Tension Testing: The Effect of Lubrication on K Factor
Surprise results testing bolt tension with different lubricants.
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 J429||Material Strength (psi)||Force (lbs)||Note|
|Proof Strength||85,000||39,000||Max safe load without inducing permanent deformation|
|Yield Strength||92,000||42,500||Max load before inducing permanent deformation|
|Ultimate Strength||120,000||55,400||Max load before necking and fracture|
|Clamp Load||64,000||29,400||Commonly recommended target load |
(75% of proof load)
Predicted Results Before Testing
Using standard formula (Clamp Load) = (Torque) / (Diameter * K)
|Clamp Force (lbs)||K Factor||Note|
|27,429||0.20||Generally accepted value for dry threads|
|36,571||0.15||Generally accepted value for lubricated threads|
|Link to Video|
|WD-40 spray oil||28,000||0.20||https://youtu.be/I3sNZw6L1L0|
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.
- 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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