Hex nut with a nylon-filled collar at its back end. When a screw reaches the collar, the threads and nylon form a tight, frictional fit, restricting movement of the screw when it is subjected to vibration. The nylon insert comes in various colors. Designed to be used with like-material machine screws and bolts. It is able to be reused more times than a two-way reversible nut. Steel nylon insert stop nuts are less expensive than grade-C automation lock nuts. However, nylon will begin to deteriorate at temperatures from 150˚-350˚ F.