Siege How to Build a Torsion Bundle

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How to Build a Torsion Bundle      
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Torsion bundles are basically large amounts of twisted rope. These provide the motive force behind Onagers, Ballistas. Most of our torsion machines are still quite small. The materials you use will dictate the size of machine you can build.

It is important to remember that when building the frame for your torsion machine that the force imparted at the end of your arm is quite small compared to the crushing force imparted on your frame. On our machine Onager Jr, for example, the arm only generates about 200lbs of force at the throwing end, but the inward pull of the rope bundle exceeded 1500lbs of force. (The amount needed to permanently deform the rope we were using.)


Much of this terminology is greek, and we found it first at the Knights Armoury, but we also found this terminology in a book on siege engines by Eric Marsden.

This roughly translates to `washer', and it holds the Epizygis in place, and bears against the hole carrier beam.
This roughly translates to `lever'. It is an iron bar that the rope is wrapped over. It's width should be about 1.5 times the diameter of the rope you are using.
Rope sleeve
Historically there were no rope sleeves, but we decided to add one to protect our rope which is the most expensive part of our machine.
This is the rope that is wrapped into the spring which forms the engine of a torsion machine.
This roughly translates to `hole carrier'. A term used for the load bearing beams in the machine's frame that carries a hole through which the rope is run.

Strands of a Skein

Torsion bundles depend on the stretchiness of the rope you use. A Hemp style rope, or a static climbing rope, would not be very good for use in a skein. If you tighten up the rope bundles with the machine at rest, it will take a huge amount of force to cock the machine. The difference in force from fully cocked to at rest is significant. This means your projectile gets a jolt at the start, then quickly overtakes the arm which is not being pushed as hard as it rotates. This reduces the effective acceleration curve of your machine and reduces your throwing distance. It also requires that you use reasonably solid projectiles.

A stretchy rope like nylon, polyester, or even polypropylene keeps the force imparted at the arm closer to constant between cocked and at rest. This is because the stored force when the machine is fully cocked is not too much greater than when the machine is at rest. As a result you can add a lot of preload to the arm, and get a more consistent acceleration curve.

It is a good idea to keep the thickness of your epizygis between 1.5 - 2 times the diameter of the strands of the rope in your skein. Anything smaller can cause the epizygis to act more like a knife than you may like. In the picture our epizygis is too narrow.

It is better to have many small strands, than just a few very large strands. The result is a more even load spread across the bundle. The different strands must be wound at equal tension as well. While some settling will occur evening things out, too much difference can result in a rope breaking as it takes a greater load than the others. The rope will fail critically in this case. Surprisingly, such disaster can go unnoticed beyond a sudden drop of performance.

Very Small Skeins

These skeins are 1 inch in diameter.

For very small machines, such as Baby Onager or Baby Ballista, simple materials can be used to build the modiolus and epizygis. Large machines require that after the skein is tightened that the modiolus be pinned to the hole carrier to prevent it from unwinding.

Machines of this size can have a large bearing surface between the modiolus and hole carrier because it is easy to overcome the friction when hand while tightening. Once wound, friction will keep it from unwinding.

While wood on wood provides good friction, adding this thin metal plate under the modiolus reduces friction. This make it easier to wind it up more. To tighten it beyond what the friction in this case can withstand, a nail can be added to pin it down.

In my third torsion model, I augmented the wooden modiolus with a metal plate of the same circumference. The metal prevented fatigue in the plywood where the epizygis digs in. I welded the epizygis onto the metal disk, preventing slippage. The wood-on-wood friction also prevents the need for pins. The extra long epizygis allows a pipe to be fitted over the rod for easy tightening.

Mid Sized Skeins

Medium sized skeins (here at three inches) are similar to those for the smaller machines. A wide base to the modiolus means lots of friction against the hole carrier. This image is from Ballista Jr.

For our first machine, we built 3 different setups before we settled on this one. All our other machines used very small epizygis which just bent in. Eventually Greg built this setup for us (See very first image on this page for all the pieces.)

The Modiolus is a piece of 6 inch channel that has been cut roughly square. A hole large enough for our rope sleeve (made from a PVC pipe coupler) was put in the center. Lastly, two cuts in the flanges of the channel were made so that the epizygis would fit. The epizygis is 1/4 inch steel from an I beam Eric found near a bridge construction. To cut down on some of the friction, the washer on our hole carrier were two thin strips of metal bolted to the frame. We then wrapped the machine using Polypropolene rope.

Unfortunately, the modiolus would bend over the washer plates on the hole carrier. Eventually, this machine imploded when our hole carriers were crushed.

After Ballista Jr died in battle Eric built Onager Jr and recycled the setup from the ballista. This time the narrow washer plates were ejected, and a full sized metal plate was bolted onto the side of the onager. Much larger beams were used for the hole carriers as well, which has kept the machine alive.

