The effects of lube on bolt torque / tension doityourself

[wsmeyer]

When I was learning to work on cars I was always told that unless it is specified otherwise, never lube threads on bolts you'll be torquing to a specified value.

I was always told this but had never read anything where it had been quantified so after bringing this up in another thread I decided to do some Googling and see what I could find.

Fundamentals of Torque-Tension and Coefficient of Friction Testing -
Understanding the Basics of Tightening Threaded Fasteners

I find stuff like this very interesting but for those that don't I'll summarize the findings:

Basically they tightened a nut onto a bolt to the same torque under different conditions and measured the resulting tension in the bolt.

• Only 10-20% of the torque applied to a nut is transferred to tension in the bolt. 80-90% is lost to friction.
• Lubricating just the threads resulted in an 18% increase in tension.
• Lubricating just the mating surface between the bolt and material also resulted in an 18% increase in bolt tension.
• Lubricating both the threads and the mating surface resulted in a whopping 88% increase in bolt tension.

[Avetiso]

So basically, if we lube it and then torque to a factory torque (for example, 87 ft-lbs for wheels), we will apply more tension to the bolt than intended because friction steals less of the torque than it would have if it wasn't lubed?

[RITmusic2k]

That's very interesting, William!

I'm a bit rusty on my mechanics and materials studies, but I wonder if there's any practical trade-off between the increased bolt tension and reduced friction due to the use of lube. Like, would some of that friction in an un-lubed system actually contribute to the bolt's holding capacity / does the increased tension in the lubed system actually correspond to a linear increase in holding capacity when it's friction is reduced?

[danewilson77]

So basically, if we lube it and then torque to a factory torque (for example, 87 ft-lbs for wheels), we will apply more tension to the bolt than intended because friction steals less of the torque than it would have if it wasn't lubed?

Yes.

[M0nk3y]

Good thread.

A detailed search via google on dry and wet torque will explain key differences

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[wsmeyer]

I'm a bit rusty on my mechanics and materials studies, but I wonder if there's any practical trade-off between the increased bolt tension and reduced friction due to the use of lube. Like, would some of that friction in an un-lubed system actually contribute to the bolt's holding capacity / does the increased tension in the lubed system actually correspond to a linear increase in holding capacity when it's friction is reduced?

Like you I ended up with new questions after reading the report.

It's friction that keeps the bolt from backing off so is that friction being reduced? The friction force would be the coefficient of friction of the mating surface times the bolt tension. The lube reduces the coefficient of friction but increases the bolt tension so does it end up being the same?


In my experience the only time you lube a bolt before assembly is in a situation where it's going to be lubed indefinitely, like inside an engine.

Otherwise,

*remember when you're turning a bolt 80-90% of the force is overcoming the friction, that friction being the bolt tension x coefficient of friction.

If you lubed a bolt and torqued it to a specified value. As the lube breaks down or evaporates, the coefficient of friction will increase back to the un-lubed value and the force required to loosen it could easilly be so high now that you end up snapping the bolt.

[danewilson77]

Then there's always clamping force and temperature/environmental affect on the fasteners as well.


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