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Tension Testing: The Effect of Lubrication on K Factor

Surprise results testing bolt tension with different lubricants.

Test Setup

Bolt and Nut Used for the Tests

Components used: bolt and nut 7/8″-9 thread, grade 5

Torque used for all tests: 400 ft-lbs

SAE J429Material Strength (psi)Force (lbs) Note
Proof Strength85,00039,000Max safe load without inducing permanent deformation
Yield Strength92,00042,500Max load before inducing permanent deformation
Ultimate Strength120,00055,400Max load before necking and fracture
Clamp Load64,00029,400Commonly recommended target load
(75% of proof load)

Predicted Results Before Testing

Using standard formula (Clamp Load) = (Torque) / (Diameter * K)

Clamp Force (lbs)K FactorNote
27,4290.20Generally accepted value for dry threads
36,5710.15 Generally accepted value for lubricated threads

Actual Results

Clamp Force
K Factor
Link to Video
Anti-seize paste
Anti-seize spray
WD-40 spray oil

0.13 is very low!

Surprise Results – This is astonishingly low and very much unexpected. The dirt used for this test was gathered from the hole on a SAG mill in a gold mining operation. Normally we’d expect dirt to increase the K value, but this material created the opposite effect. For an actual installation, if this dirt is left on the thread, the bolt could be stretched beyond it’s yield point as shown in the experiment.

The Takeaways

  • It’s important to achieve the correct clamp force, as we’ve shown in previous posts (clamp force blog post link).
  • It’s difficult to predict the bolt clamp force as we’ve shown with these results.
  • For optimal results, tests that match as closely as possible to actual conditions should be done to determine the torque required to achieve the desired clamp load.
  • It’s difficult to get it right every time. Use a Security Locknut to protect your assembly when you don’t.

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(847) 970-4050