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Comment by blamazon | Hacker News Reader

I’m a mechanical engineer - smartbolts are cool but rarely used in my experience. Much more commonly used are DTI washers: https://en.m.wikipedia.org/wiki/Direct_tension_indicator

Due to the washer style can be used with a wide range of fasteners. The most common type, in my experience, due to the ease of use, has a little bit of colored goo inside the arched cells. The goo squirts out when the specified torque is reached.

That being said, there are a lot of approaches to this same problem. Another common technique is breakaway bolts that are driven with a piece that snaps away when a certain torque is reached: https://youtu.be/lrCoi3gaLfU

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Smartbolts seem to be useful in applications where there is an expectation the bolts will loosen over time/the maintenance lifetime, reducing/easing inspection time.

Similar to the tell-tales used on truck lug nuts: https://www.dealsonwheels.co.nz/trucks/features/1302/new-tyr...

Installing to a known torque and painting a line on the fastener is a lot less expensive, which becomes important when your project has potentially tens of thousands of key fasteners, like a bridge.

But for something like an MRI machine where there’s fewer fasteners and higher stakes - would make a lot of sense to use Smartbolt I think.

Tangential: related to trucks, there is a somewhat similar product to easily monitor tire pressure visually, which is popular: https://www.linkmfg.com/products/specialty-products/cats-eye

A problem with installation is that torque is only a proxy for tension. Torque alone is not incredibly accurate, so there are a lot of cases that could benefit from an easier way to accurately indicate tension than by externally measuring the stretch of the bolt (which often is not an option)

True, but this caveat is generally pre-empted in design phase through safety factor principles - multiply certain design parameters by a coefficient greater than 1 to account for the inaccuracy in things like torque-tension correlation.

I’m not getting your point here, won’t there always be inherent trade offs?

If you scale the specified torque, you risk over-tensioning.

If you scale the dimensions of the fastener, that has lots of consequences.

Is there some safety factor here you can add “for free” that I’m not seeing?

Otherwise, why is it that you “generally” pre-empt this caveat with safety factor versus coming up with a more reliable way to achieve the right tension?

The general idea is that proper bolt installation is a solved problem…at least in so far as ordinary industrial/commercial/construction applications are concerned. There’s an informed experienced trade base, a well established supply chain, and sound engineering practices already.

To put it another way, the right tension includes a safety factor. And engineering practice specifies testing and inspection and proven materials.

That safety factor and testing and inspection has costs associated with it.

Basically, it is a "solved problem" by designing things stronger than they have to be, and spending more on testing and inspection than necessary.

I'm sure the cases where these things are economical are fairly few, at least today, but if everyone took your attitude no progress would happen at all.

Safety factors; testing; and inspection are engineering.

The Smartbolt doesn't eliminate engineering.

More importantly, safety factors; testing; and inspection are progress. We have robust assemblies built first time based on rational analysis and formal processes. We have sky scrapers, suspension bridges, and space telescopes.

Obviously it doesn't eliminate all engineering. What it could do, though, is reduce costs. The idea is you could potentially achieve the same level of safety with less material or less testing.

Those things are progress in a sense, sure, but...

It's as if someone finds a way to make an alloy less prone to metal fatigue, and you just say "but you can use thicker metal, and test more." Sure, but.... well I don't see the point explaining this further to you, but I assume people reading this that don't see everything in black and white already understand.

Have you ever taken direct personal legal liability for structural fasteners as a licensed professional?

I have.

Does that give you the ability to have nuanced, big-picture, long-term view thinking?

You might as well argue that someone who works in a bank must understand macro-economics, or that someone in the military must have an excellent grasp of foreign policy.

You are steeped in the status quo. Your thinking on this is expected.

There's a fantastic set of two textbooks covering bolted joint design by John H. Bickford. One for gasketed assemblies and another for non-gasketed assemblies, if you want to really _really_ dive deep into it.

At the end of the day you get the final clamping force by multiplying a bunch of modification factors together that have been empirically measured and the bolting procedure has been verified. When you step outside any of the parameters you've verified in the past, you have to substantiate the math with experimental proof. The size of the fastener, fastener style (e.g., stud vs. bolt), fastener length, thread pitch, torquing method, presence of a washer, presence and type of lubricant, fastener material, base part material, and bolt arrangement ALL play into the final bolt assembly procedure. A lot of these are well-known and published.

