A lock nut used for fastening to a bolt when mechanically joining materials together.
The nylon material located at the top of the nut prevent loosening from vibration or back off.
The insert deforms elastically over the threads of the screw, but threads are not cut into the nylon.
The nylon insert locks the nut in two ways. First, it forces the bottom face of the screw threads against the top face of the nut threads, increasing the friction between the two. Second, the nylon applies a compressive force against the screw itself.
