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Old 11-03-2003, 10:46 AM
JimSmith JimSmith is offline
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,596
This is a very interesting topic which will remain confusing for quite some time. Most of the confusion arises because the torque specifications are an awkward and often innacurate translation/estimation of bolt stretch, which is what is desired. When you tighten a fastener to a torque value the engineer providing the torque value was trying to achieve a specified stretch of the bolt to deliver the calculated clamping force needed to overcome operating loads without subjecting the fastener to fatigue. If the clamping torque is insufficient to prevent the fastener from being cycled while the machine is in operation (by cycled I mean being loaded then unloaded to the point where the fastener relaxes over and over), the material will experience conditions similar to a wire clothes hanger when you bend it back and forth. It breaks.

The target for most fastener designs is to have the bolt or stud preloaded to 65% to 80% of its yield strength. If you need more clamping force you use a larger fastener. The clamping force is transferred to the joint member(s) by a washer under the moving member of the fastener during assembly. In some cases there can be a through bolt or stud with a potentially moving member on each side, in which case there is a washer on both sides of the joint. The washer is really intended to provide a replacement bearing surface, usually of a material that is not as hard as the fastener, so that at the next assembly the joint bearing surface can be replaced (or, for those of us without a box of new washers for every job, resurfaced using a sanding block to remove grooves and galled washer material) without much trouble. In most cases the washer is of a larger diameter than the fastener, which can also serve to spread the load somewhat under the fastener, however, unless the washer is unusually thick, this is not likely a significant aspect of the joint design.

The amount of axial load that is generated by torque on the fastener is controlled by the coefficient of friction between the various moving surfaces of the fasteners, which is controlled by too many aspects of the fastener system to be reasonably accurately known, even in new conditions. Thus you see many newer assembly bolt torque specifications based on bolt or nut turning degrees after a joint seating step based on torque (where the coefficient of friction has a limited effect on the fasteners as there is no real axial load jamming the surfaces together). This is because the telling feature, the actual coefficient of friction between the moving surfaces ranges by nearly an order of magnitude in new fasteners (actual surface finish in the load bearing areas, how the lubricant was applied, how clean the parts and lubricant are, any coatings and their actual dimensions, the perpendicularity of the joint flanges and fastener, etc.). Setting fastener preload by turns is more accurate because the thread dimensions (threads per unit of length) can accurately determine axial stretch.

Using a lubricant, and how it is applied, is a critical aspect establishing a coefficient of friction, and therefore of how torque is turned into axial stretch. If the lubricant is not used when it is called for, the coefficient of friction will increase to the point where the fastener is likely not sufficiently preloaded - you will reach the torque value before the fastener is stretched because you are consuming the torque overcoming the friction. If the fastener is lubricated when it was not supposed to be lubricated, the specified torque will likely cause yielding, or even snap the stud or bolt as more turns will be achieved due to the lower coefficient of friction. Reusing washers that are munged up is another way to jeopardize the integrity of the joint design. At the very least, turn the washer over to present the better condition surface to the bolt head or nut. I typically sand off the washer face marks and any galled material with sand paper wrapped around a block with flat surfaces.

In general, torque specifications are provided where assembly is sensitive to cyclic loads, and assembly conditions can be practically controlled. This typically means conditions in a shop or garage.

Wheel lug nuts/bolts, for instance, have to work with any driver's physical abilities, given they follow instructions to change the tire in the manual and achieve a safe installation when the tire goes flat in dirty, dusty or muddy, rainy conditions. Thus, these fasteners do not typically require lubrication, as no car manufacturer can reasonably expect Joe or Jane Average to have a wire brush to clean the threads, and thread lubricant in the car, or have the presence of mind to know how to apply it, especially if the manufacturer does not supply this in the car standard issue tool kit. Such instructions are also not in any car owner's manual I have seen, while there is a page or two of legal disclaimers these days so if these special steps were needed I fully expect they would be spelled out in detail in the manual.

Also, while a torque value may be given, the car manufacturer does not supply a torque wrench with the car. You get a tool that is designed to be able to be used by an ape or a smaller than average person and achieve a safe wheel reinstallation. The joint design is generally not designed to be lubricated, but given the tools and materials, it apparently works well with dirty fasteners and cleaned and lubricated ones. All in all the wheel lug nut/bolt joint is a pretty robust design, and I would be reluctant to add anything to the procedure that is not explicitly in the owner's manual (I do not lubricate these fasteners, but I do clean them off if I get any dirt on them by dropping them or laying them on the ground).

As for other fasteners, I carefully clean and lubricate them, then tighten using feel, or, if a torque specification is given, I use a torque wrench until I develop a feel on a routine maintenance joint. When there is a gasket involved, depending on the type and configuration, sometimes you need to add torque increments and repeat them in patterns to achieve a proper assembly.

I hope this helps explain why there is confusion, and how hard it is to clear the issue up. I share your frustration though - Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Last edited by JimSmith; 11-03-2003 at 10:53 AM.
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