Okay, it's been more years than I want to admit since I learned all
about metalurgy (sp?) and structures while earning my Aerospace
Engineering degree (I have since learned that computers are more fun
and make my living doing that :-) ...), but here goes (sorry this is
going to be long)...
On the subject of grade 8 bolts (seems to be mentioned here often);
Chris Kent Kentarjiev and Philip J Ethier were discussing wheel studs
and changing the size from 7/16 to 3/8, and Philip J Ethier wrote (not
to single him out, many others have expressed similar comments
concerning grade eight bolts):
> ... I put in 1/2-inch grade 8 bolts. This
>is the same grade steel that Dorman boasts is used in their studs. ...
Bolt grades indicate a bolt's ultimate tensile strength (pulling
loads) and do not indicate the bolts shear strength (sideways loading,
tends to "cut" the bolt). A grade 8 bolt has a higher tensile strength
(pulling loads), but tends to be more brittle, have less fatigue
strength, and less shear strength than a similar quality grade 5 bolt.
The strength of a bolt is indeed dependent upon the type of the steel
used in manufacturing the bolt, however, the difference between grade
5 and grade 8 comes more from the heat treating during the
manufacturing process than any differrence in the type of steel (you
*can* use high quality steel to manufacture grade 5 bolts).
Under normal conditions, a grade 5 bolt will "yield" (stretch) long
befor the ultimate breaking point is reached; the yield point is
*significantly* lower than the breaking point. On the other hand, a
grade 8 bolt, while able to withstand a higher tensile loading, has a
yield point closer to it's breaking point. The result is that a grade
5 bolt will "bend" or "stretch" when it's load capability is exceeded,
A grade 8 bolt will just simply "break" (and suddenly at that) when
it's (higher) load capability is exceeded. In addition, a grade 8
bolt will fatigue and become even more brittle as it is subjected to
vibrations and shock loading. Ever take a piece of metal and bend it
back and forth until it "breaks"? The softer the metal the more
bending it takes (and the "gentler" the breaking), the "better" the
metal, the less bending necessary to break it (and it will break more
suddenly).
As an aside, remember the thread about sockets and impact wrenches? A
nice shiny *thin-walled* Craftsman (or who ever's) socket it usually
hardened, to make it stronger (the difference between good tools and
cheap tools). This "strength" is what can make it break when used
with an impact (high shock loading), and is why the dull, thick "soft"
impact sockets don't break.
What does all this mean? Simple:
*NEVER* use grade 8 bolts for *ANY* application subjected to high
vibration OR shock loading (such as suspension components, lug nuts,
etc.).
Incidentally, the reason the Philip's lugs are doing well probably has
more to due to the fact that the size increased (and high quality lugs
were used) than the change in grade. But for you others, remember,
the lugs don't have to hold much tensile load (they usually only need
to be tourqued to about 25 ft-lbs), but they do have to withstand a
lot of shear loading.
Tim Mullen mullent@ymv5.ymp.gov
72 Elan Sprint
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