Hi Andrea, Welcome to this site. It a great community for help in building this open source robot arm.
I have sent you the ART4 TramsmissionColum design in a step file that I created from Fusion 360 for you to also try. It will help you print all parts on your printer.
P.S I am also looking and designing an Idle Gear for the other side of the transmission column, this is based on the redesign that was done by Danny Van Den Heuvel. I am printing now and going to assemble to test later this week.