In his treatise on catapult design, Belopoeica, the ancient Greek author Philon tells us this about successful designs, “The man who wants to shoot far must try to put on as much spring-cord as possible: of course, not only do we believe that the secret of power lies particularly in this, but all others believe it, too. But, since the spring cord passes through the holes of the hole-carrier, he who intends to put in more spring-cord must make the hole in the hole-carrier larger (otherwise they will not take more), so that the surrounding edges are left extremely thin and are naturally weak. It is impossible to make the hole-carrier broader, for it will then exceed its dimensional scale”. In other words, the impulse to make the spring bundles larger in diameter is limited by the size of the holes in the frame through which they must pass. There comes a point when that desire to make larger springs requires the whole frame to become bigger, and then of course we are talking about making a different machine than the one originally envisaged.

 

With the Orsova reconstruction the original artifacts presented a bit of a dilemma.   The following model should help illustrate the problem.

 

 

 The end caps in the field frames have 3 1/8” holes for the spring bundles and do not have any kind of recess cut in them for the bronze tightening washers to ride in.  Absent some other component, that would mean that the flange on the bottom of the washer would need to fit down inside the holes of the end cap to allow the washer to run concentrically in the frame as the pre-load tightening procedure is being applied. The wall thickness of such a flange would need to be at least 1/4”, therefore the effective hole diameter for the spring would naturally reduce the room available for the spring bundle down to 2 5/8”.  This represents a 16% reduction in spring size. An unthinkable amount for an obsessive catapultier lusting after more foot pounds of energy. Either we accept that the Romans devised a way of torquing up the springs in their ballistas without having the washer’s axial movement controlled mechanically, or they were unconcerned about the washer drifting around on the end cap of the field frame until it was pinned in place. Possibly two strong men with a tightening lever that projected from either side of the washer could control the axial drift by sheer muscle power, while a third popped the locking pins in at the appropriate place. Would they still be able to pre-load the springs with as much torque as a system that had some kind of flange projecting from the underside of the washer. Probably not. Especially considering that any frictional braking that might impede maximum torque with a flange based system would be caused by bronze rubbing against iron, a pairing of materials well known to prevent galling and reduce friction. Moreover, from the earliest of times, torsion engines had relied on a flange riding in a groove to allow maximum pre-loading of the springs. Probably this part of their past designs they would have little incentive to change.

 

How then to use a flange based system for tightening the washers, when the original field frames unearthed in Orsova had no recess in the end caps for the flange to run in?  Enter my proposal for the “vernier” plate.  A simple 3/8” thick plate that had four projecting pins to lock it into the end cap and a vernier based hole pattern for the washer’s locking pins. With a 3 5/8” through hole it would naturally providing a recess for the flange on the underside of the washer to ride in. Now we can have the spring bundles at the maximum diameter allowed by the artifacts (around 3”), and all we need to accept to get there is that,  like so many of the other parts of the original machine, with the ravages of time, they just turned up missing. I will favor this interpretation as it gets us to a point of maximum performance, with a minimum of incredulity.

 

As a side note, we might speculate that the forging of a very simple plate containing the hole patterns described above,  would have great utility in the ancient workshops as it would allow an easy way to mix and match the various field frames and washers they might have on hand.  Such an approach would obviate the need for much of the close tolerancing of the hole patterns between the field frames and washers (esp. radially), as final assembly is now controlled by the fit-up work on a single separate component.   In my experience, this is a fairly common strategy when large amounts of handwork are involved.

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