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Some Flanges Leak Due To Bolt Tension Variations
Over 50 web pages on torque, tension, & bolted joints.


Flange designers know that their flange seal may leak, if their flange bolt tensions vary too much between each bolt. Yet, most installations are done with a torque wrench, not a tension wrench.  SureBolt is a new technology that measures actual flange bolt tension more reliably. It is not just flange bolt torque meter. Steel flange, pipe flange, flange adapter, flange mount, flange gasket, gasketed & non-gasketed mechanical seals, ..., rely on tension, not torque.

After all the engineering classes on flange design, why do some flanges leak? Here is a list of information that may be useful for a flange engineer or flange designer. Verify your flange design.

1.  Flange seals sometimes leak. What is the most common cause of flange gasket leaks (as stated by
bolting experts and flange joint designers)?  Lack of proper - evenly distributed - pre-load tension. What are some of the factors that cause pre-load tension errors, see this link

These all affect a flange design, especially the assumptions of the gasket tension being applied if only a torque wrench is used. 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% bolt to bolt.  How can you tighten each flange bolt to closer tolerances?  How can you use SureBolt on a flange?

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

2.   Flange 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 flange design often relies on consistent pre-load tension. See "friction".

3.   Measure tension errors. How can a flange designer measure torque wrench errors?  Do these bolt tension errors matter? 

4.   Friction. Flange 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? 

What about different lubricants, multiple passes, different tightening sequences, ... etc?

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

Credit John Bickford for this graph.
He mislabeled the first pass (it should show 100 Pound-Feet instead of 200 Pound-Feet). This is corrected in his new edition. Mr. Bickford is probably the world's expert on bolted joints.

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

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

6.   Bolt gage. Your flange's bolt 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.

7.   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 their prices similar? (Yes)

8.   Flange Engineers. Some engineers use a strain gauge to measure bolt tension because they know a torque wrench is not directly measuring tension. Yet their flange design software may assume a limited amount of tension error (+- 25%) when calculating bolt tension from bolt torque.

9.  Tension errors. What are some of the tension errors associated with using a torque wrench? Does your flange design software let you enter the tension error ranges?

10.  Flange engineering. This site is a flange engineering resource for torque versus tension in bolted joints. SureBolt is a nondestructive testing - NDT - method of measuring bolt tension. Non destructive testing includes the use of ultrasonics.



SureBolt's Whole Echo patented technology greatly increases reliability by using the whole echo instead of just one point. 

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

Bolt gages have been around for over 20 years. Their "Peak Jumping" problems and difficult learning curves, 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. 

Example, with a lot of training and laboratory testing, NASA has used 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 ET (External Tank umbilical bolts) measurements 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 ultrasonic pulser and digitizer are built into the CD ROM and PCMCIA slots. Click here to see the actual SureBolt screens. See the animated DEMO page to see how an ultrasonic bolt gage can measure tension. SureBolt has been used on 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