…..So anyway:

From Marsden’s second volume on Ancient Greek and Roman Artillery, we read Philon’s advice for the more perfect maintenance of high powered torsion engines:

“There follows something else that is altogether inconvenient, unworkmanlike, and detrimental to the range. In the heat of shooting and pulling back, the string experiences a slackening and needs tightening again. The range of the shooting deteriorates because of the relaxation. But those who wish to tighten it cannot apply the stretching vertically and in a straight line, but do it by extra-twisting, imparting an extra twist unnaturally greater than is suitable. They think they are doing good, but they are actually doing the tension great harm and are reducing, I maintain, the velocity of the shooting and its force on impact. The engine looses its springiness because the strings are huddled up into a thick spiral and the spring, becoming askew, is robbed of its force and liveliness through the excessive, extra twisting.”

In the experience of this ancient master, it is far better to tension the spring bundle of a torsion engine, by pulling on its ends to cause it to stretch out longer, much like stretching a rubber band, than it is to rotate the washers and thereby twist the spring bundle into an ever more tightly wound spiral. This latter technique he roundly disclaims as being an unfortunate, lowbrow way of cranking up the power on a torsion engine. My personal experience with over 500 shots on the old Gallwey ballista (see earlier postings) indicates the fundamental importance of keeping the springs as straight as possible.

While I do not doubt these general principles that Philon lays out, I cannot quite shake the idea that maximum power will be achieved by twisting these torsion beasties up to the point we wring out every last drop of slackness.  Only then will they ever give up all their foot pounds of energy to us.  I believe that both tensioning techniques are appropriate, if they are sequenced properly and if the preload is carefully monitored. In terms of maintenance on these machines, it is very important that the linear expansion of the crossbars always be performed first, before applying any rotational tension.  In this sense, rotational tensioning of a ballista is not necessarily a bad thing.  It just has to be done in the right order and to the right degree.  In short, straight springs provide the best foundation for generating power in a torsion engine. However, in my experience,  you can always zip things up a bit by applying an appropriate level of twist at the end.   And come to think of it, Philon seems to be suggesting the same thing when he warns against “excessive” twisting.   Which, of course, is not the same as “no” twisting.

On another note:  Did I just talk myself into converting the crossbars on our machine into a wedge based system?………Old Philon would be so proud.

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