Nuts and Bolts: Keeping it Tight

It’s not much of a stretch to say that without nuts and bolts, the world would fall apart. Bolted connections are everywhere, from the frame of your DIY 3D printer to the lug nuts holding the wheels on your car. Though the penalty for failure is certainly higher in the latter than in the former, self-loosening of nuts and bolts is rarely a good thing. Engineers have come up with dozens of ways to make sure the world doesn’t fall apart, and some work better than others. Let’s explore a few of these methods and find out what works, what doesn’t work, and in the process maybe we’ll learn a little about how these fascinating fasteners work.

What Doesn’t Work

Transverse vibration leads to self-loosening. Source: BoltScience.com

There are plenty of ways for a bolted joint to fail, but vibration-induced self-loosening is perhaps the most insidious. Anyone who has ever pounded on a stuck bolt or used an impact wrench to remove a rusty nut knows that vibration really helps. Put that same joint into service and subject it to the right kind of vibration, and there’s a good chance the connection will self-loosen and cause the joint to fail.

In the 1960s, German engineer Gerhard Junker studied self-loosening and came to the conclusion that transverse vibrations were responsible for the failure of bolted connections. He devised a simple test apparatus that provided rapid transverse vibrations while monitoring fastener preload tension with a load cell. Graphing preload as a function the number of vibratory cycles yielded clues as to the effectiveness of various locking methods. The test became known as the Junker test and as standard DIN 65151, it remains the gold standard for testing self-loosening.

There are some fascinating videos out there showing Junker tests in action, and some are downright scary. Typically, we’ll throw a simple helical spring lock washer on a stud or bolt, torque down the nut nice and snug, and call it a day, feeling like we’ve made a secure joint. But nothing could be further from the truth. In fact, the video below shows that not only do lock washers add very little security to bolted connections, none of the other common methods — plain washers, nylon insert nuts, and stacked nuts — provide much help either.

What Does Work

Jam nut in action. Source: BoltScience.com

Obviously, the video above is aimed at marketing the company’s fancy wedge-locking washers, and it’s pretty clear that they work well. But why do they work when a simple lock washer fails? To answer that, it pays to look at what else works, something that wasn’t tested in the video — a properly installed jam nut.

A jam nut is a low-profile nut, typically about half the height of a standard nut, that’s installed below the larger nut. When the jam nut is installed, it’s tightened only to about a quarter to a half of the full final torque. The thick nut is installed next and torqued to the final value while the jam nut is held in place with a wrench. This effectively pulls the bolt up through the jam nut. The threads of the bolt are then in contact with the top flanks of the threads within the jam nut, while simultaneously contacting the upper or pressure flanks of the top nut. With the top and bottom nuts providing opposing forces on the bolt, the nuts are far less likely to self-loosen.

A similar mechanism is at work in the wedge-locking washers. The two halves of the washer have interlocking wedges, the angle of which exceeds the pitch of the bolt threads. As the bolt is tightened, the higher pitch of the washers pulls the bolt back upwards, providing an opposing force to jam the threads and prevent the fastener from self-loosening.

If we look at all the locking methods that fail, they all have something in common: they all rely on friction. Jam nuts and wedge-lock washers work by providing tension to oppose the transverse vibrations that lead to self-loosening and are therefore much more effective.

Of course, there are other methods of locking threaded fasteners. Adhesive thread lockers come to mind, as do more complicated methods like wired nuts and tabbed washers, and they can be very effective methods. But for low cost and ease of installation, it’s hard to beat a simple jam nut to keep the world from falling apart.

Featured image source: Nord-LockGroup

72 thoughts on “Nuts and Bolts: Keeping it Tight

  1. “It’s not much of a stretch to say that without nuts and bolts, the world would fall apart.”

    Another episode of that soap opera, “As the nut turns” sponsored by Procter and Gamble.

  2. How does the ‘jam-nut’ differ from the double-nut that they tested?
    What difference does making the lower nut half the width make? Given that they coated the threads in grease, I wouldn’t have expected a jam-nut to have performed any differently.

