DPG
Well-known member
Our structural engineers specify no more than the thickness of the nut.
How many threads past the nut?
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ntsqd
Well-known member
I use the 1X fastener diameter because those come with a built-in gauge. If the protruding threads aren't "square" in footprint then there's either too much or too little.
For critical joins I assume only three threads are actually uniform enough to be load-bearing. The others are along for the ride if/until the loading goes past the design expectation at which point the threads deform enough for others to engage. Ever notice that MIL spec thin nuts don't reduce the max torque for that size fastener? At least no torque spec that I've found does. A real, live Rocket Engineer taught me that.
PVC air plumbing and lock-washers belong in the same dust bin.
For critical joins I assume only three threads are actually uniform enough to be load-bearing. The others are along for the ride if/until the loading goes past the design expectation at which point the threads deform enough for others to engage. Ever notice that MIL spec thin nuts don't reduce the max torque for that size fastener? At least no torque spec that I've found does. A real, live Rocket Engineer taught me that.
PVC air plumbing and lock-washers belong in the same dust bin.
Bill T
Well-known member
One of the documents frequently referenced in my industry, at least for Civil applications, is the AISC Guide to Design Criteria for Bolted and Riveted Connections. In this document it does mention that in testing, the max capacity is often achieved with less than full thread engagement, but, for consistency, assume a full nut.
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38Chevy454
Well-known member
Finn has it right. *ALL* bolts stretch when tightened. It is related to the torque, but torque is a poor indicator of actual stretch since other variables influence it: friction of threads, friction under bolt or nut face, material(s) being bolted together, as examples. But torque is easy to measure and works for most applications to get proper clamping load. Clamping is what a bolted joint does, it holds the pieces together with spring force that is a result of elastic modulus (E) of the bolt. Most bolts being steel the elastic modulus is approx 30E6 psi (30 million psi). Doesn't matter on the strength of the bolt, E does not change with heat treat. Higher strength bolts do have less plastic deformation (stretch, work hardening) than lower strength, but the higher strength bolts have higher yield. Almost all bolted joints use the bolts in the elastic deformation range, meaning returns back to original length when unbolted. Torque to yield bolts are designed to yield slightly, this is because that value of clamp load is very consistent bolt-to-bolt. It takes torque variability out of the process. Equal clamping load is a good thing for most joints with multiple fasteners.
finn
Well-known member
The NHRA three thread rule has nothing to do with actual load on the clamped joint. It’s simply an easy visual check to insure lug nuts aren’t missed during the installation of the wheel.
If the inspector doesn’t see at least three threads, his suspicions that the wheel isn’t properly attached are raised.
They could have just as easily said ten threads, or one, and it wouldn’t change wheel retention. Three is enough for a visual flag that something is wrong.
kbeefy
Well-known member
3 threads past nut !
one if our racing partners was a nut & bolt guy ( owner a bolt company) and he always was taking about torque specs - dry or lubed and 3 threads past the nut minimum!
Now this was back in the mid / late 90’s
so I’m sure that changed, But I follow that rule
knobby
Well-known member
If the fitness equipment that i have been assembling lately is any indication the current chinese standard is negative two or three. Might just be thick powder coating and poor engineering meeting material minimizing tho.
andyvh1959
Well-known member
the only logic I can apply is the making of threads may cause the 1st thread form (at the threaded end of the fastener) to be incomplete, by taper on the end, or where the full depth thread actually starts. So by spec'ing 1-1/2 to three threads past the installed length of the connection insure full thread engagement, and the easy visual confirmation of proper assembly.
Carguy99
Well-known member
I was have used this..my dad said it should stick out at least the diameter of the bolt. Personally I think 3 would be Adequate but in my work I see a lot of tapered bolts. So… more needs to stick out.
Pretty sure the 1,2 or 3 threads is just a visual thing. Someone had to come up with something other than "the bolt must protrude past the nut". 3 threads exposed isn't any stronger than 1 thread, but someone made a rule about it.
speed bump
Well-known member
According to Shigleys 1-2 threads past the end of the bolt. According to AISC minimum is flush but structural code for bolting is also primarily concerned with different loading patterns than mechanical applications.
RoninB4
Well-known member
-It seems that I haven't worded my reply very well. The machine tool industry rule of thumb applies to how many threads are actually engaged, not how much is protruding past a nut. So if you have a 1/2" diameter fastener (bolt, screw, etc.) then there should be 3/4" of thread engaged. If the nut being used, particularly the thinner jamb nuts, is less than 3/4" thick then some scrutiny for the application is in order. Yes I know most of the standard nuts out there are only 1 times the diameter, some estimation of the application applies here. Any threads protruding past the nut don't do anything for strength at all, it's all about how many threads are actually engaged against other threads.
