FYI, if you want a GT2 tensioner (see  http://www.robotdigg.com/product/41/Timing-Belt-Tensioner-Spring ) and you don't want

FYI, if you want a GT2 tensioner (see http://www.robotdigg.com/product/41/Timing-Belt-Tensioner-Spring ) and you don’t want to pay/wait for it, find a wooden clothes pin. That spring in the middle looks an AWFUL lot like the same thing.

That’s exactly where the idea behind those tensioners came from :smiley:
Though adding springs to you belts might not be a good idea under certain circumstances. They do reduce backlash under near-static conditions, but also increase the tendency of your axis to oscillate and reduce the overall stiffness of the belt drive, possibly causing increased overshoot artifacts on the outside corners of your prints.

There’s also a huge range of spring constants on those things, speaking as someone who briefly used torsion springs to put compression on an extruder bearing.

@Thomas_Sanladerer has got it right. Don’t use these. At anything over about 40mm/s they actually introduce lash into the system and can cause oscillations unless your acceleration is turned way way down (which in turn leads to blobbing on corners, etc)

Hehe, good hint.