    1. Oh, and in trying to work out an answer myself I see on the ‘jam-nut’ wiki page, it links to: http://www.boltscience.com/pages/twonuts.htm which contains a couple of load graphs from Junker testing with the small nut above and below the main (large) nut, albeit with ‘cycles’ along the bottom rather than ‘time’ that the marketing video does (also we never got to see the installation method for the double-nut).

    2. (Haven’t watched the video)

      Jam-nut: you don’t do it up tight, and you hold it in place when tightening the ‘main’ nut

      Double-nut: the lower nut is allowed to turn when you apply the upper nut, so both nuts are (probably) engaged on the lower face of their threads.

  3. If I understood this well, not enough friction is the cause of loosening problem. How about pouring abrasive dust into threads just before final tightening? It would require replacing of nut and bolt after each unlocking, but at least it would be easier and faster to remove them then to remove e.g. a rivet.

      1. This actually works quite well, unless the thing is torqued to F-tight you can easily get enough torque on it to undo it.

        You could even put a spot of weld on the join, just grind it back if you need to undo it.

    1. Aside from the abrasive getting into things other than your fastener junction?

      Depending on the abrasive, it may crush under the pressure/vibration, having abraded the fastener surfaces when installing. The combination would probably make the fastener actually loosen sooner I suspect.

    2. There are already products specifically made for this purpose. Is called thread locker and is a kind of glue that becomes hard in absence of air, when you buy a bottle of it, it is always half empty so the air inside keep the glue in a liquid state.

    1. …Or a drop of ScrewGrab, the stuff you use when you have a bolt or screw with a stripped-out slot. It appears to be micro crushed particles of tungsten carbide in a suspension liquid and it bites between the screwdriver tip and the slot . It’s the closest thing to magic in a tube, having saved me a number of times. No connection to the manufacturer but just a fan of the product, I completely recommend it to anyone. I bought a small tube 15 years ago and it will probably last my lifetime.

  4. This is a false generalization, as it tests a case where the two bolted surfaces are free to slide against each other. This is an incorrect design, as the bolt itself is never supposed to carry the transverse load – it’s merely supposed to press the two pieces together.

    In a properly designed bolted joint, the tension on the bolt is enough to hold any forces that might cause the two surfaces to slide and the joint to move, so the transverse vibration depicted in the little gif animation simply will not happen. If it does, you fire the engineer who designed the joint, or the technician who installed it and forgot to tighten it properly.

    In some structures this kind of loosening cannot be avoided, as the materials flex and there may be significant deformations, such as in car brake discs for example. In those cases, you drill a hole through the bolt and put a wire or a pin through it to make sure it doesn’t unwind.

    1. Furthermore:

      -spring washers or wave washers are almost always completely useless. When you tighten it completely, the spring goes flat and it acts just like a regular washer. They’re meant to be used for mechanisms where e.g. the bolt forms the pin of a hinge, so the spring washer provides some light tension to it.

      -washers in general are detrimental, because instead of one friction surface you now have two that may move. They’re used if the material to be bolted is too soft to bear the tension in the bolt, so the washer spreads the load over a larger surface area. If the material is not soft, no washer should be used.

      -Washers are also used when the two pieces to be joined are not thick enough. The material of the bolt acts like a tensioned spring, and if the spring is too short then it’s too easy to over/under-tighten it, or it may loose tension when the clamped materials deform under pressure because it’s only stretched by a few micrometers, so the washer gives it a bit of extra length and extra travel. If the pieces to be joined are not thin, no washer should be used.

      1. I am familiar with riveting for aircraft and the clamping provided by the rivet is the important part. A proper rivet job will not have any transverse forces on the rivet. Likewise bolts are really very strong springs and tightening them is all about treating them properly as springs. For example, over torquing head bolts can exceed their elastic limits and they are no longer springs and must be replaced.

        However, I don’t follow that part about washers; maybe because the real does not behave like the ideal. In the ideal, friction force has no mention of surface area. Friction is just normal force times coefficient of friction. A nut and a washer will have the same friction as a nut alone. The friction between the part being fastened and a washer is the same as between the washer and the nut, which in turn is the same as between a nut alone and the part. It does not matter how many washers you stack. What is happening to keep this from being true in the real world?

