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  #221  
Old 04-09-2019, 03:11 PM
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Quote:
Originally Posted by oldskewel View Post
https://www.youtube.com/watch?v=fvzdehnJA9k

I'm trying to stay out or at least limited here, but here are some general and specific comments. Not trying to give any answers, just information for those interested.

There is a ton of bad info on the internet. It's a challenge to sort through some of it.

Stress-strain of steel (and other similar materials) is a very important and pretty well studied and understood science. Important for it to be well understood for building bridges, pressure vessels, pipelines, etc.

That science can be extended and applied to help understanding of the specifics of threaded fasteners - nuts vs. bolts, yield, proof load, torsional vs. axial stress, etc. That's where things can get a little complicated and it may seem to be purely empirical and more black magic than it really is. Thinking back to the basic materials science of stress-strain helps bridge the gap.

For example, "yield" means some of the steel plastically deforms, which is a permanent change in shape, and strengthens the material. From that point on, it can still function elastically, including returning to the previously reached deformation point repeatedly in an elastic state, with the same elastic modulus but a now higher limit before yield will occur. But pushing beyond that point has its limits, and will eventually lead to fracture.
https://en.wikipedia.org/wiki/Torque-to-yield_fastener

Those were the general comments. Now for some specific stuff:

Be careful if torquing with a non-flanged nut and bolt. The BMW ones are both flanged. Due to the increased area and torquing radius vs. non-flanged fasteners, and the fact that about 50% (depends, of course on surface prep, smoothness, etc.) of tightening torque is used to overcome that surface friction, using non-flanged fasteners will provide more stretch (and I use that term without distinguishing between plastic and elastic deformation) than will flanged ones. https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Structure/deformation.htm

Similar to how using really well lubed surfaces and threads might result in excessive tightening.

So that could mean if you're using non-flanged fasteners, even on the initial installation you are over torquing the 56 Nm stage, possibly causing plastic deformation where it would otherwise not occur. Lubing bolts is no longer recommended because of potential over torquing and wider variations of torque.


Different thread pitch
If you try using SAE fasteners or a fine thread, the bolt stretch you get from applying a 90* angle will be different. The torque will be different too, but that is less clear.


Variable nut-thread loading
That is a basic fact of threaded fasteners. It happens regardless of any plastic deformation.

Fasteners are generally designed to one one hand have the nut be more compliant than the bolt to allow for better load distribution, trying to reduce this effect.

And on the other hand, the nut is always designed to be stronger than the bolt. The reason for that is that a nut would fail by having the threads progressively strip out, vs. the bolt would just fracture. You need to worry about the first nut thread stripping out, because if it starts to strip it will pass that load down the line and all the threads will strip out. Vs. the bolt fracture would most likely happen during assembly which is not such a big problem, vs. having a nut progressively strip out would happen literally "down the road" when it would be more of a problem.

For similar reasons, you should never use a weaker nut than bolt if there is any concern for nut failure. So using a 10.9 nut on a 12.9 bolt is not something to do casually. Doing that, you will be ensuring that the nut will fail before the bolt, which is the opposite of what is safer. Here's a pretty good reference for that and other things: Frequently Asked Questions on Bolting Matters

On the other hand, my M54 engine uses Class 12.9 head bolts, which do yield along their whole 110 mm length, threaded into an aluminum engine block ... or during a typical head gasket repair threaded into a ~15-thread long steel insert, which is probably not Class 12.9. So if it's designed carefully, it can be made to work.

When terms like plastic deformation, yield, proof, ultimate, etc. are used, they sometimes refer to total system failure (like "ultimate tensile strength" tells you when the bolt will snap [system failure] when increasing load is applied).

And sometimes they refer to localized material effects - where for example, the first thread in a nut may plastically deform very slightly - and it's not a problem at all, but it did deform. BTW, by the time the nut threads noticeably plastically deform, the bolt will have deformed a lot more.

