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Terminology
siege-engine.com's Torsion page
How Torsion Bundles Work
A torsion bundle uses energy stored in the stretched rope. Once the bundle is loaded by twisting both ends in the forward direction (The direction that the arm rotates when throwing) a torque is applied to the arm. The increased tension in the bundle from the twisting also holds the arm in place. Then the arm is released, after being rotated backwards, the bundle will turn is forwards providing the power for the throw.
Use
Onagers
Onagers have one torsion bundle parallel to the ground which swings the arm in a vertical plane.
Ballista
Ballista have two torsion bundles which turn the arms to function like a cross bow.
Types of Rope
Weaves
Woven
Woven rope has some advantages over other types. First it is usually slightly stronger, and does not have a tendency to untwist like twisted rope; however, woven rope can be more expensive.
Twisted
Twisted rope is usually cheapest , but in some cases can unwind slightly, or the slack gets all twisted around itself during the winding process.
Materials
Nylon
Moderate elasticity and very high strength.
Polyester
Mainly valued for its low elasticity while being almost as strong as nylon.
Polypropylene
Polypropylene is very cheap, but weak, and deforms easily. Polypropylene also has very high elasticity.
Elasticity
If a bundle has too much elasticity it will require a huge amount of twisting to get it up to the desired tension, and the arm rotation will only use a small portion of the stored energy. The full effects of this are still debated. This does provide a smoother acceleration curve. Longer bundles effectively increase elasticity.
Performance
Torque and Rotation Speed
Torque generates rotation speed, however a higher static torque does not always produce a higher rotation speed. This is because there are limits on how fast the bundle can rotate. This is caused by the properties of the rope such as the recovery rate. The torque drops as the rotation accelerates, which means that in some cases longer arms can increase efficiency.
Tensile Strength
The tensile strength of rope is how much load it can take before breaking. It is important to note that this is different from working load which is calculated from the tensile strength. The working load is usually around 1/4 to 1/5 of the tensile strength but it varies. Sometimes it is under 1/10 of the tensile strength. It is not uncommon to see the same rope for sale labeled with different working loads, sometimes different by over a factor five. The load in the bundle relative to its tensile strength is important because it effects the type of stretch.
Types of Stretch
Structural
Structural is when the rope is lengthened by a tightening of the weave, or other movement of the strands in the rope. Structural elongation involves a lot of friction and the recovery rate is very low. Torsion bundles should avoid structural elongation by operating at tensions high enough to achieve mainly material elongation. This can be done my removing rope, or by increasing tension.
Material Elongation
The point at which the stretch is due to material elongation is not easily determined. Material elongation stretch has fast and efficient recovery rates.
Design
For the direct effects of varying specific characteristics see Tuning