainless steel sleeve over a 10-24 rod for my pulleys. From my work with it there seems to be a power advantage when doing so but have no measurements to back my claim.
     
  8. JoergS

    JoergS Administrator

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    I have said it a few times before, here and elsewhere.

    Anyway.

    Muscle operated non-storing weapons (bows, slingshots, blow guns...) have a limitation on projectile energy: Your bodily strength. You can only get out what you invest.

    In an ideal world, your muscle energy is 100% converted into kinetic energy. But we are NOT living in a perfect world. Rubber itself isn't 100% effective, some of the energy you invest when you pull out gets lost when you release. But rubber performs really well in comparison with most other materials I know.

    Rubber has one big disadvantage: The speed of the retraction is limited. The more you stretch it, the faster it will retract though. Record band sets are extremely tapered, allowing the part of the bands close to the pouch to be stretched very close to the breaking point. Such bands can generate 500 fps or even slightly more on a warm day, or in heated condition. Then they break (after three to five shots).

    With a bandset that lasts, say, 200 shots, it is almost impossible to exceed 250 fps. 200 is more likely.

    You can, in theory, exceed this by means of mechanical contraptions. A simple lever system (think two mouse traps) can increase the speed. A cam system can allow the shooter to handle higher draw weights due to the let off effect (the shooter only has to overcome the peak draw weight and then it goes down to a fraction of that).

    But such mechanisms come with a price. Friction has to be overcome. Additional mass has to be moved. All the energy for this has to come from your muscle energy and won't be converted into the shot energy.

    So, yes, the contraptions CAN make it happen, you CAN increase the max speed of rubber. The Raptor slingshot is the best try I have seen so far.

    [ame]http://www.youtube.com/watch?v=kHxS2eiEZ3M[/ame]

    But even this professionally made masterpiece reveals the shortcoming of the concept: It only works with tiny, lightweight BBs.

    Why? Well, there isn't enough energy left after so much of it has been invested into friction and mass movement.

    Yes, a Raptor shoots a tiny BB faster than a conventional slingshot. But use a big steel or lead ball, and the conventional slingshot outperforms the Raptor by a huge margin.

    Long story, I know. But if you want to go down that route, you must understand all this.

    Simple message:

    1. Reduce friction.
    2. Reduce mass.
    3. Polish.
    4. Live with the downsides.
    5. Accept the fact that butterfly shooters will still outperform you with the most simple slingshot in their hands.

    Good luck! And by all means keep us updated.
     
  9. Chase

    Chase New Member

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    It is not all about performance, although it is very important))
    Anyway, if some mechanism ever will be able to be as effective as butterfly using only half draw, why not make it two times larger?))

    Everything you say is true, of course. There really is the limit - the energy human can put in a draw.
    So I completely agree with you on that point.

    The only thing I'm saying - we aren't even close to that limit. I'm not very experienced in making slingshots, but form creating bows from different types of wood and using different techologies, I know that high velocity of projectile is the hardest thing to achieve.

    If the one picked the wrong wood type, allthough the bow will have same shape and same draw power, (and in theory, will accumulate same amount of energy), it will discharge slowly and that's terrible.

    Just in theory, we can imagine a person drawing a bow (25kg prestretched, 35 - fully drawn) for about 80cm between left hand and chin. Theoretically, 100% of energy is 240 J, and that is not even a very tough bow, medieval archers and myself used even more powerfull ones.
    And the record shot of slingshot, is about half of that.

    I believe a man can put a lot more energy in a shot than 240J, btw.

    Also, as we know from history, english longbowmen used heavy arrows, about 400grams and the velocity was somewhere between 30 and 40 mps, but modern compound bows prove themselves to be more effective against unarmored targets.

    And as I see no armored catchboxes, or anything else, It would be great to put the energy to the velocity instead)

    Joerg! I shot classical wooden broadhead arrows with rubber, they were pretty heavy and I believe I can achieve velocity about 40 mps by adding more rubber and muscles)) and TECHNICALLY, that overperforms even best slingshots with lighter ammo. as the energy will be about 300 joules.

    BUT! what's fun in that??)

    And thank you again for great theory and advices. I'll try to do my best.

    uberfreak9, your JG940 is something very interesting, and thank you for your advices. I'll also try to make pulley as light as I can and use some bearings to reduce friction.
     
