Mista Ballista's engine operates with 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 alternate means of twisting is needed.

The twisted ropes are held in place via a Modiolus and Epizygis. You can read about how we built them on the Modiolus and Epizygis page.

Come-Along Twisting

At the 2002 Punkin Chunk, and for initial winding at the 2003 Punkin Chunk, winding the rope bundles was done via this come-along. We could chain one end and then use the come-along to add torsion power to the machine.

Using a 2 ton come-along is very limiting, and does not provide much torque. But it's useful for initial tightening to get the arms held in place.

Hydraulic Ram Twisting

For the 2003 Punkin Chunk, we opted to use a Hydraulic Ram to do the tightening for us. See the chunk page for stories on the disasters that ensued. This section describes how we did it.

First, Dave added some Clevis Hooks onto the chain loops used last year. This let us dangle long chains from the ram, and hook onto them at convenient locations.

A hydraulic ram was then strapped into place. The chain on both sides let us twist both rope bundles at the same time. You can see this same ram on the mystery parts page during initial testing.

We had two of these red rams. One on the bottom, and this one which was lofted up on top of the rope bundles. Here Jeff is configuring the chains used to tighten the bundles on the top.

A testament to the strength of the twisted ropes, Dave stands on the arm while working the hydraulic twister.

After a hard weekend of chunking, we unwound the rope bundles, and took out the arms. The rope had stretched so much the modiolus fell right out the bottom.

Torsion extension arms

For the 2004 chunk, we built two torsion arms. The arms had special teeth that interfaced with the modiolus pin-down holes, and also bolted into place, forming a very strong bond to the modiolus. The hydraulics from last year could then be used to tighten them.

To make them interface, we needed to get the three pegs to line up with the modiolus holes. Jeff made eccentric pegs which fit into the rough-cut holes in a piece of channel. They would then be forced into the holes of a modiolus and twisted to get a perfect fit. This was then tack-welded into place.

Here Jeff is adjusting a pin so that it will mesh into the modiolus.

Eventually, the top of the channel was cut off, and welded onto an extension beam. The extension beam then had a hole on the end for attaching the hydraulic ram. Here you can see the pins have grabbed onto the holes in the modiolus, and a bolt is used to hold it on.

Here the hydraulic cylinder is attached to a torsion arm on one side, and the scutula on the other. The hydraulic hose is running up to it. Dave is checking the fit.

In 2005, we decided it was important to add some flexibility to the pin-point for the hydraulic cylinder. At the 2004 chunk, it took some careful adjustments of the hydraulics to get stuff to line up. This year, we got some mega-chain from a scrap yard, and cut it into short lengths. These will be shackled onto the torsion arms, and make it easier to bolt onto the hydraulic rams.

We even got the torsion arms painted. Huzzah!

Video

With hydraulic winding comes problems. We found that the rams were much stronger than other materials in the system. This video shows just how much force is being applied to break the chains, and the scariness of the rope bundle unwinding.

No one in this video was hurt by the flying metal shards, nor rapidly spinning modiolus. After the bundle was unwound and idle, the arm fell, and konked Dave and Jeff on the head.

Google Video Service Mista Ballista - Failure of first torsioning system

Information: At the 2003 Punkin Chunk, Mista Ballista was using a new hydraulic system for tightening the rope bundles. This shows the system in action, then breaking to disastrous effect. 8 sec - Nov 20, 2007

Later we improved our winding using specially made torsioning arms.

Google Video Service Winding Mista Ballista

Information: Roger and Dave working on winding the machine with the hydraulic ram. Shot during 2004 Punkin Chunk 9 sec - Jun 25, 2006

Next, we added trunnion mount hydraulics, and a remote way of controlling the hydraulic valves.

Google Video Service 2007 - Adding Torsion to Mista Ballista

Information: Using hydraulics with an electric valve on a long lead, Roger can tighten our rope bundles and get a good view of when the holes line up. 9 sec - Nov 5, 2007

Additional Pages
Mista Ballista : Modiolus and Epizygis	The Modiolus and Epizygis used in Mista Ballista account for a majority of the time spent. The size and detail needed to construct these items in a way that could reliably contain the force of so much rope worried us, and we over engineered them.
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 : Arms	Building an arm for a torsion machine this large has been very challenging. A list of our failures in this area might provide some hints on what not to do.
Mista Ballista : Trailer	One of our goals for the 2003 season is to acquire a dedicated trailer for Mista Ballista.
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 : 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 : Torsion (This Page)	Mista Ballista's engine operates with 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 alternate means of twisting is needed.
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 : 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.