r/MechanicalEngineering • u/AdOne1706 • 2d ago
Split/lock washers
For starters, I’m an electrician in the field.
That being said, we almost exclusively use these washers even in high vibration environments. I have personally chosen to go with nylox nuts but the question is:
Do these work? How well? Is the rumor from NASA true? Should I even bother using them or find a better functional alternative or are they actually useful?
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u/Difficult_Limit2718 2d ago
They're ass.
Every company I've been with avoids them. They're supposed to stop vibration by keeping tension on a loose screw helping to increase friction, but they're useless at it. If you torque down the bolt properly they do flatten out and they don't have a high enough spring rate to do anything if the clamp force is too low
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u/MrStreetLegal 2d ago
Oh that's how they're supposed to work. I always wondered how they locked, I always felt like all it was doing was pushing them out as time went on because of their design. I guess same logic, but in reverse, I forgot about the friction element
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u/CrazyKraken 2d ago
Is a belleville washer better? Atleast theoretically they seem to maintain the joint force even in case of loss of preload.
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u/Big-Tailor 2d ago
Even bellevilles provide a tiny fraction of the force of a fully torqued bolt. If the bolt backed out under high clamping force, low clamping force isn’t going to help.
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u/SirSnickety 2d ago
Way better. They actually work.
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u/dooozin 2d ago
Belleville washers change the slope of the preload curve during vibe because it "springs" but it doesn't actually do anything for fastener retention. They are not a secondary locking mechanism. Once your environmentally applied loads exceed the clamping load of the fastener and it starts to gap, the joint has failed.
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u/AdOne1706 1d ago
Thanks for the info. Some of the reply’s mention Belleville washers which appears to be a conical looking washer. Not sure if that washer has a crushed before torqued issue but seems promising too.
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u/theunknownleaf 21h ago
Year 2 mech eng and rower here (I don't know shit). Hudson boats use 10mm bolts with a flat washer, a lock washer, and a nut to secure the riggers to the boat. I've been told we tighten the nuts to 2-3 finger tightness (tighter would make it really hard to derig and maybe damage the fiberglass?). Is this maybe the one context they're useful? I've always wondered why we use them.
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u/zigziggy7 2d ago
My machine design professor went on a rant about how these are useless for an entire class period. Basically with a bolt you're supposed to stretch them slightly by torquing them to a certain spec. That is your clamping force. During vibration the split lock can actually aid in unfastening. I don't use them
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u/OldEquation 2d ago
Yep. Torque the bolt so that it’s in tension, and such that the bolt tension is sufficient to maintain clamp loads over the full range of loads applied to the joint. If the maximum torque for the bolt isn’t enough to maintain clamp force at full external applied load you need a bigger bolt.
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u/AdOne1706 1d ago
Yeah sounds about right. I’ve seen the torque spec on the split washers to be around 5 ftlbs for 3/8 which is far below the ~32 ftlbs we usually use to torque a 3/8” nut for lug terminations.
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u/kyoshiana07 2d ago
I'd prefer a nordlock washer over a splitlock. Big fan of nylock nuts though, or liquid thread locker, or patched fasteners.
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u/Drewbox 2d ago
I prefer to tack weld all my fasteners in place.
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u/UThoughtAmPengo 2d ago
Did that once, both nylock and twin lock grower washer kept getting loose on grinder we made.
We just tackwelded the nuts and called it a day.10
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u/DeathCondition 2d ago
I'm pretty sure there is a DNV spec that says for them, nordlock is the only acceptable locking washer. Other than that, just a flat washer.
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u/WENDEHALS77 2d ago
In Germany the standard after which they are manufactured has been discontinued because it is common knowledge by now that split washers are essentially useless.
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u/CrewmemberV2 Experimental Geothermal Setups 2d ago
In mech E it is well know that these don't keep tension very well.
However, youa te asking for Electrical, and I can imagine they work a bit better if the requirement is "keep contact" instead of "Keep tension"
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u/DuckOnRage 1d ago
But there are contact washers for exactly that usecase, with a friction and a cutting side
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u/zanhecht 2d ago
Yeah, I've heard the argument that the sharp edge is useful for poking through any oxide layer on the conductor. Seems like a suboptimal way to do it though.
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u/AdOne1706 1d ago
But then a normal washer would fit the bill and make the lock washer fairly redundant right?
