Assembly torque for drilled socket head screws

[conrad1]

Hello,

I´m looking in vain for assembly torque values for NAS1351 (drilled socket head) screws. The assembly torque for undrilled heads damages the alan key far to early before achieving the desired assembly torque. The damage occurs in a way that it´s impossible tighten or untighten the screw at all.

Does anyone know a standard or a official source for such values?

Thanks in advance

 

[Tmoose]

What is the screw size, and what is your "desired assembly torque" ?
Where did that torque value come from?
What was behind the decision to use NAS1351 fasteners? Just the fact they are drilled for safety wire?

Is it the hex key/Allen wrench being damaged? Or, is it the hex socket in the screw that is 'rounding out"?
In any case Please describe the damage and provide pictures of damaged screws and hex key tool used.

Once a hex key is rounded at all, it is useless. It will slip in the fsstener hex and ruin it. Replace the hex key, or grind it back to full sharp corners keeping the tool cool to the touch. After grinding, excessive chamfering of the end corners can reduce the "purchase" of the hex key in the socket head, and round out the socket as well.

============
This semi-official source suggests NAS1351 fasteners are 160 KSI Minimum Ultimate Tensile Fasteners. A far cry from "real" US socket head cap screws. And basic torque specs are mighty low compared to "real" SHCS.

https://www.fire.lacounty.gov/wp-co...andard-Practices-Manual-Chapter-02-Torque.pdf

 

[conrad1]

Hello,

the screw size is 0,1640-36 (dash no.8 of NAS1351). The assembly torque is corresponding to 90% utilisation of yield strength of the stress crosssection. Afaik that´s common practice and fits in most of the cases. For the drilled heads this seems to be too much torque.

The choice to use NAS1351 was indeed the drilled head.

The allen key is fine but as you mentioned the hex socket gets round until the torque can´t get transferred anymore. Unfortunately I can´t provide pictures but the case exactly like the picture here:

https://static1.squarespace.com/sta...1e3df28392a87f7c6/1469302861306/?format=1500w

 

[rb1957]

NAS1351 spec says "locking element within "M" region must develop required torque when tested in accordance with MIL-DTL-18240."
90% fty is typical for structural tension applications. Not sure I'd use a #8 for "structural tension". It sounds like this is too much, surprised it's an issue with the drilled head (you've tried undrilled heads and they work ok ?).

Tmoose, nice attachment, but you may want to disguise where you work (you wouldn't want the county of LA to get into to trouble with Bell Helicopters ...).

another day in paradise, or is paradise one day closer ?

 

[conrad1]

I agree, 90% of fty seems to be too much. But what would be the correct torque? I didn´t try this exactly on a NAS1351 screw with undrilled head but many times with comparable sizes of other socket head screws and it worked fine.

Thanks for the attachment Tmoose but i think the tabulated torque values apply only if you tighten the nut on the screw.

 

[WKTaylor]

conrad1...

See link to SPS brochure for socket head fasteners. It includes recommended torques... but also provides Cap Screw use CAUTIONs and WARNINGs that are eye-opening for NAS1351 & NAS1352 etc.

https://files.engineering.com/getfi...f9&file=SPS_Socket_Head_Fastener_Brochure.pdf

Other useful references...

FF-S-86 Cap Screw, Socket head

ASME B18.3 Socket Cap, Shoulder, Set Screws, and Hex Keys (Inch Series)

ASTM A574 Standard Specification for Alloy Steel Socket-Head Cap Screws

NOTE. I have had problems with rounding-out internal hex drives due to poor-fitting or wrong-size ['close metric'] wrenches... and 'slap-ass-wore-out' hex-drive [Allen] wrench-ends.

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]

 

[3DDave]

Especially in the small sizes, really small variations in the recess and the wrenches make a lot of difference. In the past I have gone to metric keys for some inch sizes and vice-versa due to excess variation in the produced heads.

 

[conrad1]

Thank you WKTaylor. I´ll do a few tries with these torques to verify.

 

[WKTaylor]

All... been in the business awhile...

I really like the emerging 6-lobe [Torx] recess drive design for ALL RECESS DRIVE fasteners... will probably replace the classic cruciform, off-set cruciform and Hi-Torque/dovetail/coin-slot recesses over the next few years... at least for new production and replacements.

https://www.google.com/search?q=tor...PG3qjcAhVpz1QKHc0MB-oQsAQIJg&biw=1680&bih=881

NAS fasteners with 6-lobe [Torx] recess drive designs...

NAS1798 BIT, SIX LOBE DRIVE, INTERNAL RECESS, FOR POWER AND HAND TOOLS

NAS1799 BIT, SIX LOBE RECESS DRIVER, SPECIFICATION

NAS1800 RECESS, SIX LOBE DRIVE, INTERNAL, DIMENSIONS FOR RECESS AND GAGES

NAS5310 SCREW, SHEAR, 130 DEG FLUSH HEAD, CLOSE TOLERANCE, SELF-LOCKING AND NON-LOCKING, NAS1800 SIX LOBE RECESS, 95 KSI Fsu

