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I've been using these for a while, and had to re-print one today that broke. Something I noticed this time: the arm, if printed upright as in the STL, does need supports in one spot if you want a perfect print: Take a look at the vertical hole through which the filament passes. Where the filament emerges from the arm to make contact with the gear and then goes back into the arm, there are un-supported horizontal overhangs on either side of the vertical hole where it re-enters above the bearing/gear.

Also, I would suggest tapering the inlet hole on the bottom, to make filament loading easier.