Mechanical advantage to shoot projectiles faster: 2:1 pulley system, vs lever arm system

Discussion in 'Slingshot Crossbows' started by jonathanfv, Dec 24, 2018.

  1. jonathanfv

    jonathanfv Member

    So, I've been asking myself a question and I was wondering what you guys think the answer would be.

    This is a 2:1 pulley system to be able to shoot projectiles faster than rubber bands would allow under normal conditions:

    And there are "flipper" types crossbows, what use a lever arm to convert a short but strong pull into a weaker, but faster and longer pull.

    What do you think are the pros and cons of each type of system?

    2:1 Pulley:
    Pro: the pulleys can be made very lightweight so they don't create a lot of losses of energy
    Pro: the whole system can be kept very compact
    Con: the ratio has to remain 2:1, there aren't really any benefits in adding more pulleys

    Lever arm:
    Pro: can be made to different ratios
    Pro: possibly less finicky than pulleys
    Con: higher mass than a pulley, so likely harder to accelerate and loses more energy
    Con: much less compact, and the length of the power stroke would depend partly on the length of the lever arms, which is a limitation

    Overall, I think the pulleys are a better system. I'm also wondering if using heavy steel springs could be good enough to use in colder weather, vs rubber. I'm pretty sure rubber is faster than steel springs, and also quieter. But could steel springs generate a good amount of power, or are they too slow to use with a 2:1 ratio and should be used with lever arms only?
  2. Markus Oswald

    Markus Oswald New Member

    Hello Jonathan, the different pully systems remind me about my own brainstorming design phase.
    I share the same problem of using rubber bands and want to switch to more compact compression springs.
    (By use of pre compressed springs the whole system could be signifficantly shortened).
    Unfortunetly steel springs have certainly lower specific energy density than glassfiber -> so it will not be the optimal solution for high speed bolts. I have 4/1 ratio -> so maybe this option would be still "ok" , ...but I'am looking for better alternatives.

    As many glassfiber limbs are used in combination with heavy cams this also has disadvantages (see last video from Jörg , mounting two compound bows side by side without big gain in speed due to inefficient system mass)

    Optimum would be glassfibre springs (fibres oriented for torsion) -> i think there is development ongoing -> some car producer are going to take these for suspension,..etc...
    So maybe we get opportunity of these compact high specific enegery storing springs soon.