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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.
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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Contact: Team Tormentum Copyright © 2000-2018 Eric M. Ludlam All rights reserved. Twas' brillig, and the slithy toves did gyre and gimble in the wabe... | Last Modified: 05/10/09 |
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.
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.
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.
