Prevailing torque is the extra turning resistance built into a locking nut that must be overcome before any clamp load is created. It comes from the nut’s locking feature (deformed threads/slots or an insert) and is separate from your joint’s tightening (installation) torque.
How prevailing torque works
In all-metal locknuts, prevailing torque is usually generated by a controlled deformation at the top of the nut. Traditional designs rely on axial distortion of the top threads (commonly seen on double-slotted nuts), creating friction that resists rotation as the fastener advances.
Auto-Lok double-slotted nuts use a distinctive vertical deformation method that concentrates the locking action while minimizing damage to the bolt’s threads. Result: reliable locking with lower prevailing torque and better reusability compared to many standard axial designs.
- Enhanced security: Maintains resistance to loosening under vibration and thermal cycling.
- Thread-friendly: Vertical deformation reduces galling on the male thread.
- Reusable: Designed to meet ISO 2320 prevailing-torque windows across multiple cycles.
Prevailing torque vs tightening torque (they’re not the same)
Prevailing torque is the baseline resistance of the locknut’s feature (e.g., slots), measured without intent to generate clamp load. Tightening torque is what you apply to achieve the target clamp load in your joint. A common estimator is T ≈ K × F × d (where K is the friction factor, F the desired clamp load, and d the nominal bolt diameter).
For safety-critical applications, select installation torque from your engineering spec and confirm that the nut’s prevailing torque stays within the ISO 2320 limits over reuse cycles (1st turn, 1st removal, 5th removal).
Quick glossary
- 1st turn (MAX): Maximum acceptable prevailing torque on first installation.
- 1st deturn (MIN): Minimum acceptable prevailing torque on first removal.
- 5th deturn (MIN): Minimum acceptable prevailing torque on fifth removal (reusability check).