Lock nut, also referred to as a nylon-insert lock nut, polymer-insert lock nut, or elastic stop nut, is a kind of locknut with a nylon collar insert that resists turning.
The plastic insert is placed at the end of the nut, with an inner diameter (ID) slightly smaller than the major diameter of the screw. The insert deforms elastically over the threads of the screw, without having threads in turn cut into it. The nylon insert locks the nut by friction against the screw under the applied radial compressive force. Nyloc nuts retain their locking ability up to 250 °F (121 °C)
Flange Nuts DIN 6923
Serrated flange nut or Flange lock nut has a flange shape above the hexagonal body surface which increases its bearing surface allowing them to be used with irregular oversized holes.
The ﬂange offers protection for the bolted component where the driving tool may otherwise scratch it’s surface. The ﬂange also lessens the need to use a washer.