The key to developing power in a torsion engine is to pre-load the springs with as much torque as is consistent with the capacity of the spring material to utilize without failure. In the case of the Orsova reconstruction, we are looking at a 3” diameter bundle of nylon line. The amount of tensile force required to break a bundle that thick is around 40 tons,  however the important number for a ballista would be the point at which the spring fails to develop full recovery.   No doubt this would be considerably less than the final breaking strength.  However,  nylon, like sinew, is renown for its elasticity and so it seems reasonable to suggest that even a puny 3” spring can be quite powerful if we are not too timid in how it is pre-loaded.   Locking in maximum torque while the machine is at rest is largely dependent on the number of locking stations available to the tightening washer.   The Gallwey reconstruction has 32 notches , 11 ¼ degrees of rotation per locking station.  Using a 5 foot spanner I often find myself straining to get that one extra notch of tightening.  It follows that the closer together the notches are, the more pre-load can be put into the springs without having to let the washer slip back to the preceding locking station.  Field trials with the Gallwey showed how important it was to balance the torque between the springs to get good accuracy.  I found with that machine, if I used good body mechanics each time, my maximum physical exertion with a 5 foot spanner was a pretty accurate way of judging the torque applied to the springs. 

 

  

 

 

The value of introducing high levels of pre-load into the springs was well understood by the ancients. The first record of a “vernier” system can be traced to the hole pattern evident in the tightening washer of a second century ballista excavated in Ampurias, Spain just before World War 1.  It allowed the ancient engineers to lock in the amount of twist introduced to the springs to a fine 7 ½ degrees of rotation.   This level of rotational control also allows the two spring bundles to be more closely matched in power than was possible with my Gallwey ballista.   Clearly this was the way to go with the Orsova reconstruction.  With a spanner maybe eight or ten feet long, and the field frames locked into some kind of fixture mounted on the ground, it should be possible to take that 3″ bundle of nylon and torque it up to some ungodly level.  Taking it to the extreme I’m contemplating,  the rest of the machine will need to be as stout as possible.  I guess I’ll know I’ve gone too far when something breaks.

 

 

It takes about one minute for an old man to cock this contraption to the yellow mark on the side plate.  It seems like some of these notches are better than others in the accuracy department.  Need more work to test this premise for sure.  Sand bags are used to bed the rear end of the machine when actual accuracy testing is in progress.  Full power testing requires more elaborate safety protocols (i.e. no civilians in the area and a thick plywood pavis) than is apparent with this relatively tepid series of shots. 

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