A hex nut pre-assembled with a free spinning conical washer. A high tension is maintained in the screw and nut assembly as the nut is tightened against the bearing surface.
Having the washer pre-assembled with the nut shortens installation time. Conical washer assemblies can compensate for joint contraction and expansion and can span slightly oversized holes.
This is the most popular type of locknut because of its versatility, cost and ease of installation. Doesn't gall screw threads.