Nobody is out there on assemblies like this just slinging unsubstantiated numbers around, at least if they're doing their due diligence.

Even where you can't measure the stretch directly, torque-plus-angle is a better way to achieve consistent stretch than torque alone.

In case anyone else needs to look this up, torque-plus-angle means seating the threaded fastener to a base torque, and achieving the target tension from there by further turning the fastener a specified angle. So super duper simple and something that many of us have done instinctively or from instructions, now we know there is a term for this general technique. Cf https://dannysengineportal.com/torque-specification-torque-t...

So "tighten till you hear a crack, then back a quarter turn" has merit after all?

Yes, though instead of the bolt or part cracking it's the click of the torque wrench, and you're going a quarter turn extra not back.

So really the exact opposite.

Well often nowadays it is torque+angle:

https://www.srtorque.com/error-proofing-tools/digital-torque...

This is a common problem is aerospace with obvious potential for bad consequences. The general approach is safety wiring.[1]

[1] https://en.m.wikipedia.org/wiki/Safety_wire

Yeah, a quality torque wrench isn't cheap (particularly the kind you find in fixed installations like auto repair shops and factories) but there's got to be some crossover when you are buying enough bolts that you're better spending money on tools that are more idiot proof.

I install automated and semi-automated equipment in factories, sometimes including threaded fasteners, and the overwhelming majority of QC issues I observe (both on the devices being manufactured and on the mechanical equipment I'm delivering and writing code for) are not inaccuracies in torque or clamping load, but instead, failure to torque a bolt at all. The cycle was started but someone hit the E-stop mid cycle, and the fasteners were barely finger tight but you can't tell by looking at it.

A torque wrench is better than a Smartbolt once it's put on the fastener, but the Smartbolt can be measured just by looking at it.

An HN company does a similar thing with RFID tags for grease zerks:

https://greaseboss.com.au/

Again, it's not so much about measuring that the precisely right amount of grease was dispensed, but that every one of the 37 grease points was at least near the tip of the gun once every 30 days for the life of the equipment; the operator didn't forget about that one zerk that's hard to see and grease only 36 zerks for 5 years...

A problem with many torque wrenches is that they click (or beep) when the set torque is reached, but they do not prevent tightening beyond that, nor do they have any way to indicate how much overtightened the fastener might be. It seems to me that these bolts have the same issue.

Ever wonder why the tire shop has tightened your wheel lug nuts so tight that you can't get them off with the hand wrench when you need to change a tire? They spin them on with a pneumatic impact wrench, and then "check" the torque with a torque wrench. Of course it clicks, because the nuts are already way too tight.

Powered torque wrenches like Milwaulkee's (designed for electricians) and assembly line fastener drivers absolutely stop at the proper torque.

If you think dealership/tire shop/corner shop garage techs touch a torque wrench for lug nuts, you're dreaming. They blast each nut on with the gun and walk away.

Techs never are incentivized to get everything done as fast as possible because they're paid by book rate, and there's zero disincentive for not doing it properly.

Torque sticks were an attempt to address the problem but most techs can't even be bothered with those.

Proper torque for lug nuts is multi-step anyway. Star pattern partial tightening, with the wheel placed correctly on the hub if it doesn't have a concentric ring - then full tightening, then re-checking after the vehicle has been driven for a few minutes, because of fretting between the wheel, rotor, and hub surfaces...even if you've carefully prepped the surfaces, which again, no automotive tech does.

That explains a lot. Had a tire shop tighten the bolts on my old car so extremely tight that the threading on the bolts were broken. Every time I changed the tires after that I had to use a wrench with a looong bar attached to loosen the bolts (In Norwegian we call this a “latmannsarm which directly translates to lazy man arm, not sure what the English word would be). No matter how much grease or how much care I took to not tighten too hard the bolts were stuck next season. Glad the car is sold and not my problem anymore.

