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:690090 that uses the original spring to tension the the stock pulley. This version uses an 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 in on the belt and do the bolt up until it sounds like a bass string when plucked! Dont use too long a bolt or it will catch on the side (the one I used is 20mm) 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. Once I made this I actually remixed the ball bearing versions, which gives a much smoother x axis. Print it on a raft with 100% fill. Depending on the accuracy of your printer you will need to sand the grooves on the bottom, if only to clean up after the raft. You may also find the axle slot makes a slight ridge.