True "stretch bolts" or TTY bolts - by which I mean bolts that are made specially for a TTY application, rather than regular bolts that are used in a TTA/TTY(?) application (as we have here) - are typically designed with relatively oversized heads and threaded sections, but with a long, thinner, uniform portion that is designed to yield, and to have all the yield there, and none anywhere else. Those are easier to analyze.
Do we know for sure the bolts are regular bolts? TTA is a process. TTY bolt uses TTA process and are for one time use.
On figuring out whether your previously used nuts and bolts have deformed enough to significantly affect reinstallation by requiring more torque to overcome the bad fit caused the deformation ... We have two questions to answer. Do the bolts break after some number of TTA sequences. The other question is does the clamping power change with each torquing sequence. Clamping power is only about 10-15 percent of torque. At some point of decline in clamping power the bolts may loosen over some period of time.

One pretty simple test is to see if you can finger-thread the nut onto the bolt. If you can do that, your torque wrench will probably not be fighting too hard to overcome that, and the measured tightening torque will have the same clamping effect as when they were initially used.
There are formulas to answer these questions without tests but we would have to know the specifications of the bolt and nut and have someone that can wade through them.

One other thought--what we are posting is not necessarily about the same bolts. I suspect not all models/years used the same bolt?
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Last edited by bcredliner; 04-09-2019 at 03:17 PM.
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  #222  
Old 04-09-2019, 03:20 PM
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Quote:
Originally Posted by OptimusPriM5 View Post
Cant believe this discussion is still going on had to unsubscribe as this has become as painful as an oil thread

Carry on

It's a 20 yr old discussion but a couple weeks old to me. I found no obective input to help determine one way or the other what is going on with these bolts.

What I've detemined is that the vast majority of people including pro mechanic have been deciding without knowing what they are doing to reduce the effectiveness of the bolts by a factor of four.

I like to figure out why something is done before I reengineer it in case there's a reason for the design in the first place.

I'm planning to do both a torsion and parallelogram experiment before I put wife's plate back on.
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  #223  
Old 04-09-2019, 03:26 PM
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@bc: doesn't have to be the same bolts to have the same rules, any bolt that is over torqued would be in the same line of reasoning.
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  #224  
Old 04-09-2019, 04:48 PM
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Quote:
Originally Posted by andrewwynn View Post
@bc: doesn't have to be the same bolts to have the same rules, any bolt that is over torqued would be in the same line of reasoning.
Correct
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  #225  
Old 04-11-2019, 05:20 PM
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Quote:
Originally Posted by cn90 View Post
1+,

I tighten to 56 Nm + 45.
That is good enough for me.
The 90-degree thing may be too much, actually 56 Nm + 90) will bring the torque to the same as a typical wheel lug, which is usually M14, not M10.

Anyway, for me, it is 56 Nm + 45...
I have done this several times, maybe 4-5 times, zero problems.
No problems *yet*; the 56 Nm is pretty close to advised torque. When torquing to 90, I noticed that by the 3rd or 4th test, i could feel the bolt go plastic deformation pretty close to 45.

I think you came up with a very solid compromise that I would likely come up with but basing on some actual testing not just guessing.

The 56+45 will get the bolt past proof load every time so it just means it will take more times to break the bolt; the problem is this: if you didn't get the bolt to snap during install it may just snap when you hit a pot hole.

As i've covered these bolts are not mission critical, the the worse case scenario NOT involving a crash is that the sway bar gets loose and will start banging around like hell; you'll know you'll pull over, it won't do any damage.

I'm going to test the 45 TTA and see just how much less clamp force is generated. It will surely be above spec for the bolt which is 7500# and going full 90 the math worked to 9000, so it should be somewhere in between those two which I am good with.

The 'standard practice' of 'two three taps of impact' is very close to worthless in getting these bolts to do their intended job (hold the aluminum from sliding sideways), The people that do that are installing a skid plate not a shear plate.

The point of installing them TTA/TTY is to get consistent forces in the field. BMW wants to have 9000# force each bolt when installed. Using seat-of-pants methods, as demonstrated when i removed my wife's plate, achieved somewhere between 9000 and maybe 1000 if that; her car was not protected against an offset impact; both the front bolts were from a 10MM bolt perspective 'finger tight'

as you mentioned the torque similar to wheel, that's exactly right and as mentioned above on relatively new bolts, I measured over 100 Nm close to even 120 in one case; a simple way to confirm your bolts haven't hit their useful life span is to use a torque wrench when going through the 90 swing; set the torque wrench to 100 Nm (75 ftlb), and if you get a click before you get to about 60 of the swing, the bolts are still usable.

