SPRING 2022

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THE DISTRIBUTOR’S LINK
Rob LaPointe AIM TESTING LABORATORY
Rob LaPointe is a noted authority in materials and fastener technology. With extensive experience
in the management and science of materials testing laboratories combined with master’s degrees in
physics and education, he excels at bringing solutions to the client. Working specifically in the fastener
testing industry, he has developed expertise in mechanical, nondestructive, metallurgical and chemical
testing. With a background of 20 years in physics education, Rob is effective at communicating complex
ideas in a simple and understandable manner, communicating well with clients enabling them to make
informed decisions about their products and business. AIM is located at 1920 Cordell Court #101, El
Cajon, CA, 92020. Tel: 909-254-1278, email: sales@aimtestlab.com or online at www.aimtestlab.com
FASTENER SCIENCE:
HOW TIGHT IS RIGHT TIGHT?
If you’ve spent a large amount of time while growing
up, or as an adult, wrenching on things like engines,
automobiles, motorcycles, equipment, or other various
mechanical devices, you’ve come to know a bit about
how tight is right-tight. For fasteners from 6-32 up to
about 3/4-10, I have an internal torque-sense that
keeps me in the sweet spot of fastener tightness most
of the time.
I’ve gained this torque-sense through wrenching many
fasteners to the point of “Uh-oh…. I turned this one too
far.” By either taking the bolt or screw into yield (a point
where the fastener stretches and deforms permanently)
or all the way to ultimate tensile failure (the point
where the fastener breaks into two pieces), you get a
good sense of how tight a fastener should be, or more
particularly, how much torque (rotational force) should be
applied to the fastener to get to a tension (elastic force
caused by stretching) between 50 – 80 % of its ultimate
tensile value. Bear in mind that I’m not often measuring
to confirm this tension for applications where a specified
torque value is not required, but that it’s a feel that has
been calibrated by many failures and occasionally by
comparing to actual data acquired through measuring
torque-tension relationships in the laboratory.
Acquiring this internal sense of right-tightness can
also lead to other helpful “by feel” sensations. One can
gain a sense of the performance of a particular metal or
of a hardened material’s characteristics as it progresses
from yield to ultimate tensile. For example, stainless
TECHNICAL ARTICLE
FIGURE 1. TIGHTENING A BOLT IS OFTEN A MATTER OF “FEEL.”
steel has a great deal of stretch (yield) before it ultimately
breaks, and high hardness alloy steels have only a small
amount of stretch between yield and breaking. For me,
this experience has been calibrated by seeing hundreds
of tensile tests with a variety of metals in the laboratory.
It’s amazing to see the stress-strain curve of a tensile test
and gain a sense of comparison to what you feel when
you tighten a nut and bolt made form that material. It is
being sensitive to a very real feeling and comparing that
to knowledge about how metals behave under stress that
enables right-tight by feel.
For most installations, where the correct tension
is critical to the application or installations where
“tension by torque feel” is neither acceptable nor
legitimate, we need another more quantitative method
for relating torque to tension. Typically, the correct
tension for fasteners is somewhere between 50-80% of
the fastener’s ultimate tensile strength.
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