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Bolt & Nut Tension, not bolt & nut torque


Which choice of: Bolt pattern, Bolt Torque specification, Bolt Heater, Bolt grade, Shear bolt, High strength bolt, Stainless bolt, Aluminum bolt? Why all these choices, just to get tension into a joint?

Engineers take classes on bolt & nut specs, so why do some bolts fail? Because getting the correct pre-load tension into a bolt and nut is even harder to implement, than it is to calculate.  And if you think calculating the correct torque is easy, just see either of these two web pages (no affiliation with FAST-DAQ or any bolt gage manufacturer), "nut factors" or "torque is not easy". With SureBolt, instead of calculating the torque to get you the correct tension, you just measure the tension directly. You can even use SureBolt to determine the torque and its variation in tension.

Verify your bolted joint design. Bolt and nut selection are affected by many factors:

1.   Friction. Engineers know that bolt and nut friction consume over 80% of the torque measurement.  But, how does the actual tension vary bolt to bolt?  What if you use the same type of bolt? It still varies more than 20% (flange bolts vary even more: see #7).

NDT Update says:  
" SureBolt outperformed the one-point bolt gauges on every bolt and every test in reliability and accuracy."

2.   Bolt failures. What do bolted joint designers say the number one reason, stated by bolting experts, for bolt failures? Lack of proper pre-load tension. What are some of the factors that cause pre-load tension errors? Why are the bolt torque specifications not enough? Can't I just choose a good wheel bolt pattern?

3.   Measure Tension errors. How can an engineer measure torque wrench errors?  Do these bolt tension errors matter? And If you think calculating the correct torque is easy, just see either of these two web pages
(no affiliation with FAST-DAQ). Do you think nut factors or torque calculations are easy? With SureBolt, instead of calculating the torque, to get you the correct tension, you just measure the tension directly. You can even use SureBolt to determine the torque. 

4.   Bolt gage. Your bolt's stress analysis can be verified using a bolt gage. What is an ultrasonic bolt gage?  See an animated demonstration of an ultrasonic echo inside of a bolt.  Mechanical engineering labs rarely have enough resources to let each engineer learn ultrasonics. Are bolt gages reliable? Are they easy to use? New patented technology has finally made ultrasonic bolt gages reliable and easy to use.

5.   Bolt tension, not bolt torque. Tension not torque. You want to put a known amount of tension into your nut and bolt combination, not just a known amount of torque. Your bolted joint design often relies on proper pre-load tension.

6.   Stress, strain & yield. A stress strain curve can be plotted using SureBolt.  Stress strain and yield.  SureBolt can help a engineer verify his bolted joint design.  Did his bolt reach yield?

7.   Flanges. Flange joints sometimes leak.  Why does the tension in a flange bolt vary so much, bolt to bolt?  Tightening one bolt loosens some others. No matter how many passes, the tension can vary over 50%.  How can you tighten each flange bolt to closer tolerances?  How can you use SureBolt on a flange

Notice the variation in bolt tensions on each of three passes with a torque wrench.

Credit Integra Technologies for this graph. They have taken data on 94 joints.
They mislabeled the first pass (it should show 100 Pound-Feet instead of 200 Pound-Feet)



X-axis = bolt position. 16 different bolts
Y-axis = bolt stretch (tension). 
Large tension variations are typical with torque control. Notice the > 250% variations.

8.   SureBolt. How does SureBolt differ from any other ultrasonic bolt gauge? What is the difference between a one-point bolt gage and SureBolt's patented whole echo method? Are the prices similar? (Yes)

9.   Engineering. Some engineers use a strain gauge to measure bolt tension because they know a torque wrench is not directly measuring tension. This site is an engineering resource for torque versus tension in bolted joints. 



SureBolt's Whole Echo patented technology greatly increases the reliability over all other bolt gages, by using the patented whole echo method, instead of just one point. 

See NASA story. All other bolt gages use just "One Point" (one zero crossing), that leaves you susceptible to peak jumping (20% error, and much more).  Surebolt is a more reliable bolt gage due to this patented DSP technology. See comparison table.

Bolt gages have been around for over 20 years. Their "Peak Jumping" problems and all the training required to take a reliable measurement, have kept them from being widely used. When your bolted joint fails or flange leaks, then you find out your one-point bolt gage may have jumped peaks. You cannot be sure, because you did not record the whole echo.

Example, with a lot of training and laboratory testing, NASA has used one-point bolt gages for years. NASA physically takes Polaroid pictures of the echoes before and after tensioning. Then they use their trained human experience to determine if the echo has distorted "too-much". Even after all this, they have to throw away approximately one third of the measurements (ET umbilical bolts) because they are not reliable. When NASA used the SureBolt prototype, no measurements had to be thrown away.

SureBolt gives you a 13.3" TFT, portable PC (Panasonic Toughbook Model 72). The bolt gage is built into the CD ROM and PCMCIA slots. Click here to see the actual SureBolt screens. See the animated DEMO page to see how SureBolt can measure tension. SureBolt has been used on a variety of bolts, from special high strength titanium bolts to standard "off the shelf" bolts.

For more information, see the main SureBoltTM navigation screen.



SureBolt FLASH Video Why NASA Used Video Download Animated Demo Tutorial On-Line Torque Errors Technical Info Torque Wrench Specifications+ Prices & Compare Services News

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"Echo Time".  See animation below of how to ultrasonically measure the change in bolt tension using echo time.
This animation requires FLASH (version 4 or higher).



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 No more guessing as to which "one point" is the right point.


The FIRST whole echo method (patented DSP Technique).

SureBolt picture

Built into a 
Panasonic Toughbook

Weather proof keyboard.

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Last Modified: September 27, 2011