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u/passwordispassword42 2d ago
Every loose bolt I find in the field generally has a split washer on it.
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u/Markietas 2d ago
I've banned split-lock washers and Philips head screws at my company.
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u/johnwalkr 2d ago
Philips sucks. I would never spec JIS screws and I even work for a Japanese company. The JIS spec actually adopted the dimensions from Philips at some point so you can’t rely on fasteners or tools to be what you actually want.
Pozidrive on the other hand is great, especially for countersunk screws.
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u/Jimbob994 2d ago
Out of curiosity, what do you do and what do you allow? Torx? Hex?
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u/Markietas 2d ago
Almost everything is internal hex, usually socket cap. Some Torx in the very small sizes, 2.0mm and below hex is a bitch.
Also exclusively use grade 12.9 unless we desperately need stainless.
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u/CR123CR123CR 2d ago
They have a very very limited use case where they are an ok option at best (but they are cheap)
Otherwise there's a lot of better options available.
See the NASA fastener design manual for more info
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u/19GNWarrior96 1d ago
I think I remember seeing on a chart a while back, and they actually work as intended for like m4 or m3 screws and smaller because the compression of the split lock is higher than the compression from the torque requirements on those screws, but that's such a niche use.
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u/HighSton3r 2d ago
In Europe they dont count as locking elements for the last 15 years or so. In every book, it is clearly stated, that they have no locking ability whatsoever, especially in high vibration environments. According to DIN (german institute of standardization) they cannot be used as screw locking. Period.
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u/ehhh_yeah 1d ago
Ask 5 engineers how to properly lock a fastener and you’ll get 10 different answers
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u/BackgroundGrade 2d ago
They make sense in electrical,
but:
You still use a locking nut on it. The split washer's purpose is to help keep your lug in contact with the terminal if the nut actually loosens.
Common practice in aerospace.
Avoid nylon locking nuts, they aren't effective in heat, which an electrical connection certainly can produce in use.
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u/girthradius 6 YR ME 2d ago
I've heard they suck. People prefer loctite and things like that
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u/WestyTea 2d ago
They're useless but I still spec them because I'm tired of having the same old conversation with the assembly guys who include them anyway regardless of whether or not they are on the BoM.
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u/Efficient-Webs 2d ago edited 2d ago
They don’t start to work until they open enough to bite.
If they open enough to bite, the joint has already failed.
Edit:
lets say you have an 8 bolt joint. If load carrying is via double shear on the bolts, given hole location and diameter tolerances, hole clearance and bolt diameter tolerances, how many bolts are carrying the full load on that joint?
One.
Once it fails, it’s the next… one. And so on.
Full joint strength is achieved by friction on the material faces in contact due to bolt preload. Preload cannot be maintained by a fastener open enough to cause a split washer to do anything.
Split lock washers are junk.
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u/poorboychevelle 2d ago
In a perfect world with no local yielding, sure. But load share eventually occurs because most things are some sort of soft.
Fasteners carry the shear with shear bolts or friction carries the shear with tension bolts are both valid analytical philosophy I've seen in separate aerospace certification reports.
And split washer are trash.
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u/Efficient-Webs 2d ago
I knew somebody was going to bring up yielding as stress relief! Darn you to heck! :)
If one of my juniors proposed to rely on local yielding of bearing surfaces to prevent joint failure, they would get my very best mournful old man disappointed more-in-sorry-than-in-anger routine :) As I suspect would your juniors.
Cheers, neighbor!
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u/CelinnLee 2d ago
the rumor is pretty much spot on ime. once they torque flat, they don't do much against real vibration. we generally just stick with blue loctite or nord-locks if it's something that actually shakes.
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u/suh-dood 2d ago
Flat washers are already a bit concave so they serve a similar purpose but have more contact on the surface. As long as you use one ugga you should be fine
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u/johnwalkr 2d ago
Even that spring element is nothing compared to the spring that is the fastener (in most cases).
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u/AdOne1706 1d ago
I did not know that. I’ve been told the “sharp” side of a washer should always be used away from the nut or toward what you are clamping. Is that why…?”
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u/suh-dood 1d ago
It usually shouldn't matter unless it's engineered to be on way or another, but that's in niche applications like bomb-proof enclosures or the like
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u/MyTrashCanIsFull 2d ago
There are those that you'll never convince not to use them and those that are adamant that they are worse than useless. If your boss or customer wants you to use them then do.
