Grade 8 vs Grade 5 Bolts

Hey Chief,

I'm with you about #8 being more brittle than #5 but I can't prove it. Supposedly brittleness is not part of the SAE grade 8 or grade 5 spec.

Try googling sci.engr.mech and sci.engr.metallury for the subject "Rockwell Hardness as a predictor of toughness/brittleness?" I liked the answers I got from a guy posting as Pittsburgh Pete - that his answers backed me up had nothing to do with it. :-) Most of the sci.engr.mech replies preferred Grade 5.

I'd say that tightening down the fasteners enough and evenly is more important than Grade 8 vs 5.

And Grade 5 probably is spec'd due to lower cost most of the time.

Is road salt used in your part of the world? Is it very cold? Supposedly Grade 8 is more vulnerable to stress-corrosion cracking.

Jay the Pig.

Same size grade 5 and 8 have different ratings for tension and shear obviously.

What is your tensile load? What is the shear load? What is your safety factor? What is the temperature, chemical environment? Salt? What is the fatigue life with vibration? What is the design life?

I'm glad engineers design elevator cables. Do you want your children to drive at highway speeds on a congested freeway directly behind your loaded trailer with a hitch that you designed and fabricated?

Umm, honestly I think the biggest safety issue is the nut behind the wheel. Typically wielding a cell phone, not a torque wrench.

I would strongly suggest that Chief obtain a copy of "Nuts, Bolts, Fasteners and Plumbing" by Carroll Smith. AKA "Screw to Win."

Possibly the single best practical book on the subject.

Jim

So far the replies have pretty much ignored the application that the fasteners are in, which has a big impact on what choices to make. Carroll Smiths recommendations are for race cars, which is a big step from a trailer hitch. Even he says that in a lot of applications (even on race cars) the fasteners are so big that a nail will work. On my current race car I have an application where I have a 5/8" diameter bolt in double shear that will see a total load of less than 1,000 lbs - typical engineering overkill from

SAE graded bolts, either grade 5 or grade 8, are tension bolts not primarily designed for shear applications. The MS bolts so favored by Smith are shear bolts not primarily designed for tension applications. The biggest advantage to MS bolts is the quality control - you know that you will get a quality bolt every single time, while you don't get that with industrial quality bolts that are SAE graded. Grade 8 bolts are not brittle - I have bent them in pretzels to demonstrate this. On the other hand, anything will fail if you use it wrong, and putting a high strength bolt of any description under incorrect loads will result in failure.

What I would do if I were faced with your dilemma is A), use grade 8 plated bolts with grade 8 plated self-locking nuts, grade 8 plated washers and no lock-washers, B) consider the load type and try hard to have no threaded portions of the bolt under a shear load, C) calculate the expected gross load that the hitch will see in service, decide the load type (shear or tension), use about a 5X safety margin, and see if the bolt strength is adequate using 150,000 psi for strength in tension and 90,000 psi for strength in single shear (SAE does not specify shear strength, only tensile strength). If I wanted an upgrade from that I would go to HoloKrome plated SHCS with grade 8 self-locking nuts. HK SHCS are the only ones that I can readily buy that are available plated for corrosion resistance.

As an example, a 1/2" bolt will have a tensile strength of 29,000 Lbs and a shear strength of 17,600, maybe use 8 of them to attach your hitch, and you will have a gross strength of 232,000 pounds. If you use them all in single shear you will be able to tow 141,000 pounds. No doubt more than you need.

Brian

If you're really concerned, use grade 16. If you're really concerned and have money, use NAS bolts. NAS means National Aerospace Standard, and that hardware can't be beat for quality and strength. NAS has a tensile of around 190,000 psi. Most aircraft have at least a few in them. The standard AN (Airforce-Navy, now known as MS, or Military Standard) aircraft bolt is roughly as strong as grade 8, but is made of 2330 nickel steel, making it more corrosion resistant and much less brittle. AN stuff isn't much more money than grade 8, and the thread fits are better. A local aircraft parts supplier can fix you up.

Dan

According to Coast Fabrication,(where I buy my mil-spec fasteners, actually) MS bolts are 125KPSI, a lot closer to Grade 5. Reminds me that I need to call up and order some NAS 6303 bolts for my racecar...

Brian

Brian

||I am mounting a trailer hitch on 1-ton truck. Trailer weight will be up to ||15,000 lbs.. My first thought is to use Grade 8 bolts. (Tension and shear) ||But I notice most applications use Grade 5. Is this a matter of cost? I ||will only be using 8 or 10 bolts some the price difference does not matter. ||I know #8 is supposed to be better than #5, but is that true in all ||situations? Something in the back of my brain tells me #8 might be more ||brittle than #5. Also worried about vibration and shock from potholes. Any ||thoughts? Thanks, Chief

Hitches are designed with at least a 2-to-1 safety margin using the fasteners they ship with, so you should be fine with Grade 5. The current thinking on this an some racing applications is that a Grade 5 will fail gradually, showing signs of elongation and distress long bfore it fails altogether. A Gr 8 will often snap with little warning. I'd be OK with Grade 5, but make sure it's a true grade 5 and not some lowest-bid item that just happens to have 3 marks on the head. You should be OK with Dorman or Rockford, for example. Also, fasten all the bolts loose, then but some pull tension on the ball mount to take up the slack while you to the final torque (with a torque wrench) You might consider also welding a short bead in a couple of places that are readilu visible. Check the weld periodically. If it cracks you have a problem with the hitch trying to shift. Texas Parts Guy

The off road gang argues about this regularly for tow hooks. Concensus: grade 8 are stonger, grade 5 fail more gracefully.

Here is a decent chart showing the minimum specs on the various bolts:

S> I am mounting a trailer hitch on 1-ton truck. Trailer weight will be up to

Somehow this does not give me a warm and fuzzy feeling....................???

Sorry, but I work with alot of engineers, both mechanical and electrical, every day. Suffice to say, simply stating "engineers designed it" does not help me believe in anything.

Tom