        1. For the friction, there’s mechanical friction which is the uneven surfaces locking against each other, and there’s chemical friction or more properly adhesion which works mainly by electrostatic forces, and then there’s surface deformation which has to do with the mechanical friction. When the same force is spread over a larger area, the surfaces where the hills and valleys touch are deformed less and the friction of the joint becomes less, so it’s easier for it to slide.

          You can think of it like a kids’ plastic sled on snow. If the sled is small, the kid sitting on it will cause it to sink into the snow and it won’t slide very much, but if the sled is long and wide, it won’t break the surface and it’s easy to slide. Push it and it keeps going, whereas in the first case you push it and as soon as you let go, it stops.

          But counterintuitively, racing cars use fat tires -because- they offer more surface area, because the hot rubber against the road is adhesive and offers more grip, especially when the tire starts to skid sideways. Skinny wheels have almost no dynamic traction sideways, so those old cigar shaped racers with motorcycle wheels were real deathtraps – slip a bit and it’s out.

          So the highschool version of friction is not entirely accurate. It works at moderate surface pressures, but when you’re dealing with nuts and bolts that will be tightened near the elastic limit, they will bite into each other and the simple description of friction no longer applies.

          1. I would only say that if the skinny tires are made of the same material, they have the same friction as the fat ones, but the surface contact area is so small that the forces can more easily exceed the strength of the tire material and the rubber strips off in a skid. More square inches in contact means less force on the material per square inch, and hopefully, no tearing. Plus modern race cars have high down force that greatly increases the friction.

            The normal force times coefficient of friction is why rock climbing shoes work as well on a little “chicken head” as on a nice flat spot. That is, unless the bit of rock is so small that the forces will tear the material of the sole.

            From your description of bolts and washers, I don’t see why a stack of washers will be any different from a single washer or a nut with the same surface area as a washer.

        2. “over torquing head bolts can exceed their elastic limits and they are no longer springs ”

          That’s not entirely accurate. They are still springs, but they’ll have permanent elongation and going over the elastic limit will cause work hardening which changes the properties of the steel, so you now got one bolt that’s different from the rest by an unkown amount and the head can warp as it gets hot.

          Sometimes bolts are deliberately overtensioned, I forget why exactly.

          1. That is actually why you are supposed to change your head-bolts when you remove them to work on the heads. The torque specifications they give “stretch” them enough that they are no longer to spec.

          2. They sometimes are overtightened because that is the most force they can take, any more and you’re just making them longer and narrower ;-)

        3. Caution: Useful simplifications ahead.

          Putting a fastener into the plastic range mainly changes the unloaded length, it doesn’t change the spring rate, that is, it doesn’t change the amount the fastener will shorten when the load is relieved. What confuses this is that the material tensile testing that is done is based on an original length, so that when the material begins to yield the slope of the stress/strain curve starts to flatten out. Early in the process and before the fastener necks down significantly, releasing the tension would show a straight line relation between stress and strain that is parallel to the original elastic curve.

          The inherent springiness of metal is due to atoms in crystals changing their distances while retaining their bonds. As long as there are crystals, metal will be elastic. What happens in plastic deformation (of metals) is the atoms in the crystals start shifting to neighbors. They are still attached, just to new locations.

          In particular for head bolts are the torque to yield installed ones.

          One tightening method measures turn vs torque for a continuously tightened fastener and detects the change in turn/torque as a sign the fastener is beginning to yield, which means it is loaded to the maximum load it can handle. This is a valuable method because it doesn’t matter what the friction is and it doesn’t leave any tension capacity unused. Since the elasticity is a bulk material property, the yield strength can be tested by sampling, and the diameter can be controlled, this leaves the joint with a very closely and uniformly controlled fastener tension. Often these fasteners will have a precision ground diameter where the plastic deformation will occur.

          There are some cases where the installer skips the torque vs turn equipment and generates instructions that are guaranteed to plastically deform the fastener. This is a cruder method, but it gets the same end result, except it doesn’t minimize neck-down.The instructions will be to torque to some moderate value and then put a certain number turns, which will be enough turns to yield the bolt.

          As mentioned, this is all great but the fastener necks down increasingly, each time the method is used. Depending on the sensitivity of the turn vs torque measurement and especially in the torque and turn-count method, this can be a significant amount; hence not reusing them.

