What grade is this bolt?

[loononastick]

People refer to the hardened bolts as grade 8. What grade are the standard bolts, and what does "grade" mean? I know it has to do with hardness, but exactly how much more hard is a grade?

[Richard J Kinch]

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

[tbasc]

Thanks R.K. That's a keeper. TB

[nicksanspam]

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.

Nick

[Edwin Pawlowski]

Not every bolt uses a nut.

[Harry K]

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, not tension.

Harry K

[J. Clarke]

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.

[bob kater]

[Larry Jaques]

On Sun, 09 Jul 2006 04:02:07 -0500, with neither quill nor qualm, [e-mail] quickly quoth:

Grade 5 is about 3 times stronger than an unmarked bolt. Grade 8 is about 4 times stronger @ 130kpsi.

[Nick Pine]

Hmmm. That might double this Pi((0.427/2)^2-(0.407/2)^2)120K = 1572 pounds.

Nick

[mm]

Doesn't this formula assume that only 1 circle of thread is engaged in the nut, when it's almost always more than 1 and could be 5 or 6 or more.

[Richard J Kinch]

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.