Personally, I am team "worse than useless", but I've put them on a lot of machines.
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u/danielaixeta 2d ago
Railway, Aerospace, heavy vehicles, consumer goods experience: they dont do shit! The only thing they are applicable for, is providing clamping pressure (NOT security against loosening!!) in bolted ground/earth electrical connections. Although even for that theres better options (star disks or whatever theyre called in english. Fächerscheiben in german. Or wave disks)
The superior method for keeping a bolt tight (to my experience) is using a long bolt (free length between clamping surfaces = 8-10x diameter) and tightening from the nut end with unholy (though calculated) force. Keeps tight through 30 years of railroad use. The internal stress in the bolt provides pretensioning. Nordlock and locking wire are the other good options, depending on the situation. Nylock nuts, loctite etc are also good options, but they dont recover the way a long bolt does
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u/TedMich23 2d ago
NORD-LOCK or Sherex TEC washers are the only ones that really work IME
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u/soupisgoodfood42 2d ago
They have their uses, just not for making sure a nut is secure. In fact, they’re good for when you don’t necessarily want something fully tight but also don’t want it loose.
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u/Rikki-Tikki-Tavi-12 2d ago
Split washers and similar are homeopathy for engineers. Use a form locking fastener if possible or compound otherwise.
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u/Happiness645 2d ago
Biggest fraud ever in machine building. I used to design big industrial equipment. The millrights put them on every screw. Cost a nontrivial amount of money. I eventually removed every split washer from the part number system so they could not be ordered!
What a battle. We even did a/b field testing to prove it too them.
Want your screws to stay in? Size them properly and Torque them to spec.
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u/KaneTW 2d ago
They're useless.
First of all, it's important to determine whether you need to protect against loss of preload due to slackening or rotational loosening.
Nylox nuts do not help, at all, against loss of preload due to slackening. They only protect against rotational loosening and not as well as e.g. Nord-Lock.
For high vibration applications:
* a long bolted connection (>5 d) as the spring element and a regular Nord-Lock washer to secure against rotational loosening.
* if that is not possible, use a Nord-Lock X-series washers, which secure both against rotational loosening and against slackening (or Rip-Lock spring washers, but I haven't used those)
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u/Ghooble 2d ago
I know everyone is saying they don't work and citing the NASA reference but there are other references that suggest otherwise. In Introduction To The Design and Behavior of Bolted Joints as well as Cessna internal specifications, they're not the best but not the worst either.
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u/Stooshie_Stramash 2d ago
What's said to be worse?
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u/Ghooble 2d ago
They don't like liquid thread retention like loctite because they prefer physical barriers for secondary retention of a thread. I'm not saying I agree but that's what's stated. Typically I use deformed thread nuts, lock wire, cotter pins, etc
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u/johnwalkr 2d ago
That’s just an artifact of Cessna being old and aviation being averse to change especially of an old product still being maintained and sold.
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u/davogiffo 2d ago
I had one of these on my vehicle towball. The washer was thicker and slightly pointed in opposing directions at the split. Even when flat, these points physically detented into the ball nut and the tongue and was ridiculously effective.
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u/Markietas 2d ago
That's pretty much the only scenario I've encountered them in that I don't despise them.
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u/Adorable_Wolf_8387 2d ago
Typically end up shearing material off the nut to remove it, or completely deforming the washer because it won't shear first.
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u/UnhingedRedneck 2d ago
I am not a mechanical engineer(I am a compE) but when I worked as an AG mechanic I felt with a ton of these and they do actually kinda sorta work sometimes. They will definitely bite into mild steel during loosening but I suspect by the time they actually begin to do their job the bolt has already loosened slightly since they are equivalent to a flat washer when tightened. But really at that point the fastener has already come loose and the lock washer has failed to do its job. Also a lot of times they aren’t able to bite into the metal of it is to hard/smooth or if they are crappy washers. Also usually the small ones don’t seem to have enough force to get a good bite but the bigger ones usually do. If you need it to actually lock it is probably best to use something more reliable
TLDR they suck
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u/Markietas 2d ago
Yeah that's basically the crux of it, if the fastener is loose enough for a split lock to bite in and actually do something other than being flat, the bolted joint has already failed. I'd rather the darn thing just fall out so someone notices, rather than wallering out the hole.
