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Tensile strength comparison

Stewbronco

Well Known Member
Ok...so I should be able to figure this ..BUT ... what would be the comparative tensile strength between 3 #8 structural steel screws( MS24694 old AN509 ) and 2 AN3 bolts ...?? Shear strength ?

Many thanks. Stew
 
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Search and download MIL-HDBK-5J. Threaded fasteners are in chapter 8.

Remember, there is a lot more to joint design than just fastener choice.
 
These are not exactly an apples to apples comparison, since the MS24694 screw is a flush-head screw and the AN3 is a hex-head bolt. I'd avoid using a flush-headed fastener in a tension application if possible, especially in aluminum, since you won't develop the full strength of it. However, I don't have data to support that.

A better choice would be the MS27039 screw, which has a protruding head. It's available at Spruce, isn't expensive, and has the same 125 ksi alloy steel strength. Its tension strength is 1,750 pounds and its shear strength is 1,585 pounds. But you should know that I don't have data on the tensile strength of a typical kit nutplate and that you probably won't be able to carry this load with that, so it's moot.

The same data for AN3 bolts is tension strength of 2,250 ponds and shear strength of 2,125 pounds.

BIG CAVEAT: DanH was right, of course. Joint design is very involved and complicated. Back before I retired, I had a form that I used to analyze joints and that thing was a very dense 7 pages of analysis. MIL-HDBK-5J is a good beginning reference but not much of a guide and omits some information, so be careful. You sort of really need to know what you're doing here. In addition, if you're planning on having the fasteners carry the load sequentially rather than being evenly-loaded, you could build yourself a zipper, where the first fastener fails and then the second fails and finally the third. Another complication is that we don't know the tensile strength of the #8 nutplates and they are probably considerably lower strength than an AN365 nut would be, so you wouldn't get all the tensile strength you want.

All in all, my suggestion would be to build it per the plans and don't worry about it. A day's annoyance now might save you some real agony down the road.

Dave
Retired Stress Analysis Engineer
 
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