Quote:
Originally Posted by jacobsed
Are you sure about that copper anit seize? I thought you were supposed to put them in dry to get the correct torque spec.
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The Service Manual specifically states to use a light coating of copper based anti-seize on the plug threads, or I wouldn't have included it in my statement. But I'm sure there will be replies by the usual 'internet' mechanics/engineers that feel they know better than the BMW engineers that designed the car
I've worked as a Mechanical Design Engineer in several industries (including automotive) since 1982. For what it's worth, based on my experience:
There are no absolute rules where thread lubricant is concerned. When a lubricant is required it is usually specified in the shop manual (as in this case). As a general rule head bolts engaging cast iron should be lubricated with oil, and
ANY thread engaging aluminum should be lubricated with an anti-seize compound - typically because there is a corrosion or galling issue-as is the case with spark plugs with their fine thread pitch and dissimilar metals in intimate contact .
- 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.
-Conversly-
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 buggered 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.
Don't gob the stuff on, a very small ribbon down the length of the threads is sufficient...