|U type / style nut/clip, clip-on nut, is a type of locknut with two sheet metal prongs that act as one thread. They are made from spring steel.
The fastener serves the functions of both a lock washer and a nut. As the fastener is tightened in the nut the prongs are drawn inward until they exert pressure on the root of the thread on the fastener. When the fastener is tightened, the base of the nut, which is arched, elastically deforms and applies a force to the fastener, which locks it from loosening under vibrations.