Self-locking or Irreversible Worm Gearing.
Neglecting friction in the bearings, worm gearing is irreversible when the lead angle is equal to or less than the angle of friction. When worm gearing is self-locking or irreversible, this means that the worm-gear cannot drive the worm. Since the angle of friction changes rapidly with the rubbing speed, and the static angle of friction may be reduced by external vibration, it is usually impracticable to design irreversible worm gearing with any security. If irreversibility is desired, it is recommended that some form of brake be employed.