    Last edited: Feb 11, 2014
  10. JoergS

    JoergS Administrator

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    Chase, I have never seen a bow shooting an arrow at 240 Joule. The fastest bows I know of do 350 fps with a 350 grain arrow. That is about 125 Joule. Not quite as much as I can do with my strongest slingshot (a starship of course). But same ballpark.

    If you can double that, then I recommend making a video about it and putting it on YouTube. It will be a hit. Even the really powerful crossbows, the models you can only cock back with mechanical aid systems, won't hit 240 Joule.

    If you want to achieve 300 Joule with 40 m/s, then your arrow has to weigh 375 grams. Not grains. One gram is 16 grains. 375 grams is one third of a kilogram. 0,85 lb. 17 times more heavy than a modern day arrow for compound bows. Do you realize that?

    Throwing a 375 gram arrow at 40 m/s with an 80 cm draw would need a Hulk (the green one, not the wrestler). I estimate the draw of such a bow to be just under 200 lb.

    But hey, maybe you can really do it! I have my doubts though. Maybe you just made a calc error somewhere.

    I find this article very helpful:

    http://www.huntersfriend.com/bow-review-400-fps-bow/400-fps-compound-bow.htm

    Basically it says that a 400 fps bow (350 GRAIN arrow) at a 22" power stroke and 70 lb draw weight is theoretically possible - but in reality it won't work, and even if it would, there is no more room for "comfort" such as a let off effect.

    There is only one way to get to insanely high energy values: Increase your bodily strength. That is what English longbowmen did in medieval times. They practiced very hard all their lives. You can still see the effects of that grizzly routine on their skeletons.
     
  11. Chase

    Chase New Member

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    yes, I meant grams not grains, and now, after your message, i have huge doubts.

    I never actually measured the weight of my arrows, just read somewhere that heavy broadhead could weight 400 grams. anyway, the least weight i can imagine for my arrows is 200 grams for a boodkin, but now I have noting to prove it, so I'll just shut up))

    Yes, modern arrows are very, very light comparing with the ones used in medieval battles.

    I apologize for probably being wrong with my calculations))

    but, about 200lbs, 90kgs or aproximately 900N of draw force, may I ask how did you count that? is there a formula or is that an aproximation?
     
  12. JoergS

    JoergS Administrator

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    Medieval war arrows with bodkin tips weigh between 60 and 95 grams, usually. 200 gram and 400 gram arrows are pretty much unheard of. I believe you do mix up grains and grams. A 200 grain broadhead tip would be considered very heavy indeed these days.

    Several tests were made with replica longbows, the strongest one had 160 lb draw force (710 N) and achieved 160 Joule. That bow was operated by a robot though as no man could draw it. Man operated longbow replicas always stayed well under 100 Joule.

    How did I come to that estimation of about 200 lb draw? Just by interpolation. Look at modern bows and draw up a chart, showing the relation of draw weight vs. energy. Then just extend the curve. Voila.

    Of course my estimation is only true for a modern compound bow. A longbow is much less efficient and would have to be much heavier to achieve 300 Joule.
     
  13. BoyntonStu

    BoyntonStu Member

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    Using the rubber and keeping it happy and long lived is my goal.

    Let me remind everyone, what a 'slow' 330 gn arrow thrown from a 40 pound bow can achieve.

    [ame]http://www.youtube.com/watch?v=LsqrlaIef2o[/ame]

    If you want to kill rubber, pull it as far as it can be stretched.

    The rubber will not be happy and it will commit suicide after living a short fast life.

    So why chase speed, if rubber can live to a ripe old age if it stays in the slow lane at 150 fps or less?

    Nuts going though cans at 400 fps is child's play compared to an arrow going through a deer at 150 fps.

    However, watching a shot zing at high speed is a thrill in itself, and I am not discouraging that goal.

    The only way that I know it can be effectively achieved with rubber is by compounding.

    I can remember Joerg reviewing an extremely fast (395 fps?) crossbow slingshot that used multiple rubber loops to store energy.

    (I could not find the video)

    Joerg, the link please.

    My $0.02.
     
