Request for recommendation for self locking nut with highest resistance to removal

Dear John2005:

Better add some Belleville washers to the stackup...

... because you want something that must be thrown away when it fails?

Not good for wet or high UV environments. Least likely to gall and fail the fastener.

Failure mode includes galling and losing the bolt / screw with them.

Use two or more Belleville washers, then your concerns are mitigated somewhat. Inspection will let you know when it is time to tighten it back up.

David A. Smith

A plain washer under a plain nut will hold up for ever in non rotating, vibration free settings. The stiffest lock nut you can apply is the thin one that's tightened over a regular plain nut.

Nylocs are unsuitable in high temp applications - disapproved in aero engine compartments for example. Any lock nut is unsuitable where there is relative rotation in the axis of the bolt. Here, there should be a pinned nut at least, or a washer with a tab in a channel, or a side fixing, and ears to engage the flats of the nut (automobile style) or a center hole that engages the hex flats of the nut.It may come as a surprise that despite lock nuts all getting easier to torque with use, the FAA criterion for reuse of a lock nut is that it cannot be turned on the bolt by the unaided fingers alone.

The unit force due to vibration is rather small, and it is only the many cycles of vibrations that run the nut looose on the bolt.

The split spring washer is effective, so is the star, and so is the nut with trapped star washer, which does not drop and get itself lost.

Brian W

Thanks for the replies guys,

Hey Brian,

If I use a regular non hardened hex nut (maybe a grade 2 or a brass nut) could I tap a hole through the side (through one of the nut flats to the center of the nut) that would allow me to use a cone point set screw to lock the nut in place after it's snugged down, or would tapping the nut in this way mess up the nut threads so that it would not thread onto the bolt ?

If it could work, tapping the nut would be easy and set screws are cheap. Plus I would save time cleaning the bolts, applying primer, and waiting for the loc-tite to dry.

Once the cone point set screw digs in, I don't think it would go anywhere. It's just a matter of whether it can be done without messing up the nut threads.

Do you think it would likely work ?

This is not relevant to my question because my nut must rotate with the bolt, but I have read that split lock washers are no more effective than a plain round flat washer...

From the NASA Fastener Design Manual RP-1228:

"The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is usless for locking".

From theNaval Ships' Technical Manual, Chapter 75:

"Although lockwashers may be encountered, using the flat washers with selflocking nuts, self-locking fasteners, self-locking inserts, or thread sealants such as MIL-S-22473 anaerobic compounds is preferable.

If loosening has been a problem, however, replace the lockwashers with self-locking fasteners.

The helical spring lockwasher (Figure 075-5-11) is flattened when the bolt is torqued down. Once compressed, it acts as a flat washer, contributing normal friction between the nut or bolt and the bearing surface during tightening".

One locking washer that I have found that looks like it would work well is the Nord lock washer...

///

///

Though I would stand corrected by the NASA avoidance of split spring lock washers, my concern would be at the damage they inflict on the two facing surfaces, which provide the basis of their locking action.

Aircraft mechanics are taught to drill a cross hole through both a nut and its threaded stud with a cobalt drill bit to pin or safety wire, if necessary. Nuts are designed to be stronger than the studs in which they engage, so drilling a hole is not the issue - but a set screw will mar the stud's threads. And a tapped hole for a set screw would call for a bigger transverse hole, so a long nut to maintain safety margin might do well, if that's an issue.

Unless the application is very critical, a squirt of solvent before applying thread-locker would provide the nut lock you want - unless the surfaces rotate, and it's starting to sound like they do. Do they?

Brian W

Hey Brian,

Loc-tite works OK, but I was just looking for a mechanical solution that may be better from an assembly standpoint and not require a waiting time for the loc-tite to dry. Also with loc-tite you have to make sure the nuts and bolts are clean and in most cases apply lco- tite primer.

The nut "rotates with" the bolt as one unit, as the bolt is rotated to adjust a slider. There is a Delrin washer under the nut, so unless the friction between the nut and the delrin washer is greater than the force required to loosen the nut, the nut will stay in place. During adjustment, there's not much axial force on the bolt, so I would think that it would not take much friction to keep the nut from backing off.

As far as the set screw marring the bolt threads, that won't really matter because once it's installed, the nut is never repositioned or removed.

I guess I could drill a hole through both the nut and the bolt after assembly and install a roll or spring pin, but then I would have to fixture the whole assembly in a drill press or clamp the assembly down to a bench and drill it with a hand drill. Something about drilling on the assembly after it's put together did not appeal to me, but it might be viable.

Thanks John

I would think you should know what the installed position of the nut on the screw should be so drill the srew/bolt beforehand and the use of a castellated nut should account for any variability, a technique use on motorcycle axles for 40+ years.

That's my thought too.

Brian W

A castle nut sounds like the best method to me to. I'm going to give the castle nut a try, I found one at mcmaster that looks like it will work. Every thing is in a very confining space but I think I can make room for the cotter pin, and/or smash it down so it's not a problem.

Thanks guys, John

Actually, even with the castle nut I will probably have to drill the hole in the bolt after the unit is assembled.

This is because I can't have any axial play in the bolt and if I pre drill the hole in the bolt, then I may have to back off the castle nut a little to get the cotter pin to align with the hole.

If a distorted thread lock nut is not reliable enough, it looks like I will probably have to stick with loc-tite, go to a different faster drying adhesive, or tack weld it.

Using a lock nut and loc-tite together may be an option since the lock nut would just have to hold the nut in place while the loc-tite dries, then once the loc-tite dries it's not going anywhere.

Dear John2005:

This will gall the threads, and fail the nut and bolt.

For the second time, put belleville washers in your stackup. Then you don't need to worry about turning the nut a little forward or backward.

David A. Smith

Peening the bolt end to hold the chassis nut in position was a method used, surprisingly, by the first rate auto engineer, Royce, in his first products.

Brian W

I think you were planning a 1/4 inch fixing. If you used a 1/4 x 27 size and cross-drilled the stud twice at a multiple of 30 degrees angular spacing: the maximal gap would be

Brian W

Use of a plain washer,a spring waser and a nut with a thickness much smaller than main nut should do for locking,Steel nuts may do.

I will give the belleville washer's a try David.

Actually, with the castle nut even if I have to back the nut off to align the cotter pin with the hole in the bolt, If I can just snug the nut back down that should bend the 1/6" OD cotter pin and then I don't suppose it will go anywhere. It's just a matter of how far off the misalignment between the hole and cotter pin will be but I think the cotter pin should have some give with a good wrench on the nut and bolt. A belleville washer under the nut would be added assurance.

This is not related to my problem but just out of curiosity, regarding the comments from NASA on the split spring lock washer's, do you think that belleville washer's are better than split spring lock washers, or do they also (as NASA stated about the split spring lock washers ) basically have no locking ability once they are flattened by the nut ?

Dear John2005:

... which I have seen installed even in mechanical gyroscopes ...

They *only* provide axial tension in the bolt, and a "gentle" increase/decrease in that tension as the nut is tightened or loosened. They do nothing for a locking feature.

They usually spring right back, unless you remove / dispace metal or overheat them.

Bellevilles and locktite. Castellated nuts and drills if you are into S&M.

David A. Smith

Followed by safety wired nut and bolts.

snipped-for-privacy@mine.net wrote, On 7/27/2008 6:40 PM:

Then weld everything together...

..or if not weld, you could braze - like the nut I brazed on a steel v block clamp for the clamping screw, this weekend. :-)

Brian W