Bearing Spacing on Y-Gantry-Carriage

[WandrinAndy]

Am thinking of using wider spacing of the bearings blocks only on the top/front rail of my Y-Carriage as this doesn't limit travel and should increase rigidity... but am wondering whether this is worthwhile doing and whether it might create other problems? In the pic, the wider design is on the left, with the previous design on the right.

[wilfy]

oh nice i like that idea... to a noobie like me that makes sense as i'd imagine the top rail is what takes most of the stress so the more stability you offer there must be a help.. as i said though i'ma noobie and really dont know if its good or not

[Lee Roberts]

Ideally i think you should be looking at 200mm spacing, JB will be along shortly to explain why :).

[Jonathan]

The force on each bearing block is inversely proportional to the spacing, so increasing the spacing of the top two blocks reduces the force on each. The deflection of each bearing relative to the rail is proportional to the applied force (common sense), hence increasing the spacing reduces the deflection and you end up with a more rigid assembly. The formulas to explain this can be found in this document by Hiwin. Also it should be noted that there's a limit to how far you should increase the spacing since there becomes a point where the gain is smaller than the deflection caused by the material they're mounted on bending in between the bearing blocks. You're not approaching that though, so it's fine. When I make my new gantry the Y-axis bearings will be about 450mm apart, but there will be 3 on each rail...

If anything I think the forces on the lower blocks would be greater since they are closer to the tool, so it would be wise to space them out further too, however clearly that doesn't fit well with the rest of the design.

I hope you are adding plates to the sides to stiffen the joint between the aluminium extrusion and X bearing mounts.

[WandrinAndy]

Thanks Jonathan, I neeeded that confirmation.

Yep plenty of plate on the sides, but hid it from this image.

[D.C.]

I'm sure Jon/irving will correct me if I'm wrong but I think with your first extra wide design you could always extend the top rail out past the vertical supports of the gantry to give yourself more travel.

Heavy industrial gantry designs do that quite a lot to maximise travel, see here:

heavy lift dockship Dock Express 10, London, photo 1 | Flickr - Photo Sharing!