Shear strength of fasteners

[winifredevw]

I found this handy article with the formulas and a chart, pretty informative.

http://www.rockcrawler.com/2003-04/fast ... iscussion/

[helowrench]

The easiest way to stick with a known quality is to use aircraft hardware.
the stuff is found online for Lowes prices, and gives a level of performance that I can look up easily.
NAS 1100/1200/etc series bolts are cheap and easy.
Unfortunately by definition, they are only available in inch sizes.

[Ol'fogasaurus]

There are at least two classes to Fed (NAS, AN, MIL, AND, STB, etc.) Standards (Standards include fasteners; e.g., nuts, bolts, screws, washers, nut plates, studs, etc., specifications, processes, hold patterns, extrusions/shapes either "squirted of formed, etc.) flight ready and tool grade applications; both are very different in specifications. Standards also describe the application such as single shear, double shear, tension, general duty, etc. I have never worked with Tool Grade (mock-up, tools such as jigs) so I don't know the exact differences other than tool grade is not to be used in other than tools meaning not flight approved. The same thing with off-shore built standards... do you really know if they meet the specs and the rigorous testing and documenting that has to be done to meet the standards. I think I have ranted about this several times... sorry!

I didn't now Lowes could get better than "box store" grades. When I was first into fasteners they were not to be out for general purpose and you could get your fanny in real trouble for a lot of what is now being done by companies now days.

Lee

[helowrench]

Foggy.
The AN and MIL systems are being phased out where bolts nuts washers are concerned.
NAS is still currently widely used.

[Ol'fogasaurus]

I remember when the AN960 washers were changed to NAS1149 washers so I knew something was probably going on. You won't find out much on STB as that is really a small section dealing mostly with punched notches and slots. MS is where a lot of the electrical holes are as well as some other processes and hard parts. MIL was not used much but there was also a MIL- as I remember.

I wonder what they are going to mess with the AN 37° fittings and the AND and MIL fittings? Hmmm Been retired too long so I haven't been kept up-to-date. Still, what I said is true... some businesses are giving away the store . Even some people I know don't really understand the concept of security, only $$$!

[fusername]

probably not for a long time on the AN fittings I would assume. My new job has me playing w/ some military power electronics, and they REALLY dont like changing things on the ships, all our upgrades need to be drop in re-using old plumbing, so I figure that momentum will be a long time to break unless there is a real need. Can't blame em either, but cmon guys, let us give you a better rack!

nut and bolt standards however can come and go on the fly as long as they FFF drop in replace!

[transporterjr]

The table may well be correct - but note that the quoted section below is incorrect - i.e. - the cross sectional area calculated is incorrect.
All fasteners have a stress area - this is a tabled value, but there are formulas to calculate the stress area as well.

"Using a .250-inch diameter grade 8 fastener gives you the following shear capability:
A = Cross-sectional area of the fastener size (since bolt bodies/shanks have circular cross-sections, use area of a circle) = Pi x r2 where R (radius) = .250/2 = .125, therefore A = Pi x (.125)2 = .0491 square inches (in2)
Capability in shear = 91,000 lbs / in2 x .0491 in2 = 4468 lbs"

Just about all metric bolts go by ISO standards today. ISO, and most other organizations, do NOT publish shear strengths, only ultimate tensile strengths. A safe value in terms of MPa (or ksi for inch bolts) is 50% of the tensile strength, but it can be as high as 75% with 60% being often used. Note that the stronger the tensile stress is, the harder the bolt is, the less ductile it is, so the shear factor is actually less for a Property Class 10.9 bolt that it would be for an unhardened bolt. The higher property class bolt will still have a higher shear strength, it's just that the shear strength will not increase by the same ratio as the tensile strength.

The section saying manufactures use Grade 5 instead of Grade 8 because of cost, also is not correct. First, metric has been on just about every car since approx 1977, but applying the same logic to PC 8.8 and PC 10.9 bolts, the cost is often identical. With fasteners into aluminum, almost every auto manufacturer prefers that the bolt fail before the hole strips. Therefore, the strength of the bolt is chosen based on the engagement depth into the aluminum. That's why you will see a mix of 8.8 and 10.9 bolts on any modern engine.

[Ol'fogasaurus]

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[fusername]

An interesting tidbit on actual tensile strength of a bolted assembly. When you have a bolt that can handle say 2500 lbs of pull on it in tension. that doesn't mean you can hold up 2500 lbs with it bolted to the roof of your shop. When you torque your single bolt to the ceiling in this example, you use up some of that 2500 lbs based on the tension of the bolt being tightens in place. This is obvious to you or me when you look at it, but what is not intuitive is how MUCH do you use up. it is black magick to get an exact number since torque is so hard to translate to tension directly, so the error is many percent, however a proper torqued bolt uses up about 75% of its holding strength on JUST the preload of the assembly being put together.

So when you have a 1/4-20 grade 5 bolt,
1/4 inch bolt has .03 in^2 cross section area
if it has a proof strength of 85000 psi then
85000 psi * .03 in^2 = 2500 lbs <- Max strength of the bolt in tension before you are getting into dangerous territory
but now you are going to torque this bolt to the recomended 7 ft-lbs
7 ft lbs = 2000 lbs of clamping force!
2500lbs max on the bolt - 2000 lbs clamping force = 500 lbs left for the loop hanging from your ceiling.


now I think that is as clear as I know how to make it, hopefully this helps someone understand why a bolt seems so strong on paper but you always need so many more than you may think based on the numbers.