SmartBolts change color when they're tight enough

Smartbolts have little discs in their heads that change color as the right amount of tension is applied to them. I recently installed a child safety gate with a similar mechanism: a red, spongy washer between the bolt and the frame; once the right amount of tension was on the bolt, the washer was squished down so much it disappeared and you could stop tightening.
Unfortunately, a torque-wrench does not measure bolt tension accurately, usually only about +/- 30%, because it does not take friction into account. The friction depends on bolt, nut, washer-material, plating, surface smoothness, machining accuracy, degree of lubrication and the number of times a bolt has been tightened. Fastener manufacturers often provide information for determining torque requirements for tightening various bolts, accounting for friction and other effects. However, in field applications, this information is often not available, practical or administered poorly.
SmartBolts (via Red Ferret)


  1. Getting the right torque on a bolt is easy. Tighten it down till it goes “ping” then back it off a quarter of a turn.

  2. I’m not really seeing the applications for this…

    Automotive, why? Currently you either have one time use torque stretch bolts, or bolts that are more than strong enough to handle the torque needed for their application.

    Aerospace, maybe. But this field could actually spend the money to calculate the exact torque based for friction, material, and design.

    Consumer, seems overkill. How many times do I actually break out my torque wrench when I’m not working on my car? And even then common sense and a little know how works 99% of the time.

  3. These do not indicate fastening torque, rather they indicate the bolt’s tension. They have a hollow center filled with a nearly opaque colored liquid (like the magic 8 ball). As the tension in the bolt increases the bolt stretches slightly changing the thickness of the gap between the window and a colored dot. Since they cost about $20 each, they are only used for applications where the tension is critical.

  4. Wow. This is a lot of work for something that that I can’t find an application for. . .

    Car wheels, maybe, I guess? Space Shuttle? Bullet train?

    Oh well, if ya’ just gotta’ get a patent on something before you die I suppose this is better than yet another new kind of smart bomb. . .

  5. is it not the case that a torque setting is application rather than fastening specific anyway? i could use these bolts in 2 different situations which would require 2 settings.
    maybe these come in different grades?

    my thoughts would be

    a: graduated colour change is not a reliable index. 2: don’t use under unusual lighting conditions.
    iii: don’t use under normal financial conditions.
    four: why aren’t they shiny?

  6. @#5: It’s easy to get the right torque, but not to know what the right torque is. For example, do you know how the torque requirements of your lug nuts/bolts change if they’ve got an anti-seize or lubricating compound on them? It could be 80 ft·lb dry and 120 ft·lb oiled.

    Still, measuring bolt tension doesn’t help much with the aforementioned problem. It _would_ be helpful in place of stretch bolts, I suppose. For example, in a cylinder head where you want all the bolts to be pressing down with equal force.

  7. The application this is crying out for is in surgery – we sometimes plate and screw fractured bones – obviously they have to be tight enough, but half a turn too much and you crush the bone and (much) worsen the fracture. This is what surgeons have been waiting for!

  8. Unless it’s the same price as a normal bolt, not interested. I don’t need nails with little levels built into them either.

  9. They have an impressive list of customers. Apparently the military uses them in Apache Helicopters. I think this is really cool tech that doesn’t have much use outside of industrial applications, unless the cost bottoms out. Additionally, I wonder if they could incorporate this technology directly into studs.

  10. Critical aircraft bolts have a cavity with fluorescent dye in it. If the bolt cracks, the dye leaks and creates a big day-glo stain for inspections.

    I heard the MTA was considering those dye bolts for buses in Baltimore. They had a slight problem with the wheels falling off (and rolling away into nearby McDonalds!)

  11. Of course, if you are using a socket, you can’t tell until it’s too late. Therefore I propose bolts with a little sound chip like greeting cards: “AAHHH, stop, that’s enough, o god please stop!!!”

