Buzz's Trebuchet

Now that the sling pouch is done I can move on the the larger parts of the trebuchet. A trebuchet is designed around the throwing arm. Well, being the lazy person that I am I didn't want to do a whole lot of cutting so I started with a readily available piece of lumber for the thronging arm; a 16' piece from my local lumber store. Unfortunately, medieval trebuchet's were built from felled trees like oak and douglas fir which are expensive to buy so I stuck with the SYP (Southern Yellow Pine) that was available from the local store. It isn't as strong but it is much lighter and with a little engineering you can mitigate the weakness of the wood. Also, don't go with the cheap stuff on the shelf. Get Prime #2 or better to help prevent lateral splitting. Also, I chose to use 2"x6" pieces instead of 2"x4"'s because that extra 2" of cross sectional area will help to add strength for only an extra dollar or so per piece.

	Here is the first load of lumber to get me started. You can see that there are several different lengths depending upon where they will be used. The longest beam will eventually be the throwing arm. The shorter ones will be used for uprights and supports. When building a trebuchet you always want to keep the forces involved inline with the grain of the wood and you will want the wood to be in compression more than in shear, torsion, or tension. Unfortunately, the fulcrum of the main lever arm can only be in shear so we will add two more shorter beams to create a laminate.
Here is another concession that I had to make when building my trebuchet. Because the wood at the local store isn't designed to be used in shear there was no way that I could make a main axle out of it similar to the axles used in many medieval trebuchets like King Edwards I WarWolf. However, after doing some research I found that there are records of some medieval engineers using iron axles. The problem with iron axles was that they were extremely heavy by comparison and would rust and weaken over an extended period of time. However, by the time the axle was rusted too much to be of use the rest of the wood had rotted as well making the whole thing useless. I am using a 1.25" 316 Stainless Steel axle for the fulcrum. This piece is 54" long and weighs almost 20 pounds. I use another shorter one for the pivot point between the lever arm and the counterweight bucket.
	To make sure that the beam moves smoothly I use some oil impregnated bronze bushings. I could just as easily have gone without them but I wanted to make sure that the rotation of the main axle didn't wear out the wood prematurely.