If anyone is interested this device [www.dwbowen.com] uses ABS 3.5" plastic discs connected to 4 inch nylon webbing. The conveyor is advanced by a 1 rpm AC gearhead motor. The webbing is stretched using a tensioning device and it seems to stay put during printing... at least enough for my purposes. The total length of the conveyor for this device is roughly 20'. I have also successfully used a 40' version.
Each time the device does a new print it centers itself. Before it begins printing the onion plant it puts down a 2" x 2" raft of ABS. This helps the prints stick to the discs as they get taller. Hope this helps. -Bowen - http://dev.forums.reprap.org/read.php?171,36933 - , quoted by --Sebastien Bailard 08:02, 24 February 2010 (UTC)