Have you taken a look at Legio XX Valeria Victrix Ballistaria's siege artilery page?
http://198.144.2.125/Siege/siege.htmIt's about REAL ballistas. Including the stone throwing ballista one of their members built,
it's not small and build as a reconstruction. They also have some articles and other helpful
info about ballistas.
I also suggest "Greek and Roman Artillery : Historical Development" and Greek and Roman
Artillery : Technical Treatises" both by Eric Marsden.
This is all good info on the use and construction of Ballistas. Now it's great for a part by
part reconstruction, but that's not what we want. We can get the function and design
specifics need to help build an SCA combat version. Which is what I'm trying to do. The
Greeks had designed a chart of ratios based of the diameter of the spring hole which is
determined by the wieght of the projectile for a stone thrower and the length of the
projectile for an arrow shooter. I have completed an Excel spreadsheet based on the table of
ratios for the stone thower, which is in the files area of the SCA-SiegeDesign list at yahoogroups, and am working on the arrow shooter verion now. When I am done I plan to study both and try to
simplify the design of the ballista. Makeing an exact reconstruction is great, but for
economy and ease of maintenance a simpler design (less parts) that functions the same is all
that is needed. That's what I'm doing. This way everything should be proportional and
function well. Now that I've gone way over your head and off the question, I'll try to get
back on track.
Look at the references above. Look at as many engines as you can. Ask these people what they
found that worked, didn't work and what they changed. Take a look at some of the online
plans. Like the Spruce Moose Ballista (
http://home.austin.rr.com/dmiller/ballista/ballista.html ). I think it's inefficient, as the
torsion box is wider than it is tall. I have a friend that build one, it didn't have as much
power as I though it should. It does have some good ideas for construction if your designing
from scratch. Look at the release trigger on the Legio Ballista. I'm going to try that. I
think it would work much better that the Spruce Moose gate latch option. Also take a look at
The Realm of Taergerasmob (
http://andy.trebuchet.com/plan_archive.htm ), look under taxing
and mayhem. Look at the Desktop Ballista, also there are the Spruce Moose and Legio plans.
http://www.legionxxiv.org/lrgballistacrew/One of the questions that I'm looking for an answer for is the string issue. It would
have to be something I can make the length I want and not strech. Someone on the SCA-SiegeDesign list said they used the construction technique in "The Book of the Crossbow" for making a crossbow string. One thing to remember is
that the uprights in the torsion box, both the outer and inner, are what stops the arms
motion not the string. Otherwise the string might snap after a few shots, unless you use
something really heavy duty.