Alan Myers wrote:
> I can answer part of your questions.
> The bolts with 6 hash marks are Grade 8, the highest grade and strongest
> generally available.There are markings on the nuts, as well, but I'm not
> sure how to describe them: a dot, a hash mark and a percent sign, sort
> of. Ask at a hardware store for a sample. Grade 8 will handle the
> highest torque.
> The bolts with 3 hash marks are Grade 5. This is a moderately strong
> Bolts with no markings are ungraded and the weakest of the bunch, most
> likely to fail under stress and can take the least torque.
All the above is correct, and only needs to be fleshed out a bit. Most
grade 8 nuts have a dot or a dimple at the apex of each two flats
(making six dots). The bolts with three lines on them are a grade 5, but
this is also known as "commercial grade," so if you ask for that, you
should get a grade 5. Four marks are rarely used, but indicate a grade
6. Two marks indicate a grade 3, also rarely used. No markings indicate
grades 0-2. There are bolts available higher than grade 8, but these are
generally used only for aircraft. Unbrako is one trade name for a
superalloy bolt, and these are available in 12-point or socket head
types, and are roughly equivalent to about a grade 11 or 12.
Other considerations. Avoid using deformed-thread prevailing torque
nuts. Some hardware suppliers will suggest them in high-vibration areas,
and they are to be avoided, since they get chewed up when they're
installed, and their thread engagement is lower than that of standard or
nylock nuts, which translates to less holding power. For high-vibration
applications, use a nylock grade 8 nut, full height--more thread
engagement means it will last longer and be less likely to fail. Another
little-known need-to-do about fasteners is that for any prevailing
torque design, whether nut or bolt, once it's removed, throw it away and
install a new one. Even nylocks do not hold well once they've been
installed and removed.