Marshall Space Flight Center,
This small picture is one of the final reports (press on the
picture to enlarge). Notice the repeatability and sensitivity of
NASA needed to verify
that their Space Station Node design would be safe. A big part of the
verification depended on the special bolts used to hold the CBM Attach Ring to
the Nodes. The years of service of the Space Station would be strongly dependent on the
fatigue and tension characteristics of these bolts. Each hatch (6 hatches per Node) uses
approximately 225 bolts.
Tests: How do you measure any tension change in these
bolts as the Node is pressurized? How do you make sure the extra tension is
distributed evenly? How do you measure the preload tension? Normally you rely on
a torque wrench for preloads. But a torque wrench is known to only be +- 20%
reliable or worse.
A more accurate way is to use an ultrasonic bolt gage. Ultrasonic bolt
gages (at about $20,000 each) have been around for many decades. But they do not work
very well on small bolts like these 2.25 inch length Space Station bolts. But, NASA needed these measurements. So
a new type of ultrasonic bolt gage was invented for this difficult
NASA needed more
reliability, more measurements (they had to measure 8 bolts at a time), and the
ability to read remotely (from 400 feet away) while the Node was being pressurized. Plus or minus 20% error would be
too much. New technology would have to be used.
Measure 8 bolts at a
time remotely (from 400 feet away), while the Node was being pressurized.
Increase reliability if possible and monitor each measurement's reliability.
Coupling to the
threaded non-head end of the bolt,
Control the oscilloscope remotely and download the waveforms,
These bolts are so small (diameter), that the signal strength
is low even with the best piezo-electric transducers,
Remotely control the standard (one-point) bolt gage's gain, hold-off, and
Display and analyze the data
Develop a digital signal processing technique
(DSP) and compare it to
the standard Bolt Gage reading. (This turned out to be the first SureBoltTM
Use the one-point remote bolt gage measurement for comparison.
The old technology
one-point bolt gage
was the only instrument that was officially calibrated.
Recreate the normal
bolt gage technique in software to compare to the old technology one-point bolt gage and the
digital signal processing technique (SureBoltTM
These challenges were met and exceeded. The
prototype worked well on 100% of all tensions on all bolts. The new digital
signal processing (DSP) technique was the most reliable
and accurate, but it was not "officially" accepted as calibrated until the 3rd
3rd Node test. 100%
reliability. The design engineers now wanted the tension from the DSP technique
along with the old technology one-point bolt-gage tension measurements.
4th Node test. The
engineers only wanted the tension from the new DSP technique. The old
bolt-gage tensions were ignored.
super-sensitivity test. The
Huntsville engineers were so pleased with the reliability and precision of the
prototype, that they wanted to verify another Space Station design. They
wanted to know how much the tension would change in a bolt while its adjacent bolt was being
tensioned. The new SureBoltTM
prototype was able to detect CHANGES smaller than plus or minus
50 pounds (out of a preload tension of 3600 pounds). The new instrument was so
sensitive that the radiant heat from the operator caused the temperature sensor
bolt to change which looked like 10 to 20 pounds of tension until the operator
walked away. Our main source of
error became temperature measurement.
Next to the Space
Station we installed; one oscilloscope (set to one billion samples per
second), RS422 communication line converters, a GPIB extender, a "one point" bolt
gage used as only a pulser, a custom designed switcher remotely
400 feet away in the
control room we installed: two full sized desktop computers, printer, GPIB extension box,
RS422 serial converters, and cables.
Now ARVC has developed the technology to the extent that the SureBoltTM single channel unit fits into a customized Panasonic
Toughbook Model 72 laptop computer.
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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