Grade 8 vs. Stainless?

[Rezarf]

Guys, I am considering using stainless over grade 8 on the structural parts of my trailer, like mounting the cargo box to the frame.

What is the strength difference? I am leaning toward the stainless.

Thanks!

Drew

 

[nuclearlemon]

stainless is weaker

 

[Rezarf]

Ige, do you know by how much? Is it more like a grade 5 bolt?

 

[BogusBlake]

Well, for one, SAE Grade 8 is not a material but rather a strength rating (they can withstand a maximum tensile stress of 150k psi). The "Grade 8" fasteners you buy at the hardware store are medium carbon steel that has been tempered for strength and hardness then yellow zinc plated for corrosion resistance.

SAE Grade 8 fasteners are STRONG. But they are also sort of brittle. The ultimate strength is much closer to the yield strength than lower grade fasteners. What this means is that a Grade 5 bolt will stretch or bend a little before breaks, giving you a little bit of a warning.

Here's a good reference for determining what load rating of fastener you have or need: Fastener guide

CRES (Corrosion-Resistant Steel) used for MANY structural fasteners in aerospace and military applications typically has an ultimate tensile strength of 80k psi or more. To put this in perspective: A #10 screw (.190" diameter) of this material will hold 2000 lbs (a Volkswagen) in tension. This is similar to the "Stainless Steel" you're probably considering. The strength of fasteners made from CRES is typically a little less than SAE Grade 5.

These numbers are for tensile loading. The materials can typically withstand about 80% of their ultimate tensile strength when loaded in shear.
Also keep in mind that an overtorqued fastener is worthless- it's already deformed past it's yield point.

The choice really comes down to your requirements. Is the joint you're going to use this bolt on critical to the safety and function of your trailer? How many of these joints have to fail for something bad to happen? How is the fastener loaded (in tension= good, in shear= bad)? Is it in a spot where you can replace it easily? Is it next to other materials that will induce corrosion via a galvanic couple?

If it were me, I'd use SAE Grade 8 fasteners for things like suspension attachment and the CRES screws for things "in the weather" or where Ill see them often. For mounting the cargo box to the frame of your trailer, you'd probably be safe with the CRES fasteners, but if they're not in a corrosion-vulnerable spot, the SAE Grade 5 or 8 would probably be fine too.

 

[Rezarf]

So is that bogus Blake?

Great info man. I have been overbuilding this thing.I am trying to hold down basically a metal "topper" like a truck bed cover. Total wieght with RTT around 250#.

That said, where is a good fastern supply house in Louisville/Boulder? Someone that lets you bug them until you find what you need?

Thanks!

 

[BogusBlake]

I have been overbuilding this thing.

Nothing wrong with that! Unless you have a serious weight budget requirement, you don't need to push the limits of your fasteners. There's always shock loads and fatigue to consider, also the unquantifiable "solid feeling" you get when something is HFS.

In AZ, we had Copper State Nut and Bolt. They had everything. Surely there's some place like that up here. If not, there's always McMaster-Carr.

 

[ArloGuthroJeep]

So is that bogus Blake?

Great info man. I have been overbuilding this thing.I am trying to hold down basically a metal "topper" like a truck bed cover. Total wieght with RTT around 250#.

That said, where is a good fastern supply house in Louisville/Boulder? Someone that lets you bug them until you find what you need?

Thanks!

For a place you can walk into and actually bug a knowledgable person, Mcguckins is your best bet as far as I know...Their selection is by far bigger then any other hardware store I know of.

 

[Uncle Ben]

For a place you can walk into and actually bug a knowledgable person, Mcguckins is your best bet as far as I know...Their selection is by far bigger then any other hardware store I know of.

X2!

 

[Rezarf]

McGuckins came through. Cheaper than my Lowes score too. I love that store. I am going to go with the stainless. I am only talking a few hundred pounds over 10, 3/8" bolts.

thanks!

 

[Rezarf]

McGuckins came through. Cheaper than my Lowes score too. I love that store. I am going to go with the stainless. I am only talking a few hundred pounds over 10, 3/8" bolts.

thanks!

Nope, changed my mind, going for the sure thing over asthetics...

Can anyone speak to a good supply house in town? UB, Bruce or Nathaniel?

Looking for nuts and bolts.

Drew

 

[wesintl]

I used stainless on my m416 but they really weren't structural components. They hold the bed down on the frame, fenders, id tags, etc. One place I did have to reuse rust bolts were on the tail lights to the frame since they wouldn't ground properly. You should be able to use grade 8 zinc coated which are out there just about everywhere.

 

[Red_Chili]

What about stainless gun barrels? Doesn't it kinda depend on the alloy whether it can be hardened or not?

 

[BogusBlake]

Stainless fasteners are not graded, since they can't be hardened.

According to my notes, both A2 (metric stainless) and 18-8 (SAE stainless) go as:
1/4" thru 5/8" are 80-90 ksi yield, 100-125 ksi tensile
3/4" thru 1" are 45-70 ksi yield, 100 ksi tensile
>1" are 45-70 ksi, 80-90 ksi tensile

This would put stainless roughly just higher than Grade 2.

All true, according to my Machine Design book from college. So Dave's right- the CRES fasteners are closer to the strength of SAE Grade 2 than 5.

Also, even though most CRES fasteners have cut threads, you can get them with rolled threads which locally work hardens them for just a little more strength (if you really need to push it).



Hijack for trivia: Who knows the technical difference between a screw and a bolt?

 

[BogusBlake]

Well geez, even a clueless sparky can guess at that one. A screw is a fastener designed to be turned and a bolt is a fastener designed to have a nut turned onto it while its held motionless. Am I close?

You must have gone to engineering school.

 

[Rezarf]

Wow, this is getting close to an engineering pissing contest!

Seriously, that is the first real number I have heard of, thanks for the help guys and gals.

FWIW, I returned all the stainless today, and I will start tracking down the grade 8 I need.

Drew <><

 

[Hulk]

Well geez, even a clueless sparky can guess at that one. A screw is a fastener designed to be turned and a bolt is a fastener designed to have a nut turned onto it while its held motionless. Am I close?

So if I turn a bolt into a captive nut, it's suddenly a screw?

 

[corsair23]

So if I turn a bolt into a captive nut, it's suddenly a screw?

Nah...that would be scolt or a brew...I prefer the latter myself

 

[Uncle Ben]

So if I turn a bolt into a captive nut, it's suddenly a screw?

Sir....you have a screw loose!

 

[Groucho]

If McGuckins don't have it, I'd be stumped...

Nope, changed my mind, going for the sure thing over asthetics...

Can anyone speak to a good supply house in town? UB, Bruce or Nathaniel?

Looking for nuts and bolts.

Drew

Give me a PM, I might have what you are looking for. If you want to get stuff in town, Fastenal is where we get all of our G8 SAE hardware. Anything they can't get right away we get from McMasterCarr. Metric we go through Colorado Fasteners Metric in Longmont. But seriously, any 3/8-16 and 1/2-13 stuff give me a PM, I might have some of it.

 

[Beater]

a and i bolt in denver