The tension of the belt presses it against the large washer of unsupported end of the pulley. Rubber moving against metal is a perfect brake that can do to lose steps in the motors. Simply adding two new additional standard washers of the same outer diameter of large original washers, but with larger hole, can make a more perfect pulley bearing. In the next pics is explained the assembly.
https://drive.google.com/file/d/0BycCP-4CirzmMEo5WHZ1dlVHTmc/edit?usp=sharing
Hi asanchez,
those secondary washers look like standard size M10 washers, right?
To reduce friction and wear between the belt and the washers, you can also bend the alu bracket a bit to adjust the angle of the bearing.
In addition to your nice solution (I like its simplicity), it’s probably also possible to adopt the idler design from other RepRap machines, e.g. pairs of flanged ball bearings. F625 bearings do exist, however, the flange is only 1 mm high, so a good adjustment of the bearings towards the belt would still be necessary.
Cheers,
kuraasu