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From: Albin43SDtr (no email)
Date: Wed Dec 01 2004 - 10:38:07 EST
Michael,
Parts are seldom DESIGNED to be held together solely by bolts in shear. The
way most fixed parts are held together to keep them from turning is by the
clamping force applied by the bolt. This clamping force creates friction
between the clamped surfaces. Of course, the bolt also provides some
strength in the shear axis, but that is not the primary source of the
designed structural integrity of the mated parts. The bolt usually provides
all of the tensile strength.
I would imagine that you are bottoming out on the end of the THREADS (not
necessarily the bottom of the hole). As a result, even though the bolts are
tight, if they are too long, they are not creating the necessary clamping
force.
Things like some alternators use bolts placed in shear, as the bolts are
used as the hinge point. Even then, most are tightened to provide a
clamping force, but some are made in a manner that precludes clamping them,
and only the adjusting bar is clamped, using a bolt.
If you are really having to use the shear strength of these bolts, I would
recommend Grade 8, as their tensile and shear strength are greater. While
you can tighten Grade 8 bolts to a much higher torque, you cannot exceed
the capabilities of the female threaded part.
One of the more common reasons for breaking bolts is that the mounting
surfaces are not flat, so that the strength of the bolt is used to pull the
parts towards each other over a high spot. Even after the parts are pulled
down so that there is good friction from the clamping forces, through use,
the bolts start to stretch and the mating surfaces separate. This results
in the parts being held solely by the bolts in shear. This is another good
use for Grade 8 bolts - they won't easily stretch. Again, one must not
exceed the capabilities of the "non-Grade 8" threads.
Take care, be safe, and good luck.
Wayne
M/V Celestial
Albin43 Sundeck
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