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Baby Scissor Trebuchet (Scissor Jack Near-Linear Motion Trebuchet)
The Scissor Jack Near-Linear Motion Trebuchet is an adaptation of the common Scissor Jack as found for lifting automobiles. The motion found in a Scissor can be adapted to a near-straight drop trebuchet. The straight drop is an idea used in a a FAT (Floating Arm Trebuchet), or in our Baby ASOK (Arm Slides Over Cam).
Linear drop trebuchets are a modern way to add more power from a smaller counter weight. Having a weight drop straight down maximizes its efficiency. Holding a big weight up in the air requires a lot of structure. The thought with the scissor treb is that the structure itself could be used as counterweight.
My other goal in building this machine was to construct something completely off the wall that would cause spectators to scratch their heads and say "How the hell does that thing work?"
The basic concept is to replicate the scissor jack, but leave one edge of the base free. A line tied between the two mid-height axles provides the pull that causes the arm to spin over the top. A counter weight on top provides the push needed to set the system in motion.
In this picture, the system is being held up, ready to fire, by a block of wood on the skid-track. The counterweight disks are made of mild steel, and originally appeared on the Cardan Gear Trebuchet.
The scissor itself fits onto the track with close tolerances. A dowel rests over the top of the track, while part of the scissor rests in the track, forming a stable platform.
The track, and interfacing blocks were sanded as smooth as I could manage. Two coats of tung oil was added to the track to help smooth things out and reduce friction. This turned out to be not such a good idea. (See end of story for why.)
The cotter pins and nylon washers were used between all pivoting wood surfaces to hold the machine together.
Attached to the arm itself, the white line is what will pull the arm over. This rope goes over the bridge, and into a pair of pulleys. The rope crosses twice between the mid-height fulcrums. When the machine drops, the mid-height fulcrums separate from about 7 inches apart to 24 inches apart. This provides 17 inches of pull on the rope. This hauls the arm around about 140 degrees.
Note that the counterweight is not in this picture. Removing it makes it easier to fiddle with the machine to get pictures.
Of important note in this image is that the bridge is folding. This is one of the more complex aspects of the machine. For the counterweight to work, the counterweight side must be shorter than the projectile side, and it must have some length or it wont work. If length was added directly to the arm, that beam would strike the inner workings of the scissor, and most likely trash it. To prevent this, the counterweight extension is pivoted, and held in place by a second line (the brown string.) Once the arm has rotated to this point, the bridge starts to fold over preventing internal damage. The pull line then changes what it is pulling against.
After operating this machine once, the original pine top-fulcrum broke under the mass of the counterweight. With the whole machine collapsing with counterweight attached, it makes a mighty thud that scares away cats and causes tools sitting on the bench nearby to jump.
After breaking the main pine fulcrum, I picked up a mild steel dowel of the same size, cut it to length, and drilled two small holes for cotter pins. This metal dowel has held up very well.
My first tests before I started taking pictures had the machine flinging the koosh about 24 feet. Roughly equivalent to the Cardan gear trebuchet. After adding tung oil to help make things slipperier, I found that the machine had an easier time starting up, but was, on the whole, pathetic in function with distances ranging about 5 to 10 feet.
Another problem with this implementation include the arm bridge hinge digging into the lower frame after collapsing.
I found it a bit difficult to get the rope length just right. I did a lot of fancy math to determine bridge size, and eventually scrapped that and resized things by inspection and trial and error instead.
This machine could be greatly improved by the following tasks:
Here is a video I made of the scissor treb in operation. I tried a few things suggested on the Catapult Message Board, but with little luck improving the performance.
You can see several points in the drop where the machine pauses with a small bounce. While trying to get this video, I managed to break the lower running axle which I replaced with more steel rod. Step through this video to see folding bridge in action, and all the pull lines getting the tension pulled out of them.
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Twas' brillig, and the slithy toves did gyre and gimble in the wabe...
|Last Modified: 05/10/09|