On Mon, 14 Jan 2008 11:11:00 -0500, Bob Engelhardt
wrote:

Scott S. Logan wrote:
Actually, it's 900 pounds of THRUST on the carriage.

900 lbs! So, if you rigged some pulleys, the lathe could lift itself
off the ground? That might be convenient for getting it onto skates for
moving BG

As far as lead screw torque: an 1/8" pitch, 3/4" diameter leadscrew has
a PI*3/4 / 1/8 mechanical advantage. About 19:1. So 900 lbs thrust
means 900/19 = 47 lbs tangentially on the screw. 47 * 3/8 (radius) = 17
in-lbs torque. I've ignored the friction between the leadscrew and its
half nut, as Mach. Handbook gives a value of .04 for worm gear at 250 fpm.

You can't ignore friction. The efficiency of a 3/4-8 acme screw is
perhaps 30% - a consequence of the fact that the load is carried near
the OD of the screw.

As you've already figured, the OD of your screw travels 19x the
distance the load moves, and the effect of the friction is multiplied
as well - 1-(19 x .04) = .24 efficiency

Assuming the load is actually carried at the pitch diameter, and
guesstimating the the PD of your screw as .68 inch -
1-(17 x .04) = .32 efficiency

That's a grossly simplified analysis, but it works reasonably well for
acme screws, less well for 60 degree threads.

--
Ned Simmons