I could feel the bolts start to fail 2 uses before they actually failed. On the time it broke, it broke at about 120 but it was spongy by about 45. Using a kinder, gentler 45 the whole time, i suspect the re-use count might be as high as 10 or 20 and that should get anybody through the lifetime of their X5, so as much as it was a seat-of-pants guess to use that method, I'm pretty confident that once i actually test how many times it takes to fail a bolt with that method, it will be proven to be a damn fine guess. (It also was my first thought of how i would re-use them).

Also, odd are VERY small that any previous mechanic ever re-tightened to spec, so basically if you ever start to use 56+45 it's almost certainly 'the second use' no mater how old.

I have 12 to test just from mine and wife's cars, I re-tightened all four i put back on wife's car to 90 since I'm replacing them anyhow it would be a good test and all four held well over 100 Nm during the TTA phase and are doing their job as designed. One is broken and i lost one nut so i'll have about 10 to use for destructive testing i'm going to do half at 90 and half at 45 to get some solid numbers figured out.

The torque didn't drop below 100 until the 3rd re-tighten, so i think that's a really solid # to work with and i'll test with new bolts at some point to figure out if 95 is even ok, that seemed to be the point where the bolts started to fail. For now, a dry torque of 100 Nm at 90 worked great for a 2nd use, and I prefer to keep the design spec of full clamp force myself, but don't feel bad about de-tuning a bit to extend the life either.

The bolts start to stretch at 7500#, they are full stretch at 9000# and NEW will break about 10000#. You'd literally have to crash cars to even know the answer to how much difference 7500 and 9000 would even matter, the key to get above 7500 is you NEED to stretch the bolt if even just a little so the 45 idea is the best way to do that. The fastenal paper showed that torque value is worthless to achieve clamp force on re-use of a bolt; it took 60% more force to get to the same torque by the 4th or 5th use! tap-tap-tap with an impact is so random it'd be far better to just use a ratchet and a 'good hard tug'.

case-in-point; the four bolts i just re-used and torqued to 90 varied from about 105 to 120 Nm torque to achieve the same 9000# ish force. Each re-use the cross-section of the bolt gets smaller so maybe the first time it's 9050 the second time 9000, the third time 8050, you get the picture; it's in the same ballpark until the cross section narrows quickly and the bolt stretches very easily.

Anybody with a spare hour on their hands and wants to learn a bit more about bolts, repeat the experiment with any bolt doesn't need to be this one; I put a bolt through a socket for something to squeeze, i used a breaker bar in a vice to hold one side and another breaker bar and my digital torque adapter on the other side, and did the tighten process a few times, and what became clear was that when you get to the point of yield you can clearly feel the force drop off; every bolt will have a different set up of pretension and TTA to get you to yield but you'll feel it; the torque will climb linearly until you get to yield and then it takes the same torque to keep going, it feels very odd that it stops getting harder.

The happened at just about 75-80 the first three tests, then at maybe 60 then maybe 45, the pre-failure was very obvious and dramatic, it felt like the bolt turned to lead from steel on the last test.

So, the summary if, sure the bolts can be reused. they can even pull their design force, just about 4 or 5 times, but i'd pay attention to the torque during install, and consider the 45 method especially if you remove the shear plate frequently.
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  #226  
Old 04-11-2019, 06:34 PM
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As always, thanks andrew. Very informative. Will order up some of those clone bolts and new nuts after one last plate drop as I've had mine off around 4 or 5 times since I've owned it already.

I'd be interested if someone made a X brace that replaced the steel plate as it seems it could save some weight.
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Old 04-11-2019, 06:55 PM
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Not enough weight to make it worth it.

You could take some 1" steel tube crush it flat on the ends and middle and make a functional brace you'd also want to go across the back. Then you can just leave it on. (and have instant access to engine bottom. If you don't go off road not much to loose just a little more under belly turbulence.

