I can show you some 4 pitch 60 degree vee style threads that will hold up the left side of the World, too.
Back when I learned about threads and thread stresses, it's true, the more wraps you have in a given length creating more cross section area at the pitch line, the more strength you will gain in the thread design. It gets more technical than this. I've seen threads destroy themselves by rolling over at the pitch line. I've seen them shear out, the nut thread, at the major diameter of the screw and that one is neat. I have one of them around here somewhere. Of course, 99.9% of all of the threads I deal with daily, you won't find them listed in any standard book. Most of them are static loaded, a few are dynamic or static loaded with torque. Threads are interested when you get above the text book variety one's most people use everyday.
As Jim mentioned above, the nut is nearly 2" long. Actually the thread on the screw is around 2" in length. The nut has much longer thread in it from what recall on my vise. The nut is also cast iron, so it is kind of sort of self lubricating which helps in transmitting torque into force to clamp something in the vise. The thread used is about 1-1/8" OD x 6 or 7 pitch. It may even be metric. I haven't measured one to verify that. Again, from the number of turns of full engagement and crunching the numbers, you'll be surprised how much the thread will handle before it hits its yield limit in shear. It will be greater that the screw cross section at root of the thread. Got to remember, there is two areas that have to be calculated to determine which one is weaker in a male or female thread.
Ken
But the nut is about at least 2 diameters long, so plenty of engagement.
