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Mista Ballista : Modiolus and Epizygis
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In 2008, Dave devised a new system for managing the torsion in Mista Ballista with his friends Karl Hamm and Kevin Cheney. This represents a large investment in our machine in these custom parts.

Fresh from Martin Sprockets, 4 large sprockets arrive on a pallet in Dave's truck, ready for modification. They are series 140 sprockets milled from 1" thick plate.


We then extracted these 2x5 solid steel beams from Dave's stash. (Dave is the only one I know who might stash such a thing in case they ever come in handy.) These will be converted into brackets that will hold the epizygii.


Also in the mix are these shiny donuts. These were recently milled out of a 1.5" thick x 16" diameter billet of steel, and are to be welded to the bottom of our giant sprockets. These will fit into the scutula plates.


In this picture (taken by Dave) we see a wooden template fit to the center hole of the sprocket. The shiny donut is fit onto that, and the donut is then welded onto the sprocket.


The center of the sprocket was then cut out, and the welded hub gained a special groove the correct size to accept a layer of oil impregnated nylon which will be used as a thrust bearing. Next to the complete modiolus are the 4 very large epizygis made from the 2x5 stock Dave had brought. They were milled to a 1" radius to make our rope happy.


Here Roger is roughing out the oil impregnated nylon discs. These discs will later be set onto a lathe, and cut down to exactly fit the grooves in the modiolii.


Trevor helps out by cleaning the scutulas on Mr. B. When using oil impregnated nylon, you don't want to have any grease on the surface, as it inhibits the ability of the nylon to do it's job. Getting these surfaces clean was a big job after several years of using a metal-on-metal grease bearing surface.


After trimming the nylon down on the lathe, we get a perfect fit into the bearing groove of the new modioli.


The new modiolii were milled on a CNC machine whose long reach and associated deflection resulted in a slightly tapered hub. Our scutulas had a nice straight internal bore. Several hours of careful grinding were needed for each scutula hole to get the new modiolii to fit without seizing.


We then assembled the new modiolii, now each fit with an epizygis, and epizygis holders. We used a big fat strap to connect the two sides to hold everything in place while we try to fit stuff together. The epizygii are bolted on the sides, but the load is taken by the face of the modiolus.


The modiolii were bolted into place for the trip down to Delaware for the 2008 chunk. In this picture you can also see the wooden shims we made. The shims are to prevent the rope from splaying during tensioning. If the lower level of rope were to splay, then the upper ropes would sink in. This would make the bundle looser, and decrease our available power.


Visit the 2008 Chunk page for details on the final build out to get these modioli working for us. Also see the Torsion page for details on how these modioli work for creating torsion for Mista Ballista.
Additional Pages for Mista Ballista
Mista Ballista Mista Ballista is _Team Tormentum's_ *Torsion Division* competition catapult.
Mista Ballista : Modiolus and Epizygis
(This Page)
In 2008, Dave devised a new system for managing the torsion in Mista Ballista with his friends Karl Hamm and Kevin Cheney. This represents a large investment in our machine in these custom parts.
Mista Ballista : Bowstring The bowstring has been one of the most challenging pieces of our torsion engine. It is the last piece to get right, and has been the most likely part to fail in any given year.
Mista Ballista : Rope Bundles The rope bundles are the main engine of the machine. The framework that holds everything together is about 24 ft long, and 6 feet tall.
Mista Ballista : 2005 Rope bundle Upgrade At the 2004 chunk the main stanchions for the torsion frames were bent by the impact of the arms. You can see the animation of when this happened on the Mista Ballista Arms page.
Mista Ballista : 2006 Rope Bundle Reconfiguration In 2005 we broke our fancy fiberglass arms. In 2006 we got the new carbon fiber arms and video taped them in action in the summer at our 2006 History Chunk. This led us to discover how much they bounced around the outer stanchions. We were getting multiple recoils after every shot, sometimes back at least 30 degrees. That was also just with 2000 lbs of pullback, which is much less than we expect to use in competition.
Mista Ballista : Frame The framework for the Ballista had to be built strong enough to resist the pullback, and to hold up the 2500 pounds we currently estimate of our engine. It must also push it up 16 feet in the air!
Mista Ballista : Trailer One of our goals for the 2003 season is to acquire a dedicated trailer for Mista Ballista.
Mista Ballista : Torsion Mista Ballista's engine operates on torsion from twisted rope bundles. One of the biggest challenges of torsion for this machine has been adding the twist. An onager is pretty straight forward in that a large lever and gravity can be used. With our ballista, the direction of twist is sideways, so an alternate means of twisting is needed.
Mista Ballista : Arms After the failure of our arms in 2007, a new tactic was needed. Dave contacted his friends Karl Hamm and Kevin Cheney about designing and building a set of ballista arms out of aluminum. Aluminum was chosen for its strength and light weight.
Mista Ballista : Deployment To travel to different chunkin' locations, we need to pack the system down onto our trailer. Deploying from the folded up position is challenging and time consuming, taking a day and a half at the 2002 chunk. This year at the 2003 Punkin Chunk, we were done in about 4 hours.
Mista Ballista : Hydraulic and Electric Power Mista Ballista uses hydraulics for lifting the engine to a 45 degree angle for firing, and also for winding the cord bundles. We also like having electricity on hand since Mr. B has a tendency to break, and need on-field repairs. This page describes what we are using to power up the system.
Mista Ballista : Mystery Parts All winter during 2003 we have been collecting the parts we need to accomplish our 2003 chunk goals. Here are a bunch of pictures of these random parts. Can you guess what they are for?
Mista Ballista : Modiolus and Epizygis 2007 This page describes the Modiolus and Epizygis system we used from 2002 through 2007. In 2008 we developed a new system for twisting up Mista Ballista.
Mista Ballista : Torsion 2007 All torsion catapults depend on twisting the rope bundles to create the engine to drive the catapult. Mista Ballista went through many phases as we attempted to add more power every year to our throw. This page describes the obsolete systems we'd used in the past.
Mista Ballista : Arms 2006 - 2007 This page describes the construction of our dearly departed Carbon Fiber composite arms. We miss them.
Mista Ballista : Failed Arms Building an arm for a torsion machine this large has proven to be very challenging. Based on the experiences of our competitors, who also keep breaking arms, we find that we are not alone in this dilemma.


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