Back on Friday, January 8, 2010,  I posted this photo of a wedge from a rather clumsy attempt at a linear tensioning system I came up with for Firefly.  (That posting is in the archive for any deranged catapult nerds that might want to ponder on it.)

 

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This wedge was placed under the crossbar rather than on top of it ala Philon.  Note the impressed grooves in it’s surface where it contacted the underside of the crossbar.  This particular crossbar also had a full radius on it’s bottom just like it’s top, so that’s what left these marks.  Such are the forces of compression in the springs on an Orsova sized machine.  (Well, at least our version of it.)  The shallow “S” bend in the wedge was caused by the spiral in the spring that starts out on the underside of the crossbar.  Of course with a Philon type wedge system the wedges are placed on top of the crossbar where the spring is highly controlled and there is no spiral to worry about.

This bent wedge protruded enough into the smooth path of the spring that it quickly chaffed through the cords after fifty shots or so.  This was my first lesson in the relative fragility of spring cord.   The cord has to be treated with great respect or it will quickly wear through on a high-power machine like Firefly.  Then we end up with a mess like this:

 

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Or like this:

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To the uninitiated, you may take it as gospel that right angled edges on the top of the crossbar would be pretty much fatal for the spring cord in high-powered ballistas.  It is extremely doubtful that sinew would fair any better when forced to contend with focused stress points than nylon does.  In my working with sinew I have yet to encounter any magical properties that would make it immune to chaffing.  It is only a soft fiber after all.   Just saying.

 

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