NAS1351 Minimum Strength Note Question 1

The answer spans spec NAS1351 and the procurement spec FF-S-86...

1. NAS1351...

Table II Minimum Breaking Strength (LBF)
'Alloy steel' [column]
Diameters -4, -5, -6, -7, -8 [-8 = 1/2"-Dia]

6,200 6,550 /h/
9,850 10,400 /h/
14,900 15,800 /h/
20,200 21,400 /h/
27,000 28,800 /h/

/h/ MINIMUM BREAKING STRENGTH VALUES ARE BASED ON 180 KSI HEAT TREATMENT. LINED THROUGH STRENGTH VALUES WERE BASED ON 170 KSI.

2. Per the procurement specification FF-S-86...

3.2 Mechanical properties.
3.2.1 Alloy steel cylindrical head cap screws. Ultimate tensile strength
and hardness of alloy steel cylindrical head cap screws shall conform to the
following:
(a) Ultimate tensile strength
(1) Sizes .060 through .500 inch •••••••••••••• 180,000 PSI min
(2) Sizes over .500 inch •••••••••••••••••• l70,000 PSI min

3. Summary/Conclusions...

3.1 IF the NAS1351 are to-be made/qualified per FF-S-86, then it is mandatory for screws made from alloy steel to 'meet' ultimate tensile [breaking] strength per FF-S-86 Table II.

3.2 For all NAS1351 screws 0.500-Dia, and smaller Dia, it is mandatory for them to attain UTS of 180-KSI

3.3 For all NAS1351 screws larger-than 0.500-Dia, it is mandatory for them to attain UTS of 170-KSI

3.4 Obviously for screws 0.500-Dia and smaller there was a 'awsh*t', when spec was introduced... and these smaller Diameters only required [in the NAS1351 spec] to meet UTS of 170-KSI... which did NOT conform with long-standing MANDATORY requirements of FF-S-86 to meet UTS of 180-KSI.

3.5 AIA/NAS elected to leave the UTS 170-KSI values on Table II as 'a record of the prior requirement'... but 'struck-thru these values' making them no longer valid/applicable... and added the appropriate [UTS 180-KSI] values that current production fasteners MUST attain.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Thank you Wil! This makes sense. It didn't occur to me that 'lined through' referred to the crossed out data. Your insight is much appreciated!

Something similar happened [or should happen] with Titanium bolts...

1. Hex Bolts... example...

Procurement spec NAS4004 does NOT include diameters greater than 0.7500 [Nom/X/Y], so sizes 14, 16, 18 and 20 were 'lined-out' with flag note explanation.

NAS6403-to-6420 hex head bolts, short thread, Ti, nom/X/Y Dia

NAS6803-to-6820 hex head bolts, short thread, Ti, nom/X/Y Dia

2. Flush head Bolts... odd contradiction... Dias NOT lined-out when they should be lined-out... example...

Procurement spec NAS4004 does NOT include diameters greater than 0.7500 [Nom/X/Y]... but, interestingly/oddly, sizes 14 and 16 have NOT been 'lined-out' [as of latest spec revisions that I have] in these specs...

I doubt if the 14 or 16 Dias would even be available, due to spec contradictions...

NAS7303-to-7316 FT head bolts, long thread, Ti, nom/X/Y Dia

NAS7603-to-7616 FT head bolts, short thread, Ti, nom/X/Y Dia

NAS8802-to-8816 FS head bolts, short thread, Ti, nom/X/Y Dia



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Thank you again! Great info.