  14. Chase

    Chase New Member

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    Okay, thank you again, I'll just wait for a warmer weather and try it myself, just to make sure)) it's not easy to breakdown a misbelief of several years, even thought I never tried to check if it is true))
     
  15. JoergS

    JoergS Administrator

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    Stu is absolutely right, the most effective weapon that can be made with rubber throws a heavy, pointy, sharp projectile at reasonable speed and causes massive damage through momentum.

    This hunt for speed is pretty stupid, EXCEPT if for some reason you need to shoot things from very far away. Up to 30 meters, no need for anything over 250 fps.

    150 fps is a bit too slow for my taste. And there is no need to go THAT low. 200 fps can be achieved without much efforts and with long lasting band sets.

    So I'd much rather optimize the system towards "how heavy an object can I throw at 200 fps with a compact enough slingshot crossbow"?

    That, nothing else, is the road to effectiveness.
     
  16. ruthiexxxx

    ruthiexxxx ruthiexxxx

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    Just a couple of minor points. Regarding the idea of arrows of 200 grammes plus: I do have something of a predeliction for arrows that are I suppose 'silly big'. I'm talking of well over a metre of thick, heavy bamboo with a massive nail at one end. But the biggest of these weighed in at only 110 grammes and I would be the first to admit that they are simply not very practical. OK they have some serious momentum when shot from Theratube Silver or purple DubDub but it is a helluva draw and I can see no real advantage over a more sensible arrow at half that weight.

    The other point concerns the power that is applied. Yes, we can increase efficiency in the bow or slingbow. But, as Joerg points out, it all comes down to muscular strength. His world record with a slingshot results from his being able to pull a helluva lot of rubber out to full butterfly. He could not do this if he was not a power lifter who does a lot of training with heavy weights.
    For anyone wanting to seriously push the power boundaries of a rubber powered weapon it has got to come down to sheer body strength. And our strength CAN be increased. All it takes is progressively increased resistance and good protein diet.

    But I have heard it suggested that the way to do this is simply to increase the rubber on one's SS gradually until one can pull the big stuff successfully. I would like to suggest that this is NOT the way to go ! To develop in such a unilateral way is to invite serious problems with the spine...quite apart from developing a Quasimodo-esque shape ! If one wishes to shoot more powerful slings or bows then exercise needs to be BILATERAL. Any decent gym will have a machine that mimics the action of drawing a SS quite closely with weights that may be built up over time. It must be used on BOTH sides equally for the sake of one's back. The exercise can be done at home with heavy tubes anchored to a wall...but again it needs to be done EQUALLY on both sides. And better still to have a resistance training routine, whether weights or otherwise, that develops ALL parts of the body. To increase the strength of some parts whilst neglecting others is a sure recipe for injury!
     
  17. BoyntonStu

    BoyntonStu Member

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    Do I have your permission to laminate your quote to carry with me in my wallet?

    (It should be a sticky)

    I am using TB Black tubing. At 3.5 X it shoots ~130 fps.

    I also want 200 fps but I also want tube life.

    Is TB Black tubing capable of a long life at 200 fps, and if so, what is the stretch ratio and poundage required to achieve that speed?

    Would a pseudo taper with a short double tube section at the crossbow fork help?
     
  18. JoergS

    JoergS Administrator

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    Ruthie is correct of course, you must increase your entire body strength.

    But you must also work hard with the slingshot itself. Just the gym won't do it. Both workouts are required if you want to maximize your shot energy.

    It will take years of workouts and practice to reach your individual maximum, defined by your genetics.

    Good news is that powerful slingshot shooting isn't just for the youngsters. In fact, you keep your strength longer than your speed. Rubber converts strength into speed. So even at my age you can still compete with the young folks.
     
  19. JoergS

    JoergS Administrator

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    Stu, those are more your words than mine, as you know!

    I am a bit disappointed by my latest shipment of Thera Tube Black. It is thinner now, and behaves funny. Lacks the speed and strength it used to have. I am not sure if it is just a bad batch or if Hygenics changed the recipe for the worse.

    I recommend using Thera Band Gold instead. If you do not taper it and never stretch it to more than factor four, it will last a looong time and also easily get to 200 fps.
     
  20. BoyntonStu

    BoyntonStu Member

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    "how heavy an object can I throw at 200 fps with a compact enough slingshot crossbow"?

    Crossbows or slingshots?

    Slingshots require strength.

    Crossbows may require strength; but brains can reduce the required strength.