Siege Engine.com: Baby Scissor Trebuchet (Scissor Jack Near-Linear Motion Trebuchet)

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Baby Scissor Trebuchet (Scissor Jack Near-Linear Motion Trebuchet)
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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 skid-track is shaped like a bit letter I. The wood block with the small bit of molded metal resting at one end is where one side of the scissor is screwed down. Each skid track has a square notch cut down it's entire length. They are held apart by two lengths of wood on the front and back, end-screwed on. These lengths also add left/right stability to the machine.

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 scissor mechanism is screwed on with the metal plate in this picture. The metal is thin, and was merely beaten into shape with a hammer and vice. The scissor base fulcrum was drilled with two holes which use cotter pins to prevent the beams from sliding off the axle. Nylon washers cut down on pivoting friction.

The cotter pins and nylon washers were used between all pivoting wood surfaces to hold the machine together.


The articulation mechanism that causes the arm to spin over has several pieces that must all work together to function properly.

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.


Here the machine is partially collapsed, and the arm has spun a ways around. You can see the two mid point axles have separated a long ways, and the right side on the base has slid down the skid track.

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.


Fully collapsed, the arm is not upright, and the projectile has been sent on its way. The bridge has almost fully folded itself up out of the way. You can see the small bridge fold block on top of the left side of the machine.

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.


The counterweight was scavanged off the Cardan Gear Trebuchet. This machine needs weight of a similar style, so I used those. Fortunately I had time to paint them before using them in this machine. The two small hitch pins are used so that I can easily exchange weights.


Here it is fully assembled and ready to launch. I have not yet constructed a sling for this machine, and am using my traditional koosh on a string. The platform this machine sits on does not provide a good place for a projectile to sit as it awaits its fate. A side effect is that the projectile sometime catches on the frame and slows things down.

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:

  • Improve bridge folding point, moving it closer to the fulcrum, and recessing the pivot to avoid damaging the machine.
  • Use a better oil on the skid-track to reduce friction.
  • Adjust where the main rotation of the arm occurs in relation to percentage of collapse. Allow more drop time before pulling starts.
  • Spring load the base to give it a kick start in falling.


Video Clip

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.


Google Video Service
Siege-engine.com: Scissor Trebuchet Model


Information:
Launch a koosh with the Scissor Trebuchet model. Learn more about this odd treb at: http://www.siege-engine.com/ScissorTreb.shtml
2 sec - Jan 19, 2007


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