OK, so I found this on thingiverse and made a little testprint myself.
Basically you take at least three ball-bearings and group them around a plain steel rod - but skewed.
That skew will generate a linear motion when the rod is turned.
Pros of this approach are:
+ Almost no backlash.
+ Use stuff that you have laying around / can obtain for cheap to make something you usually have to order.
+ Travel-per-turn is variable! It depends on the skew angle and the diameter of rod and bearings (not sure on the exact math here). It seems that 20° will travel about double the same bearing-rod-combo would at 10° skew.
+ You can use multiple "speeds" on one rod at the same time.
+ Travel-direction is determined by nut-orientation. You can have multiple nuts with opposing directions on one rod.
- Holding-force is obviously much lower than with a thread-based system.
I made two different ballnuts with what i had laying around.
Shaft is a 4mm Fischertechnik axle, the bearings are 3x7x3mm and 4x8x3mm RC-hobby bearings.
I printed the centerpieces on my UP! mini at 0.25mm layer-height. Threads for the screws are cut directly into the PLA.
The purple one has a skew angle of 10°, the transparent one 20°
Embedding videos here is crap by the way…
Step 1: Some Files…
OK, by popular demand - the STLs for the three ballnuts you see above…