  12. Load-indicating washers (LIWs) have been used in steel-framed buildings for years. They have raised areas on one face that smush down when the correct tension is reached. Sort of like the spongy washer Cory described in the post, but made of steel and calibrated for thousands of pounds.

    LIWs aren’t as easy to read as a dot on the head of a bolt, but they’re comparatively cheap and work with standard bolts.

  13. use explosive bolts and let natural selection remove those mechanics from the gene pool that tend to overtighten.

  14. Hows about using TC (tension control) bolts instead? I see them used on bridges to replace rivets and they are certainly cheaper. I get cask of 300 3/4″ inch diameter x 2″ long with nuts and washers for $250 – thats less than $1 each. I used them in a restoration project because the round head on the bolt looks like a button head rivet of old.

    A TC bolt has a splined end that extends beyond the threaded portion of the bolt and an annular groove between the threaded portion of the bolt and the splined end. The special wrench required to install these bolts has two coaxial chucks—an inner chuck that engages the splined end and an outer chuck that envelopes the nut. The two chucks turn opposite to one another to tighten the bolt. At some point, the torque developed by the friction between the nut and bolt threads and at the nut-washer interface equals the shear resistance of the bolt material at the annual groove. The splined end of the bolt then shears off at the groove.

  15. interesting design – and topical, but as commercial as one can get to the line. The twin tab washer: is that a one-use? I imagine unreliability would develop if the tabs were repeatedly compressed and popped up.

  16. Takuan @ 19 – if the charges are carefully gauged, we could be left with a generation of careful, unidextrous mechanics… it might just work…

  17. I, as an amateur installer of washing machines etc, would certainly welcome washers which indicated when a joint had been sufficiently tightened. I’m always afraid of over-tightening plastic joints, and have been twice rewarded with slow leaks.

  18. ok, i’ll try… but in the last 20 years I’ve seen most of the independent ironmongers near me go down, and most of the big DIY chains available to me have distilled into kit-shops- i.e. they decide what you might want to do, and provide a bag of bits with which you can do it. Long gone are the days (here) where an amateur could buy a single 3/8″ nut and bolt, teflon tape I have yet to try for…

  19. grrrr! Methinks a local web based affinity group that shares cooperative trade suppliers is in order. Failing that, mail order from across the Pond. A roll of tape is a dollar and postage the same.

  20. Multi use fastener. The tabs do not compress down. The following instruction sheet might help to understand better. Thanks for your input!


    TO USE:
    -Place the bolt through holes of the joining materials
    -Slide the TAB-LOCK WASHER on the bolt, aligning the key of the washer with the keyway of the bolt.
    -Put the nut on the bolt and tighten to desired torque.
    -Place RING WASHER on over the nut, then turn nut counter clockwise to engage the two washers. Note: The RING WASHER has slots spaced 90 degrees apart so that it can engage the tabs of the TAB-LOCK WASHER every 30 degrees. If the slots of the RING WASHER don’t align on the first try lift the RING WASHER off the nut and rotate the RING WASHER to the next flat of the nut (60 degrees). Pick the closest alignment and then rotate the nut either clockwise or counter clockwise to allow the RING WASHER to slide all the way on. It is recommended to adjust the nut in the clockwise direction whenever possible to make the alignment.


    -The RING WASHER and TAB-LOCK WASHER are made of a 1050 spring steel heat treated to RW 34.
    -The nut and bolt are a standard 5/8″-11 grade 5 , with the addition of a longitudinal locking channel located on the bolt.

  21. Neat system Phil. I wonder about the keyway in the bolt. Does that have to be cut into the bolt as a step in manufacturing?

  22. Your right, it might be best to have the keyway cut prior to any heat treating or galvanizing process. The bolts shown on our web site are a 5/8″ standard thread grade 5 purchased off the shelf with no keyway until we had them cut out ourselves. This post bolt manufacturing keyway cut process seems to work well though. Thanks for your input.


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