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
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
What is your tensile load? What is the shear load? What is your safety
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.
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
30 years ago. A modern race car will use a 5/16" bolt in the same
application, and submit it to 5 times the load - yes, it had better be a
pretty good bolt to reliably take that load, and anything SAE need not apply
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.
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.
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...
||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:
grade 5 bolts are 92kpsi yield and 120kpsi tensile, grade 8 are 130kpsi
yield and 150kpsi tensile. So grade 8 is quite a bit stronger. But the
difference between tensile and yield is quite a bit smaller on the grade
8 and the PERCENTAGE difference between grade 8 and grade 5 is about
half. Net: when grade 8 fails, it fails. When grade 5 fails, it tends to
bend and distort much more than grade 8. Gives you a chance to take it
out of servie.
Since most trailer hitches are bolted into some relatively thin frame
materials, I suspect that the number of bolts,location, and the fit in
the holes is much more important than grade 5 versus grade 8. YMMV
Chief McGee wrote:
Somehow this does not give me a warm and fuzzy feeling....................???
Sorry, but I work with alot of engineers, both mechanical and electrical, every
Suffice to say, simply stating "engineers designed it" does not help me believe