And the #1 reason for improper bolt pre-load? Relying on torque, not tension.
A torque wrench only measures bolt torque (friction) not bolt tension. This
friction varies so much, bolt to bolt, that the final error
is +/- 20% and not uncommon to have 50% error. Lubricants, calculations,
training, ASTM procedures, ... cannot help but so much.
Note: New ultrasonic method cannot be fooled by friction (torque)
Each society (American Society of Mechanical Engineers, American Society of
Testing and Materials, American Society of Civil Engineering, American Society
of Safety Engineering, ...) tries to help its members design safe structures.
Update says: " SureBolt outperformed the one-point bolt gauges
on every bolt and every test in reliability and accuracy."
Verify your design.
Engineers can use SureBolt to verify that their bolted joint has enough "over design"
to be safe.
Test, test, and test some more. But look what happens ---
Notice the variation in bolt tensions (stud stretch) on each of three passes with a
Why doesn't all the testing work?
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
Y-axis = bolt stretch (tension).
Large tension variations are typical with torque control. Notice
the > 250% variations.
Useful Information and links to more details:
1. Bolt failures. What do engineers 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 engineer's bolted joint design.
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 an engineer's structural analysis.
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 engineer learn ultrasonics.
5. Friction. 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?
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. 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
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"
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).
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