Summary

This might not be a print-in-place model but it was the strongest, lowest-friction design I could come up with for a ball bearing or roller bearing in this case since keeping all the layer lines parallel would reduce friction. You will need to cut 7 small pieces of filament around 6mm length to use as the axles that hold the rollers in place and you will need to very carefully super glue the racks and ring halves together so that you don't accidentally seize any rollers or join anything out of alignment. Lubricate with dry silicone spray.