Every spool of filament I buy seems to be a different width, and I wanted to find a solution that would work with the PrintrBot Simple Tower. The expansion piece http://www.thingiverse.com/thing:221509 allows wider (95mm) spools to fit on the bearings, but it doesn't work for narrower spools. These rollers fit over the bearings, so they turn easily, and allow any width of spool (up to 95mm) to fit without having to adjust anything. The rollers have a camber, which helps to centre the spool. The OpenSCAD file is included, with all sizes defined as variables, so it would be simple to create rollers of different length or diameter. Designed to be printed vertically without supports. I used a generous (3mm) brim to make sure it stuck to the bed well, and didn't have any problems.