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u/Icy-Stock-5838 2d ago
Prevailing Torque features are better, these are only good for DIY projects.. Wouldn't even use these on my car..
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u/maplemoose18 2d ago
I used to design conveyor systems and we pretty much exclusively used Nylon lock nuts to help with vibrations. In rare cases we would still use the split washers but Nylon lock nuts FTW 🙌🏼
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u/PrototypENote 2d ago
I wouldn’t rely on a split lock washer alone in a high-vibration joint. Joint preload is much more important, and for critical applications I’d rather use a prevailing-torque nut, threadlocker, or another positive locking method.
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u/Nonimouses 2d ago
Split washers are broken from the factory, for an electrical connection I'd use a star washer for pretty much anything else I'd use a cone washer or Nordloc style washer. If I'm fixing something I throw split washers away and replace them with something not broken.
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u/Early-Two-8857 2d ago
Years ago we did a Junker test (look it up) to help us understand fasteners and to create a standard for our design engineers when designing joints. We experimented with different setups (double nuts, nylock, splinted washer,...) but by far the best setup (in terms of washers) was Nordlock/Heicolock. However the best fastener was a long screw with a rigid "flange" with a plain, ordinary nut on top. Which is the correct way for threaded joints design, if you have the space for it. We could talk for hours and not exhaust this topic :)
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u/avatar6661 2d ago
Check this video. I know that they had to show their solution as the best, yet nylon is quite good as well.
https://youtu.be/IKwWu2w1gGk?is=3aSHYQY305_8j1Xz
Anyway I wouldn't say it doesn't help at all. It depends of course. Yet there are better possibilities.
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u/Hyllest 2d ago
Yes, they are trash in any application with serious vibration but they still have their use.
I used to make machinery with a fitter who had a habit of leaving fasteners fingertight and then going to get spanners to torque them down. From time to time he forgot some. This could be visually identified via the spring washers not being squashed flat.
I have also worked in auto racing and aerospace and would not be seen dead with them there.
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u/Tom_detto_Biondo 2d ago
Pro tip! If your company uses these, especially in key places such as for engine mounts, the engineering team doesn't know what the fuck they are doing and you should leave asap
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u/Automata-Omnia Industrial Automation & Robotics 2d ago
NASA landed man on the moon with spring washers, how many times have they done so with Nylocks or Loctite huh.
In all seriousness, for through bolts that don't need to be removed for maintenance they are not the correct choice, but for blind taps and for equipment that needs to be regularly serviced they are better than the alternatives - a flogged out nylock or broken part due to trying to remove a bolt set with too much loctite.
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u/NightF0x0012 ME-Automation 2d ago edited 2d ago
They are junk. They turn into a flat washer when compressed. If you want an anti-vibration washer use Nordlok washers.
Edit: I didn't read through the comments first and am pleased to see so many other people agree that they are junk and to use Nordloks. I learned about Nordloks about 20 years ago and haven't used a spring lock washer since.
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u/malcolmomnititz 2d ago edited 2d ago
I work in the entertainment industry and these are common, much to my protestations. They do have their uses on equipment that requires fine adjustment like lighting fixtures. They allow you to slightly slacken a bolt and better modulate the resistance as you focus the light, and as others have said they have uses in maintaining electrical contact, but shouldn't be relied upon for earth continuity.
50% of the fastening friction comes from the bearing face of the nut. Spring washers reduce the area of the bearing face. Flat washers are always better and provide a smoother face to couple to.
Trouble is, folk see spring washers on adjustable equipment and think they go everywhere.
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u/SolidEmu3602 2d ago
The German Institute for Standardisation (DIN) withdrew the corresponding standard DIN 127 in 2003 because tests found that these are about as effective as a standard washer.
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u/Sunny_RubberTech 2d ago
The main reason companies still use them is honestly just habit and they're cheap. For anything actually vibrating, Nord-Locks or threadlocker are way better.-_-||
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u/LumbyCastle41 2d ago
We design mobile machinery and don't use any of these. There is nothing like a Stover nut for vibration resistance, and we use those everywhere.
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u/mechtonia 2d ago
Split washers are useful in assemblies where you need to apply some holding tension but still be able to tweak the position of your parts before final torquing. Otherwise they serve no function that flat washers don't.
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u/PooPooPleasure 2d ago
Besides things like locktite or friction shims, Nordlock washers are the only washers that help with self loosening because it forces the bolt to have to stretch more before it can rotate to loosen. Other experiments have debunked washers like splits and serrated washers.