If the fastener is threaded into something AND a nut is used on what protrudes past it then the nut is only there to lock the threads, preventing it from backing out. The machine tool industry doesn't use through bolts and a nut very often, this is mostly used on automobiles, appliances, or on small electronic components. The 1-1/2 times the fastener diameter is mostly for threading into something (ie; how long a fastener should be used?).
There are other considerations for excess fastener length (alignment, jacking, clocking, adjustment, etc.) but I've made this a long winded reply. I need to learn how to be less wordy.
OP
bluedog225
Well-known member
I wonder if there is additional strength at the point of failure. That is, when all the threads are loaded to the point of deformation. Does the additional material beyond the last engaged thread provide support that would otherwise be absent?
American Locomotive
Well-known member
No. Any spec that requires 2 or 3 threads beyond the nut is basically just doing it for "good practice". It allows for easy visual checks, and guarentees you are passed any "lead in" or partially formed threads.
I forget the specifics, but in general due to friction and stretch of the bolt, the first couple of fully-engaged threads inside a nut carry the majority of the load. You can achieve the maximum clamping capacity of a bolt without it even going all the way through the nut.
I forget the specifics, but in general due to friction and stretch of the bolt, the first couple of fully-engaged threads inside a nut carry the majority of the load. You can achieve the maximum clamping capacity of a bolt without it even going all the way through the nut.
JSGAuto
Well-known member
That's the first time I've seen Shigley referenced outside of the work.
I love these threads. Clearly there are very technical people adding their experience....and some not so much "self certified" experience. Hopefully it's being recognized and the proper advice taken.
Spud McGee
Well-known member
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This is very good news for the fellow in the other thread who's car lift got installed with some of the nuts still proud of the ends of the anchors.
ntsqd
Well-known member
In my first post in this thread I almost posted what a real, live Rocket Engineer told me; that due to thread imperfection you can't count on more than three threads to be engaged at full load. It is only when the loading exceeds the design spec that the threads distort enough for all of the threads to become load bearing. And by then some of them have entered the Yielding phase of the Stress-Strain curve.
My class didn't use Shigley's book, we used another author. I think that I got gypped.
My class didn't use Shigley's book, we used another author. I think that I got gypped.
niget2002
Well-known member
I've always done 2-3. I can't remember where I heard it, but that's what I've stuck to. I will confess to have designed/built quite a few things that barely had full thread engagement on the nut.
tinmanwpk
Well-known member
andyvh1959
Well-known member
Really? Tapered BOLTS? What spec is that? I have never seen tapered threads for anything other than water, air, or hydraulic plumbing. And none of those are structural fasteners.
Lassen Forge
Well-known member
Depends on the type and grade of the bolt... When we did structural bridge assemblies, using A-325 bolts, nuts, and washers, the maximum allowable overage was 1 full width of the nut, using one A-325 washer under the nut and torqued to the proper foot pound + degree angle of torque... Using non-matching or non-compliant fasteners or washers was not permitted...
ntsqd
Well-known member
In some automotive applications the start of the threads is a tapered spiral. My hypothesis is that these bolts make it so that on a fast moving assembly line you need only get the nut somewhere close to the spinning bolt for it to pick-up the threads and start to tighten. More common to see them holding on a body panel with some form of fixed female thread. In this case I guess the purpose is to get the bolt started when the bolt holes don't align perfectly. Again in a fast paced assembly line.
finn
Well-known member
Self tappers? Some of the larger hex headed ones are beyond what I would consider screws.
finn
Well-known member
I think those beehive type bolts are actually an extreme version of a self tapper. GM once used them for body assembly. They screw into a relatively thick plate.
ntsqd
Well-known member
Not that I've noticed they're not self-tapping. The bosses that they screw into appear to have been machine tapped and the bolts have no cutting relief like fine thread self-tapping bolts usually have. Fine thread as opposed to a lag bolt or a sheet metal screw, not necessarily UNF threads.
whitesco
Well-known member
Sooooo… are the capped lug nuts gonna come flying off my car or not? 
Yes. Maybe. Perhaps. Probably not.Sooooo… are the capped lug nuts gonna come flying off my car or not?
If you were a joint designer, you'd also know that lock washers are not relics. They work in certain applications, and do not work on other applications. Hardened steel housing with 12.9 grade high tension preloaded bolts? Yeah, they're only a potential failure point in such cases. Classic lock washers are perfectly fine for non-preloaded bolt assemblies. When you just snug up a bolt. Like if you want to fasten a sheet metal plate to a massive welded steel frame. You can't properly use preloaded bolts in such applications (not enough flange thickness, it would require you to use really small diameter fasteners that complicates stuff way too much).
