Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4960578.
Dual Z leadscrew setups, especially when fitted aftermarket, usually need a timing belt to keep both Z axis in sync. A belt is even a good thing to have if your dual Z setup incorporates a second stepper motor - because both Z axis might get out of sync when the printer is not powered (and you work on the gantries, or even just move the printer around)
So, these setups usually come with a belt tensioner. But - these tensioners normally adjust on one side of the belt only, resulting in one very large stretch of belt where the tensioner does not touch it and, opposite, to shorter stretches of belt with the tension idler touching it somewhere - ideally in the middle, but YMMV.
This certainly leads to uneven tension which in turn might affect timing, shortening the life of your belt and quite possible leading to other problems, hence my (now revised) design of a symmetrical belt tensioning device applying even tension on both lengths of the timing belt.
What has changed to yesterday's original version?
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back mounted bracket, keeps the top clean :)
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tensioner arm moved closer to top extrusion (21 mm is the correct distance to leadscrew center axis)
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prints without supports
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bigger idlers (based on Rickysol's design of his Tronxy pulley (https://www.thingiverse.com/thing:4167751), running on standard 16mm V-roller bearings
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alternative bracket included for those who prefer a (lock) nut to attach the tension bar