How many threads past the nut?

[ntsqd]

snip.....
Classic lock washers are perfectly fine for non-preloaded bolt assemblies.

Based on both my education (Theoretical and Applied) and my ~40-ish years of experience in a range of fields I'm going to disagree with this. And leave it at that as I've no intent of turning this into a lock-washer thread.

 

[Echohunter]

How many threads before or after the nut are completely and totally irrelevant ....what matters is getting it tight enough, and by tight, I mean you tighten it down til she spins free ...than back'r off a quarter turn and you're golden!!

 

[finn]

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.

The more correct way to design the joint would be to figure out how to incorporate a longer bolt, of the correct shank diameter and spring rate, so there’s adequate stretch to provide a predictable preload and clamping force, rather than to rely on some janky, half assed lock washer to deform the surface.

Ever compare the head bolt design of a forties or fifties car engine with a modern design that uses longer, smaller cross section bolts and countersunk holes? There’s a reason for that evolution.

 

[Pexto]

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.

A butted spoke for a bicycle or motorcycle is another pretty good example of this principle.

 

[F-22]

The more correct way to design the joint would be to figure out how to incorporate a longer bolt, of the correct shank diameter and spring rate, so there’s adequate stretch to provide a predictable preload and clamping force, rather than to rely on some janky, half assed lock washer to deform the surface.

Ever compare the head bolt design of a forties or fifties car engine with a modern design that uses longer, smaller cross section bolts and countersunk holes? There’s a reason for that evolution.

I totally agree that would be the more correct way. It's also the considerably more expensive way that also takes way more space to achieve.
Engines mainly use preloaded bolts. But the fact is that this is usually not the case.

If we stay at cars, how would you mount a door hinge to the frame of the car with a properly preloaded bolt? Rule of thumb is you need the flange height of at least 5 times the screw diameter for it to stretch and be self-locking. If a regular car uses M12 or M10 screws, that means half the hinge would be 50mm tall. Just makes no sense. Even if we go to half that, a 25mm fat hinge would be totally overkill for the task. Usually they're 3-5mm thick sheet metal. Sometimes they're cast/forged, and I think they may use tapered head fasteners for that (get a bit of locking action from that too, but also since they can get flush with the body), but they all generally also use thread lockers or some other locking device like a nylock nut or serrated washers.
Or, how would you mount a plastic car bumper to the car with a properly engineered bolted connection? Or even a steel one? Most fenders I've removed used some kind of nylock nut or crushed metal nut that wouldn't make a preloaded connection but would prevent the nut from unscrewing just due to vibrations.

If you use a properly engineered bolted connection, there is no need for lock washers, nor other types like tab retentions, safety wire, nylock nuts, castellated nuts, serrated washers, schnorr washers, nordlock, loctite, disc spring washers, flanged nuts and bolt heads... They exist because they were/are needed for certain tasks.

Based on both my education (Theoretical and Applied) and my ~40-ish years of experience in a range of fields I'm going to disagree with this. And leave it at that as I've no intent of turning this into a lock-washer thread.

Well, based on my theoretical and applied knowledge and examples I'm also sure I'm correct, but I agree that this type of a discussion will lead us nowhere

 

[cad70]

NHRA say 3 for lug nuts ?

NHRA is as follows.... doesnt have to protrude out of the lug nut.

The thread engagement on all wheel studs to the lug nut, or lug bolts to wheel hubs, must be equivalent to or greater than the diameter of the stud/bolt. Length of the stud/bolt does not determine permissibility. (Example: A 7/16-inch stud must be thoroughly engaged through the threads in the hex portion of the lug a minimum of 7/16-inch.)

 

[Cruzan80]

Since we are on the topic, curious about the reasoning behind the front rotor nut on the '04 F150 (2WD). Calls for not reusing the nut, as it has a nylon-style insert, torqued to 295 ft-lbs, then has a keyed style cap over it with a pin thru it (cotter pin style). Fist, this seems artificially high (almost like they spec-ed the torque higher than a standard 1/2" torque wrench on purpose), but a nylon insert and a cotter pin seems extra redundant. Yes, I understand the wheel not wanting to come off when driving, but this seems excessive. I can look and count the threads that are exposed if needed, I think 3-5 threads.

 

[Ole Slewfoot]

It's about preloading the bearing assembly, which takes much greater force than just holding it on.

 

[Cruzan80]

I guess I am not questioning the torque number, as much as nylon insert and a cotter pin together. If the nylon "locking" portion lets go, I doubt the small metal pin will hold the preload...

Or it is to help the guys who reuse it, even with "DO NOT REUSE" stamped in it...

 

[Ole Slewfoot]

In event of a bearing failure;
The nylon can melt.
The anti rotation hat may keep the wheel on the vehicle long enough to pull off the road.

 

[Cruzan80]

Gotcha. That makes sense. To be clear, I was sure there was a reason, just wasn't apparent to me what that reason was. Thanks!

 

[The Tool Tyrant]

Almost 100 posts on "how many threads past the nut"...ONLY on Garage Journal will you find these in- depth discussions and debates.

 

[kwb]

There is a lot to a proper bolted joint.

If you haven't actually had to develop one to live thorough a products life it can be brushed off as being nerdy

 

[sz0k30]

Specific industries/companies may have their own standards. I'm just a guy who plays with cars, So generally, most of the bolts I work with are 3/8 max. So if I can, I just eyeball roughly 1/8-1/4" & I'm good.

But normally the number of threads past the nut end is irrelevant, as long you have full nut engagement. It all depends on the lengths that particular bolt comes in. So If the length you have, puts the nut at the end of the thread, your next length may be 1/2" longer. What are you going to do? Cut 3/8" off just so you have 3 threads sticking out?

 

[finn]

I totally agree that would be the more correct way. It's also the considerably more expensive way that also takes way more space to achieve.
Engines mainly use preloaded bolts. But the fact is that this is usually not the case.

If we stay at cars, how would you mount a door hinge to the frame of the car with a properly preloaded bolt? Rule of thumb is you need the flange height of at least 5 times the screw diameter for it to stretch and be self-locking. If a regular car uses M12 or M10 screws, that means half the hinge would be 50mm tall. Just makes no sense. Even if we go to half that, a 25mm fat hinge would be totally overkill for the task. Usually they're 3-5mm thick sheet metal. Sometimes they're cast/forged, and I think they may use tapered head fasteners for that (get a bit of locking action from that too, but also since they can get flush with the body), but they all generally also use thread lockers or some other locking device like a nylock nut or serrated washers.
Or, how would you mount a plastic car bumper to the car with a properly engineered bolted connection? Or even a steel one? Most fenders I've removed used some kind of nylock nut or crushed metal nut that wouldn't make a preloaded connection but would prevent the nut from unscrewing just due to vibrations.

If you use a properly engineered bolted connection, there is no need for lock washers, nor other types like tab retentions, safety wire, nylock nuts, castellated nuts, serrated washers, schnorr washers, nordlock, loctite, disc spring washers, flanged nuts and bolt heads... They exist because they were/are needed for certain tasks.


Well, based on my theoretical and applied knowledge and examples I'm also sure I'm correct, but I agree that this type of a discussion will lead us nowhere

Maybe that’s why many door hinges are welded to the body since the 80’s?

In any event, I haven’t seen lock washers on cars in any significant quantities since the fifties.