ACU-3S gets loose - how to fix?

[turboscrew]

I tore the focuser into pieces originally, because the coarse focusing knob next to the fine focusing knob was rotating free. I decided to do an overhaul now that it was in pieces on my table. I greased it with lithium grease, and put it back together. I tightened the tightener nut such that the fine focus would turn the big knobs even if there was some resistance. After just some tries, the fine focuser became loose and had no effect. So I tore it apart again, tightened the nut, and put it back again. It felt good - for a while. Then it became loose again.

Any ideas why does it get loose? And should there be some kind of lock nut?

 

[turboscrew]

Hmm, there doesn't seem to be threads left for a lock nut. Maybe the mistake was pushing the axle bearing too deep such that the outer rim of the bearing made contact with the nut. Let's see...