What moves the ribbon on a Teletype 43?

I just got some new ribbons for my Teletype 43 and the print was (at first) nice and dark. But it soon started to fade. I watched the ribbon while printing and it didn't move! (There are two rollers on the left side of the typing unit which allow rotation only in one direction, and the ribbon is a continuous loop reinked as it passes through its cartridge, mounted at the right side). So obviously when *something* moves the ribbon, the ratchet action on the rollers prevents it from reversing direction, so there is net motion of the loop in one direction. It looks like the ribbon is intended to move during each carriage return. But what actually moves it? I don't see any source of friction other than where the ribbon passes over the printhead which is quite smooth. There is also very little tension on the ribbon (there is a spring mechanism that holds the rollers to the left). I don't have a service manual, just the basic operator's manual, and can't find one online. thanks for any assistance. -Charles