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  #16  
Old 05-09-2002, 10:07 AM
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GSXR, have you seen the instructions on the head bolts where MB says take up to torque specs THEN TURN AN ADDITIONAL 90 DEGREES ? I have never seen this anywhere else... but they have a special 'degree wheel' to do it with .... this does not really effect our torque discussion but thought I would mention to those that have not seen the factory manual...Greg

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  #17  
Old 05-09-2002, 10:50 AM
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Yep- on my OM603, your torque to 20, then 35 (I think), then 90 degrees, wait 10 minutes, then another 90 degrees. That's for "torque-to-yield" bolts that are designed to stretch slightly. MB also specifially states to coat both the threads and the underside of the bolt head with engine oil before inserting the bolts (but not too much...)


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  #18  
Old 05-09-2002, 11:03 AM
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Well, this has been an interesting thread. I have to agree with leathermang (Greg) on the issue of how to make the torque wrench readings most consistent and meaningful.

The coefficient of friction between a tapped hole and a threaded fastener can vary by nearly an order of magnitude even if lubricants are applied due to the condition of the load bearing surfaces of the male and female part threads, and the surfaces under the head of the bolt or nut. The point of applying a fixed torque value is to establish an elastic preload by stretching the male fastener a prescribed amount. The stretched fastener then acts like a compressed spring holding the joint together. If the preload is high enough that the joint does not move during operation of the machine, the fasteners never see the varying loads due to pressure, thermal or other factors and will have an essentially infinite fatique life (the only load cycle they see or feel is the initial assembly). For most cyclically loaded joints this is the design goal.

Torque happens to be the most convenient way to correlate the assembly of a mechanical joint to this bolt stretch as you can use the same tool to assemble the parts as you use to "measure" the fastener stretch. But it is also the most innacurate method as it relies on an estimation of relationship of relative condition of the threaded parts instead of making a measurement. In cases where the coefficient of friction is known by experiment to vary enough to make it unlikely the desired preload will be achieved with a torque wrench (eg: the low end of the coefficient of friction range will result in a yielded or broken fastener if you used the high end of the range calculated torque value to achieve the desired stretch, or, there would be inadequate preload - stretch - on the fastener if the low end of the coefficient of friction range torque value was applied to the high end of the range installation) the fastener design will include means to measure actual stretch.

Anyway, the majority of fasteners should be cleaned and lubricated with the prescribed assembly lubricant, or engine oil if nothing is prescribed, at assembly to make sure the coefficient of friction is closer to the nominal design value for the closure assembly design. As things age and corrosion and other stuff gets into the threads, or on the bearing surfaces under the head of the bolt or nut, there is no point in using a torque wrench if you are not going to clean and lubricate the threads/bearing surfaces. There is no correlation between bunged up fasteners, stretch, and torque so if you are not going to use good assembly practices and clean things up, you might as well not use anything more precise than the one or two grunt method for estimating whether or not the fastener is properly seated and stretched.

About the only joints I am clear on that require no lubricants are the wheel lugs as the manual in most cars tells you not to add any lubricants to the threads on these fasterners. Any other fasteners that do not require lubricants also usually say so, especially if an assembly torque value is specified.

Hope this helps, Jim
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Own:
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

Owned:
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)
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  #19  
Old 05-09-2002, 02:34 PM
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WOW, Jim, Great answer. And I am not just saying that because it supports my position, although I do also appreciate your visible support (your check is in the mail ).... , Greg
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  #20  
Old 05-09-2002, 02:45 PM
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"Torque to Yield " Bolts ....?

I have seen this before .....which I consider a funny image because for the bolt to 'yield' as compared to strech pretty much meant " Torque till they Break " .... but assuming they are not an oxymoronic joke.... what exactly are they ? I have read that Rolls Royce used "waisted" bolts... where the shaft between the threads and the bolt cap was slightly less diameter than ( the diameter measured between the bottoms of the threads ) ... and that they did not reuse them . And that they used shorter length of threaded shaft than others... This makes sense to me because it gives a longer area in which that strech can occur... and the longer the bolt the less (later not wanted) fatigue stretch occurs... in other words ... a longer bolt... torqued the same as a shorter one will keep more pressure on the item .
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  #21  
Old 05-09-2002, 03:26 PM
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Greg,

Torque to yield is a commercial practice that does leave you with a fastener that is not truly reuseable, or has very limited number of re-use cycles that are determined based on some precise length and/or diameter measurements. Most bolt materials have pretty good elasticity, as measured in the reduction in cross sectional area when you yield them until they break under tension. This characteristic means when the bolt yields slightly you get a large measure of the yield strength holding the joint closed, but the material properties of the bolt can become a little uncertain, as well as its geometry. In my experience we like to stay at 80% of yield. This leads to designs with longer fasteners as you noted to achieve a practical assembly. Longer is better as it reduces the effects of non-parallelism between bearing surfaces and perpendicularity of the tapped hole to the bearing surface. But it makes for more expensive fasteners.

Jim
__________________
Own:
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

Owned:
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)
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  #22  
Old 05-09-2002, 03:50 PM
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Thanks Jim, Glad my impression was in the ball park,,, your second check is in the mail.... Would it be impolite to ask where you got all this knowledge ? I think it would be pretty hard to get where you are without formal engineering training... this does not really sound like " hobby reading material" (but could be...) Greg
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  #23  
Old 05-09-2002, 04:42 PM
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Greg,

I have been an engineering director running the propulsion plant machinery development for the US Navy's nuclear submarines of the Seawolf and Virginia Class, followed by advanced electric propulsion research and development for a major defense contractor. The rules for bolted closures that hold seawater out of the "people tank" or the primary coolant inside the reactor pressure vessel and other components are pretty stringently enforced. You have to understand them to actually incorporate them as part of the design process. It is never good to get too far along and start trying to change things for fastener considerations - one such experience will last a lifetime.

There are some other considerations, such as after a number of thermal and pressure cycles, even a "yielded" fastener will "shakedown" to about 70%-80% of the original yield strength left as the preload. Most of this stuff gets boiled into some design guidelines that result in certain critical fasteners from one company all looking about the same, and being assembled about the same. Not a bad practice, but eventually that is all that is passed to the next generation, and they do not understand the basic principles of engineering that defined the guidelines. Which leads to lore that does not agree with things like F=ma, and that is bad if it makes it into a product.

Well, hope we helped some guys. Jim
__________________
Own:
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

Owned:
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)
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  #24  
Old 05-09-2002, 05:38 PM
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JimSmith,Your last answer does not surprise me at all. The depth of your understanding of mechanical and electrical things has always shown in your answers...I do think it is good to share credentials like that... saves a lot of web space when people know they are going to have egg on their face if they contradict you without doing good research first.... I am sure many new to this area have already been helped. Thanks, Greg

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