Trebuchet / Building the arm

Click on photos to enlarge.

John and Reid with the nearly-complete counterweight arm.

John attaching the counterweight barrel to the arm.

John screwing together the long spars of the throwing arm. Each spar is 3/4 by 3/4 inches by 12 feet; we rip-cut them from Select-quality 1x6's. The arm is 28 feet long in total, but the longest boards of this quality (required because we couldn't have any knots at all) we could find were 12 feet long, necessitating two joints instead of the one we have planned on. This task required drilling holes for and screwing in 64 machine screws and took John several hours.

Reid working on the throwing arm. At the base the throwing arm is 17-1/2" square, tapering to 3-1/2" at the tip.

The skeleton of the throwing arm after glueing.

Reid with the partially-covered throwing arm. The arm is sheathed with corrugated plastic to provide stiffness and prevent buckling. The difference between the sheathed and unsheathed arm is quite dramatic. The unsheathed arm was extremely delicate and we hesitated to move it; the sheathed arm's design load is 1,500 pounds of compression with a safety factor of eight or so.

John welding an eye bolt shut. Our theory is that by closing the circle this will enhance the strength of the part.

John testing the fit of the arm's 5/16" cable. Arranged like so (the cable also runs to the tip of the throwing arm), the cable bears only tension and the spars bear only compression. An equivalently strong solid wood beam would be 4x16 inches.