Shape and Structure: The main body is cylindrical. Usually, 2 to 4 symmetrical notches (or holes) are machined on the outer circle, which need to be tightened / disassembled by a hook-shaped wrench (or a special plug tool). Some inner walls of the circular nuts are machined with "locking grooves", which are used in conjunction with locking washers to further prevent loosening; the threads are mostly fine-toothed (such as M20×1.5), and the fine-tooth structure has better self-locking performance, suitable for precise positioning of shafts.

Application Fields: Only used for shaft components, such as motor shafts (fixed bearings, gears), machine tool spindles (locking couplings), reducer shafts (fixed worm gears), etc. They need to be used in conjunction with shaft shoulders and locking washers to achieve "bidirectional axial fixation".