Improved Tensioner for 3Dfused Rail Kits

The idlers included with the 3Dfused linear rail kits are wobbly and twisty, and the idler they include grips the GT2 belts too tightly causing the belt to twist and walk towards the outside of the idler. This can introduce backlash like artifacts.

These idlers beef up the anchoring by extending the anchor piece to capture the tensioner piece, preventing all movement except for the direction it should move. It also re-uses the existing CR-10s smooth idler (the two smooth lipped bearings stack