Summary

This is a an idler for the Z axis belt of your Prism Reprap (design by by Buback)

Its two primary purposes are : to be able to use a longer closed belt, like found for instance in original Mendel belts set, and to dampen the wobbling given by printed Z pulleys (sometimes a little oval).

The side effect bonuses are : better belt path with more teeth engaged on the driving and driven pulleys, easy belt tension adjustment.

Instructions

Usage :
1) You may want to modify the size in openscad depending on your belt length,
the stl file is for a 100 to 110 cm belt.
2) Print.
3) Assemble a 4mm Internal diameter bearing (I used a 624) onto the part with
appropriate washers - I think a 20 mm M4 bolt is fit.
4) Mount your Z motor support on the other side of its extrusion (out of the frame).
5) Mount this idler on the internal side of the extrusion next to the Z pulley.
6) Adjust the tension by sliding it sideways (and possibly the motor mount too).
7) Tighten everything.

The downside view picture here on thingiverse could be helpful if you do not get how it goes together.

You might have to experiment a bit to find the wall thickness/infill parameter which will give you enough springiness in the horizontal direction but not too much flexing in the vertical. The close-up view of the mounted idler here shows my first attempt, which was flexing too much :)

Prism Z belt springy idler