The design for this sucks. There is a metal axle rubbing against a plastic slot. It will eventually wear through and cause the belt to loosen. Following the standard procedure to tighten the belt will only fix it for a short while. This problem has been getting worse steadily, and finally got to a point where the belt was always skipping teeth during each pass back and forth. I seem to have fixed it and am glad to share how I did it.
Step 1: Testing the Damage
First loosen the 4 hex screws on the stepper motor on the opposite side of this roller. As in the pictures, if I press against the loose roller, the worn damage is so bad that the roller easily slips forward into this hideous angle, which is the source of the belt skipping noise of print failure.
Step 2: Remove Belt Roller
If you look closely, you can see that the metal has warn away at this black plastic.
Step 3: Buy These Washers
I needed only the #6-1/8, but was forced to buy the entire econo pack. You will need 6 of them.
Step 4: Disassemble the Carriage and Carve a Bigger Notch
Follow the procedure on the Makerbot tech support for removing or replacing the upper XY carriage. basically had to remove all of the tiny screws that pop out the top of the Makerbot and carefully pop this black plastic part out with finger pressure.
I carved out this notch by placing one of the washers over the spot I wanted, and tracing it with an exacto blade. I then ground the U shape out with a rotary tool and cleaned it up again with the Exacto blade, testing the washer fit the whole time. I only went the depth of one washer.
Repeated on the other side.
Step 5: Place Washers
Adding 3 washers to each side, and a bit of white grease between the plastic and the first washers, slide the much stronger part back into place.
Tools used shown in background.
Step 6: Reassemble
After putting it all back together, I ran a 10 hour print that was all round shapes, and it printed more smoothly and better aligned than I can remember. Printed several things since and it is still perfectly aligned.
The same part, white print shows before when the belt is loose, how badly a cylinder shape printed.
The black was printed after the belt roller fix, showing the way the part is supposed to be.