Z axis sync solution for Anet A8 printers
Problem: the two z axis screws can easily become miss-aligned. Any pressure on one side will create an offset, requiring the other side to be adjusted every time you print.
Solution: Sync the two screws with a pulley and belt system.
You will need two bearings (688ZZ, 16x8x5mm), couple of M3 nuts and screws, and ~800mm of 6mm GT2 drive belt, same as used on the x and y axis. Get the fibre reinforced belt, not the steel reinforced, and cyanoacrylate glue (super glue).
Or you can buy a closed loop belt, 400t (800mm).
Update April 2019: I've modified the bearing housing, now has a thread, and a nut to keep it in place. I've also re-written the belt splicing instructions, hopefully easier to follow now.
Glue the pulley halves together, push the nuts into the slots.
Push the bearings into the holders. Update: I strongly suggest using the V2 bearing housings, they have a nut to hold them in place.
You'll need to remove the top frame plates, one at a time, slide the pulley over the screw, then the bearing cap (might be a tight fit), and replace the top frame plates.
Use the tool provided to cut one end of the belt at an angle, then stretch around the two pulleys and mark where the ends overlap.
Take the belt off and align the mark with the end of the tool and make the second cut. Then put both ends in the middle of the tool so they meet, and put a drop of superglue on the join (helps if you smear the joining tool with Vaseline or butter, to stop it sticking to the tool). Wipe any excess off quickly with kitchen paper.
Take the top plates off again, one at a time and place the belt around the screws, replace the top plates .Fit the belt over the two pulleys, hopefully will be the right length. Leave the set screw loose on one side.
Finally, set the height of the x axis to the same height at both ends, tighten the set screw on the other pulley.
Now when one z screw turns, the other turns too. Set your bed height, that's it. Happy printing!
Update: It's been nearly 18 months since I installed this, and yesterday the belt join failed. I probably used inferior glue, and it was my first attempt, so tbh, I'm surprised it lasted as long as it did. Still. many hundreds of hours of happy printing in between. So I fitted new belt, took a bit more care over the cutting, used good quality glue, and the end result looks much better. Hopefully, this will last indefinitely.
Update: You can buy a 400t closed loop belt that will fit perfectly, so you don't have to glue the ends together!