A system for Z-axis movement with hoist action.
This is Version 2 of my "Bed_Hoist_for_a_3DPrinter (D-Bot, Core_XY)", https://www.thingiverse.com/thing:2846764.
Difference: one 10mm belt, simpler parts.
For a time I left out one hoist and belt in the first version and that worked ok, even somewhat better than with 2 belts.
So if you already have made version 1, perhaps you can leave this version for what it is and try it with 1 bed hoist: just move one hoist to the middle, uncouple the other one and look how that goes. ;-)
But if you are new to this concept or want to try other worm gears, forget version 1 and go for this thing.
In my search for a solution of a "2 mm wave"-error in the print, i.e. a recurring vertical irregularity with a periodicity of 2 mm, with different worm gears and pulleys I found that I could simplify things using GT2 40T pulleys, because these hold the belt on a nice distance from the rails.
One can use a 6 mm belt with all these same parts too, but I went for a 10 mm belt, just to be on the safe side, although that could be just too much caution, like when I thought that I needed 2 belts.
Be warned: I still found no real solution for the wave which happens with each turn of the worm/stepper.
At this moment I use a M1 20T worm wheel, as in version 1, with GT2 40T pulleys, which produces a 4 mm wave.
I suppose that these waves are dependent on the quality of the worm gear and the way they are made ("precision casting", hobbing, forging, whatever).
So despite the fact that I cannot offer a real solution yet(?) I publish this second version, because it does things simpler.
Ideas are very welcome (also a reason for publishing). ;-)
I made this thing so I can use it with my D-Bot_Linear_Z-guide, https://www.thingiverse.com/thing:2779260, but it is independent of it.
I made this to get rid of the lead screws, therefore good Z-guides are necessary.
No good using it in a system with wheels for the Z-axis, because without the support of the lead screws the bed angles so much downwards that this cannot be compensated for with the usual bed supports. Just too much play in those wheel systems, whatever I tried with 3 of 4 wheels. So be warned. ;-)
No such problem with the linear Z-guides.
What remains too in this version is that this thing is much simpler and quieter than the lead screw configurations I tried.
As accurate also, I think.
Compared to the loads they are made for, I feel this load won't stretch the belt so as to give wrong printing heights.
Somewhere (can't find it any more) I saw a pre-load advised by the manufacturer of some 2kg for a 6mm belt and this ca 330x330mm bed assy weighs around 4kg, only 2 times that pre-load. And all that with very low accelerations.
Though, of course, it doesn't harm to check when height is of the essence with your print.
For materials, assembling specifics, more images and more considerations, see the "Notes on a Bed Hoist.pdf" in the "Thing Files".
Edit (23-04-19 bis) Entered the result of the tryout of a M1-40T-wormgear (which gave no improvement in the wave) -> "Notes on a Bed Hoist-V1b.pdf".
I don' t know whether it is a problem of quality or an inherent problem with wormgears, so at the moment I don' t know what to do. ;-(
I suspect it is a bit of both and that the cost of a worm drive which has an acceptable level of transmission error would probably be prohibitive for our purpose anyway.
Edit (27-04-19): According to the_digital_dentist (see comments) it is possible to have perfect prints with a worm drive, so the quality is the determining factor.
Alas the cost is a bit steep.
Edit (01-05-19): Added a remark about cutting the belt in the Notes -> "Notes on a Bed Hoist-V1c.pdf".