There is no "standard". Perhaps you mean "no mark", which could mean an
SAE Grade 1 bolt (33 Kpsi proof strength) or could be just an ungraded
cheap import that is much weaker. The grade is all about the tensile
strength of the material, not the hardness (although harder usually
means stronger). Proof strengths run from SAE grade 1 at 33 Kpsi, 2 at
55, 4 at 65, 5 at 85, 7 at 105, and 8 at 120. Not sure what happened to
grades 3 and 6. The proof strength is determined by the carbon content
and (in grades 6 and up) alloy content of the steel.
To figure the actual strength of a bolt in pounds, you multiply the
cross sectional area of the minor diameter in inches times the proof
strength of the material in psi. So a grade 8 1/2-13 UNC bolt, having a
minor diameter of 0.407 inches, has pi*(0.407/2)^2 * 120,000 ~15,000 lbs proof strength; grade 1 would be 4300 lbs.
The common "B" mark on the bolt head (three ticks at 12/4/8 o'clock)
indicates SAE grade 5, and the "I" mark (six ticks at 12/2/4/6/8/10
o'clock) grade 8. The Handbook has a dozen different such marks for
various SAE and ASTM grades.
A higher grade bolt is not necessarily better. Besides cost, if you
ever have to drill it out, you want the lowest grade bolt that holds up
to the application.
"Bolts, screws, and other fasteners are marked on the head with a symbol
that identifies the grade of the fastener. The grade specification
establishes the minimum mechanical properties that the fastener must
meet." -- Machinery's Handbook, 26th ed., p 1488
Nice answer :-) Looks like we add about 20K psi per grade.
Hmmm. I woulda thunk the bolt would fail when the threads slip, eg when
the force exceeds the area of the ring between minor and major diameters
times the tensile strength. So if that bolt had a 0.427" major diameter,
it might fail at Pi((0.427/2)^2-(0.407/2)^2)120K = 1572 pounds, but
maybe that isn't true, if the nut is thick enough.
We built an experimental agricultural subsoiler in the shop I used to
work. Shanks were hardened (forget they alloy type), all shank bolts
were 3/4" grade 8. They failed on the first test. The bolts
broke...come to think of it, those would have all been in sheer load,
If possible one should avoid loading a bolt in tension.
Note that you need to consider the kind of loading as well as the total
force--higher strength bolts are also more brittle--they'll do better under
a continuous load but not necessarily as well under an impact load.
Thread failure is a possible failure mode, but that depends on the length
of thread engagement. Typically enough threads are engaged that the shank
pulls apart before the threads pull out. The Handbook tells you how to
analyze and calculate all that.
On Sun, 09 Jul 2006 04:02:07 -0500, with neither quill nor qualm,
email@example.com quickly quoth:
Grade 5 is about 3 times stronger than an unmarked bolt.
Grade 8 is about 4 times stronger @ 130kpsi.
When love and skill work together, expect a miracle.
--John Ruskin (1819-1900)
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