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Old 04-11-2019, 11:09 PM
oldskewel oldskewel is offline
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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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