Metric nylon lock nuts has a nylon polymer insert to prevent back-off by vibration. Also known as a elastic stop nut what has a plastic insert that becomes threaded as the screw to bolt is threaded onto to the nut. This is called deforming the elastic insert. The nylon insert locks the nut to the screw by forcing the bottom face of the screw to the top face of the nut threads increasing the friction at both ends and applies force against the screw or bolt. Nylon lock nuts are not made to be back-off after being applied which creates cross threading of nylon insert. This will create lock-up.