A modified version of donnyb99's great tensioner design. As some mentioned in the comments, the alignment of the pulley ends up raising the belt a little causing misalignment of the belt path and rubs against the v-slot aluminum extrusion. This remix drops the pulley to avoid this misalignment and some other minor changes for better fit (at least on my machine).
You will need a M5x25 hex head bolt and matching nut to install in puller and a M4x25 socket cap bolt and matching nut to mount pulley in puller. I originally used a nylon lock nut on the M5 bolt and have recently come into a situation where tightening/loosening the nut has put enough torque on the bolt head to strip the nut trap in the puller part. Not sure if standard nuts would help in avoiding that or not.
I installed this without disconnecting the belt from the carriage, which requires removal of the belt from the stepper gear and once the tensioner is installed you can roll the belt over the stepper gear, eg putting a chain on your bicycle sprocket. Once the belt is back in place, apply tension with knob (pic shows just a nut).
Quick animation of assembly can be seen here
- 07/6/2017: Added knob that is flipped to have the outer part larger, idea from Phiippe.