That looks familiar! How come you've gone for 7x08 bearings not 7207? I can guarantee that you wont have problems with 35mm bore bearings.

A few points about the design:

  • 40mm (or 35mm) main portion of shaft needs to be machined for a 'just sliding' fit on the shaft. Must not be a press fit otherwise the housing must incorporate the means to pre-load the bearings, which is more difficult.
  • Be careful with the 62mm dimension - the tolerance on flange of the ball-nuts isn't especially consistent so you probably want it 0.1mm oversize, plus it's critical that this feature is concentric to the main shaft.
  • The 2mm shoulder can be reduced to 0.2mm to save an, admittedly negligible, amount of material, but it also helps stop swarf getting on to the adjacent bearing.
  • Use aluminium for the shaft since the moment of inertia will be about 2.9 times less, and it's plenty strong enough.
  • Similarly, reducing the diameter to 35mm means you can use a smaller pulley, again reducing the inertia. I make the hole through the center just over 27mm, mainly because I have a 27mm drill, so if reducing the external diameter I'd make the hole 27mm to add a little strength. This wont affect the moment of inertia much at all since it's close to the center.
  • Since the shaft rotates in both directions, you need room for two M40x1.5 (or M35x1.5) locknuts so that they can be locked against each other to ensure they don't unscrew. Standard M40 locknuts are 9mm wide. Alternatively you could a single locknut with grubscrews, or some other means of securing it, in which case make the thread as short as you can, as again there's no point having excessive thread when you could be reducing the moment of inertia!
  • If you can incorporate a seal to wipe the swarf off the ballscrew as it runs along that could significantly improve the life of the ballnuts.