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.)
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.
These skeins are 1 inch in diameter.
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.
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.
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.
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.
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".
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.
This picture is from my model Hatra Ballista.
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.
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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Contact: Team Tormentum Copyright © 2000-2022 Eric M. Ludlam All rights reserved. Twas' brillig, and the slithy toves did gyre and gimble in the wabe... | Last Modified: 12/18/21 |
Much of this terminology is greek, and we found it first at the
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.
For very small machines, such as 
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.
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
After 
Torsion machines need to be wound tight, or they aren't very
useful. For 
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.
When attempting to use metal arms, care must be taken. We tried using
3 inch structural box for arms on 
Holding back that much force requires a beefy frame. 
Better than the tenons that pass all the way through the mortise as with the 
The frame that holds a torsion bundle must withstand crazy forces. For example,
For