[JesseK]

It's possible on a rudimentary level to figure out the torque requirements of a lead screw by figuring out the angle of the threads as if projected on a flat plane. I figured this out as we went along in 2011, so I don't know if it's the best way to do it:

e.g. There are 2" of travel in 1 turn, PI*0.625" of circumference rotation in 1 turn, thus Theta = Tan(2/(PI*0.625)) should give the angle of thread incline in radians. Sin(Theta) will then correlate to the amount of force due to gravity the motor must overcome just to move the load. You'll also want to add in a buffer for the losses within the threaded nut, thrust bearings and the gearbox. If I remember correctly, I did it as a ratio [sin(Theta)/(sin(Theta)+cos(Theta)) * weight of load] was the force at the tip of the sprocket on the threaded rod, and it seemed to be close to what we experimented with under heavy load.

Note that the '2" of travel in 1 turn' is the rating after the # of starts is taken into account.

As far as how fast an acme rod can spin under load -- if the ends have thrust bearings and the radial bearings are also mounted properly, then there's no real reason to not go right up to the rod's rated limits (we went beyond it in 2011, yet that was due to a very light load that year). One end's radial bearing should have a little play (ours was in a block mount that used a 1/4" bolts in a oversized 3/8" hole) so that when it gets to the end it doesn't seize/bend the rod.