          Washer stacking can be a problem if the fastener does not apply enough load to force the washer faces into intimate contact. If there are gaps there is the chance for movement and movement will be in a direction that eliminates pre-load. The more washers, the more gaps one can have. A single thick spacer is a better choice.

          There have been some great advances in understanding how friction works. The best theory is this – that every surface has small protrusions called asperities and they bump against other surfaces. It used to be thought that they mechanically interlocked, but then why would friction levels vary? The current theory, one that fits with experiment, is that they act more like tuning forks, plucking each other to cause them to vibrate. Higher friction is due to engaging and causing larger vibration of the asperities. The friction is the energy lost to heat as the material dampens the vibrations to a stop.

          Lower contact areas means that those areas that are in contact have much higher engagement, which gives the appearance that friction is dependent only on normal force. For ordinary use this simplification is true.

          This only works to the point where the surfaces are so close together they can share electrons, and the Van der Waals forces take over and the surfaces begin to bond – seen in wringing gage blocks together and in gecko toe hairs on walls and windows. At higher levels galling occurs along with material transfer.

          Otherwise this friction theory fits well enough – polishing tends to remove the longer asperities which would take a lot of energy to vibrate and polished surfaces tend towards lower friction. Harder materials have stiffer asperities, so they don’t deform as much and don’t store much energy and they also have lower friction. Rubber stretches and can store a lot of energy – so lots of friction. Graphite is weak across layers and stiff within layers – so not much friction. (Did I mention this was greatly simplified? Just a reminder.)

          1. Nice. I thought they lost some temper or young’s modulus changed or something like that when they yielded. I had students who often twisted the heads off class 8 bolts used to compress chemicals into disks for an IR spectrometer. I must have equated it to what happens when you start to strip threads.

            Still can’t picture the stacking problem. The force will be the same on all the interfaces, and equal to force on a single washer. It isn’t like the force gets divided up among the washers, otherwise it would be zero on the top washer and max on the bottom. Testing with several hands in a vice will show this is not true.

          2. Each washer has an uneven surface, which creates a small gap. When you tighten two washers together, they don’t fit perfectly and vibration tends to let them stick and slip around.

    2. The reason why the bolted joint must be designed tight enough to hold the pieces together by friction, is because if the joint is designed “loose” such that the pieces may slide, it’s subject to fretting where the pieces rub against each other under load and that very quickly leads to the surfaces forming cracks and soon the joint fails by fatigue even if the bolt is locked in place and cannot move.

      It’s possible and sometimes permissible to design a joint that keeps together by shearing forces on the bolt itself, but it’s all kinds of stupid to do so under dynamic loads. If you think you might need locking washers or jam nuts against the bolt getting undone, go back to the drawing board and design a better joint.

    3. This, the bolts are just for maintaining clamping, if there’s likely to be lateral movement, spigots, dowels, hollow dowels, keying features etc should be taking the load from this.

  5. I always feel a bit uneasy when bolting the safety ground cable to a amplifier chassis. Can you recommend the safest way to install a bolt to single sheet metal that is going to vibrate without it coming loose?

    1. For higher production consider PEM fasteners. They crimp into the sheet metal in a reliable way, but do require an appropriate installation tool, making them difficult for a single use. I would also fix the cable to prevent the loads from vibration allowing them to be applied to the joint.

    1. Are they fully engaged with the thread? In the video, it looked suspect to me, like the bolt was a little short so the nylon insert wasn’t gripping the major diameter fully. They’re also supposedly only good for a couple of bolting cycles as the nylon gets deformed so they’re not great for frequent unfastening.

  6. Pretty much the definitive (readable ) treatise on the matter of fasteners is Carol Smith’s fourth book, following, Tune to Win, Drive to Win, Prepare to Win, the aptly names “Nuts, Bolts, Fasteners and Plumbing Handbook”. It will tell you more than you ever wanted to know about fasteners. And scare the pants off of you when you see how things are maintained in the Real World.

    Carol wanted to title it “Screw to Win” but the publisher balked. If you bought a copy from Carol, it came with a sticker in the correct style, etc, to put on the cover.