Lock washers employ a few principles of operation. If you fasten a soft 5.8 grade nut onto an aluminium flange, a lock washer is supposed to be harder than both of them and will bite into the material of the nut and the flange, and that wedging will prevent it from unscrewing. With quality locknuts this is very noticeable, unscrewing it will require more force than just snugging it up. The spring function is a lot less meaningful but also prevents unscrewing. On high tension fasteners, they did not merely realize they don't work 50 years ago - pretty sure they knew that since they were invented, but lots of engineers saw their widespread use and decided to use them solely based on experience, so that's how they sometimes ended up in places where they shouldn't be.
Preloaded bolts are supposed to squeeze the flanges together so hard, that the actual friction between the flange surfaces prevent them from slipping and turning. On a non-preloaded bolt, the shear strength of the bolt itself prevents it from turning (the flange holes make contact with the bolt).
This is also false. Bolts stretch. The first few threads hold the force, anything beyond a certain point makes no difference at all unless the first few threads already fail (at which point, the rest will fail too if you want the same clamping force - of course if the first threads fail, the clamping force will drop cause the bolt will be able to stretch further). The truly effective threads are in the ballpark of ~2 thread diameters - e.g. on an M10 screw, I'm quite certain 20mm of thread engagement will be enough to transfer all the force the thread is capable of transferring (probably even something like 1.5 thread diameter, so ~15mm, would be enough). And even at that, the load on the threads drops, the first 3 or 4 engaged threads take up the bulk of it, everything beyond that isn't helping that much. That's why standard nuts are not very thick, they're chosen as the best compromise for most strength in the smallest length. I think their length is around 70-80% of the thread diameter.
Edit: in regard to engineers working from experience instead of theory only to be proven wrong later... Happens all the time. All those mechanics tightening car lug nuts to 200Nm when the engineers prescribed 110Nm have no idea what they're doing, but they worked on tractors in the past and know tighter is surely better! Even if you bend rims, hubs, warp discs and stretch lug studs/screws to the point that they hold less than they should.
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rattle_snake
Well-known member
My understanding is that the first thread engaged holds most of the force and by the 4th thread it's negligible, because the bolt stretches.
Ah I see I'm a bit late...
Ah I see I'm a bit late...
Ole Slewfoot
Well-known member
Y'all must be using some crappy taps, most or maybe all of the stretched bolts I've removed, the stretched part is between the bolt had and where the female thread came to. Dozens and dozens of head bolts come to mind.
By the way, when engineers want a bolt that performs exactly as needed, they use special bolts with a thinner shaft, to eliminate the inconsistencies and notching effects associated with the head and the thread. Such a bolt stretches only in the middle (in a meaningful way...). You find these on conrods or cylinder heads.
In Aviation (general rule of thumb) we normally go 2 to 3 threads showing past the nut with two being optimum.
Any more than 3 either needs washers (2 maximum) or a shorter fastener.
Using the correct grip length fastener is optimal but not always feasible or available.
”Standard Aircraft Handbook“ states 1 to 3 threads. (lock nut).
Any more than 3 either needs washers (2 maximum) or a shorter fastener.
Using the correct grip length fastener is optimal but not always feasible or available.
”Standard Aircraft Handbook“ states 1 to 3 threads. (lock nut).
3baygarage
Well-known member
Most nuts pass the thread if it’s outside of Free Parking.
I would also guess that if you can see 3 threads, then that is also a good indication it hasn't been cross-threaded. on coarser thread nuts&bolts where someone sticks the wrong nut on a bolt, like metric on SAE, I've seen it spin on two rotations before it finally jams on the 3rd rotation.
By the way, another thing to consider - I know we're talking about nuts which are too thin for such considerations, but when making threaded through holes in castings (especially alloy), the typical recommendation is to avoid the screw from going all the way through. The exposed threads may form corrosion and upon removal, you can strip out the threaded hole. If you use deep nuts like the DIN 6334, leaving the screw 2-3 threads inside wouldn't weaken the connection but would prevent a lot of the issues with rust and dirt piling up on the exposed end.
What you are seeing is the plastic deformation from severe over tension loading. Long before you reach those plastic limits, materials (in this case fasteners) stretch elasitcally, not only in the area between nut and bolt head, but inside of the threaded area of the nut. It is this elastic deformation that typically loads the first 3 threads of a consistent nut and bolt joint. The thread deeper in aren't loaded because the bolt is no longer being "pulled" by the threads past the first 3 (in which each of the threads in the NUT must deform elastically to carry their load). It is that very forgiving nature of materials to be stressed and strained within their elastic limits that makes them so predictably and useful.