The last great hurdle to get this Orsova reconstruction ready for its first trial shot, is the manufacture of the four bronze washers that will hold the cross bars that the line for the torsion springs wraps around. Several of these washers have been recovered at other sites from earlier styles of ballista.   Seen below is a washer found at the Hatra site in Iraq.  Dated to the third century, it is barely one hundred years older than the Orsova artifacts. Like the Orsova ballista, the Hatra machine also had a wide squat frame and so there is every likelihood that it too was an inswinger.  


Photo copied from, Recent Finds in Ancient Artillery, by Prof. Dietwulf Baatz.


The tapered reinforcing projections  seen at the bottom of the notches for the cross bars would help resist the massive pressure applied to the bottom of the notch from the cross bars as the torsion springs squeeze down on them from the top.   The sixteen holes for the locking pins are clearly visible.