This is a modification of belt tensioner by chopmeister to fit the QU-BD RXL printer belts. I have one each fitted to each Y Axis belt (my X Axis was already OK). On another RXL we fitted this to all three belts. Be careful to mount the tensioner in a position where it does not collide with the drives or rollers. On the Y axis it does touch the thread of the Z-axis (notice the oily black mark in one picture). This does not seem to effect any operation. The issue I had at the time was the wave in prints as the hot-end changed direction. See the gallery, a photo shows closeup of two prints the right one was with loose belts showing the ripple effect on the print as the hot-end changed direction. The left print is after the belt tensioners were added.