The locknut doesn't come off, ( I even add Zap-a-Dap- Goo to the end of the nut). The shaft breaks off at the nut ! Vibratrion ,I guess. And no, I don't over tighten the nut. This has only happended once. Easy fix, replaced the broken shaft with some " All Thread" rod with a lock nut on each end.

Ugo