The plate is not two dimensional so there is some flex resistance that would be lost going to x brace.
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Old 04-11-2019, 07:08 PM
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True, maybe 10lbs going from the heavy steel plate to a lighter aluminum? But greater access is the more driving force. Would take a few removals of the bolts/nuts out of the equation for normal service I would imagine.
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  #229  
Old 04-11-2019, 11:09 PM
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Quote:
Originally Posted by andrewwynn View Post
great stuff, especially how of course the angle changes directly with respect to the thread pitch when using TTA; the specs are only for one exact bolt. Also with the details about lubricate torque; my 'torqueometer' shows both the dry and lubricated torque.
Thanks for reading carefully. I was hoping a few people would benefit from this discussion one day.

Quote:
Originally Posted by andrewwynn View Post
re: hand thread nut; of course with these nuts you can't hand-thread them because they are oval in the center self-locking nuts, so you won't be able to tell that way. I was concerned that the squashed nuts would affect the torque numbers but i guess once the nut gets started it's much easier to turn because i got no reading on the torque adapter from the nut before snugging tight.
A few of mine can be finger threaded together, so maybe the oval self locking gets worked out too. ??? But one way or another if you've got a nut and bolt that can be threaded together very easily, then there will be no extra torque needed to overcome any misfitting while torquing it down, and it should be like new in this regard.

Quote:
Originally Posted by andrewwynn View Post
I'm a little confused by the 'stronger nut' vs. Fastenal quote above saying that the nut is 'softer' I believe that means that the metal of the nut is a lower ksi than the bolt, but because the nut 'melds' into the bolt it holds stronger or something, i would love to have some clarification on the seemingly contradictory things.
The few people that actually do the specs and design the manufacturing processes to meet them make it so that for example a Class 10.9 bolt and Class 10.9 nut are matched to provide the extra compliance in the nut. Use them together and you're fine. So roughly the 10 there refers to the ultimate tensile stress and the 9 refers to the yield stress being 90% of that, but actually things are a little different.

The nut will also have a bit of a bevel on the first (or more?) threads, which makes it less likely to fail. Also, the nut threads have a bigger radius from the center, so more volume, so more strength than the bolt threads they match up with.

The key is to prevent the first thread from stripping out, because if it does, it is likely for the rest of them to follow as the load that is no longer carried by the first thread is passed on.

In general though, if you consider the total effective area of the nut that would have to yield (roughly the circumference of the nut/bolt interface times the length of the nut = pi*r*2*h) vs. the corresponding are of the bolt (the cross sectional area of that = pi*r^2) you'll see the odds are stacked in favor of the nut. Those areas are equal at around nut height (h) = bolt diameter / 4 (= r/2). So forgetting the variable loading, a pretty thin nut should work fine. Vs. it's more typical for steel to have the nut height close to the same as the bolt diameter. So with the extra nut height, the variable nut thread loading issue turns out to be manageable.

And that all means that it is much more likely for the bolt to fracture than for the nut threads to strip. And that would likely happen during tightening ...

even more so because of another factor that has not been mentioned ... hate to throw fuel on this fire, but ...

When torquing the bolt, the bolt will be in torsion as well as being stretched. Those two stresses combine as an equivalent, "von Mises" stress, and will cause yield at a point before just the stress due to stretch will. After the tightening operation is finished, somehow this dissipates (mystery to me).

And when considering the torsional stress, the surface friction between the bolt or nut head and the clamping surface does not count. So the torque wrench value would need to subtract out that surface frictional torque, but not the torque due to the threaded surfaces.

But that's another factor that makes it more likely for a bolt to fail.

Still, matching Class or Grade for nut and bolt is the safest practice when there is any doubt about failure.

And if you're wondering where all of these factors go in those nice geometric-looking formulas you see in the torque tables, well it's in those coefficients that appear pulled out of the air. I love when they do stuff like that - all mathematical and physical and geometric and precise and oh yeah, here's a factor of 0.18. move along, nothing to see here. Don't ask don't tell.
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