Locking Nut Torque: 4 Simple Factors That Matter
Locking nuts are designed to stay tight under vibration. But their maximum tightening torque changes with repeated use. Here are the four main factors that affect it — explained simply.
1. Material Hardening (or Softening)
When you tighten and loosen a locking nut many times, the material can get harder (cyclic strain hardening) or softer (cyclic strain softening).
Example: Alloy steel hardens with use.
Effect: Harder material increases elastic recovery force, which increases the tightening torque.
Simple rule: Harder material = higher torque.
2. Friction Wear
Friction is what keeps a locking nut from loosening. But every time you use the nut, the contact surfaces rub together.
What happens: Rough spots and sharp edges get smoothed out.
Effect: The friction coefficient decreases, so the maximum tightening torque drops over time.
Simple rule: More use = smoother surfaces = less friction = lower torque.
3. Manufacturing Tolerances (Break-In Effect)
No nut is perfect. Small imperfections — like sharp thread edges or tiny size mismatches — exist due to manufacturing limits.
During first use: Torque may fluctuate or feel inconsistent.
After a few cycles: Surfaces wear in, and torque becomes stable.
Simple rule: Run the nut a few times before measuring its "real" reuse torque.
4. Closing Value (How Much It's Squeezed)
The closing value is how much the locking nut is pre-compressed (for example, an oval or offset thread). This is a design feature that creates prevailing torque.
Larger closing value causes two opposite effects:
More elastic force → increases torque (good)
Smaller friction area → decreases torque (bad)
Also reduces fatigue life
Result: The net effect is unpredictable without testing
Simple rule: Closing value is tricky. Always test reused nuts — don't guess.
Practical Tips
Single use only (safety-critical)? → Always use a new locking nut.
Reusing? → Run it in a few times first, then measure torque.
Don't rely on theory for closing value → ask for test data.
Final Takeaway
Four factors affect locking nut torque: material hardening, friction wear, break-in, and closing value. Closing value is the hardest to predict — always test. For reliable joints, know how each factor behaves with repeated use.