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Baby ASOK (Cam Drive Trebuchet)
This is a Cam Drive Trebuchet I'm calling an ASOK (Arm Slides Over Kam.) This small machine stands 2' tall and has 4 lbs of counterweight. At the moment it can hurl a small key-chain koosh ball about 20-35 feet.
Here you can see one of the two small counterweight boxes. The axle the box is attached to slides straight down the upright channel. This maximizes the use of gravity. The arm slides over the cam (hidden by the plywood shield) which causes a change in the pivot point as the weight falls. The Koosh has a very short sling-line attached to it. I have not yet concocted a sling that works well for this machine.
I designed the cam by first picking two points. Point 1 was a location where I could prevent the arm from digging into the ground. Point 2 was a location near where the counterweight would fall, which made a nice inflection point. I then hand drew a curve between the points which looked nice, and appeared to provide a smooth transition.
Before building these boxes, I had put small simple weights on the fulcrum, and watched the movement of system. I noticed that there was a stall point where the weights would almost stop before continuing down. This point I assumed was where the weights were doing the most work. As such, I build the boxes height to be roughly equal to the distance from the ground to this stall point minus a 2 inches. The two inch buffer was so the weights would push pas this point more easily.
I suspect that I should have shifted the cam farther back so that the arm at rest is nearly upright. I should also provide more room for the arm to move forward past the uprights. This could cut down on the high arc I've seen from this machine so far.
I performed some tests for sling length on this trebuchet. On a traditional trebuchet, I usually make my slings slightly shorter than the distance from fulcrum to pin. As there is nothing like that here, I performed some experiments. The beam length is 28 inches. The projectile is the small koosh as seen in some of the pictures.
Here is a video clip made in early October 2003. The white thing tied to the end of the arm is the throwing pouch. The small gap between the rails makes using the pouch impossible, so I went with my old standby, tying an extension line to the end of the koosh.
You can barely see it, but in this picture, the small clamps from the previous pictures have been replaced with hitch-pins. This is a better design I've used in other trebuchets as well.
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|Last Modified: 05/10/09|