Lubricating lug nuts and bolts
Luke -
Firstly, the forum is the best source of technical expertise and just plain common horse sense for MB issues ever.
While I do not profess to be omniscient about automotive threaded fasteners, I feel compelled to take issue with your response on using anti-seize on lug bolts as I believe your response, while well intended, does not adequately address all issues associated with threaded fastener preload via torquing.
I do have some expertise in designing and specifying threaded fasteners, particularly in deriving a designated pre-load by using a specific torque range on aerospace and nuclear applications.
1 - Common engineering practice is to design threaded fasteners such that, in an over-torque situation, the bolt head would break off due to a combination of shear and tensile stress before stripping out threads (mostly a shear failure). This hopefully results in some bolt body being left to allow removal of said bolt should overtorquing break the head off.
1a - However, that being said, my observations of my 300TE lug bolt thread engagement does indicate that the length of thread engagement (about 8-10 threads) would result in thread stripping before bolt head failure. While I have not run the stress calculations, another good engineering practice is to design the threaded fastener such that the bolt (external) threads would strip out leaving the hub (internal) threads. The reason being that the bolt is the cheaper item to replace.
1b - To add another curve to this, the hub appears to be a high carbon low alloy steel (that's why they rust) and it is quite likely that is has a yield strength significantly lower than the high alloy low carbon Corrosion Resistant Steel (CRES, aka Stainless Steel) lug bolt. I would need to know the material composition of both items to determine the yield stress and ultimate stress to allow accurate stress/strain calculations. One would hope the superior engineering qualities of MB would accommodate the aforementioned common engineering practices.
2 - Not using a lubricant on threaded fasteners results in other issues.
2a - The friction in a threaded fastener can absorb a wide range of torque load, depending on thread pitch, class, quality, geometry, condition, lubrication, and foreign material (dirt, rust, etc.). Controlled condition tests using stud tensioners shows this loss can range from 5 to 95% of torque. By far, the best way to obtain consistent preload from a threaded fastener is to keep the threads clean, in good condition, and to use an appropriate lubricant. This will typically limit the loss of torque load (due to thread friction) to a general range of 15 to 25 %. I am hard pressed to believe that MB would ignore the importance of clean, lubricated threads in their threaded fastener and torque specifications.
2b - Again, I would have to run calculations, but from general observation, I would wager that the components of the lug bolt fastener system (the lug bolt, internal and external threads, as well as the compressed area of the wheel material) could easily withstand 150 to 200 lb-ft of torque before reaching the elastic limit (i.e., yield stress) of any component of the fastener system.
2c - Another issue is galling of material. Wheel lug nuts are probably cycled more than any other threaded fastener on an automobile. Tribology (the study of interaction of materials smearing across one another) shows that the more a threaded fastener is used, the more galling takes place. Proper lubrication reduces, and can eliminate, this galling.
2b - Lubricant material is another issue. Given that most never seize has a copper base, galvanic corrosion can be an issue, particularly where the threaded fastener materials have a high level of galvanic separation. For example, common automotive never seize should not be applied to spark plugs threaded into an aluminum head (unless appropriate threaded inserts, such as helicoils, are used). The copper in the never seize provides a high galvanic potential with the aluminum, and the aluminum head threads can actually corrode into the spark plug threads, particularly if the spark plug thread material is CRES.
However, while most lug bolts are CRES, this should not present a problem with the forged high carbon steel wheel hub threads. Conversely, there could be some issues with aluminum wheels, particularly if the finish (paint, anodize) is damaged where the shoulder of the lug bolt mates with the wheel hole chamfer. Never-the-less, a marine grade never seize is the better choice as their lubricant base is usually a tin/lead rather than copper.
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