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.
Kevin Cheney's blueprints for the arms laid out on Karl's welding table in preparation for starting construction.
These arms were to have stays on the back of the arms to aid in deceleration, and hopefully prevent breakage.
The material started as a giant sheet of 3/8" thick aluminum plate. The next step was to cut it up into the different pieces that were to be
used in welding the arms together.
Here are the main lengths (top and bottom) of the arms. The big round holes are the tips of the arms, and will hold aluminum pins to act as the bowstring connection points. The prominent shoulder seen near the butt of the arm will prevent the arms from sliding inward, through the bundle, when the bowstring goes taught at the end of a throw.
Here is an arm mostly assembled excluding the backside. Inside you can see a series of bulkheads which prevent the base of the arm
from being crushed inside the rope and add overall strength. There is also a series of small holes through the bulkheads. These holes will later hold a steel cable that will run
the entire length of the arm. The cable is our safety in case the arm fails catastrophically, and will prevent parts from flying off
and hitting our neighbors.
Historically, the cable was made of leather, and was tied to the front of the arm, so this isn't a new idea.
Here are the stays before they are welded on. The stays will go on the back of the arm, and reduce forward deflection as the arms
decelerate.
Karl just after grinding and sanding down the rough spots on the new arms. The back stays are now installed and the arms are ready to go.
And here are the two completed arms, ready to go for the 2008 Punkin Chunk.
Here they are about to throw a pumpkin a bit over 400 ft.
Unfortunately, the back stays failed, and the arm cracked near the shoulder.
After breaking the arms in 2008, they needed to be repaired, and Karl Hamm and Kevin Cheney came through for us again.
The broken bit was cut apart, and welded back together. New plate was added to beef up that area on the arm, which can be seen
here as a diamond shaped layer under Karl's feet. We opted to remove the stays and go with a different method of stopping the arms, such that the arms would not be stressed so much during deceleration.
Here is an aluminum post welded half way out. This post is to prevent the new arm stop rope from riding up the length of the arm
allowing it to more effectively stop the arms before they bend around the outer stanchion.
Additional Pages for Mista Ballista
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Mista Ballista
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Mista Ballista is _Team Tormentum's_ *Torsion Division* competition catapult.
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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.
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Mista Ballista : Bowstring
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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.
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Mista Ballista : Rope Bundles
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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.
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Mista Ballista : 2005 Rope bundle Upgrade
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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.
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Mista Ballista : 2006 Rope Bundle Reconfiguration
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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.
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Mista Ballista : Frame
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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!
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Mista Ballista : Trailer
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One of our goals for the 2003 season is to acquire a dedicated trailer for Mista Ballista.
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Mista Ballista : Torsion
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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.
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Mista Ballista : Arms
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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.
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Mista Ballista : Deployment
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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.
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Mista Ballista : Hydraulic and Electric Power
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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.
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Mista Ballista : Mystery Parts
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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?
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Mista Ballista : Modiolus and Epizygis 2007
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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.
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Mista Ballista : Torsion 2007
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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.
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Mista Ballista : Arms 2006 - 2007
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This page describes the construction of our dearly departed Carbon Fiber composite arms. We miss them.
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Mista Ballista : Failed Arms
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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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