How many threads are engaged in a typical large (>3/4") nut and bolt? At what point does it achieve optimal/max strenght?
According to Fastenal, a standard 3/4-10 NC nut is .656" thick. so call it 6 full threads and a little extra. (I note that's a little short of the 1-1.5x diameter my book says for making threads, but again {shrug} I'm not an expert by any means.) As far as max strength, I assume that each added thread adds some strength, but the optimal thread engagement is where diminishing returns really take over. Optimal strength of a threaded joint is probably usage specific.
Sure -- but what's the diameter of that breech cap? It had what, maybe four or five threads engaged?
I know in automotive suspension work, the commercial truck spring place where I get U-bolts for my Jeep leaf springs supplies the U-bolts with double-height nuts -- twice the height (and number of threads, obviously) of a standard hardware store nut of the same diameter and thread pitch. I assume they do that for a reason.
I think we've (the internet) have been saying it's about an 1.5 diameter, but I don't know how the fact that it's a donut plays into that. It's not like a solid 1.5" bolt.
On the leaf spring thing I just put a lift on my F150, and it too has tall nuts for the leaf spring u-bolts. Tall, grade 8 at that. I'd be interested in why that got speced actually, I suspect that the pull on those bolts is really high, and uneven, trying to resist lateral movement of the axle when off camber or cornering. (the leaf spring would try and tilt on the axle with a lateral load, and one half of the bolt would be in compression, while the other half is in tension) That may have some crazy loads, but the math is beyond me.
To be clear, I'm not arguing the RN-50 has "enough" threads, or that 4 is indeed the "right" number of threads like the video said. I'm more trying to learn more through the discussion, and the extra research the discussion sends me on.
For another data point Howitzers still use interrupted threads to close cannon breeches. Here is the open breech of an M109 155m Paladin:
Even halving the number because of the interruptions, that's more than 4 threads.