Hi Rich, I used a maxxon motor with the gearing built in, I needed around 35 kg of torque, the jack screw is 8 mm.
here are three links to videos of the gear operating
http://www.youtube.com/watch?v=A6rsxI5X90o
http://www.youtube.com/watch?v=dviXDxnz_BI

http://www.youtube.com/user/meepamda#p/u/5/g3haltUkvPM

the last link, gives a reasonable view of the mechanism setup.
Mike