Timeline for answer to How to construct a re-arming mechanism for a medieval crossbow using medieval technology? by JBH
Current License: CC BY-SA 4.0
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11 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| 11 hours ago | comment | added | Nuclear Hoagie | @Nosajimiki If the force is sufficient to pull the string back, it doesn't matter how long it takes. Imagine replacing the spring with just a weight hung from the bowstring - as soon as the applied weight exceeds the draw weight, the bow will draw fully (it'll just draw faster with more hung weight). It's only the biomechanics of the human body that make it tiring to hold a drawn bow. But you can pull a bowstring halfway back with a spring and leave it there forever with no additional energy input. | |
| 11 hours ago | comment | added | JBH | @Nosajimiki I may be, too. But my understanding is that springs require the same energy to tighten them as they release (ignoring inefficiencies due to imperfect manufacturing and circumstances). I think we're confusing Joules with Watts. Whether a slow draw or a fast draw, the same amount of Joules is expended - but there's a considerable difference in the number of Watts used to draw vs. the Watts to launch. | |
| 11 hours ago | comment | added | Nosajimiki | @JBH I might be missing something , but I'm pretty sure there is a force over time issue. The longer you spend drawing a bow, the longer you need to spend overcoming the resistance which means the more total energy you need to expend. A crossbow that you can draw and lock in 1 second might require about 90J of energy but if it takes a coil spring 10 seconds to draw, the bow will exert 900J of resistance over the full time of the draw. | |
| 12 hours ago | comment | added | JBH | @Nosajimiki Ah. That makes them good at pulling the string back (the OP didn't say how fast the re-pull had to be) but not, as Alex suggests, at pushing the bolt directly. I'm learning new things! | |
| 12 hours ago | comment | added | JBH | @NuclearHoagie Therein lies the reason the phrase "handwave" appears in the answer. Modern metallurgy and spring technology could produce such a spring... but I'd have to do more research to determine if it could have been done in the early days of mainsprings... and then backtrack to see if it could have been pulled far enough back to meet the OP's needs. | |
| 12 hours ago | comment | added | JBH | @AlexP That's a good point. The unwinding spring must push that hard, but might not be capable of pushing that fast. | |
| 14 hours ago | comment | added | Nosajimiki | Coil springs are generally not efficient for bursts of kinetic energy. They are good at slow, controlled releases of energy, but not so much powerful ones. | |
| 14 hours ago | comment | added | Nuclear Hoagie | Early coiled springs were used for clocks, where they only had to store a small amount of energy sufficient to overcome the internal friction of the mechanism. I agree one large enough to store the kinetic energy of multiple crossbow bolts would be quite unwieldy, if it's even possible at all. | |
| 15 hours ago | comment | added | ccprog | Re weight: who says you have to carry the weapon in your hands while firing? Late medieval hand cannons were often fired while bracing them on a pronged stick or other support. | |
| 16 hours ago | comment | added | AlexP | "Why waste time pulling back the bowstring?" Maybe in order to accumulate energy at a lower power rating for release at a higher power rating. A crossbow typically pushes some 200 J into the bolt in say 10 milliseconds, for a power of about 20 kW. | |
| yesterday | history | answered | JBH | CC BY-SA 4.0 |