(I could of course have bought new bolts, but I always forgot about it after spending 2 hours changing the tires …)

in English we call that a Cheater bar, so it seems like that has carried across pretty well :)

https://en.wikipedia.org/wiki/Cheater_bar

Not mentioned on that page, but you can make one out of two ordinary wrenches, which is both practical and may impress your friends! All I found online describing this was a video: https://youtu.be/5DShRItYxLA

AKA Cheater Pipe in Texas.

A cheater bar or breaker bar.

Slipper torque wrenches prevent over tightening

Adding on to that, tensioning bolts using torque is convenient and inexpensive but not very accurate at all. Some data reproduced in Shigley's Mechanical Engineering Design showed a +/- 30% variation in tension for properly torqued bolts, if my memory is correct.

Emphasis on properly torqued: in the field threads or bolt heads might be dirty, or torque wrenches might be out of calibration, adding even more uncertainty.

Torquing to a known torque and painting marks is fine...except there's no guarantee the clamp load hasn't changed due to all the mating surfaces wearing.

There's also too many variables in the real world if the application is critical enough. Clamp load changes with how many times the fasteners have been assembled due to the threads getting polished, how clean and dry the fasteners are, etc.

> painting a line

what if the bolt doesn't actually turn any, but the material itself stretches and loses tension?

If you look at a tire from the side, you can precisely see the bulge of the tire, which I do all the time to check the pressure.

This isn't generally true. Run flat tires have stiffer sidewalls and don't bulge.

Obviously, I don't have run flat tires. My tires bulge when the pressure is low.

I need these in my garage door. I wonder how much more expensive they are.

Edit: way too expensive.

https://www.mcmaster.com/smart-bolts/

$26+ each? Now I am curious which applications it does make sense to use these in.

Ah; so that's what those are for. I've seen them on lorry wheels, but never got around to formulating a sensible query for a search engine. These becoming commonplace seems like a recent thing (as in last decade or so).

Except that black == good == red + dirt/grime/grease.

I'm not an ME or anything, but it would seem washers attest to pressure (tension) at some point in time (when the nut was tightened -unless the washer is "springy", which I doubt as it seems it simply yields at some point), whereas smartbolts attest to the tension when you look at them --so during maintenance and not only during installation, so from that PoV, they are more useful (informative).

That’s true, but you can have a qualified inspector mark bolts with goo with a paint line that straddles to the fastening surface. Then you can just inspect the paint line for movement, which gets you most of the same functionality at significantly lower cost.

Of course, that requires a trustworthy inspector, which is one application where smartbolts become useful, if you don’t have a trustworthy inspector available at install time. But many projects that have importantly torqued fasteners (bridges, buildings, rollercoasters) have certified inspectors anyway… which is why smartbolts are a niche product I think.

Speaking to the trust problem - if you trust the operator of the torquing install device, you don’t need fancy torque indication at all, just mark the paint line at install. That is the most cost effective.

That line is fine if the bolt winding itself loose is your only failure scenario, but some might have others as well. What of they clamp a part that can subtly deform in certain exceptional situations? Then you could have a bolt with zero tension that is still perfectly aligned with the marker line from when it was properly torqued.

What I find somewhat surprising is that they make the "ok" indication black. The cry for attention state in red seems nicely intuitive at first glance, but given the market they serve they should be far more concerned with false negatives than with false positives and I imagine that it would be very easy for dirt or bad lighting to make a red indicator remain unnoticed in an inspection. Perhaps the color choice is an inherent property of the chemistry involved?

It's not chemical. It's mechanical. It basically covers the red with a black liquid when it's good. A different color probably wouldn't cover the red as well. The bolt is basically hollow to do this.

The paint line is the most common method I have seen. I was an inspector and for fasteners in a critical load path we would use an ultrasound machine to measure the length of the bolt (these were installed in flange like connections). We would measure them when first installed and compare measurements after that piece of equipment was used.

For really critical components we would disassemble them after every use and would perform multiple tests on each piece including fasteners.

Totally agree. And two cents to add. The so called smart bolt adds movable parts in side a part supposed to be inertia and at the same time might compromise the strength in comparison with ones with the same profile and body material. And when faulty, it actually more dangerous than ordinary ones: now your torque wrench and the bolt gives different readings, which one to trust?

