The standard item has a spring and a pathetic screw to keep the tension on the belt. I found the screw would tire and then the belt would loosen under load as the spring let it go. This is a remix of the belt tensioner http://www.thingiverse.com/thing:805215 that uses 2 x MR63ZZ (3x6x2.5mm) ball bearings mounted on the stock pulley and the original spring. This version uses a 20mm M6 bolt through the end, it's much easier to fit - Simply loosen the bolt right off, push it all the way in, slot the idler pulley + bearings in on the belt and do the bolt up until it sounds like a bass string when plucked! This version with bearings is much smoother than the stock item. I did a version with a captive M6 nut, but it's much more difficult to fit, the thread ramps in the hole work just as solidly and help stop the bolt undoing.print it on a raft with a solid infill