    For those who don’t know Carol, he was an incredible aerospace and automotive engineer who won in every form of automotive racing. Truely an amazing guy. I was lucky enough to spend a few hours with him and the other Carol (Shelby) many years ago. Incredible time I will never forget.

    1. “Engineer to Win” is a favorite of mine. It has great sections on bolting and metallurgy, and is just a joy to read in general. I’m jealous that you got to hang out with both Carols!

  7. nord locks are a godsend on things which move due to thermal cycling, i.e. turbocharger / manifold joins .. outside of that a correctly engineered joint that has no movement will be held firm with a correctly torqued and specked plain ol’ nut.. I like a nylock or pinned nut as if the fastener comes a bit losse itll rattle / performance drop etc but not come completely off its mounting so while, say, a suspension arm may rattle about, it wont completely fall off. the mylon part (or pinning etc) is like a second level of safety. my background is purely automotive so forgive my ignorance outside that.

  8. For all intense purposes all this nord lock does it prevent the vibration of the nut off the bolt when it is clamped or bolted together like lets say tire nuts to the wheel(for simplification)… correct me if iam wrong(since that is what they demonstrated and what they said you cannot correct me)….but all this does is make sure the nut doesnt vibrate off.
    And at a point in the video they say …and iam confused on this….”Adhesives are seen as ‘safe’, how ever repetitive tests their performance can vary significantly in maintaining clamp load…these solutions depend heavily on the operator”… How can Lock Tite, Lock Tight… how ever you want to spell it be any different than the Nord Lock? Frankly they have different strengths from light to permanent(I have had to use a torch to heat up a bolt using the permanent one broke 2 wrenches without it) LT i dont ee a problem with heat doesnt affect it, used it on my engine 300-500deg, and the torch i had to get the bolt to well over 1k to even get it a smidgen to loosen at all. I figure the Lss perm one will be fine. And yet no one tested these.
    The LT would be cheaper, you can use it ore times from a bottle or tube, cost effective in that is is cheaper to use than a nord lock due to I bet you dimes to dollars, that you only get a packet of 3-10 for about what 5-10$? So you can only use them once? From the looks of it they werent hardened they got squished the bottom and top threads looks warped or at least flattened, they would hold maybe 3 times tops if lucky but i guess only 2 times max, but they seem to be a 1 times use over all. So then if you have to replace them every time(or worry as well if used a 2nd)…then why use them at all and not LT when you have to reuse it ever time anyways when you remove a bolt/nut. Yeh You can prob buy them in bulk, but even then I use 1-3 drops of LT on my bolts all over my truck, from the engine to my wheels, and I never have needed to ever redo them ever.

    So all in all, how can LT reduce clam load if it doesnt move at all?
    The bolt as someone said above does stretch, over time, which is physics…You keep either way using the NL or LT doesnt matter it will stretch period…. but this has NOTHING to do with vibration…vibration, and stretch are 2 diff things.

    As for the “depend heavily on the operator” WTH? Arent you supposed to rely on your ‘operator’ or ‘technician’ since that is their job?
    (Generalization) So you are going to hire an idiot to do your bolting who is not trained in remember doing things like putting LT on a bolt. That is why we have technicians to remember and trust them…I trust a 100KV electrical wire above my head is not going to fall on me and electrocute me cause I put my trust in the city to hire somone who knows what they are doing and to at least put LT on a bolt so it doesnt fall on my head.
    What they are saying if i get it right, we should NOW be hiring idiots now untrained people to install things and dont know their touccas from a hole in th ground to install things all over the place with these NL things cause and/or PEOPLE ARE TOO STUPID TO THEIR JOB! Is what I am getting out of it really.

    1. Psst, the phrase is “For all intents and purposes”

      However, yes, their video has lots of holes in that easily fall down to scrutiny and, like a lot of things, if they’re badly installed then pretty much anything will fail so a lot is reliant on the person doing their job with a modicum of skill and care.

      1. Yeh sorry…I was just so infuriated… didnt check or even proofread my post. Thank you.

        Least i am not totally nuts then thinking like this. I thought I had lost it somewhere(mentally), or just lost the meaning of what the thing was period.

  9. > The threads of the bolt are then in contact with the top flanks of the threads within the jam nut, while simultaneously contacting the upper or pressure flanks of the top nut.