There's no moving parts. The bolt is hollow with a pin projecting from the tip of the bolt back to the head. When put under tension, the outside of the Bolt stretches, but the inside pin doesn't. It's not as strong as a solid bolt of the same size and material. But that's something you engineer around.

Yes, that seems like a cheaper option; however, I think when you have constant stresses over time, bolts can stretch or simply parts can wear over time and a paint score or even a frozen nut won't reveal lessened tension. Though maybe in this case the issue would have become a concern way before the bolts start to indicate a problem?

Interestingly that has kind of moved upstream to a design phase consideration in modern times due to the incredible depth of understanding we have about materials. Structures can be designed such that their expected loading will not stress fasteners beyond their ‘yield strength’, which is well characterized.

Fatigue complicates the analysis, but we understand that too nowadays, although that is an understanding forged by many unfortunate consequences, such as: https://en.m.wikipedia.org/wiki/Aloha_Airlines_Flight_243

Maybe the following has been designed out; however, I've on occasion seen the following wear pattern which compromises the initial strength of a fastener (usually due to friction over time):

                 _
    _    _______| |
  _| |__/         |
 |        ______  |
 |_  ____/      |_|
   |_|

First, I love the plain text diagram! (Edit: not fatigue) This failure should be predictable during the design phase and that fastener is either wrongly specified or should have a recommended number of cycles after which it should be inspected and replaced if found to be out of specification.

Of course, many items in our built world do not receive the same level of design scrutiny as I would wish, nor the maintenance regime…

Unless I'm misunderstanding the diagram, it does not look like fatigue at all but rather yielding from shear loading.

The definition of fatigue is a small crack (in fact microscopic in the beginning) that grows through the application of cyclical loading (typically about 10k cycles is the lower bound for high cycle fatigue, which does not involve macro yielding).

Thus a fatigue failure would look like a fully ruptured part , usually with two distinct zones of the fracture face: one smooth zone from crack growth ( sometimes with characteristic "beach marks") and the final sudden rupture which is usually rough.

https://en.m.wikipedia.org/wiki/Fatigue_(material)

What I was trying to convey, which I didn’t do well in the diagram, was you have a bolt holding two plates together or a splice, and you have wear on the bolt shank -not necessarily shearing.

You are correct, I had a brain fart.

Nice video of the colored goo in action: https://youtu.be/5BMQE5XnKww?t=372

Breakaway bolts are discussed/demonstrated a few seconds earlier.

Fireball Tool is a great YouTube channel! I recommend it for any hackers interested in fabrication. The content is very approachable.

Release the schmoo!

That is pretty neat. The Wikipedia page on DTI does a pretty poor job of describing how they work, so thank you for the visual.

Thanks for the link, looks like a great channel!

Out of curiosity, on a somewhat related topic, have you tried nord-lock washers [1] in any of your applications? We looked at them for a small work boat we designed and built, but they were a bit pricey and it was just a prototype, so we ended up just using a dab of lock tight to avoid vibration. Was always curious to find someone who had practical experience with these things. I may use them on an upcoming autonomous boat project where there's nobody around to do rounds and tighten loose bolts that have shaken loose unexpectedly.

1. https://www.nord-lock.com/nord-lock/products/washers/

When I worked on railway vehicles (bogies for passengers vehicles, more accurately), Nord locks (or equivalent clones) were standard on all structural joints.

Especially on a prototype I'd recommend them. Prototypes are really expensive, a complete supply of Nord locks for your project is probably less than a single one-off CNC part.

Note that Nord locks are specced to be single use. Not that I've never reused some on prototypes, but, well, I'd never do it on a life-critical joint that is meant to be taken apart often.

> Nord locks (or equivalent clones) were standard on all structural joints.

> Especially on a prototype I'd recommend them.

> I'd never do it on a life-critical joint that is meant to be taken apart often.

Very good/helpful feedback. This thread is giving me much more confidence to make the extra upfront investment. Thanks!

