Pratt Truss Arm
Pratt Truss Arm

What you are looking at here is my new attempt
at a throwing arm for my fiffer trebuchet my large one I’m going with a Pratt truss
style arm hoping that I can get this to work I’ve tried to make engineered lumber arms
before a laminated beam one and it broke apart on my first throw hopefully this one holds
together better It’s all made out of ash according to my area moment of inertia calculations
at sixteen inches tall with nominal two by fours on top and bottom this arm should be
about twice as strong as an eight by eight timber and weigh just a little over half as
much down on this end this block in the middle
is where the throwing arm or the axle hole will be this is left is the
counterweight end and then down here is the projectile end where the hook will go as you can see I added a little bit of extra
bracing around the axle hole most of the bracing along the way is just to transfer the shear
forces from the top and bottom chords but here with the axle being through here it could potentially pull this way and that
way and I didn’t think Oh I forgot to mention I’m going to put half inch plywood on both
sides of this to hold it all together so that half inch piece of plywood you know would
have seven and a half inches here seven and a half inches there you know then on both
sides to hold it together I wasn’t sure if that would be enough so along with my truss
bracing I added these pieces here that way when it pulls along it has to it’s going to
create a shear zone all along here and all along there help transfer that energy As I said this is a Pratt truss this will
be the top and this will be the bottom which means the vertical members are in compression
the diagonal members are in tension if I were to flip it around the other way it would be
a Howe truss and it would be the opposite way with these in tension and these in compression
but I decided to make the shortest members in compression so I don’t have to worry about
deflection there’s such a short length here even these are just a little over eleven inches
long inch and a half by inch and a half that I
don’t think deflection would have been a problem but I wanted it this way the tension joints are the hardest to create…
or to hold secure so I am going to be putting some construction adhesive on all this and
then screwing it down so the tension members are the hardest ones the compression members
just from the bearing surface will be held together I had thought I had considered that I might
not even need these tension members in there and that the strength of the plywood on either
side would transfer all of the tension but I figured this being my first one I would
make this as conservative as I could so that if it breaks I know I did everything possible
to make it hold together I did leave a little bit of area over here un-webbed you know I
could have put one more these are eight inch sections here to make these corners at forty-five
eight eight and I could have put one more in there but theoretically the strength the
beam needs to be decreases by the cube of the length so I think this will be plenty
strong with just plywood around the outside I am a little concerned about any sort of
torque that will be created when I put my hook out the end here but I think I will be
ok I will let you know if it works when I know

2 thoughts on “Pratt Truss Arm”

  1. MrTMaciejewski says:

    How about using steel?

  2. beardedone85 says:

    @Mr T If I had metal working equipment I might use steel, but based on my successful results I may be able to make a new truss design that would actually be lighter than steel and much easier to work with and modify.

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