NAS517 tensile strength 1

[dcascap]

Hi,

For NAS517 screws, which it suppose that they have a min tensile strength of 160 ksi according to the spec.
Can I use this value as max strength of the screw in tension? Or should I somehow calculate also if the countersunk head fails before the shank?
The later is quite difficult in the case of a countersunk, even though I could make some kind of approximation.
I attach to the post the link of the specs.

https://www.aircraftspruce.com/catalog/pdf/NAS517%20TDS.pdf

 

[3DDave]

There is going to be a stress concentration at the transition, so that will likely cause the shank to fail at that location, though there is a second stress concentration where the threads end.

I would caution that this is not a tensile rated fastener - there isn't a guaranteed load for each size of screw. If it was reasonable to do so, they would have done it. The fastener material strength is mainly a quality control and indication of resistance to stripping the recess.

 

[dcascap]

Thanks for your answer!
What about using the spec MIL-S-7839, which is the procurement spec of the screw? Then I can use the shear allowable from NAS498 and tension for a low alloy steel corresponding to MIL-S-7839 (which is 110 ksi instead of 160 ksi so it should be conservative).

 

[3DDave]

If it is critical, get a sample of them and do a pull test with the joint geometry you intend to use and see how they do.

 

[rb1957]

I'd probably go with Ftu over the min thread area. I might try to calibrate this with other fasteners or by simple test. I wouldn't use 110ksi for 160ksi.

 

[dcascap]

Thanks for the answers I'll check how critical it is, even though it should not as it is mostly for shear loading...

 

[Sparweb]

Since when does any CSK head screw develop full tension strength in its joints?
I'll tell you: when the materials being fastened are very thick and very stiff.
Browse through MMPDS Chapter 8 and take note of when CSK fasteners develop full shear strength in the joint.
Tension is worse.
The head pries the hole open and tears through.
Developing full tensile strength of the fastener with a CSK head is possible, but you'll be surprised by the thickness of the joint that can do it.

 

[WKTaylor]

One quick point... 160-to-180-KSI is the HEAT TREAT of the alloy steel... not necessarily the tensile strength rating for the entire fastener

Refer to the specifications for 'spec min required tensile/shear strength'. A LOT of elements go into the static and fatigue strength of fasteners.

 

[dcascap]

Yes, I'm aware about how thick plates need to be to develop the full strength of a CSK, it's a very good point, that I have already kind of consider it.
Still my issue is that I'm not able to find the spec of min required tensile strength that easily, according to NAS517 MIL-S-7839 (only for tension, for shear another spec applies), which gives the following attached image, the only issue there is that the values are based on a low alloy steel of 125 KSI, I would think that then using this values it would be conservative, is this assumption correct? or Am I missing something?

Anyhow, it's a CSK screw, it should not be critical in tension, if I see it critical then I need to check with the designers of the part...
Thanks!

 

[WKTaylor]

FYI... NAS517 is an antique cross-recess flush-tension head [XR-FTH] long-thread [LT] low alloy steel [LAS] bolt [1950s]

Obviously You have a need for a structural XR-FTH-LT-LAS Bolt... why not a newer spec fastener? Is this for homebuilts?

Oh Well

 

[Sparweb]

Those fastener tables with aluminum bolts in them - they're so quaint, aren't they?
Don't see that often any more.

The last time I saw an aluminum bolts was actually a custom-made electrical bus-bar. I found out the fastener was aluminum when I....
... well that's another story, of course.

 

[dcascap]

Thanks for the comments, no it's not for a home built, I was asked to evaluate this design, that had this type of fastener in it. Already checked and in the end tension was not critical at all, which was expected.

If I would like to get/choose a new spec fastener, how can I do this? (Then I could raise this issue to the designers of the part) I find it difficult to search for new equivalent bolts etc... maybe Will or somebody else can point me in the right direction? It will be highly appreciated!
Thanks!

 

[Ng2020]

For conventional airframe fasteners - once you understand the requirements of your specific- a reasonable starting point is the pictorial guides in faa ac43-13b, use this to develop a shortlist of potential fastener specs for your specific application. Since some of the the material in ac43 13 is quite old, balance this against what is presently stocked by reviewing suppliers catalogues (eg skybolt, monroe, aircraft spruce, genuine aircraft hardware...), to narrow down your shortlist.
Then pull up the individual specs.

Alternatively look at other contemporary IPC's, or contemporary mods and repairs to get a sense of the types of hardware typically used.

Also take a look in chapter 51 of srm's for any medium to large aircraft and you will find fastener substution tables, which might not give you an answer, but may help point you in the right direction.

All that said, I don't think nas517 fasteners are uncommon.

 

[WKTaylor]

d... I could suggest a variety of male replacement fasteners... problem as I see it... your 'old design' seems 'comfy' with the NAS517 style inexpensive, readily available flush head bolts ['screws' generally have most of the shank fully threaded]. In this case the NAS517 has a large 100Deg flush head that fits old countersink diameters/depths... an evolution of antique flush rivet heads. So Your countersinks are deep in thicker sheet metal. OH, and yeah, an there are no repair oversizes in the '517... and the threads are very generously longer than needed... and it's all steel alloy.

Generally speaking a new properly designed structure... would cut-out at least %30 of the fastener mass by going to Flush shear head, short-thread steel alloy bolts ... and even more [+20%] if those same style bolts were made from titanium alloy. There would be NO cadmium or zinc-nickle plating nor eventual/inevitable all-over-rust to contend with... which would even invade the Phillip's-X-drive recess in the head.

Titanium is naturally impervious to corrosion... and with a baked-aluminum pigment-coating... titanium would be indefinitely inert to aluminum [never corrode] unlike all steels and SSTls.. which are high reactive to aluminum alloys.

An old engineer once advised me, RE: 160-KSI and above steel fasteners... rusted <=> busted

Just saying...

Make sure the alloy steel NAS517s get installed 'wet with primer or sealant'... shank, threads and shank and under-head/Csk... and in the X-recess.

--------------------

FYI... NASA RP-1228 Fastener Design Manual https://ntrs.nasa.gov/api/citations/19900009424/downloads/19900009424.pdf

 

[dcascap]

Thanks both of you for your answers and tips, it clarifies a bit better where and what do I need to look