Speed Testing different pulley systems

Discussion in 'Slingshot Crossbows' started by RalphG, Jun 13, 2014.

  1. FIAAO

    FIAAO Failureisalwaysanoption

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    Very interesting numbers! Great job with your testing, looking forward to see more. Keep up the awesome work!
     
  2. RalphG

    RalphG Member

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    I was cut off from the internet because some moron construction workers damaged a cable in the street. Took the Provider a full Week!!! to fix this, No telephone, no internet. :mad::mad::mad:

    Here is what happened in the meantime:

    At first I got a little sidetracked and tested some old trampoline steel springs instead of rubber. I knew these would be slower but not tha slow!
    I had to move the trigger forward because these dont give much in terms of draw length, hoked them up in series to get at least 45cm. Drawweight on the Trigger is 120lbs.

    The 37g Arrow clocked around 145fps average the 22g around 170fps, didnt pursue this further because its real slow It would only be of some use if you want something fairly indestructible or shoot in freezing temps in winter.

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    Then I tested a "modified" 2:1 setup which gives mw 160lbs on the trigger without using more rubber than the 120lbs setup. I only got about 60cm draw length because I ran out of space in my rig to draw the rubber further.
    Set up like this it was about as fast as the 120lbs unmodified. If I were able to draw it to the same length of the unmodified it would be a little faster but not by a large margin I think. So thats also a version that has to be ruled out unfortunately.
    The only thing in favour of this setup is that it has almost no recoil because the rubber pulling in 2 different directions dampens this. This self- dampening effect is probably the cause why - despite the larger draw weight - its not much faster.

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    I Still wanted to go faster so I shortened the rubber and its now about 195lbs. Because I dont trust the rigs stability anymore it gets completely covered during spanning and firing.

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    I also wanted to know if and how much the weight of the pulley system has an influence on speed. The used bearings cant take the ~200lbs on a long term so I tested a heavvier version of pulleys with Steel axles and oilite bronze bushings. These also have noticeably more friction than the bearing version but can take a higher load and be relatively maintenance free. In comparison these are about 25% heavier than the bearing version:


    [​IMG][​IMG]



    The first shot with the 37g Arrow at 195lbs and the heavier pulleys was ~277 fps

    Compared to the lighter pulleys with 120lbs at 269fps its clearly to see that despite a massive increase in draw weight we loose a great amount of speed due to the heavier pulleys.

    I swithed to the lighter pulleys to make a comparison shot at 195lbs and - well, the result was this:

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    I somehow managed to hit my trusty chrony and the rear part got shattered into numerous fragments large and small despite being made from som thick drainage pipe that can usually stand some abuse... The aluminium Arrow was bent into a pretzl and unfortunately the coil windings of the rear ring got damaged also.

    So setbacks, setbacks and no real progress:rolleyes: Well, you wanted to see me shoot something - now you have it.:p But I would rather have preferred it to be something I set up for destruction intentionally...


    It would have been interesting to see the comparison for the 195lbs with the original, light pulleys but this will have to wait until I have either reconstructed the chrony or made a new one with the phototransistors, doing away with the magnetised tips.
     

  3. BoyntonStu

    BoyntonStu Member

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    First, terrific work. A great test bed. We all thank you for your efforts I am sure.

    My push pump X-Bow using a bare single Green TB (no paracord) with about 30 lb of draw launched a 308 gn arrow at 170 fps or 20 fpe.

    Quite surprising to me.
     
  4. RalphG

    RalphG Member

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    Yes, , Steel springs are definitely not the way to go, as said I expected them to be slow but these results on the springs suggest they are not very useful when you want speed. No wonder medieval crossbows with steel prods had draw weights up to 1000lbs (at least in the late middle ages) and even then fired only slow speed. But the very heavy bolts (70-100g) did make up for this in terms of impact energy. I think steel springs are only for cold weather or durability. No great use for me. But since they were just laying aound in the workshop i thought - why not make a quick test as well? Now I know more...:D
    Its always useful to know the dead ends to avoid - same goes for the heavier pulleys - definitely seems to be a no-go. I will have to source some needle bearings which can take more punishment without breaking down in the long term. The bearings I use now are still fine after the few dozend shots I made but I guess if I were to fire a few hundred shots at 200lbs the bearings would eventually give up on me.

    Well, its not about a quick fix, I am progressing slower than antipicated but I still have to learn about how things work best, what materials to use for best results etc.etc

    I rather fancy the slow , methodical approach, I dont need this thing tomorrow and I want it to be easy and safe to use and not to fall apart after a few shots. I have to feel confident about what I eventually cobble together - otherwise I wouldnt dare to put a 200lbs homemade contraption right near my face and fire it!
     
    Last edited: Jun 25, 2014
  5. BoyntonStu

    BoyntonStu Member

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

    Is this the speed you want?
     
  6. RalphG

    RalphG Member

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    Now THAT would be great! :D:D Maybe I should build a Scorpyd clone...

    I am content if I can get a little more than 300 fps with my 570gr arrow with reasonable draw weight. I am close but it still some work to do.

    Forgot to add:
    The Idea for the "Modified" 2:1 I got from this: The ancient Chinese Chuangzi-Nu Triple Bow Arcuballista
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    [​IMG]

    The exact String setup is still debated by experts, there are 2 different options, I chose variant b) because It suits my system fine. Instead of 2 bows in the front and one in the rear I have 4 rings of rubber in the back and 2 in the front because of the added frontal rollers reversing the pull direction. The double amount on one side is necessary to get both of the rubber sides to be stretched evenly because of the pulleys effect.

    I expected more from this setup but the self-dampening effect robs it of some energy. Maybe in huge siege crossbows this dampening effect was sought after because these huge machines must have had a very large recoil. As said : I noticed almost no recoil in this setup even if its not much faster than the "regular 2:1 at lower draw weight.
     
  7. CEZ

    CEZ New Member

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    Awesome stuff, poor chrony, what went wrong? That arrow must have had a lot of energy!
    Btw. have you heard of cocktail tubing? It's like tapering for thera band (you tie 4 tubes together with say 2-3 tubes and use the lighter side at the pouch or pulley in this case), if you run out of options but still want more speed you could give that a go, it will increase your rubber speed. I can't find a good photo but hope it makes sense?

    Also if pulley weight is such an issue, maybe you should tie the tubes at the trigger side and not at the pulley, the knots are probably dead weight.
     
  8. RalphG

    RalphG Member

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    Good point on the knots CEZ! Did it out of convenience because 3 strings of paracord take less space than 3 strings of rubber at the pulley side. Pulley weight seems to be really important but I wait til I can directly compare at the same drawweight and get some confirmed results
    . Yes, heard of "cocktail tubing" its the same as "pseudo tapering" IMHO . I am not stretching the tubes to the max but I guess its also worth testing;)
     
  9. konni

    konni New Member

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    @Ralph, any news on your setup? I found your work quite inspiring! Thanks a lot. Here I drafted some 3:1 pulley design...
     
  10. Markus Oswald

    Markus Oswald New Member

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    @Ralph: Do you have efficiency values for your best system? (Eout/Ein) ->Im looking for high efficient pulley system for muscle pump-action reloading.