Self-locking nuts, also knows as lock-nuts, maintain the ability to be disassembled intentionally, while also reducing the likelihood of accidentally become disassembled. Lock-nuts typically have a polymeric component that conforms to the shape of the thread, or a washer that, when compressed, applies a load to the nut.
Nuts have a tendency to back off as a result of vibration, when not under preload. Inherently, clearance exists between a nut and bolt. When a nut is tightened, it created tension on the bolt, which compresses the nut against a surface. This compression forces the threads in the nut to contact the threads in the screw, which creates friction.
The friction keeps the nut in place. If this load is lost, vibration loosens the nut, eventually causing the assembly to fail. In addition to polymeric lock-nuts, some nuts and bolts come with a factory-applied patch of adhesive. This adhesive serves a similar purpose, conforming to the clearance between the nut and bolt, creating friction between the two.