Prevailing Torque Nut or Stiff Nut

Prevailing torque is the amount of torque needed to screw a locking nut down a thread,often referred to as Stiff nuts,Self Locking Nuts,Lock Nuts,Flexloc nuts or Anti-loosening Nuts, are one of the commonest methods of providing resistance to self-loosening. Patents for this type of nut began to appear in the 1860s and many of the principal types can be traced back over 100 years. One advantage of this type of nut is that the locking feature can be checked at the time of assembly by measuring the prevailing torque. The present standard test code (ISO 2230) specifies performance requirements to ensure that nuts meet a certain minimum standard. Key requirements are that the first prevailing torque in the tightening direction must not exceed a maximum value and that the first and fifth prevailing torque in the untightening direction must achieve certain minimum values.

Prevailing torque nut or Stiff Nut in general they can be classified into one of four categories,

I,  all-metal Nut whit normal thread

On most metal locknuts, the prevailing torque is created by distorting the top threads ( double slotted) of the nut in some way.

DIN980V and DIN980M define distorting type and additional metal element insert type to achieve prevailing torque.