NAS5311 SCREW, FLAT FILLISTER HEAD, FULL THREAD, SELF-LOCKING OR NON-LOCKING, NAS1800 SIX LOBE RECESS

NAS5312 TENSION, 100 DEG FLUSH HEAD, FULL THREAD, NAS1800 SIX LOBE RECESS, SELF-LOCKING AND NON-LOCKING

NAS5313 SCREW, SHEAR, FLAT FILLISTER HEAD, CLOSE TOLERANCE, SHORT THREAD, NAS1800 SIX LOBE RECESS, 95 KSI FSU

NAS5314 SCREW, SHEAR, PAN HEAD, CLOSE TOLERANCE, SHORT THREAD, NAS1800 SIX LOBE RECESS, 95 KSI FSU

NAS5315 SCREW, SHEAR, 100 DEG FLUSH HEAD, CLOSE TOLERANCE, SHORT THREAD, NAS1800 SIX LOBE RECESS, 95 KSI FSU

NAS5316 SCREW, SHEAR, 100° FLUSH HEAD, CLOSE TOLERANCE, SELF-LOCKING AND NON-LOCKING, NAS1800 SIX LOBE RECESS, 95 KSI FSU

NAS5317 SCREW, PAN HEAD, FULL THREAD, SELF-LOCKING, NAS1800 SIX LOBE RECESS

NAS5318 BOLT, SHEAR, FLAT PAN HEAD, CLOSE TOLERANCE, LONG THREAD, NON-LOCKING AND SELF-LOCKING, NAS1800 SIX LOBE RECESS, 95 KSI FSU

NAS5319 SCREW, SHEAR, 100 DEG FLUSH HEAD, CLOSE TOLERANCE, NAS1800 SIX LOBE RECESS, 95 KSI Fsu

NAS5320 SCREW, SHEAR, 100 DEG FLUSH REDUCED HEAD, CLOSE TOLERANCE, NAS1800 SIX LOBE RECESS, 95 KSI FSU

NAS5321 BOLT, SHEAR, 100 DEGREE FLUSH HEAD, CLOSE TOLERANCE, NAS1800 SIX LOBE RECESS, 108 KSI Fsu - Rev


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]

 

[Tmoose]

What is the installation torque, in units of in-lb, lb-inch, Nm etc ?

For my money, damaging the hex socket when tightening with a tight fitting full length hex key is a sure sign the torque spec is simply too high for the actual bolt/screw material.

Are your fasteners possibly 0,1640-32 (8-32) not 0,1640-3 6 ?

The table 2-3 for 160ksi fasteners in my first link does not even list a torque for such a small fastener.

https://www.fire.lacounty.gov/wp-co...andard-Practices-Manual-Chapter-02-Torque.pdf

Concerns about tightening the fastener VS the nut are often of no concern.
"If the thread friction torque remains the same, the torsion in the shank will be the same irrespective of whether the bolt head or the nut is tightened."

http://www.boltscience.com/pages/nutorbolttightening.htm

 

[WKTaylor]

Tmoose... hope this makes sense...

The examples You refer to are for industrial fastener installations, primarily tension pre-load, without shank in shear-bearing [IE: shear pins are typically installed between screws/bolts] in typical aerospace structure.

Bolts/screws [stl, SStl, Ti, etc] are usually installed in very tight NAS618 Class A or B holes when shear-bearing load/fatigue is critical. In these cases the heads may/may-not have a washer; but the HL collar or Typical washer-nut installations are typically applied by torque-turning the collar/nut elements.

Bolts/screws [stl, SStl, Ti, etc] turned from the head are usually installed in ‘loose’ NAS618 Class ~C holes [aluminum structure/parts, ONLY]; or NAS618 class E holes in stl, SStl, Ti, HRA structure/parts into nutplates, barrel nuts, etc.

Reason is simple: tight fastener-shanks-to-hole-walls [for shear-fatigue] are often spiral-scored when bolts/screws are torque-turned from the head... and subsequent torque-tension is far-less 'predictable' [inconsistent] over-time. In-other-words, the joints have to be intentionally designed 'loose' for male-fastener torque-turn head installations... typically into nutplates or barrel-nuts or locations where nuts/collars are inaccessible.

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]

 

[Tmoose]

Hi Will,

Thanks for the info.

I'm thinking OP Conrad1 is tightening his fasteners via Socket head, and in fact severely OVER tightening them, based on reports of socket hex damage.
So far the actual tightening value remains a vague mystery. The "90% of yield strength" Conrad1 offered yesterday, July 17, relies on me walking down down a trail, hoping it is the same one he followed. (material, which heat treat condition, is yield strength listed as well as the UTS in the Bell link) .

I wonder if "90% of yield strength" is what is stated on the assembly instructions. ** In the end, the torque value the assemblers are is using is what counts. Unless, as others mentioned, the assemblers are using ill fitting hex keys.

Attached is an image showing that NAS1351 socket dimensions are the same as full head civilian SHCS. AND, that the thread pitch is indeed 36 tpi.

** Turns out we are licensed holders for drawings from a German OEM that call out one important torque value just that way, and that is also what appears in the OEM O&M manual. Other fasteners on the same Arrangement drawing carry torque in multiple units. The material spec is hidden in an obscure location from us, and missing completely from the O&M manual. I'm undecided if it was an intentional cruel trick, or a sign the OEM manual publishing group gave up in disgust.

 

[WKTaylor]

TMoose...

I'm wayyyyy over-thinking this... dohooo...

You are positively correct about torqueing 'cap' screw/bolts.

conrad1...

Typical cap-screw installation guidance may/may-not include directions to 'lubricate threads' with a drop of oil just prior to installation. Are You doing so?

Is Your shop using a 'flat-tipped hexagon key' driver; or a 'ball-nose hexagon key' driver [for off-angle torqueing]??

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]