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    leaf spring prod question

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    twedzel
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    leaf spring prod question

    Post by twedzel on Sun Jan 12, 2014 5:56 pm

    The previous owner of my house was gracious enough to leave behind a newish looking leaf spring right in my shop! It measures about .29" thick 1.75" high and about 25" to maybe 26" of usable length (7.4mm x 44.5mm x 635mm for those of us that are fortunate enough to us metric). How appropriate is this length/thickness for a prod? I noticed it was shorter and thicker compared to the commercial prods available.  What kind of power stroke should I be building around this kind of spring? And what height would you taper the limbs to (given that it is .29" thick)?

    Rizzar
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    Re: leaf spring prod question

    Post by Rizzar on Mon Jan 13, 2014 10:44 am

    Hey!

    It would help a lot, if you post a picture of the middle section (bridle hole) with its measurements.

    A short bow with that thickness can be a tough thing I cannot suggest to beginners.

    More advanced people then have realised leaf springs are no joke and skip that phase, too.

    Rizzar
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    twedzel
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    Re: leaf spring prod question

    Post by twedzel on Mon Jan 13, 2014 2:05 pm

    I was afraid of that.

    The bridle hole is 1cm diameter and is centered lengthwise (as close as I can measure) and about .5 mm off center in height.

    I am working on a design drawing right now that I can post after my kids stop bugging me for attention.
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    twedzel
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    Re: leaf spring prod question

    Post by twedzel on Mon Jan 13, 2014 3:00 pm

    here is a WIP of the plans I am working on, they are basically ripped off from the alchemic website. The top green is the leaf spring dimensions flattened out with a hypothetical taper.


    Last edited by twedzel on Mon Jan 13, 2014 3:14 pm; edited 1 time in total (Reason for editing : imagining making images imagable)

    Rizzar
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    Re: leaf spring prod question

    Post by Rizzar on Mon Jan 13, 2014 10:02 pm

    i am sorry, I mixed my vocab, this thing is called center hole, the bridle hole is the hole you loop in your bridle in the front end of the tiller (can be seen on your drawing)

    Concerning your drawing there are some points I would reconsider if I were you (flatten the tiller top to leave a good support for the bolt, don´t do such a deep grip taper at the front bottom (material weakness)).

    But my main concern is the center hole.
    I am sorry, but I have to say that I really haven´t seen any appropriate solution to the center hole problem.
    The closest to a solution can be complete stiffening with clamp plates or cutting the prod to a shape so the hole is not existing anymore.
    If the hole stays, there will always be a very weak point making calculations and good speed nearly impossible.

    I´m a bit in a hurry, duty calls, but If the hole was not there you could be with that spring shaped to a prod at about 450-480lbs. I consider that much too hight to risk any weak points!!!

    Rizzar
    perhaps more details in 12-14hours if needed


    Last edited by Rizzar on Tue Jan 14, 2014 11:08 am; edited 2 times in total

    Hermit
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    Re: leaf spring prod question

    Post by Hermit on Tue Jan 14, 2014 11:02 am

    Twedzel,I would not try to use that leaf spring,it is much to thick.It is possible to use it,but shaping it would turn into a full time hobby,never mind building the rest of the crossbow!.28ins. of usable spring would be the minimum to aim for,and friends and wreckers yards are the places to shop.Try to find a leaf spring between 1/8 to 3/16th of an inch thick.
                                                            I believe there is an answer to getting rid of the hole that will work,and am going to make a separate posting explaining how.I agree with Rizzar,a bow of that thickness,were you to make it, would likely pull around 500lbs.The only way to cock such a bow,would be with a cranequin,or other kind of winching device,and the loads on the bow,particularly on the trigger mechanism would be problematic for an inexperienced builder.I hope this posting is of help,and good luck with your build.
                                                                     Hermit.
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    twedzel
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    Re: leaf spring prod question

    Post by twedzel on Tue Jan 14, 2014 2:27 pm

    I appreciate all the feedback I can get. I'm still frittering around with the plans and making many changes.

    But I don't think I will attempt to use my existing spring. The steel will have to go to use elsewhere. I put all my weight on it and barely moved it so I figure its draw weight (even if well shaped) would be prohibitively high for a hunting bow. I will probably take a little boo down to the local steel recyclers and see what they have in their yard.

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