I and Nick Watts have been having many good discussions on his talk page lately. Nick is currently engaged in making a new ballista based on the Elenovo find (see Kayumov & Minchev 2010), and while doing that he thought of combining ideas from Philon's wedge machine and incorporating them into a "standard" ballista. The basic idea is that the top surface of the washer bars is flat, and a pair of iron wedges is mounted on top of each one. Spring cord is then wound into the torsion springs, and each cord is individually pretensioned using a stretcher. So in the end we have a torsion spring frame full of cords which pass over the crossbars and the wedges on top of them. This idea is pretty ingenious, because it solves two very real and serious issues in known archaelogical artefacts.
The first problem is that the currently known late-Roman iron field frames and washers typically have too few holes to allow exactly matching the power output of two spring by rotating the washers. In particular, Gornea hole carriers have four round holes (Gudea & Baatz 1974: 54; 1978: 15), as do those from Orsova (Gudea & Baatz 1974: 58; Baatz 1978: 9), Lyon (Baatz & Feugère 1981: 203) and Elenovo (Kayumov & Minchev 2010: 334). The Sala field frame (e.g. Iriarte 2003: 123) has additional holes, but as mentioned by Kayumov & Minchev (2010: 334), the extra holes seem to have been added later. The only washers that have been found in conjunction with a metal field frame are those from Lyon, and they have only six holes (Baatz & Feugère 1981: 203). That amount of holes (4+6) is simply too little in practice to allow exact matching of the torsion springs' power output: this has been adequately proven in practical tests conducted separately by Nick and I. One of the tricks I've used is to only use one pin to lock the washer in place, but even that does not solve the problem entirely, even in the rather slender spring of the cheiroballistra, let alone in more squat frames. It should be noted, though, that both Nick and I have so far used nylon three-ply as the spring cord. As I discussed in an earlier blog post, it is possible that sinew is much more elastic than nylon, which would mean less rotational granularity is needed when using it, among other things.
In any case, if the arms do not rotate at exactly the same rate during pullback, they will also return at a different speed, assuming they're of same weight and that their weight placement is identical. Any deviation in the arms' return speed causes the bowstring and thus the bolt's butt-end to move sideways when the ballista is shot, so that that bolt leaves the ballista at an angle. In the best case this causes slightly erratic bolt flight, and in the worst case it can cause a dry fire, which can injure or kill the operator(s) and/or damage the ballista.
The second problem is that many of the washer crossbars do not seem to be adequately rounded at top. This applies to washers from Lyon (Baatz & Feugère 1981: 203), Emporion (see Emporion ballista) and Xanten-Wardt (Schalles 2011: 20). The crossbars of Caminreal washers (Campbell 2003: 23) are different in that their top surfaces are not flat, but higher at the center and at their extremities. The same applies to the Cremona crossbars (Campbell 2003: 15). In both of these cases it seems unlikely that there could have been wedges on top of them. However, even the Cremona crossbars seem to have fairly sharp edges, i.e. very little rounding. The Caminreal washers have also been generously rounded where the spring cords pass them. However, in most cases it seems that the archaeological crossbars were not typically rounded much, if at all.
According to Nick Watts the crossbars have to be very heavily rounded or the (nylon) spring cords will be cut under tension. It did not even occur to me to not round my crossbars, so I have no direct experience on the matter, but rounding makes perfect sense even when using sinew cord. So here we have a clear discrepancy between archaelogical finds and modern, practical experience, which can be explained in a few ways:
- Sinew is less likely to get cut under pressure than nylon. In my limited experience this is the case, but I don't know how much the crossbars need to be rounded when using sinew cord.
- The crossbars had pairs of iron/steel wedges on top of them. The upper wedges were heavily rounded to minimize chafing of the spring cord. This is unlikely to apply to the Caminreal and Cremona crossbars, so the only explanation besides #1 would be that they were unfinished similarly to the the Gornea field frames.
In any case, it seems that adding steel wedges on top of the washer crossbars is an elegant solution to the above two problems. Both problems might be non-issues with real sinew cord, but practical tests are needed to see if this is the case.
In any case using wedges on top of crossbars would provide several benefits:
- Linear pretension of the springs could have been increased easily to a degree without having to restretch each cord in a stretcher. Overly high wedges would have fallen under pressure of the spring, so it is unlikely they could have entirely replaced the stretcher.
- When ballista was not in active use, the washers could have been rotated to rest position. When it was time for action, the washers would have been rotated equally to increase power output, not to tune the springs. In this use-case it does matter whether the washers can be rotated in 7.5 degree increments, or increments of 30 degrees, as long as they are rotated an equal amount. According to Nick Watts this approach actually works: if the linear pretension in each spring is equal, then equal amount of washer rotation produces even limb movement during pullback. If the spring was taken out of action for a longer period of time, the wedges could also be loosened to further reduce strain on the cords.
- Rounding the upper wedge would effectively prevent chafing of the spring cord.
- The springs could be matched in a much more fine-grained fashion than with washer rotation alone - even if there were plenty of holes in the hole carriers and washers.
It remains to be seen how well the wedges work in practice, and whether sinew can really take fairly sharp corners at the crossbars without breaking. My cheiroballistra will not get wedges in a while, because I don't have need for them, and because my crossbars are too narrow for them to work properly. Moreover, the small diameter of my cheiroballistra washers means that I need to make use of all the available space to produce maximum amount of power.