Siege Baby ASOK (Cam Drive Trebuchet)

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Baby ASOK (Cam Drive Trebuchet)
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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.

The frame is simple. Two uprights form the channel the falling fulcrum travels down. I made this only slightly wider than a 1/2" thick wood dowel. I should have spent some time smoothing these inner edges, but happen to just rely on the existing surface from the 2x4 I hacked up. The uprights and diagonal stiffeners as slotted into the base beams which are about 1 1/4" square. An old furring strip was sacrificed for the three feet on the floor.

This is the complete Cam with the cardboard template sitting on it. The plywood you see is the shield that keeps the sliding arm from jumping off the cam. The cardboard is placed about where the real cam inside resides. The extra bit of cardboard dangling from the bottom represents space inside the frame.

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.

The counterweight boxes where made from thinly sliced pine with luan sides which was then nailed together with some small nails. The inside dimensions are 3.5"x2.5"x1.5". The pivot was eventually drilled 3 inches from the bottom. I filled them with small pieces of metal that happened to come in 1.2"x6"x.125" strips. I don't know where they came from, but I have lots of them and they make good counterweight.

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.

The trigger is a piece of an old gas-grill grate beaten and ground into a pleasant shape. (I never used that dumb bread rack anyway.) Various holes in the block on top of the allow the long pin to slide through a metal hook screwed into the end of the arm. Pull on the fancy pin, and the machine fires.

Here is the complete machine with the starting position, a position half way through the drop, and at rest. What this clearly shows is that the weight drops over half the total distance while the projectile only travels about 1/4 it's total distance. This lets the weight pick up lots of kinetic energy. Once the weights drop near the cam, however the pivot point quickly shifts from near a 1:1 ratio to something closer to 10:1. That last three inch drop the weights have the maximum energy the could gain, and the lever quickly shifts the advantage causing the tip to really whip forward.

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.

Flinging Statistics

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.

Sling Length Test Shot 1 Test Shot 2
1.5" 22' 23'
8" 25' 27'
13" 35' 34'

Video Clip

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.

Google Video Service ASOK Trebuchet Model

Launching a koosh with an ASOK trebuchet model. Learn more about this trebuchet at:
1 sec - Jan 19, 2007

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