Mechanical Engineering Resource or Handbook on Bolt Failures, Torque Errors, Flange Leaks. New technology for a mecahnical engineer to ...

Mechanical engineering departments often teach their mechanical engineer to design bolted joints by calculating bolt tension, not bolt torque. Yet, mechanical engineers are often limited by bolt tension errors, not bolt torque errors. See the web's best bolted joint tutorial.

Note: New ultrasonic method cannot be fooled by friction (torque) variations.

After all the mechanical engineering classes on bolted joint design, why do some bolted joints fail? Verify your design. Mechanical engineers can verify that their bolted joint has enough "over design" to be safe. Here is a list of information that may be useful for a mechanical engineer, or mechanical engineering lab doing bolted joint design.

1. Bolt failures. What does mechanical engineering say the number one reason for bolt failures is? Lack of proper pre-load tension. What are some of the factors that cause pre-load tension errors? This affects a mechanical engineer's bolted joint design.

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

2. Bolt tension, not bolt torque. Mechanical tension not mechanical torque. You want to put a known amount of tension into your nut and bolt combination, not just a known amount of torque. Your mechanical joint analysis often relies on proper pre-load tension.

3. Measure Tension errors. How can a mechanical engineer measure torque wrench tension errors? Do these bolt tension errors matter? These errors often affect a mechanical engineer's structural analysis.

Notice the variation in bolt tensions (stud stretch) on each of three passes with a torque wrench. >>>>>

How can these errors be so large?

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.

4.   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 mechanical engineer learn ultrasonics.

5.   Friction. Mechanical engineers know that bolt head and bolt thread 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? This may affect a mechanical engineer's design.

6.   Stress, strain & yield. A stress strain curve can be plotted using SureBolt. Stress strain and yield. SureBolt can help a mechanical 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? How can you tighten each flange bolt to closer tolerances? How can you use SureBolt on a flange?

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.   Mechanical Engineers. Some engineers use a strain gauge to measure bolt tension because they know a torque wrench is not directly measuring tension.

11.  Mechanical engineering resource. This site is a mechanical 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.

12. Calculating Torque? If you think calculating the correct torque is easy, just see either of these two web pages (no affiliation with ARVC). Nut factors or torque is 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.

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

All other bolt gages use just "One Point" (one zero crossing), that leaves you susceptible to peak jumping (20% error or much more). Surebolt is a more reliable bolt gage due to a this new DSP 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 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 SureBolt^TM navigation screen.

Watch the animation below.