Bolt with built-in tensile strain gauge - photo and patent

[Jon]

Bolt with built-in tensile strain gauge.





More: US patent #5668323

Previously:

Bolt threading GIF
Turning a bolt with another bolt - GIF
D shackle nut vibrates off of bolt - video
Miniature hunting knife made from stainless steel bolt - video

[McDesign]

I dunno'. I think the cross-section is so reduced at the "cup" for the indicator that the bolt is essentially useless for a given diameter.

All the elongation will be in that area before the bolt shank starts to load.

Maybe if your trick indicator half-inch bolt was only supposed to replace a normal 5/16" bolt . . .

Forrest

[PJs]

I dunno'. I think the cross-section is so reduced at the "cup" for the indicator that the bolt is essentially useless for a given diameter.

All the elongation will be in that area before the bolt shank starts to load.

Maybe if your trick indicator half-inch bolt was only supposed to replace a normal 5/16" bolt . . .

Forrest

I kind of agree. Elongation can occur at any place in the bolt and not necessarily linearly, usually just above the threaded section or at the base of the head or nut would be starting points. With the head and upper shank compromised by counterbore and a sight window added I would question any real accuracy of readings on that scale. I would however guess these are not used on any bolt <1".

The Abstract of the Patent is full of good details about the complexity of a threaded fastener systems but a few key factors are left out imho, like Quality control (1/100, 1/1k or 1/10k and always the Monday/Friday rule), metallurgic consistency, Grading, and Thread quality. It does however point out a LOT of other patents for this type of fasteners world wide.

The only thing I would think this is good for is a rough, visual thumb to eye rule, during initial installation torquing. Personally If something is critical, Torque Standards have been around a long time and if it needs torquing for critical reason...Use a well maintained and "Calibrated" torque wrench, in proper fashion!

PJ

[McDesign]

We just had a fastener guru from a supplier (Endries) come and give a presentation on threaded fasteners at work.
His take-away was that for critical applications, frictional factors were so variable in manufacturing assembly, and such a large absorber of tightening torque as measured at the bolt head, that measuring bolt stretch is really the only way to get a precise look at fastener clamping load. Luckily, few of our applications are that critical!

[TSiArt]

A guy on youtube AvE did a video on this type of bolt and his opinion is that this a bad idea from knowing how people can be lazy. I'll try to look up the video.

[PJs]

I would have liked hearing what all he presented. I agree to measure before and after is the true test for critical application. And I can't think of too many applications either, other than NASA and deep marine apps that might be that critical, maybe high speed rail too. I dealt with a few fastener manufactures over the years but mostly distributors that would bring people in periodically. Got to know a few of the loctite guys before they were bought out and they were a wealth of knowledge and help in difficult designs and environments.

[McDesign]

Pretty much any performance engine builder will measure rod bolt stretch instead of rod bolt torque -
https://www.jegs.com/i/ARP/070/100-9942/10002/-1

[PJs]

Maybe now day builders but I only built a few (maybe 6-8) street motors and don't remember seeing them back when, but did go through a lot of Plastigage and mic'd journals and such. Good Idea though for high power/rev motors!

[McDesign]

Check these three slides from that presentation out - the first shows just how little of applied torque actually goes into stretching the bolt - remember, a bolt is a spring that has to be stretched to work.

Second slide shows the components of a 4-lubricant comparison

Third slide shows the huge variation between applied torque (first red-marked column) and the tension generated in the bolt (second red-marked column) - only variable was the lubricant or lack thereof.

Forrest

[PJs]

Check these three slides from that presentation out - the first shows just how little of applied torque actually goes into stretching the bolt - remember, a bolt is a spring that has to be stretched to work.

Second slide shows the components of a 4-lubricant comparison

Third slide shows the huge variation between applied torque (first red-marked column) and the tension generated in the bolt (second red-marked column) - only variable was the lubricant or lack thereof.

Forrest

Thanks for sharing the slides, Forrest. After looking at these for awhile, pondering and a bit of research, I do think the last slide has some interesting values as you pointed out. Particularly the thread torque to peak tension and the results of the Zinc Flake example was surprising to me. However, we would likely assume these test were done on their fasteners (with no mention of ASTM standards used in manufacture or OA length {ratio of thread to shank length}) other than Grade 5 Hex Cap, Gr2 full nut and yellow zinc washer; Nor did it state where the single washer was located (head or nut), and more curious to me was how the friction of 9050lbs was calculated or measured, and what the numbers in the 3 columns on the left (Thread, head, ref.) mean. The K-Factors seemed appropriate to ASTM standards, and the Peak ON torque (Head) seemed appropriate to me, but must we assume all these values were to achieve a Target Clamp Load of 9791 lbs. from slide 2?

Slide 2 showing their test setup, definitely looks legit and calibrated in a lab. Chart 1 though, is a bit crayon for me as it is only a resultant pie chart from, I assume a spreadsheet, that represents a "Clamp Load-Vs_Applied Torque" but gives statistics for friction, which appear to be within the ball park based on this White Paper by Ralph S. Shoberg, P.E., Director of Technology, PCB Load & Torque, Inc. The good (pertinent) stuff starts at the bottom of page 4. His other paper "Tightening Strategies for Bolted Joints" is also of interest in the discussion where he details all the complex variables, proper test procedures and outlines methodology and math for Torque-Angle Signature Analysis as shown in your slide 3 in the 2 columns next to peak tension giving insight to those numbers in the "Final Angle (deg)" column.

Sorry but I'm kind of a stickler on test methodology, Statistic and Data. My credo is/has been, Discernment in all things. Not to say I don't Jump to Incorrect assumptions sometimes, but I try...consciously.

Although this discussion rises above the average HMT/shop it does give some insights to properly choosing and applying fasteners to what ever we design or build. It says volumes to things to think about, like; if, where, and what washers are used, clearance hole sizes, material finish, grade, thread pitch, etc. and the all important question...does it really need to hold the planet together and will it hurt someone if it fails? Thankfully engineers, manufacturers, and Standards, makes it relatively simple to choose, get and use fasteners in our projects at home or professionally for every day use and the opportunities to push the boundaries of technology because we all have added to the advancements.

Thanks Jon for lighting the candle and Forrest the discussion, hopefully our members find some good value and insight to it.

I watched AVE's Vid when he first did it but will need to watch it again for any further comment, if any. Thanks TSiArt for bringing it forward here!

PJ