Summary

This is simple remix of the string tensioner posted by stewarma.

Replace the screws in the printer bed with these and some nuts and you can adjust your X axis string tension by tightening the screw.

STL includes two parts, which are mirrors of each other; one for each end of the print bed.

Instructions

This version extends the part the bolt fits through to the whole thickness of the base, so that you can use a nut on the end to hold the screw tight.

Since this design is meant to be used with a nut, the hole exists in the model for the full depth, unlike the original.

STL includes two parts, which are mirrors of each other; one for each end of the print bed.

Note
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If you don't want to lose any of your print volume in the X axis, you'll need to cut some material off of your Printrbot Simple. You'll need to cut out one or two notches, depending on if you use one or two tensioners.

I've included a pic showing the notches I've made in my Simple (even though I'm only using one tensioner on the Right side of my print bed) as well as a pic of the tensioner with the printer in use.

Disclaimer
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Any modification of your Printer is taken at your own risk, with no warranty or guarantee provided by myself nor Thingiverse; either express or implied.

That said, if you do, make sure you keep your bearings clean! Cover them before cutting.

Printrbot Simple X Tensioner v2