I have used Nord-lock washers, they are demonstrated to be one of, if not the most effective thread locking washer devices under heavy vibration, but as you say they are quite pricey. I would certainly recommend them if you have the time/cost budget, especially for something like an autonomous boat where vibration is expected and routine inspection is challenging or impossible.

When I’ve used them, it’s been for critical fasteners where ease of assembly was important, and could not be effectively decoupled from vibration, and where our recommended inspection/maintenance cycle was not guaranteed.

You might also consider something like a pinned castle nut, or safety wire, etc. These are not as simple to specify or install but more cost effective. Also, I’ve seen safety wire used as a conductive link to detect failure in a remote application.

> I would certainly recommend them

> When I’ve used them, it’s been for critical fasteners where ease of assembly was important, and could not be effectively decoupled from vibration, and where our recommended inspection/maintenance cycle was not guaranteed.

Taken to heart - sincerely appreciate the insight!

"I have used Nord-lock washers, they are demonstrated to be one of the most effective thread locking devices under heavy vibration, but as you say they are quite pricey."

Which is odd, given their simplicity and ease of manufacture, etc.

Can you get them stainless or just HDG ?

You can get them in various stainless alloys. My understanding is they have a really defensible patent.

Their patent might have expired? Im not aware of the situation but these seem to be an identical product: https://heico-lock.us/

Their promotional video on the Junker test is pretty cool: https://yewtu.be/watch?v=IKwWu2w1gGk

NordLocks are excellent - I was told about them by a very experienced racecar engineer who helped me work on my racecars - he couldn't say enough good about them, and recommended them for anything close to critical importance (or that you just wanted to stay fastened). Another engineer friend had a biz providing racecar component kits (think brakes, suspension, etc.), and also enthusiastically all his kits with NordLocks. I've had less experience but never seem them fail. Good stuff

> I’m a mechanical engineer - smartbolts are cool but rarely used in my experience. Much more commonly used are DTI washers: https://en.m.wikipedia.org/wiki/Direct_tension_indicator

Here's an (old) video showing how DTIs work:

https://www.youtube.com/watch?v=s677UKFiRdA&t=60s

The thing about torques is that they’re often written into safety standards, and safety standards often have to explicitly detail how critical values are measured. So until fancy bolts are recognised in the current standards as best practice for confirming correct torquing (are they? I dunno), it’s gonna be really hard to get them into production.

The bolts I worry most about coming off are this in my car wheels. I’m not sure I could use washers there and they are recessed in the rim so the gap isn’t inspectable. That seems to be the biggest benefit of these, that the only part of a bolt that is always inspectable/servicable is the top and that’s where the indication is?

If it worries you, buy a good torque wrench, look up the torque rating for your vehicles lug nuts and check it yourself.

That’s what I did after some garage monkey messed up my wheels.

The idea has been around since at least 1971.

https://archive.org/details/PopularMechanics1971/Popular%20M...

Just curious = how do these SmartBolts handle under full tension load AND fire conditions...

You have effectively drilled out the core of a bolt and replaced it with "goo" and you are using this as an indicator of the health of that fastening node?

So what if you burn out the goo? What if you get side/compression damage on the device -- what does the thing do when you over-tight/strip?

isnt a "goo core" less structurally strong than a solid bolt? Is a bolt tightened to a setting using a SmartBolt vs a regular bolt with tension wrench going to manage the same loads or not?

> isnt a "goo core" less structurally strong than a solid bolt?

Yes, the oil used in the centre offers zero structural support.

> Is a bolt tightened to a setting using a SmartBolt vs a regular bolt with tension wrench going to manage the same loads or not?

Depends on what the original bolt is made out of, and what the SmartBolt is made out of. Two different bolts are always going to have different properties, it up to the designer to match the size and strength of the bolt to their application. You would need to validate you design with SmartBolts to ensure they have the properties demanded by your application, you wouldn’t just swap normal bolts for SmartBolts without making sure SmartBolts were at least as strong as the original bolts.

(not an engineer) I watched the video - smartbolts seem a lot more time & labor efficient and can be tested visually?

I haven't heard anyone talk about DTI's in years. We used to call them "squirters". Takes me back to when I worked in the shop as a kid.

Made my day

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