Rotating Ballnut - design ideas

[Jonathan]

I've been thinking about how to attach bearings and a timing pulley to a ballnut such that it can be rotated with a stepper motor, instead of rotating the screw.

I've already done this on my router with an M20 screw since the long X-axis means that screw whip and inertia would have limited the speed significantly:



This is, I think, the most promising design I've come up with for rotating a ballnut. The chosen ballnut is the standard RM2510 ballnut from linearmotionbearings eBay seller (among others). The diameter of the ballnut and screw would makes angular contact bearings quite costly. At first I drew it with the ballnut inside the bearings, however the following design allows for smaller 6007 bearings and also gives the option of putting 2 ballnuts in to eliminate backlash. One ballnut attached to either side of the shaft.

Here it is:
(I'll draw it in 3D if necessary)



Black is ballnut/screw/bearings
Red is shaft, steel or aluminium - probably aluminium since lower moment of inertia.
Orange is thingy that fits on the end of the shaft and attaches to the ballnut via the 6 holes in the ballnut.
Green is some aluminum plate to hold every thing. One plate will be extended to accommodate the stepper motor.
Brown is timing pulley. I could machine the pulley on to the shaft, however that means making a new shaft if I get the number of teeth wrong.
Blue is a collar bolted to the aluminium plate to preload the bearings.

Any comments/ideas/criticism welcome!