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Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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» best type of horn to use..
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» Medieval crossbow finished
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by stuckinthemud1 Sat Dec 25, 2021 9:06 am


3 posters

    Delrin strength

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    Tim Jones
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    Delrin strength  Empty Delrin strength

    Post by Tim Jones Wed Feb 11, 2015 2:09 pm

    Hello all,

    I now have a very heavy steel prod (I suspect about 200+lbs)that I intend to mount on a 14th/15th century style prod.
    My question is, how strong is delrin when used for nuts? 

    Thanks
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    phuphuphnik
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    Delrin strength  Empty Re: Delrin strength

    Post by phuphuphnik Wed Feb 11, 2015 5:05 pm

    Tensile strength is 10000 PSI, shear is 9500. My nuts have about 1/2" total at their base and a little less where the string is. So my nuts will shear off at around 4000 psi. ಠ_ಠ
    Calculating shearing force is taking the amount of force and dividing it by the area. It is a little tricky with nuts because it is a wedge shape so you'll really need to figure out the cross section of the wedge to be precise. 

    For my big 200#+ bow I'm using steel.
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    Tim Jones
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    Post by Tim Jones Fri Feb 13, 2015 11:46 am

    I'll have to do some calculations when I know the exact draw weight....
    Just as an aside, how deep does the slot need to be? You can supposedly shoot the original style ones without anything holding the nut in. Will I have to drill a hole in the side of the stock and set the nut in that way in order to get a better hold?
    Gnome
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    Post by Gnome Fri Feb 13, 2015 2:09 pm

    Hi Tim!
    I've built 2 crossbows in the 200-220 pound range, both with Delrin nuts, and have had no problems. Okay, one problem: the first one I did, my very first build, I used white Delrin and I didn't like how it looked so I replaced it with a nut of cream-colored ABS. I haven't had any problems with the ABS, either, and it looks a lot nicer.
    Regarding the depth of the roller nut cavity, remember that due to the shape of the nut i.e. not being a complete cylinder, it will stay in place fine when rotating forward as in firing, but can be easily removed by rotating it backwards. That's why so many medieval crossbows had a hole through the tiller and the nut with a bit of string tied through it. During operation it stays in there just fine, but can fall out easily when you're marching about with your crossbow over your shoulder if it isn't retained by something.
    Good luck with the build, and let's see some pictures when you get a chance!
    Gnome
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    Tim Jones
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    Delrin strength  Empty Re: Delrin strength

    Post by Tim Jones Fri Feb 13, 2015 3:26 pm

    I haven't heard of ABS being used before but I'll look into it. I was also considering the more traditional elk horn or bone, but I'm not sure I'll be able to find bits of bone big enough....
    I drew up some orthographic projections this morning- nice long, thin stock and trigger and of course lots of bone inlays!

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