In a message dated 9/25/2001 8:49:00 PM Eastern Daylight Time,
spritenut@Exit109.com writes:
> Grade 8s tend to break without yielding when over loaded (although it
> takes more to break them, when they do fail, things tend to fall off).
>
The only thing is a Grade 8 will not be overloaded until well after a Grade 5
has "stretched" and busted. Even shear is greater for a Grade 8, since shear
is 67% of tensile strength, according to Bowman. Also, once a bolt has been
subjected to forces nearing its tensile strength,it is irrevocably stretched,
which results in a loose nut. This is responsible, again according to
Bowman, for significantly more fastener failures than surpassing the tensile
or shear strength of a Grade 8 bolt. Then we come along and see a loose
nut--what do we do? We tighten it! But what we have tightened it onto is a
distorted and weakened bolt. The key is the proper preload or torque. Under
or over-torque and you either have a loose or a permanently stretched bolt.
Just right and your fastener will not fail as long as you have installed AT
LEAST the minimum Grade that was issued for the usage. Thge higher grade nut
will afford you higher torque, resulting in greater clamping force, which is
what you are after in the beginning, because it improves the strength of the
connection.
Again, I am not and would not recommend to anyone that they replace any bolts
with anything other than the originals. Personally I would feel comfortable
with Grade 8 bolts replacing Grade 5 in stress locations.
Be interesting to hear from listers who have suffered critical broken bolts
while running, what grade the bolts were, and whether they were properly
torqued when installed. Anyone?
--David C.
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