Stretcher is a device used to pretension the spring cord of a torsion ballista. It consists of a frame to which a field-frame is attached and a winch that is used to tighten each spring cord. This article describes one type of stretcher specifically designed for the cheiroballistra and similar iron-framed ballistas. Wooden framed ballistas require a different kind of stretcher.
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Design and plans Edit
The design described here has been successfully used to arm the springs of a cheiroballistra several times over. Most details were reached through trial and error, and this version represent the third major iteration of the design, with many more minor additions and enhancements. Click on the image below to see the full-size plan:
The stretcher frame is composed of 2 wide planks forming the sides and eight round dowel spacers. The spacers are pushed into tight holes in the planks and glued to hold the frame together. At the top and bottom of the planks there's a round hole for the winch axle. Two concentric rows of holes surround the winch axle; holes in each row are 20 degrees apart, which allows locking the winch handle at 10 degree intervals using a simple locking pin. The field-frame is locked into the stretcher using four rectangular wooden lathes or crossbeams going through holes in the side planks and the pi-brackets in the field-frames. The winch is removable and consists of a thick hardwood axle and a thin steel handle. The steel handle is pushed through a tight hole near the end of the axle to bind the two together. A hole has been bored through the axle near the middle for the spring cord.
The stretcher frame is fairly high to allow tuning the spring cord by plucking it from between the winch and the field-frame. If the frame was made significantly shorter, it'd be impossible to tune the machine by measuring the sound pitch.
Making the stretcher Edit
First mark the places of the holes to the planks. Remember to also mark the two concentric rows of holes which are omitted here:
Clamp the two planks together:
Drill holes for the winch axle, the spacers and the two concentric rows of holes. Chisel the holes for the crossbeams:
Saw and plane the crossbeams and the spacers to correct dimensions:
Push the glued ends of the spacers to their respective holes in the plank:
Do the same for the other plank. After this the frame of the stretcher is ready.
Making the winch is easy. Take or make round hardwood dowel of appropriate thickness and length. Drill one hole near the middle for the spring cord, and one near the end for the winch handle. Use ~6mm steel for the axle. If you want, you can add linen bindings on both side of the handle to prevent the axle from splitting.
The round pin used to lock the winch handle should be ~6mm thick and "T" or "L"-shaped to help push it to the holes in the planks.
Here's a front view of the entire stretcher with crossbeams detached:
And a side view:
Here's a detailed view of the holes for the crossbeams. Also notice the ends of the spacers glued to the planks:
And here's a detailed view of the two concentric rims of holes and the pin used to lock the winch:
The entire apparatus shown from the top:
And finally, here's a picture of the stretcher in use:
The leather bands are used to attach a selfphone with a tuner application to the stretcher. This allows measuring the spring cord's sound pitch very easily. As can be seen, it's not necessary to (be able to) pass all four crossbeams through the pi-brackets in the field-frame.