Robodrill T14iB Z axis screw problem

Hello !

Our robo has Z axis screw like on attached picture. It actually has 2 nuts bolted together, I dont know if its for improved rigidity (iB has faster rapids compared to iA)

Problem - it makes rumbling noise and lately has started nonsmooth movement on Z axis (jumps about 0.1mm, checked with dial indicator). If I let the nut move down by weight, I can feel rumbling noise when holding screw.
And, at the ends of screw it moves a bit harder. I think because the ends are not much used during normal machining and maybe they are worn out.

My question - could there be an easy "mcgyver" fix to remove 0.1mm jumping and sticking ? I know that it needs replacement but I would like to use the old one until that (avoiding Z-sensitive work).

Maybe the balls are not circulating ? I got no experience with that. On the screw I dont see much wear or "brinelling" but of course it could be there

if the nut threading along the screw feels rough, then I doubt it would be thrust bearings making the rumbling, but those could be shot as well if the rumbling went on for long enough, think about the increased dynamic load on them from all the vibration...

could try reballing the nut, upon disassembly pay attention to size of each ball that comes out, they might be staggered in size, small/large/small/large and so on

double nut setup, apart from increased load bearing, might be to eliminate any backlash, Fehlmann had something similar, but had a belleville washer in between the nuts to tension them and allow to compensate for uneven wear of the screw

barbter said:

Is the problem definitely the screw?
"Usually", rumbling axes are because of thrust bearings which make the "harsh" sound when rapid.

Click to expand...

I just checked thrust bearings - 2 oppositely placed ball bearings (J- shaped ruynway). Dont make much noise and seems there is no movement too.
Can the bearing fail "invisibly" or is it obvious and easy to test ?

Also, can the Z-servo go haywire when there is some rumble on axis ? Sounds improbable but who knows.

I meaan, can small vibration mess with feedback so the servo "amplifies" these ?