This is a single-axle trigger mechanism for a medieval crossbow. The string is held by the two teeth as the top of the cylindrical nut, and the trigger at the base of the nut keeps the nut from rotating, which holds the string.

enter image description here

What I don't understand is that all of the sources I've read say that none of the weight of the bow is held by the axle of the nut. In fact, some designs don't even have an axle, and just use a piece of string to keep the nut from falling out of the socket.

enter image description here

I don't understand how that works, though. Given that the bottom of the nut is held by the trigger, what prevents the string from pulling the nut forward, which would apply a force to the axle? The only idea I have is that the axle is loose enough that the nut is pulled forward against the socket instead, but I don't know.

If you take the mechanism as depicted in the first image, with no socket around the nut, what forces end up acting on the axle vs. the trigger?

  • $\begingroup$ I would say the socket helps a lot. Without the socket, the axle would be under a lot of load: 300 lbs from the string and roughly another 150-300 lbs from the trigger (depending on the lever arm length ratio of the string vs trigger). $\endgroup$ Oct 13, 2023 at 18:09
  • $\begingroup$ I think this must be the case. You can think of 300 lbs of force being applied to the top and bottom of the nut "lever", both pointing toward the prod of the bow. Either the axle or the socket must provide the counter-force, and given that some examples use a string, it must be the socket. $\endgroup$
    – Rekov
    Oct 14, 2023 at 19:38


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