Tensioner for the Y axis belt on the Wanhao Duplicator 9
if you downloaded V6, please delete and re-download this, it corrects a cosmetic bad fillet on the idler holder

I've included TWO versions, one for the original 6mm belt and one for the newer 9mm belt

Simply screw the two screws at the front to move the carriage and tighten the belt!
Move each a few turns at a time only to ensure the idler holder stays fairly level.
When you’re finished tightening, ensure an even space on the left and right and you’re good to print!

This was a 2 hour print at 60% infill, 80mm/s and used <30g of filament.

suggestions welcome -

Note - Some have reported it doesn't work well with the 400 -
There's a shorter re-mix here
https://www.thingiverse.com/thing:4167171

V6 - 9mm Belt version added, thanks to A Gibson and D Griffin for providing the required measurements. I've widened the central idler holder from 12mm to 15mm which 'should' work and give the same tolerances as the 6mm....there's a small change to the larger 'surround' also to ensure the idler holder is stiff

V6 - FINAL uploaded
prints well.

You need
3 x M5 Nuts (mine were 3mm tall)
2 x M5 x 30 Cap Head screws (cap head not essential, you could get away with slightly shorter also)
1 x M5 x 25 screw - for the Idler wheel

Optional
2 x M5 Washers, the onesI had were 12.2mm diameter so I've put a 12.7mm 'hole' there for them to sit in

V5 Uploaded
Complete re-design, as 'small' as I think it can go now and still be stable,
Should fit perfectly!
You'll need two M5x30 screws for the tensioning part
You'll need an M5x25 screw for the idler
You'll need 3 x M5 nuts
I've allowed some space at the rear for M5 washers also to reduce stress on the printed part when adjusting

V3 Uploaded.
Slightly slimmer design and increase in hex nut hole sizes.
also slight depth increase for the standard 18mm diameter idler, should give 1/2mm 1mm clearance all round now.