IF you are tightening the Y Belt and you have 500 cut hours then I would first remove the Y Motor and inspect the 2 small gear box bearings. In MY opinion the reason the belt comes loose is the bearings failing and letting the big plastic gear and shaft tilt toward the center of the machine. This makes a slack belt. THREE times on different machines I did this with about 500 hours on the machines and THREE times I ended up having a Y Failure in the near future with the shaft of the big plastic gear worn thin..... Had I replaced the 2 gear box bearings before I tightened the belt... I would have been better off. LHR sells the Gear and Shaft as a replacement part.
I recommend replacing the two 5 mm x 11 mm x 5 mm bearings when you replace the Cut Motor Brushes at 250 Hours.
Just one of the MANY places that sell the bearings. I buy them in a 10 pack for 5 repairs...
http://www.avidrc.com/product/1/bear...-bearings.html
OR buy local. Stay away from Cheap eBay as they have been reported here to be under size.
AL