SAE provides "standards," but there is no quality control nor consistency in the parts. The FAA has put out numerous warnings about the use of Grade 8 hardware...weather or not one chooses to use this hardware on aircraft is beside the point; the warning is valid. The parts do not show consistency, which is why they're not approved for use on aircraft. If tensile strength were the only issue, they'd be fine, but there is no quality assurance in grade 8 hardware.
Again, do what you will for the critical operation upon which your life depends. Don't however try to hold out automotive standards as some high level of quality, because they're not. Automotive parts are made based on economy, and it's far easier to replace a defective cheap part in the automotive industry than manufactur higher quality equipment. Automotive standards are rough, often not overly exact. Rather than build a quality part for a car, usually the part is just built bigger.
Most of your hardware store bolts (or automotive store bolts, if you prefer) have tensile strengths closer to 60,000 PSI or less, with Grade 8 at a shear value of 91,000 psi and 150,000 ultimate (in theory), while AN bolts are 125,000 psi. Hardware store SAE bolts are mostly not corrosion resistant, and bend easily. AN bolts are nickle alloys, while SAE Grade 8 bolts are mild steel. AN bolts are more ductile, while SAE Grade 8 are more brittle. AN hardware and SAE hardware have very different thread load bearing properties and capabilities. AN, NAS, and MS bolts (aircraft grade hardware) are consistent in properties, qualities, and standard from lot to lot. SAE Grade 8 hardware is NOT. AN bolt threads are rolled after heat treating, whereas SAE are cut afterward.
All AN hardware has the same properties. Grade 8 hardware has vastly different properties that vary with manufacturer and lot number. Where the AN hardware is rolled, often the SAE Grade A threads are cut, and that's one of two places the bolts tend to break. Yours twisted off with torque, I believe. Did it twist off in the threads? The other place these tend to break is the head, owing both the britleness,and the inconsistency in radiusing the head and shank.
You'll note I listed alleged tensile strengths. Tensile strengths for SAE Grade 8 can only be approximated, due to the variance between lots. This is not the case for NAS, MS, or AN hardware, and that's a very important distinction.
You mumbled something about not confusing SAE hardware with hardware-store hardware. I don't want to burst your bubble (actually, I do), but hardware store hardware IS SAE. It may nor may not be Grade 8, as numerous grades exist, but all the standard bolts and sizes you find are SAE bolts...just with varying degrees and qualities of production. Grade 5 is SAE. So is Grade 8. Grade 5 has lower tensile strengths, but tends to bend more, is less brittle, and may actually do better than the Grade 8 in many cases. Of course, you need to specify weather you're after actual tensile strength (as in holding the threads or headof the bolt on) or shear strength. In either case, test out several examples purchased from different sources in the SAE, and you'll find numerous different failure points...showing clearly the lack of consistency in production.
You cannot compare SAE fasteners to just any "hardware store" bolt. The truth is, each SAE grade is manufactured to meet certain tolerances. That's why these fasteners are widely used in the automotive industry to hold critical parts together. If they weren't made to specific tolerances, the auto manufactures wouldn't risk billions of dollars in lawsuits by using them.
Actually yes, the hardware store bolts are SAE bolts. Not all grade 8, but still SAE...unless they're metric.
If they weren't made to specific tolerances, auto manufacturers wouldn't take the risk? No? Why do you suppose the FAA doesn't permit or authorize their use? Think about that a little before you reply.