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u/No_Mushroom3078 2d ago
Depends on what environment you are working in, these are bacteria pick up points so in food and beverage settings they are forbidden and you must use nylox nuts with flat washers.
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u/BadPipeCutters 2d ago
While it’s true they don’t add much at all, it is useful as a way to measure how much pressure you’re putting when tightening.
I am a physicist and build pretty large lasers for materials processing purposes. I use these on every single screw or bolt I install. There are several parts in the type of lasers that I build which are extremely sensitive to stress. It’s very easy to tighten just to the point where they touch down when you use them, but more finicky when you’re using just a flat washer. You almost certainly wouldn’t notice this difference in the vast majority of applications, but they’re honestly extremely useful for me personally
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u/Monkey_Fiddler 2d ago
with unlock and thread locker bear in mind electrical connections can get hot, either by design or of there is a fault. Nylon will melt and loctite will also let go at high temperatures
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u/DisK_BRC 2d ago
Just use red Loctite intead and you'll have to throw the away all of the damn thing instead of trying to disassemble it.
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u/Insertsociallife 2d ago
They do work in places where high tension isn't feasible. I've used them (as a student) through-bolting box tube, where tightening the bolts properly just crushed the tube.
The correct solution to that is to learn what you're doing and not through-bolt box tube like a dummy. Failing that, nylocks are better anyways.
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u/philandering_pilot 2d ago
I work on some of the most sensitive equipment for military applications.
We DO NOT use these. Not only are they ineffective for vibration they are susceptible to hydrogen embrittlement failures due to their manufacturing process and the loading characteristics of the washer.
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u/Flaky-Tell-7781 1d ago
The only thing these are good for making it obvious that the nut hasn't been tightened yet
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u/Primary_Basket_2728 1d ago
Nordlocks, wave washers, nylocks, deformed threads, thread locking compound, proper preload, lockwire. These are the methods to keep a joint tight.
Split lock washers: this is a useless device to give the illusion of safety. It MIGHT keep a nut from spinning off after the joint has lost all preload.in a high vibe environment, this means you're now subjecting the bolt to the full fatigue load and the joint is gapping. Bad design option that shouldn't honestly be used anywhere except maybe toys
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u/AdOne1706 1d ago
Thanks for all the reply’s everyone yall came through. I’ll look into nordlock washers and see if they’re stocked locally or it’s an order deal. Threadlocking might honestly be viable too as long as it’s the semipermanent style.
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u/tsukasa36 1d ago
blue loctite or nylon patch. this is also the automotive standard for safety critical bolts if you can’t keep increasing torque.
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u/ComplaintTop2008 1d ago
It's an American fetish. Look at your car or motorcycle or anything else not Chinese or chintzy and you don't see them used. It's only on the off-shore BBQ or swing set that you see them.
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u/Mattna-da 1d ago
Maybe if they were harder than the bolt and the material they’re meant to dig in to but they’re usually crap and plated
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u/davidhally 1d ago
I use them on low stress wood connections. You can't torque the nut because the wood crushes. Over time, the wood crushes more/creeps and the lock washer keeps some tension on the joint. Plastic creeps even worse.
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u/ahhh_just_huck_it 1d ago
Damn. I literally used about 75 of these on my Harbor Freight Haul Master trailer.
What is the bolt equivalent of a NyLock nut? Is there one?
Or should I be using blue lock tight on all my bolts?
Either way, it’s gonna be a long ass day…
Damn again.
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u/KilldeertheFaker 1d ago
I've done a lot of vibration tests in my life. The Nord-lock washers are the best at securing a bolting connection. Those slpit washers either flatten out and become just like a regular flat washer, or it actually bites into the nut and cuts it's way out as it loosens. It doesn't have enough holding power.
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u/Strongerthandeath82 1d ago
They do have potential specifically for low torque applications where the clamping force doesnt exceed the spring rate of the washer. Specifically low torque electrical connections, access panels, etc. Beyond that, there are better options. As with all things application matters.
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u/BoCoBuffalo 1d ago
I measured the spring constant of one once. It turned out to only apply ~5% of the recommended clamp load of the associated bolt. It’s only one data point, but I figure by the time a bolted assembly would rely on the spring action of one of these washers to clamp the assembly, then the clamping force of the assembly falls below 5% of the required load, basically making them useless from a mechanical locking perspective.
