On the last page of this thread, and numerous times before, the question of lubed vs dry threads has come up, and I’ve seen lots of opinions rendered. As a member of ASM, The Materials Information Society, I thought I’d get you a proper engineering answer to the lubricated threads issue.
I read thru several sources, but thought this one summarized the issue best in terms that most people could understand. If you really want to see the raw text, formulas and tables, I’ll try and copy sections (the file is copy protected), or you can just accept my summary & commentary. From the definitive Handbook of Bolts and Bolted Joints, edited by Bickford/Nassar, with the chapter on the use of Thread Lubricants (and why the wrong choice can screw the pooch…) written by Novak/Patel of the Fel-Pro Chemical Company.
The assembly torque of a fastener is consumed by two mechanisms: friction-free bolt stretching and overcoming friction. A portion of the applied torque on the bolt head stretches the bolt. The balance of the bolt torque overcomes thread friction and under-head (nut to wheel in this case…) friction.
General assembly practices call for the use of thread lubricants. A thread lubricant fills the spaces between contact points, thereby reducing the metal-to-metal contact and prevents galling and seizing. In effect, it acts like a microscopic ball bearing to keep surfaces separated for easy assembly/disassembly. Thread lubricants have widely varying “nut factor”, K, (coefficient of friction), and choosing the correct one for the application is extremely important. Dependencies include thread pitch, surface area & metal coatings, application to thread and/or under head region, evaporation opportunity, amount applied, etc.
The efficiency formula indicates how much of the applied torque is being used to stretch the bolt, and how much is used to overcome friction. The use of lubricants can yield an efficiency of up to 100%, and all of the torque is used to stretch the bolt. Conversely, it also serves to reduce the torque required to remove the bolt, as friction is not in play. Gasket leakage (in this case wheel damage), fastener overload, fastener loosening, and fastener fatigue failures can occur if the proper preload is not developed at initial assembly.
So that’s what the book says. My take on it is that if the manufacturer says “clean and dry threads”, then they empirically determined a torque spec that gives up a substantial amount of total torque to tread and head friction, but also uses that friction and metallic galling to keep the lug nuts in place. If they wanted you to use a lubricant, they would have:
a) Specified exactly what lubricant to use, which surfaces, and how much to apply.
b) Reduced the torque spec (considerably?) to account for near zero friction, and to protect the fastener system and the aluminum wheel.
c) Provided some kind of locking mechanism to ensure that the assembly does not self-loosen.
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