    This is quite confusing – is this the standard terminology?

    The “top” flank is the opposite of the “upper” ?!

    Or maybe a typo, and it should read “lower, or pressure,” ?

    Interesting write-up, I’m constantly being asked to put star or split washers on stuff, but since I’m the electrical guy I can’t be bothered arguing the point…

  10. The video is very interesting and also the explanation of the new invention but in the comparison they did not include a toothed washer that, for what I understand, should work on the same principle.

    1. And racing. P.S. if you’re looking to purchase a used sports car or motorcycle, look for small holes drilled in the engine and gearbox sump drain plugs, and nearby fins or other parts. Racing rules require a wire through these holes to prevent the drain plugs vibrating loose and dumping a sumpful of oil onto the track. Ergo, if your prospective purchase has these small holes, there’s a good chance it’s been raced at some point.

      1. Now, if it has been raced is that a good thing or a bad thing? If the seller doesn’t mention that, then the engine has probably been abused, yes?
        But then again, most racers are pretty good to their machines.

  11. I’m amazed there are this many comments… and no link to NASA’s fastener guide.

    The helical spring washers are called out explicitly there, on page 9. “In summary, a lockwasher of this type is useless for locking.” The nylon insert nut, though, is fine, but it only will retain ~20% of initial clamp torque, which you see on the video above. However they can only be used once.

    Serrated (tooth) washers work great for locking, but can damage surfaces.

    Jam nuts are called out in the NASA guidelines as being to unreliable, which isn’t surprising because they’re far too easy to screw up.

      1. That would possibly have saved tens of thousands of dollars at my jobsite. What went bad was several fractured bolts, and the whole machine started to destroy itself. No, I will not go into any more detail than that.

        It would have to become SOP to examine bolts everyday for indication. Is there an even more visible version of these, for bolts waaaay up high near the top of a machine?

        1. Many years ago I saw a product that caused cracked bolts to “bleed”. The bolt had a small tube drilled into it which was filled with red liquid. When the bolt cracked, the dye would leak out and be very obvious. The example given was aircraft landing gear, and it worked quite well. I don’t know if it ever went anywhere (it would have been very pricey), and there may be something similar for loosening bolts (the dye weeps back along the loosened thread).

  12. Someone mentioned the inconvenience of a nut like this only being good for a few uses. I would imagine that in a field built ul listed assembly that used these for their listing that they would be one hit wonders. If you took it apart you would apply a new washer. As noted the ridges were flattened, thus the characteristics of the washer changed and they would not work the same way again.
    But that’s not really all that uncommon of an issue for those who are required to do things according to instructions and inspections and codes.
    Aircraft mechanics spend a lot of time rewiring bolts after overhauls
    Mechanics don’t reuse oil out of a vehicle they got out of an oil change (unless their hack and dump it on their driveway)
    Wire nuts are listed as a one use product
    Pv panel installers use one use only weebs for binding panels to racking assemblies
    An icbm is not intended for reuse
    Once I eat pie I wont reuse it either

    1. “Mechanics don’t reuse oil out of a vehicle they got out of an oil change”

      But they do – they just send it out somewhere else to be filtered and cleaned, and then buy it back by the barrel.

  13. Someone mentioned the inconvenience of a nut like this only being good for a few uses. I would imagine that in a field built ul listed assembly that used these for their listing that they would be one hit wonders. If you took it apart you would apply a new washer. As noted the ridges were flattened, thus the characteristics of the washer changed and they would not work the same way again.
    But that’s not really all that uncommon of an issue for those who are required to do things according to instructions and inspections and codes.
    Aircraft mechanics spend a lot of time rewiring bolts after overhauls
    Mechanics don’t reuse oil out of a vehicle they got out of an oil change (unless their hack and dump it on their driveway)
    Wire nuts are listed as a one use product
    Pv panel installers use one use only weebs for binding panels to racking assemblies
    An icbm is not intended for reuse
    Once I eat pie I wont reuse it either

  14. A trick I’ve used a time or two is to make flat washers out of coarse sandpaper. Make two, put ’em together rough side out with a spot of glue in between.Wonder how that would hold up in the Junker test.

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