Trebuchet complete

[johnridley]

well, mostly. Everything's built. I need to adjust some stuff, I need to buy some rope and make a sling, I need to buy some more hardware to really secure the weight box. The throwing arm needs to be trimmed and some hardware added.

I had these ridiculous ideas of having some people just pull down on the sling to get the thing ready to fire. WRONG. We dry fired it with no weight in the box, and it took three of us to pull it down, and I was nervous about that. I could hold it down myself without too much trouble, but it's awkward to pull down, and the whole time you're thinking "if someone lets go, this is going to tear up my hands and hurt me."

Luckily I did buy a winch for this project. I will need to find pulleys too. Lowes only has really crappy 200 pound pulleys that I put back on the shelf, I don't trust them. I don't think I'd even trust them with their rated weight.

Ah, I see Harbor Freight has decent-looking double-pulley blocks rated 300 pounds. One more reason for a trip there tomorrow.

Oh well, this wouldn't be a berzerker project if I just showed up with it ready to rock. There'll be plenty of participation with this project.

For your enjoyment...

Comments

[erikvolson.livejournal.com]

You're going to want an eye near the end to fix the downhaul to, and eyes on both legs for the downhaul rig. I'd put a metal strap around the eye point on the firing arm, as well -- the downhaul is going to put a huge shear stress on it when the weight box is loaded. I'd have *two* down haul rigs as well, because a fast way to hurt someone is going to be when you're hauling down the arm and the downhaul rig fails. Plus, you can practice your trebuchet shanties. "Way, hey, throw the ball long...."

(Oh yeah, where are you going to get weight for the box? ;) )

There'll be plenty of participation with this project.

Where's DB? (shunk.) Oh, there he goes.

[johnridley.livejournal.com]

Yeah, eyes of various types are on the shopping list for tomorrow.

I'd thought about making steel plates to reinforce the stress points, but I figure for the first year, we'll just fire the thing until something breaks, thereby determining empirically where the stress points are.

I am trying to decide whether to mount the winch or not. Harbor Freight has some double-2-inch pulleys. Even a single pulley up high giving a 2:1 gain would probably make it pretty easy to pull down. Going with a double at 3:1 or 4:1 is a possibility too and probably still faster than using the winch.

Right now the winch is loaded with steel cable, but I'm thinking I'll take it off and put nylon rope on it for this job, IF I use it.

I may buy some different lumber for the main mount of the weight box. That's two 4-foot pieces of 4x4. Problem is, when you pull the beam down the last 30 degrees or so, the box hits the beam and cocks back. I guess that's not horrible but it looks weird and I'm not sure I like it.

[erikvolson.livejournal.com]

Two double blocks with eyes. One eye to the beam, one eye to the base. You'll need an eye splice around one of the beam block sheaves, or use wire rope and d-clips (or a single block with two eyes)

That'll give you a 3-1 purchase. Two sets means two people hauling on would provide 600 pounds of downward force on the beam, each block holding 150lbs, each person pulling 50 lbs.

[johnridley.livejournal.com]

I'm a little nervous about putting screw eyes on the arm. It's the obvious thing to do, but I don't want to drill any holes there if I can help it. It's just another point for failure.

One thing I'm not sure about is whether I've got enough room for a long enough sling. I figured a 12 foot runout on the ramp for 8 feet of arm past the pivot should be enough. We'll see.

The design I'm used to has a steel post on the end of the arm to hold the sling, but the model I'm copying just uses a loop of rope around the end. I think that should work OK.

[erikvolson.livejournal.com]

Don't drill a hole -- use a strap around the beam. You might need a couple of small screws to keep it from sliding, but that'll cost you much less strength than a through hole for a large eye.

[johnridley.livejournal.com]

What, plumbing strap? I suppose but I don't think it's nearly strong enough. I guess I could get some 1/8" flat steel stock and shape something from that, but I think it's overkill.

[erikvolson.livejournal.com]

I was thinking loading strap, but strapping iron would probably be enough. Heck, a rope hitch is probably enough.

[johnridley.livejournal.com]

Yeah, I got pulleys with fixed eyelets, and I'm thinking just secure that to the pole with rope. I got some pretty decent braided rope from harbor freight (good enough for this, I wouldn't want to go climbing with it...).

To a large extent I plan to show up with the 95% completed unit and a bunch of spare parts and get some collaboration on this.