This setup has vast amounts of friction between the modiolus and the hole carrier washer plate. Eric made the mistake of painting the washer plate (though not the bottom of the modiolus.) This makes it very difficult to tighten the skein. A side effect, however, is that it has never unwound itself because the skein cannot generate that much force either.

Very Large Skeins

This modiolus has 11 inches inside for rope, and a two foot flange. Our epizygii is 1 1/2 inches thick and 5 inches tall. The theoretical maximum inward crushing force we've predicted so far, based on maximum working load before failure on the rope we'd like to use, is 105 tons. Cool!

In this picture, we had just started winding the rope. You can see our hole circle in the modiolus with 20 1 inch holes. There are 6 holes in the scutula, two sets of three. The combination of angles we chose gives us a tiny 6 degrees of resolution when winding our bundles. Unlike the medium sized modiolus on Onager Jr, this large bundle has to be pinned in place, or it will unwind itself.

We first used a come-along, but found it too weak. We then moved to hydraulics, but now find that doing so requires a lot more thought. You can read more about our torsion mechanism on the Mista Ballista Torsion Page.

We eventually created Torsioning Arms for Mista Ballista. This makes it much safer to tighten the bundle and works very well, if slowly.
Google Video Service
Winding Mista Ballista

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


Torsion machines need to be wound tight, or they aren't very useful. For Onager Jr, we found that if we tipped it up on end, we had more room to use our wrench. Ancient wrenches were apparently custom made iron tools with special fittings on the end. We used a two by four, and scrap of rope. In this picture, Roger is sitting on the machine so that these students can put their weight into our four foot wrench. The Epizygis sticks out a ways on each side, so we wrap the rope around the errant end of the epizygis, and then lean into the modiolus. This works quite reliably for Onager Jr.


Arms for torsion machines suffer much more stress than those of trebuchets. In particular, the fast stop necessitated by the breaker beam can be very hard on an arm. They need to be beefier, yet nicely tapered to the end to help with acceleration.

To help keep the arm alive longer, Onager Jr has a long breaker beam nearly the same length as the arm itself. The beam in this image was destroyed very quickly, and was replaced with plywood and part of an old street sign. We read about this technique in The Book of the Crossbow, and WEAPONS: An International Encyclopedia from 5000 B.C to 2000 A.D.

A second technique is to wrap the arm in some sort of rope. This lashing technique is similar to that used for wooden sailboats, for masts and booms, and also for quickly repairing tools such as shovels and axes. The wrapping job in this images was replaced when Eric re-wrapped using the "Rip-wrap".

When attempting to use metal arms, care must be taken. We tried using 3 inch structural box for arms on Mista Ballista, and after relatively low power twisting, it still completely folded the metal box.

To learn more about how not to make Ballista Arms you can read about it in the Mista Ballista pages. Our newest arms are hopefully the way it should be done and will last us a long time.
You do need to be careful since arm failure can be sudden and dangerous.

Google Video Service
Mista Ballista's arm breaks

During the 2005 Punkin chunk Mista Ballista breaks its arm on the first shot.
35 sec - Jun 25, 2006

Frames holding torsion bundles

Holding back that much force requires a beefy frame. Onager Jr's frame was made from 2x10s doubled up around the hole carrier. Tenons and mortises should be used in wooded frames like this one with no nails, screws or glue. Adding a nail or screw in these situations will create a point load inside the frame, and at the massive loads you will put on your frame, it will likely cause a critical failure.

Better than the tenons that pass all the way through the mortise as with the Onager Jr picture above are hidden mortises. Basically, don't cut a hole all the way through the carrying beam, and cut the tenons only as long as the holes are deep. This will leave more material in the carrying beam (more strength) while having the same jointing effect.

This picture is from my model Hatra Ballista.

The frame that holds a torsion bundle must withstand crazy forces. For example, Onager Jr's frame holds a 3" rope bundle, though I haven't put much more that 2" in it so far. With Poly Propolyne rope, I generated about 200lbs of force at the tip of our 3 foot arm. Inside the bundle, however, the frame held back over 1500lbs of force, which is what the manufacturer claimed was the working load of our bundle (300lbs per strand.)

To handle this load, I eventually added angle-iron from an old bedframe between the layers of pine. This kept the pine flat while torsioning the machine under these loads.

For Mista Ballista, we have generated almost 1400lbs of force on the tips of our 10 foot arms before they broke which pushed our polypropolyne rope past it permanent deformation strength. With about 64 wraps of 3/4" poly-P going past the safe working load 2000lbs, that adds up to about 128,000lbs of force resisted.

A side effect is that the arms bent our wimpy 6" channel stanchions, and we needed to weld on lots of strengtheners.

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