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2 posters

    Measuring stretch and compression in a drawn bow

    basileus
    basileus
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    Join date : 2010-06-18

    Measuring stretch and compression in a drawn bow Empty Measuring stretch and compression in a drawn bow

    Post by basileus Tue Nov 23, 2010 8:49 am

    Hi,

    Does anyone know of an accurate way to measure the amount a bow's back stretches when drawn? I'm thinking of a device/method which would give 0,1mm accuracy or better. According to my calculations my steel bows' backs stretch ~0,6% (0,48cm if bow is 80cm long). I've considered a few simple options which don't seem adequate:

    A good tape measure would be accurate enough for measuring the total (or average) stretch of the back, but not good enough for measuring shorter sections. A pair of (vernier) calipers would be accurate, but the results would have to "calibrated" somehow, because the surface being measured is not flat. They might be useful for certain relative measurements, though.

    Any ideas?
    Basilisk120
    Basilisk120
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    Measuring stretch and compression in a drawn bow Empty Re: Measuring stretch and compression in a drawn bow

    Post by Basilisk120 Tue Nov 30, 2010 8:32 am

    Sounds like some good methods. Unfortunatly the calculations might be the most accurate method, unless you could get access to a CMM Very Happy
    Sounds like an intersting challenge, what is this for?



    Something Something Dark Side
    basileus
    basileus
    Crossbow Building Wiki
    Founder & Curator
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    Posts : 86
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    Measuring stretch and compression in a drawn bow Empty Re: Measuring stretch and compression in a drawn bow

    Post by basileus Tue Nov 30, 2010 11:47 pm

    I'm just trying to verify that my calculations are correct, which is hard to do with calculations Wink. Anyways, "The Practical Guide to Man-Powered Weapons and Ammunition" gives another variant of the tape measure method:

    • Tape a piece of paper the length of the bow limb to the center
    • Cock the bow
    • Measure the distance between the edge of the paper and the end of the limb

    This method has the advantage that one can be sure that one end of the "tape measure" (=paper) stays in one place thus increasing accuracy.

    I'm thinking that if I measure the stretch on each half of the limbs (inner and outer) separately, I could calculate the stretch in any part by interpolating the results... meaning that if the outer half of the limb stretches 0,5% (on average) and the inner 0,6% (on average), I could deduce the that the tips stretches less than 0,5%, whereas the center stretches more than 0,6%. More exact values could also be calculated easily. This should work ok as long as the stretch increases roughly linearly.

    I hope I'm making any sense Smile.
    Basilisk120
    Basilisk120
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    Measuring stretch and compression in a drawn bow Empty Re: Measuring stretch and compression in a drawn bow

    Post by Basilisk120 Fri Dec 03, 2010 9:49 am

    What a simple and easy idea I like it.

    I believe the stretch should be pretty uniform across the surface. I guess that would depend on if this is a straight limbed bow or one with a bit of curve to it. but i would think the change should still be rather linear. of course that is assuming a proper limb with no joints or stress concentrators in it but that really goes with out saying


    Last edited by basilisk120 on Fri Dec 03, 2010 9:50 am; edited 1 time in total (Reason for editing : added a thought)



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    Measuring stretch and compression in a drawn bow Empty Re: Measuring stretch and compression in a drawn bow

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