Adding Skate Bearings to our Acrylic Spool Holder

We believe in continual product improvement. Every single comment customers send back to us we consider putting into action. The small ones are easy: tweak a corner here, update the manual there, while the larger ones have to sit around waiting until more customers add their voices.

One of the more common ideas customers have had was adding skate bearings to our acrylic spool holder, so when the supply of acrylic rods ran low, it was a no-brainer to convert the spool holder to use skate bearings instead. Keep in mind everything we design has to work in our print farm. The spool holders are specifically designed to require no adjustment or maintenance and just work day after day and month after month. Any changes to add skate bearings couldn't affect the practical functionality. 

People just seem to want spool holders with skate bearings, but the exact reasons are unclear. I'm an engineer, which often means asking questions followed by a lot of measuring. There isn't any data out there on any reason why skate bearings would be measurably better than a simple rod on a spool holder. Theoretically, the force required to rotate the mass of a full spool of filament should dwarf the opposing force from rod friction on acrylic, especially on the large 3Kg rolls we use. But the internet runs on insecurity and Reddit forums where unless you follow the fad du jour, your prints will look terrible and people will laugh at you. 

As a small business owner there unfortunately isn't time to stop and do the physics math. Still, as the person paying for the materials, I crunched even more important numbers: a skate bearing design would reduce material costs by around 50% because acrylic rods are expensive and skate bearings in bulk are not! That settled it: acrylic rods are out and skate bearings are in. :-)  This is even more important since the spool holders come free with most of the enclosure models.  

Here is the simple design change to add the bearings: a simple cap to hold the bearings in place on either end of a 3D printed rod. The first test prints with the spool holder showed why you don't want very low friction on a spool holder: the Prusa extruder moves and tugs directly on the spool, which causes the spool to rotate. Because the skate bearings have lower friction, the spool just kept on spinning, and the filament cascaded off of the spool into a tangle. Obviously, some friction is needed in a practical spool holder.

At first I thought this occurred because direct drive printers like the Prusa have the extruder motor in the hot end, both moving and extruding simultaneously. This is where a Bowden design should shine: the force on the filament spool is constant, reducing print defects from the moving head.  Unfortunately the same thing happened when I tried the super spinning spool holder with a Prusa Mini.  

Fortunately, with this design its easy to adjust the amount of friction. If you want the lowest possible friction, leave a little space between the cap and the bearing; if you need extra friction, push the caps in closer to the bearings. It's simple to adjust by twisting the end caps are you either push or pull them for more or less friction.