Trebuchet (Joining Lumber)

[NorviewsVeteran]

Hello fellow off-season CDer's,

I'm building a trebuchet in my basement, and I want to make the swing arm out of 4x4 and 2x4, but I'm not sure how to join the two together end-to-end.

Some ideas have been tossed around, all including large bolts, some including large steel C-channel, but none of us (my brother, dad, or myself) have any real clue how to do it.

Any help would be greatly appreciated, images could validate hugs. No promises.

The attached image is what I came up with in SketchUp, feedback would be nice. The lines flanking the 2x4 are shims to keep it centered.

[MrForbes]

It would be helpful to know what the loading is, and how it is applied. Perhaps you have a drawing or sketch of the whole machine?

[Rick TYler]

It would be better to describe the piece you want and then ask how to make it. Attaching a 4x4 end-to-end with a 2x4 is almost certainly the wrong answer to the question. If -- for example -- you really wanted a 12-foot-long arm with a 4-foot 4x4 section and an 8-foot 2x4 section, I would suggest that you make the arm up from a 12-foot 2x4 with a 4-foot section of 1x4 laminated onto each side of the butt of the arm using yellow glue and a some #8 2-1/2" screws to hold things together while the glue cures. Then, you won't be compromising on the strongest part of a wood laminate structure, which is its long fibers running longitudinally. Given the nature of a treb, I would strongly consider getting the nicest possible board for your arm. A clear piece of southern yellow pine or CVG Douglas fir would be good. Avoid anything with big growth rings or lots of knots.

Have you considered using an arm that is thinner and deeper? You could make a really nice, stiff arm out of some 3/4" plywood strips 8-inches wide and laminated with carpenter's glue. Stiffness increases hugely (the 4th power?) with thickness, and the deeper (higher) your arm, the stiffer it is.

Can you post a sketch?

[NorviewsVeteran]

Almost immediately after I posted this question, my brother and I had the same idea for running a 2x4 the entire length and thickening the weighted end and axle.

The entire thing is southern yellow pine, primarily because its what they have at Home Depot and our budget is "it had better be on sale."

I went back into the SketchUp model and redid the arm, so here is a screen shot with a couple of dimensions for scale.

[Jon236]

At a Mentor Gathering following the closing ceremonies in Atlanta, we ran into a fascinating couple who participate in Pumpkin Chucking . They have teams of all ages who construct various machines and hold competitions. They have a great website http://www.punkinchunkin.com/ with forums on how to construct these machines.

Have fun and be safe!

[MrForbes]

I don't see the need for doubling the arm, or using 4x4s for posts to support the pivot....since the arm is only 7' long. If you do reinforce it, I'd suggest putting the reinforcements on the top and bottom instead of the sides. In fact you could make the whole arm from a 1x6, taper the long end so it's about 3" wide at the tip.

The pivot shaft looks a bit long and skinny though.

[hillale]

Is this thing going to stay in your basement? Or are you designing it so that you can piece it down and move it? I am reminded of my dad's friend that built a car in his basement and wound up installing an improvised car elevator (forklift hydraulics) to remove it.

[NorviewsVeteran]

The plan from the very beginning was to have it able to break down in to 5-6 major components, most of which lay flat. (minus the counterweight box of course)

We were talking about moving the sides in a little to shorten the shaft. That's 5/8" steel from Home Depot, and bushings and shaft collars from McMaster, for the record.

The problem I see with using a 1x6 for the entire arm is warping from humidity and damage in transportation, I'd feel a lot better tossing a 2-by in the back of the van and putting stuff on top of it.

[MrForbes]

Then use a 2x6? my main point is to make the piece larger in the dimensions that need it, and smaller in the dimensions that don't need strength.


Maybe larger diameter thinwall tubing instead of solid rod? although figuring out a bushing would take some effort.

[NorviewsVeteran]

I went back to the model and adjusted the arm again.

There is a 2x6 running the entire length, with a 1x4 on each side where the pivots are, however those could be easily taken out of the plan. At this point they are only there because the bearings I have are too long to have one on each side of a 2-by.

Thanks for all the input, everyone.

[Rick TYler]

The other problem with a 1x6 would be that most Home Depot-quality 1x6s are suitable for trim boards, but are structurally about as strong as Kleenex. The 2x6 makes sense to me. How heavy is your counterweight?

[MrForbes]

good point!

Might also look into buying a sheet of plywood instead of a lot of that lumber....a base plate, two triangles, and not much 2x4 could make the base structure, and it might be possible to make it easy to disassemble and lay flat.