There seems to be an STL issue with “Inner Shell”. please check if m3 heatset holes are blocked. Bambu and Orca slicer seem to not have this issue, but Prusa slicer does. Printables model viewer also has this.

These filled holes can be cut out with a negative modifier if needed.

Hardware:

• Small bearing x12 - 3mm inner diameter, 10mm outer diameter, 4mm depth
• Large bearing x1 - 20mm inner diameter, 27mm outer diameter, 4mm depth
• GT2 Timing belt x1 (GT2 540-2M-06 RD015) Maker's supply
• M3 heatset inserts x12 5mm outer diameter, 4mm length
• Nema17 Stepper motor
• GT2 aluminium pulley 16TB5W6 
• M3*8 machine screw x12
• M3*12 machine screw x4 (for mounting stepper)

Primary function as a demonstation piece. High 

Mounting holes: hexagonal shape. 40mm from center, 40mm hexagon edges, 3.2mm diameter
^ Mounting pattern on both sides

Height: 30.5mm +-0.5mm
diameter: 142mm

The belt is inverted, with the teeth gripping the outside shell.

Tips & Advice

• Use a small amount of lubricant on the smooth side of the belt
• Print outer shell with a 0.2mm nozzle. I had to re-print my outer shell at 100.05% scale for the best results. Increases the size reduces friction, but also allows the belt to skip more
• The logo can easily be removed by sinking the outer shell into the bed by 0.3mm
• Assembe rollers individually, heatset the inserts, attach all rollers (use removable threadlocker if needed), insert motor, attach aluminum pulley on motor, attach belt and tension, place large bearing over the aluminum pulley, carefully install the outer shell (make sure that it doesnt push the belt off the rollers)
• I printed the outer shell with 1 layer of black petg (on a carbon fibre build plate), then the rest with Prusament orange