There are two main forces involved in how the little ladder and little arch are stressed when the arms are rotated in the torsion springs. The first force is caused by the field frames trying to rotate as the arm is rotated. In an inswinger this means the following:
- The little arch starts bending towards the shooter, the stress being concentrated at the middle (i.e. at the center curve)
- The longer little ladder beam (at front) is squeezed (compressed) fairly directly along the longitudinal axis
- The shorter little ladder beam (at back) is pulled (stretched) fairly directly along the longitudinal axis
The second force is a bending force imparted by the field frames to the little ladder beams. Because the little ladder is attached to the bottom side of the case - that is, below the bowstring and the arms - the field frames are bent towards the shooter during pullback. As the field frames are fixed at the bottom, the only thing they can do is cant towards the shooter at the top. In effect the end of the longer little ladder beam is being pulled upwards, and the shorter one is pushed downwards.
The following images should clarify the above description quite a bit. First the little arch from top, with arrows showing the direction of the forces:
Then the little ladder, first from front, then top and finally from back:
This seems to indicate that the little ladder beams do not have to be very thick, because they're mostly stretched and compressed, and they are not bent along the longitudinal axis, like, say, a bow would be. For the little arch this means that it should be thick(er) at the middle and at the place where the forked part starts. In my experience supporting struts such as those used in Nick's Firefly are not needed in a small weapon such as the cheiroballistra, even when the power levels are pumped up relatively high. In any case, one would not need to worry about the field frames crushing the center curve. The high and fragile-looking curve in the Orsova kamarion (e.g. Baatz 1978: plate II.B) reinforces this idea: a strong push from the forked ends towards the center arch would easily cause it to collapse.
This page was originally written as a blog post.