What Is Prevailing Torque ? – Prevailing torque is the torque required to rotate a fastener (such as a self-locking nut) along its threads before any clamping force is applied. It overcomes friction and any built-in locking features.
What is Prevailing Torque?
Prevailing torque, also called running torque, measures the resistance to turning in a threaded fastener assembly independent of clamp load.
It appears in self-locking nuts and other locking fasteners. The locking mechanism creates constant friction that prevents loosening from vibration while allowing installation and removal.
How Prevailing Torque Works
In a standard nut, torque mainly builds clamp load. With prevailing torque features:
- A deformed thread, nylon insert, or other element creates interference.
- This interference produces frictional resistance as the nut runs down the bolt.
- The torque stays relatively constant during rotation (before seating).
- It provides locking even when clamp load decreases.
Engineers measure it during the first few revolutions before the fastener seats.
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Types of Prevailing Torque Fasteners
- Nylon-insert locknuts: A nylon ring deforms around bolt threads.
- All-metal prevailing torque nuts: Threads are distorted (e.g., oval or top-crimped) to create interference.
- Other locking features: Patch, pellet, or adhesive types that add friction.
Benefits and Uses
Prevailing torque fasteners resist self-loosening in high-vibration environments like automotive, aerospace, machinery, and electronics.
Benefits include:
- Reliable locking without extra components.
- Verifiable performance by measuring torque during assembly.
- Reusability (limited cycles, especially for nylon types).
They are common in critical joints where safety matters.
Prevailing Torque vs. Breakaway Torque / Installation Torque
- Prevailing (running) torque: Friction while turning with no clamp load.
- Breakaway torque: Initial torque to start movement.
- Installation torque: Total torque applied to achieve proper clamp load (includes prevailing torque + torque for tension).
Technicians often compensate for prevailing torque in automated tightening systems to reach accurate final torque values.
FAQs : What Is Prevailing Torque ?
Why is prevailing torque important?
It ensures fasteners stay secure under vibration. It also helps verify locking effectiveness during quality checks.
How do you measure prevailing torque?
Use a torque wrench or analyzer while running the nut down the free-spinning portion of the bolt (before it seats and builds clamp load).
Can you reuse prevailing torque nuts?
Limited times. Nylon inserts lose effectiveness after several cycles. All-metal types last longer but still have limits.
Does prevailing torque affect final tightening?
Yes. Assembly tools often use strategies to compensate for it and achieve the correct clamp load.
What causes variations in prevailing torque?
Manufacturing tolerances, lubrication, thread condition, temperature, and reuse.