Tuesday, July 26, 2016

Techy Tuesday - SmartBolts?

It's hard to estimate how many bolts are sold in a year, but suffice it to say there's a lot.  Bolts are so ubiquitous that no one questions their history and where they came from; they've just always been there.  Bolts as fasteners that allow assembly of complicated assemblies were around before the Industrial Revolution, perhaps as far as 400 years ago, although development sped up as the Industrial Revolution unfolded. 
The “History of the Nut and Bolt Industry in America” by W.R. Wilbur in 1905 acknowledges that the first machine for making bolts and screws was made by Besson in France in 1568, who later introduced a screw-cutting gauge or plate to be used on lathes. In 1641, the English firm, Hindley of York, improved this device and it became widely used.
In critical applications where the reliability of the bolted joint becomes important, bolts are pretensioned; that is they're stressed until they stretch a desired amount.  The restoring force of the bolt trying to return to its un-stretched length keeps the hardware tight and doing its job.  The most common way of prestressing a bolt is to torque it to some specification.  From a purist's standpoint, and one I recall hearing in classes, using a torque wrench to apply a known amount of torque is not the same as applying a known amount of stretch to pre-load the bolt.  When the torque is applied, the user is counting on that torque turning into the exact amount of stretch the system should experience, but that depends on everything else being perfect.  All the torque wrench can really tell you is how hard it is to turn the bolt, and anything from a badly threaded nut (or bolt) to a misaligned bolt/nut combo to something as simple as dirt under the bolt head can affect that.

A torque wrench can be misleading, but direct measurement of the stretch is difficult to impossible in many situations.  How does anyone know they got it right?  A company called SmartBolts has developed a product intended for verifying that the preload on a bolt is correct at a glance.  There's an indicator in the head of the bolt that changes color from a red when the bolt is completely loose to black when properly tightened (stretched).  I don't see how it could indicate the bolt was stretched too far, but understretched appears to be what it was designed to overcome.
Seems like a nifty little idea.  When you look at their website, there's more information.  As you might expect, due to the size of the components that need to go into the bolt, this is for larger sizes only.  The smallest they list is 7/16" diameter, so you're not going to see this on any 1/4-20 or 10-32 hardware for a while (if ever).   
Fastener TypesHex Flange Socket Stud Other
Diameter (Minimum) 7/16” (M10) ½” (M12) 1” (M24) ¾” (M20)  –
Diameter (Maximum) 2 ½” (M64) 2 ½” (M64) 2 ½” (M64) 2 ½” (M64)  –
Length (Minimum) 1 ¼” (30mm) 1 ¼” (30mm) 3” (75mm) 3” (75mm)  –
Most of us don't work with bolts larger than 1/2", if that, and the added complexity from the indicator is going to price this out of the reach of all but the most critical applications. As a guess, things like aircraft, possibly bridges and buildings, large trucks and other heavy equipment, especially equipment that's life-critical.  Still, I can see how this might simplify life on the assembly line and make the assembly operations simpler, faster, safer.


  1. The ARP fasteners I used to use in place of the OEM connecting rod bolts had precisely machined "dimples" on both ends so you could mic them to measure the stretch, rather than just torquing them to a specific value.

    On ALL other fasteners, I made double sure that the threads were clean, the bearing surfaces that would mate together and "twist" when torqued were clean, and ALL threads and mating surfaces were properly lubricated with either light oil, or the proper assembly lube.

    When I was at Boeing, we had two different week-long classes on torquing fasteners. One class was mechanical torque for mechanical assembly (duh....) and the other was "Electrical Torque", which was given after the "Electrical Bonding and Grounding" class. It concerned how to properly assemble threaded/bolted connections that were "mechanically redundant" as far as assembled strength, but "electrically required" so that different assemblies would all be at the same potential.

    Quite interesting stuff to learn!

  2. When I was 18 I worked in a spring shop. We made and replaced springs in cars and trucks. We even made our own U-bolts. Tempered the springs and huge vats of oil. I have a lot of stories from that job. But I remember the first time I put on big truck springs and we made the U-bolt my boss was showing me how much to tighten it. Kept that big air impact wrench tightening and the bolt stretched visibly. I doubt any bolt you could buy at Home Depot or Ace today would stand up to that.

    1. That's pretty awesome to see, isn't it?

      Those bolts are out there for sure, but I have no idea if the big retailers would carry them. I'm sure you could get them at a specialty shop, like Gainger or MSC, or maybe an auto parts place. You just have to know the words (bolt class or grade) for what you're looking for.