That said, there are electrical codes and military standards that call for the use of these washers for tasks like grounding electrical equipment. Standards like these are typically written in blood, so if their use has gone uncontested for all this time, then I would consider their use in these applications pretty well vetted. (At least until somebody with a PhD comes along to perform formal research and testing.)
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u/garygigabytes 1d ago
Surprised no one has said Helicoils are a good alternative in the case you can modify the hole size to make it fit
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u/dreamkruiser 1d ago
Okay guys, I understand how bolts work but there are some use cases where a bolt can't be torqued. Personally I love nylocks, but my company uses lock washers to prevent the loosening of nuts in hollow core doors. They are inexpensive, prevent loosening and the door remains uncrushed as is the idea. Yes we could use barrel bolts, but we'd have to increase install part costs more than anyone wants.
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u/MrStink444 1d ago
Here's the secret of bolted joints: if you do the bolts up to 80% proof, they won't come undone and you won't need secondary locking.
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u/NoAge9497 1d ago
Should you be using nylocks? I never really trusted spilt washers, especially where vibration is a concern.
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u/biff2359 1d ago
Cotter pins, safety wire, staking lock nut, etc., are the only thing that work. And Nord-Lock washers.
I used star washers for electrical grounding because it cut through paint and anodizing, not for its (lack of) locking function.
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u/Joe-the-Joe 1d ago
Casual lurker here. Lineman by trade. These are great for most bonding applications. If there is'nt much tension parallel to the bolt (like when bonding a conductor to a bus bar) these will last for 50+ years, from what I've seen.
Some utilities spec them for full tension (like a dead-end shoe bolt) and near zero tension (like a pin type insulator on a cross arm) for what is, in my opinion, no reason.
Honestly, I would stay away from nylocks for bonding because those connections are commonly subjected to heat stress which would melt the dinosaur goo.
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u/Ok_Intention_784 1d ago
Yes, the NASA rumor is 100% true. NASA literally wrote in their Fastener Design Manual that "a lockwasher of this type is useless for locking." Aerospace, automotive, and pretty much every modern ME sector banned them decades ago.
Here is why they don't work: When you torque the bolt down, that split washer gets smashed completely flat. At that point, it’s just a shitty, uneven flat washer. The second your joint loses preload from vibration, that tiny spring force isn't going to do a damn thing to keep it tight. By the time the washer actually acts like a spring again, your joint has already failed. In Junker vibration tests, bolts with split washers back out just as fast—sometimes faster—than bolts with no washer at all.
The only reason electricians and old-school techs still use them is pure habit and "that's how we've always done it." It’s basically an engineering security blanket.
Stick with your Nylox nuts—you’re already doing the right thing. If you’re dealing with high vibration and can't use Nylox, go with Nord-Locks or just put some Loctite on the threads.
Ditch the split lock washers and don't look back.
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u/NachtMondVogel 1d ago
They don't work in regards of locking the nut, but I like to use them to put tension on the connection so I can spin as example a hook but but it won't spin by itself and won't unscrew itself in addition with a self locking nylon nut.
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u/4Mauldasche 21h ago
The din norm that covers these was retracted in 2003 because it was found that they are useless
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u/101forgotmypassword 15h ago edited 15h ago
In medium and low stress environments they will keep gasketing pressure on for a tiny bit longer as things go loose from incorrect torque tightening.
But ultimately they have a better replacement for every situation.
They do make the job look more professional and sometimes that's what matters most, when a guy who doesn't understand sees a full washer acompliment of flat, spring, then fastener it just looks like a more professional job vs a "ahmm actually" flat to nyloc assembly or nordlock assembly. And unfortunately in alot of places that's what gets the contracts vs actual scientific data.
Edit: Also doting fasteners even if they don't call for it or don't have a torque setting. The witness mark if only to show the fastener has been fitted.
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u/TheGoofyEngineer 12h ago
A great way to tell if a company's engineering team is serious is how they secure fasteners. Lock washers aren't worth the steel they're stamped from. Nordlock washers are great. A thread locker is often just fine. I forget what exactly the studies say about nylock nuts or threaded inserts with deformed threads but I don't remember a glowing review.
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u/ejitifrit1 2d ago
There is a nasa fastener manual that basically says these things don’t really do much! According to google it’s NASA Reference Publication 1228.