Question on Torque Importance
 

I have done quite a few things now on my car and don't own a torque wrench. I pay attention to how hard the nut/bolt is to remove and use same approximate effort to redo (for example the nuts on the transmission pan are not stressed down very hard).....but I know that is far from precise.

Note that I am not working inside the engine.

Any comments on how important it is to torque exactly right? Even the wheel nuts have a torque requirement "by the book".

I have seen some posts where apparently experienced types talk of a one "ug" vs. two "ugs" torque concept and I have watched qualified mechanics work on my suspension without ever touching a torque wrench. So, just how important is it?

Nic
'85 300CD

I guess that depends on whether or not you care that the bolts are being damaged OR left too loose.
Have seen many damaged wheels, lug bolts, & even wheels falling off from this type of practice. ie; GOOD LUCK!!

NIC,

I'd strongly recommend getting one. As M.B.Doc said, you really need to torque alloy wheel lugnuts. You should probably torque steel wheels because the tendencay is to get them too loose and they WILL come off that way.

Mechanics who have use a torque wrench often get used to the torque that needs to be applied. Weekend hackers like me don't use it often enough and really can't just guess.

Just get you a craftsman 3/8-inch wrench which goes to 80 ft-lbs. This will do most of your general purpose stuff. I have that, a used snap-on 1/2 inch wrench, and a snap-on 1/4 inch wrench for the small stuff. I still use my craftsman alot because the ratchet mechanism is better than the snap-on IMHO.

The craftsman wrench is really not expensive at all just a bit less well built than the snap-on unit. Torque and ratchet wise, the craftsman unit is just fine.

Sholin

I rarely ever use a torque wrench. I have always relied on the 1 or 2 ug method and have never had a problem.

Oddly enough, I just posted on this subject in a different topic. It applies here too, so here ya go:

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One last tip - which may get me flamed - don't use a torque wrench for "little" bolts like this. I have gotten myself into more trouble than I care to discuss by using the proper tool. (No that's not a typo.) The extra leverage of the wrench can end up masking the true force you are applying to such a small bolt, and most of the time, it just doesn't need to be that tight, or at a very specific torque setting! Use a 1/4 or 3/8 hand ratchet, snug them all down evenly, and then a little past "snug". Use anti-sieze if possible, especially on aluminum threads. If using LocTite please ONLY use the blue stuff (the red stuff should be banned.)

If you insist on a torque wrench, make sure the threads are all clean and DRY- use NO anti-sieze or other stuff - and maybe set the wrench a little lower than spec. Also, the wrenches are most accurate at the top of their rated range, so be VERY VERY careful if using a wrench rated (for example) 20-100 lb-ft on a bolt set to 25 lb-ft!! Much better to use one rated (for example) 5-30 for a 25 lb setting. Many are off as much as 20 percent in the lower rating range.

IMO, the torque wrench should be saved for "big" bolts like head bolts, suspension stuff, etc that are about M8 or bigger. Oh, and DEFINITELY use one EVERY TIME when torquing your alloy wheel lug bolts! That is actually the #1 use of my 1/2-inch torque wrench. :)

Let the flames begin! :cool: (Nomex suit on...)


Regards,

The way I look at it, I love the way the cars are engineered, and it was the engineers who determined the various torque specs. I like using the torque wrench because it assures that everything is tightened down as it should be. I use mine on the oil drain plug to avoid cracking the pan. The torque spec. for this bolt on the 617 engines is 30 ft lbs. I definetly use it on the wheel bolts, which have a spec. of 81 ft lbs.

At my work they torque everything. They have lots of little ones, like 1/4" drives and screwdrivers, they go down to the inch/pound.

I've used moving-beam type torque wrenches for many years. Not all the time. Many bolts still get the snug and a bit more treatment. I had two wrenches. One rated to 150 ft/lbs the other rated in inch /lbs to 50 ft/lbs. These covered most jobs pretty well. As I have been buying and servicing MB cars lately, I have decided to upgade my work practices. Bought a new click type wrench (150lb) and am ordering a smaller click type for smaller jobs. Should have done this years ago. The click type make job so much easier, you are more willing to use the wrench. The one thing most of us overlook is that the torque specs call for (and most of us disregard) NEW fasteners. Some are more prone to stretch/fatigue than others, but all have probably changed after being used. This being said, I'll still probably use the wrench when possible, but I'm real big on having clean threads on both the bolt and the hole it goes to. my $0.02.

Using some method of determining the amount of friction which the bolt is requiring to turn it is very important to gasket and machine performance....... The amount of area rubbing together when you figure the sides of the threads gets large fast...as you screw it in.... Not only does it need to be measured but you can not measure anything relavant unless the THREADS ARE CLEAN AND 'LIGHTLY' LUBRICATED.... this is the standard way engineers measure this....it is important because you are not actually measureing the HOLD DOWN AMOUNT of the article being bolted on.... rather a hoperfully ' translates to' amount based on trials of similar tries in the past.... You can be using a million dollar torque wrench and one bit of maching tailing or broken thread off the last bolt cause you to get NO pull down at that location... even though the torque wrench WILL READ PERFECTLY.... as if you are set to go.... \
Therefore, it is important that one use a torque wrench AND have a clean and lightly lubed thread surface...... DON'T ARGUE WITH ME ON THIS,, TO SAVE FUTURE EMBARRASSMENT.... LOL....
The only way to do this properly... and I always do it on any head gasket situation .... is to invest in the proper tap and die for the hole you are working on... and learn to use it properly.... so as not to have to learn how to drill out a broken tap.... NO LOL.. If you are interested in this sort of stuff I suggest the three ' Machinist Bedside Readers" by Guy Latuard....... Greg

Thanks for info folks. As I expected, some difference in opinion but to be safe, and in respect for my very nice mercedes machine, I'll buy the sears wrench and use it carefully and appropriately. Will pay special attention to the small nuts and wheel lugs.

Nic

Sorry, leathermang, I'll have to argue on this one. Every source I've seen says to have the threads clean and DRY - no lubrication of any type put on the threads. If any lube is added (oil, anti-seize, etc) the torque value MUST be reduced. The exception is torque specs that specifically state lubricated threads, such as Mercedes head bolts. :p


Regards,

Machinist Bedside Readers, now there's a book for me. Now all I need is one for the bathroom:D

I thought I might get away with not having to find the backup material on this... Everyone argued with me on the Sube forum ,,, then came back and agreed with me... LOL.. will find it sometime... but the rest of you .. trust me on this one.... maybe we should have a poll on who trusts me and who does not.... LOL....If you have any of your sources handy how about posting their names.. ? Maybe I will be so busy I will just cave in.... LOL... NOT likely if you know me.... Greg

It might be best if we agree that it should come with ASME indorsement ... would that be logical ?

NIC, I don't know which T-Wrench you are talking about... but the small nuts and bolts require an inch/pound... not a ft/pound to be safe.. even more important when dealing with aluminum heads and stuff....

Just a little FYI, to convert inch pounds to foot pounds, divide by 12. For foot lbs to inch pounds, multiply.

Jim, it is very hard to find any of the " Machinist Throne Readers " but they are available, you will recognize the originals because the pages were laminated in plastic. Also, they included some plumbing tips just in case....but people shy away from buying the used ones..... an image problem I suspect....Greg

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

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...)


Regards,

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

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

"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 .

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

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

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

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