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  #1  
Old 10-28-2003, 11:04 AM
Tod Labrie
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Lubricating & Torque Specifications-Attention Engineers

I do NOT want to start an opinion-based forum similar to motor oil brands or frequency of changing etc. I suppose mechanical engineers would be best suited at answering this question.

Does anyone know if there is a steadfast rule (or even where it's printed) on thread lubrication regarding torque specifications? I know that head bolts/engine reassembly hardware should ALWAYS be lubricated and then torqued (the factory repair manuals often state that). What about other applications. Lug nuts onto wheel studs? Random fasteners on the engine that actually HAVE a torque specification? A mechanical engineer once told me that whenever a torque specification is given (regardless of how big or small the bolt is or where it is on the car), the engineers ASSUME that you as a mechanic will lubricate the threads AND the shank (or underside of the bolthead). That torque specification takes that into consideration. Is that true?

The only thing that seems to be certain is with head bolts on an engine. I've heard you should NEVER lubricate (whether it be with anti-seize or oil) lug bolts/nuts, then I have read that you should. Mike Allen of Popular Mechanics (the automotive repair editor) wrote that it's OK to lubricate the lug nut threads on a wheel and then torque them 20% beyond the torque specification to account for the lubrication. I'm all confused.

What's the standard/generally accepted rule? Is it for ALL fasteners? What type of lubrication should be used? Why do some repair manuals actually specify lubricate and torque while two-pages later, that same manual states to torque with no mention of lubrication.

Is there a website (maybe by the SAE) that clarifies this?
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  #2  
Old 10-28-2003, 01:15 PM
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There are no absolute rules. When a lubricant is required it is usually specified in the shop manual. As a general rule head bolts engaging cast iron should be lubricated with oil, and ANY thread engaging alumninum should be lubricated with an anti-seize compound.

I also use antiseize compound on wheel bolts/studs.

If you ever tour an assembly plant you will probably see than virtually all fasterners are installed without any specific lubricant.

Duke
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  #3  
Old 10-28-2003, 02:04 PM
I told you so!
 
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Location: Motor City, MI
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I've been paid good money to study fasteners and I work with people who write the specifications for wheel bolts. Duke is correct on all points. There is no hard and fast rule to cover every fastener you come across. Some head bolts are installed dry, some are dipped in oil. The manual should be specific for this type of work.

The trucking industry is instructed to place TWO drops of oil on each wheel bolt before installing and torqueing lug nuts. Antisieze compound is okay, but we've seen cases where it's dried out, more added, and has turned into a big crunchy mess. We've also seen cases where antisieze compound is slathered on every conceivable surface, raising clamp loads to alarming levels at specified torque. You've got to be reasonable about what you're doing.

Since I'm a big user of antisieze compounds, I asked a fastener engineer how my wrenching will affect clamp loads. He remarked that torque values should be dropped roughly 10% to achieve the same clamp load.

Torque values depend on variables such as grade, size, thread pitch, coating, and whether a fastener is cased, so it is difficult to come up with a on-chart-fits-all guide.
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Old 10-28-2003, 02:17 PM
engatwork's Avatar
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90% of the fastening strength bolts/nuts/washers comes from the hex head against the nut. This is why you use a washer. Lubricating a fastener should only been done if spec'd out or if there is a corrosion or galling issue.

For some more info do a search on the attached site:

http://www.eng-tips.com/viewthread.cfm?SQID=57211&SPID=68&newpid=68&page=1
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  #5  
Old 10-28-2003, 04:17 PM
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Quote:
Originally posted by Kestas


Antisieze compound is okay, but we've seen cases where it's dried out, more added, and has turned into a big crunchy mess. We've also seen cases where antisieze compound is slathered on every conceivable surface, raising clamp loads to alarming levels at specified torque. You've got to be reasonable about what you're doing.
Good points. I use antiseize by rubbing a thin ribbon along the length of the thread, then run the bolt through the mating threads a couple of times to evenly distribute it to a very thin film.
I also put a thin film on the wheel bolt seats. That squeaking sound you hear when tightening or loosening wheel bolts is galling.

I've seen fasteners (and even spark plugs) where it looked like the guy ladled the stuff on with a spoon. You only need a very thin film. Periodically I completely clean the wheel bolts and mating threads and reapply antiseize, but I don't use it on spark plugs because the threads in the head are tough to clean without all the debris dropping into the cylinder. Taper seat plugs should only be torqued to about 12 lb-ft in aluminum heads, which will not usually cause them to seize.

Most fasterners have a torque range, and when I use anti-seize, I only torque to the lowest value of the specfied range.

Duke
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  #6  
Old 11-03-2003, 10:46 AM
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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
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1988 300E 5-speed 252,000 miles,
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Last edited by JimSmith; 11-03-2003 at 10:53 AM.
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Old 11-03-2003, 10:46 AM
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