bolt strength

[Zukster]

I know this has been discussed several times and yes I tried a search, but nothing came up.

For suspension compenents, do you use grade 8 bolts, grade 5 bolts, or what?

I know grade 8 is stronger, but I've also heard that its sometimes better to use grade 5 since it will sometimes bend in a situation when a grade 8 would break.

[Steve N]

Can anyone else find my reply to this same question? I can't find it either. Bottom line use grade 8, or better bolts that are American made. (there are only two actual mfg.s) If you want a bolt that will have good ductility (bend) and exceeds the clamping force of an eight use ones such as Bowmalloy or Kar's above grade 8. I should start stocking the Bowmalloy bolts next month.

[squirrelman83]

I came home from a trip once and noticed my front bumper was hanging off the front. It doesn't really weigh that much, but it was being held on by a Grade 5 bolt. The metal on the bumper sheared the head right off the bolt. I din't even hit anything with it, it just broke off. Now, I've hit rocks with the bumper with the new Grade 8's and bent the whole brush guard and dented the front fender, but it didn't break the bolt. Just my experience with Grade 5 vs. Grade 8. I don't use, or recommend, anything other than Grade 8 now. Just my opinion. ~Steve

[BillaVista]

, but I've also heard that its sometimes better to use grade 5 since it will sometimes bend in a situation when a grade 8 would break.

What you have heard is a classic myth.

There is NO situation where a grade 5 would bend but a grade 8 would break. A grade 5 will break before a grade 8 in any situation.

The truth is, the difference between loads that will bend a bolt and break a bolt are greater in a grade 5 than grade 8, because the 8 is harder. But the 5 will bend before th8 and it will break before the 8 too.

I can think of no reason to use a grade 5 if a grade 8 is available, and for critical apllications the Bowmanalloy are a nice bolt without being horrendously expensive, like aircraft fasteners.

[JEEPRZ]

Originally posted by BillaVista:
<STRONG>I can think of no reason to use a grade 5 if a grade 8 is available, and for critical apllications the Bowmanalloy are a nice bolt without being horrendously expensive, like aircraft fasteners.</STRONG>

There are a few situations where there is no need for grade 8, cus its not cost effective. However, for everything critical, suspension, driveline, etc I use grade8

What is a Bowmanalloy? never heard of that one

[windows98]

im pretty sure you would use a weaker bolt if you want the bolt to break before something expensive does

[BillaVista]

Bowman is a fastener manufacturing company - and a good one, very popular in the big rig industry.

They make bolts of their own alloy - I don't know the component breakdown, but it has no SAE rating as their is not one high enough.

What's grade 8 again - about 120,000 PSI tensile, the Bowmanalloy is I believ 180,000 min and up to 200,000 I think.

[lostone65]

Originally posted by JEEPRZ:
<STRONG>What is a Bowman? never heard of that one</STRONG>

We Use there bolts at work. Have a full stock of them. all grade 8. Never had a problem with any of them

Got to love working in a heavy duty repair (100 ton hual trucks). Oh and all my bolts are FREE <IMG SRC="smilies/biggrin.gif" border="0"> <IMG SRC="smilies/biggrin.gif" border="0">

[hy_desert_4wheeler]

Billavista you must not work around machinery too much. I worked in a grain elevator that had grade 8 bolts holding the augers together. During harvest season we were replacing bolts everyday then we talked to a rep from one of the bolt companies. he recomended that we use grade 5 bolts instead of the grde 8's. Now they onle have to replace one or two per year. And that is in 14 different augers averaging 15 Twenty foot sections each using four 3/4 inch bolts attaching each one to the next. In my expirence it is not a myth but rather cold hard fact.

[injectedEB]

The rule of thumb I was given in school was grade 5 for sheer strength and grade 8 for tensile strength. The higher the tensile rating, the less ductile the metal is which makes it much easier to fail in a shear situation. If your using a grade 8 or higher in a sheer application, torque it down so the bolt is in tension and the friction between the parts being bolted together will hold the sheer forces.

[BillaVista]

Billavista you must not work around machinery too much

This is almost too delicious. It's funny, I read all the time guys saying stuff like "Theory is just crap" "And reading it in a book is no good" and "Practical experiece is best" blah blah blah.

Here's a great example of how practical experince may be fine, but only if you understand what's going on, otherwise you just perpetuate myth.

Now listen up sonny - I'm only gonna say this one more time.

Shear or tensile strength - A grade 8 bolt is stronger than an equivalent size grade 5 bolt always forever and ever every single fucking time. Got it!

Here - I reprint this here, without permission, just for your learnin'...mmmkay

"For a given load, any steel bolt (grade 8 and grade 5 or grade anything) will stretch the same amount, up to its yield strength (also known as its elastic limit). Remove the load, and it "unstretches" back to its original length, i.e. there will be no permanent stretch (like a spring). For a grade 8 bolt, this load (and corresponding stretch) is greater than a lower grade.

Above the yield strength, the stretch increases dramatically with very small increases in load, until the ultimate load is reached. Again, the ultimate load is higher for a grade 8 bolt than it is for a grade 5. Furthermore, if you stretch a bolt beyond its yield limit, it will not return to its original length (like a spring you've stretched out of shape).

Softer steels will stretch more than harder ones before they break, but the actual load at the break point will still be much lower for the soft steel.

I used tension examples, but shear behaves in much the same way.

Then there's fatigue strength. Bend a piece of metal back and forth a bunch of times; it may not break right at first, but it will break eventually. For any given metal, there is a fatigue limit, which is lower than the ultimate load. Below the fatigue limit, bend it as many times as you want and it won't break; above that, it will eventually break. Again, in general, the higher the ultimate strength, the higher the fatigue limit.

Finally there's impact loading. This is tested by notching a steel bar and hitting it with a hammer (very precisely, of course). This is the one case where a softer steel might do better, as harder materials tend to be more brittle. However, for most of our applications it's not that important (unless you're running a snowplow or trying to snatch a stuck Jeep out with a chain instead of an elastic nylon strap), as the suspension tends to damp out and spread loads below the sharp spike necessary for impact failure. Also, even grade 8 bolts aren't all that brittle; after they're heat treated to get the ultimate strength up they're tempered to bring back the impact strength. Impact strength might be an issue if something's loose, so that it shifts and bangs around before it hits whatever is stopping it (i.e. the loose bolt).

Conclusion: grade 8 bolts are stronger in virtually any application on a Jeep. Whether you need that strength, and want to pay extra for it, is another issue (though I bought some 5/16 grade 8's yesterday for $0.60 each; grade 5's were only ten cents cheaper). There may also be situations where you may want the bolt to fail before the expensive part it's holding breaks, but that's another issue. "

The rule of thumb I was given in school was grade 5 for sheer strength and grade 8 for tensile strength. The higher the tensile rating, the less ductile the metal is which makes it much easier to fail in a shear situation.

Get your money back from that school - shear strength is a factor of tensile strength, usually something like 60%

in a sheer application, torque it down so the bolt is in tension and the friction between the parts being bolted together will hold the sheer forces.

This is at least true, but not just for "Grade 8 or higher" but for all bolted joints - remember bolts are not supposed to be structiral pins, just clamps. BTW - there is noe SAE rating higher than 8, but there are bolts with greater ratings, just not SAE.