Self-Locking Retaining Ring When people hear Self-Locking Nut (SPN / Press-Fit Nut, Self-Locking Retaining Ring), they often assume it has internal threads like a conventional nut—but that’s not the case. It is actually a threadless elastic retaining ring that locks onto the shaft by its own spring force. Compared with traditional nuts, it is thinner, space-saving, and quick to install, especially suitable for applications where the shaft has no threads but still requires an