Bolt-nut-combinations with cotter pins 2

[conrad1]

Hi,

i usually choose the combination NAS670x (bolt) and MS17825 (nut) – and the cotter pin of course. During the stress calculations of the bolted connections, a problem raised by the low tensile strength of the MS nuts. The tensile strength of the MS nut is approximately only half of the NAS bolt. The MS9358 or AS9358 nut seems to be a better one, not least because it´s made of the same material as the bolt.
The combination between the MS9358 nut and the NAS670x bolt is good regarding very similar strengths and resistances, but not regarding mounting and locking them with cotter pins.

The distance between the beginning of the close tolerance diameter (without thread) and the cotter pin hole is nearly the same distance as the nut height. For a 5/16 UNJF thread, the distance between the slot ground of the nut and the beginning of the pin hole is only about 0,152mm (0,006”). That´s a very close range. This bolt-nut combination would require a lot of different thick washers.
I think this is not a proper bolt-nut combination, although it´s an appropriate combination regarding strengths and resistances.

Can anyone help or recommend a better combination?

 

[verymadmac]

which reg are you meeting (FAR 23,25,27,29 etc).

Is it actually subject to direct rotation if not would a stiff nut work.

You could try a locking tab such as MS9276, MS9581, MS9582.

 

[conrad1]

Hi,

i think it´s the JAR29 or CS-29.
I don´t know exactly what you mean with the stiff nut. It should be a castellated nut to ensure can´t get lost during service.
Oh i see, your thought was a locking tab with a standard nut right? Unfortunately, that´s not possible.

 

[berkshire]

I think Mac means a distressed thread or nylock nut.
B.E.

You are judged not by what you know, but by what you can do.

 

[conrad1]

Thanks,

it has to be a bolt with a cotter pin nut (DAL level A). What i found is a NSA5060 nut. Also made of A286 CRES and not as high as the the MS9358 nut. But i still wonder about the bad combination of the MS9358 and the NAS670x bolt.

 

[tbuelna]

conrad1-

I'm assuming this is an aircraft application. The combination of a castellated locknut and cotter pin is normally only used with a shear bolt that must be free to rotate (like a clevis joint). If you take a look at FAR 25.607, you'll note this requirement, "(a) Each removable bolt, screw, nut, pin, or other removable fastener must incorporate two separate locking devices....". With threaded fasteners that are axially preloaded at installation, the two separate locking functions are provided by (1) friction at the thread flank contacts from axial preload, and (2) prevailing torque friction in the threads created by the interference fit of the locking feature.

As you're probably aware, the combination of a pre-drilled bolt & castellated locknut does not allow precision indexing of the installed nut. The MS17825 nut noted only has slots spaced 60 degrees apart, which means the max angular tolerance range of the installed nut is +/- 30deg. The type of joint this fastener combination is normally used for would not be axially preloaded and some amount of axial clearance would be acceptable, so the nut slots can readily be aligned with the pre-drilled hole in the bolt. If the nut is not axially preloaded at installation, it probably is not intended for any significant axial loading. So using a castellated locknut with lower material strength than the shear bolt should not pose a problem.

As a general rule, the use of cotter pins and lock wire is discouraged in the aerospace industry due to the FOD hazard it presents. Unfortunately, in some cases there is no suitable alternative currently available.

 

[conrad1]

tbuelna,

thanks for your reply. In the meantime, i had a look on some other topics here in this forum regarding the use of cotter pins. I think you are right, it´s not possible to achieve the correct preload and align the nut slot to the pin hole of the bolt.
In your opinion, the NAS670x is a shear bolt, right? Or is there a difference between the close tolerance bolts (like NAS670x) and shear bolts?

I knew about the first locking mechanism - the friction due to preloading the bolt. I´m not sure in understanding your second locking mechanism. That would be the cotter pin, a locking tab or something else right? I can´t follow you with the prevailing friction in the threads and the interference fit, sorry.

 

[MikeHalloran]

The MS9358 or AS9358 nut seems to be a better one, not least because it´s made of the same material as the bolt.

ISTR that 'hard' nuts can't develop the full strength of a bolt in tension because most of the force is directed to one or two of the engaged threads, whereas 'soft' nuts can deform plastically to distribute the force over the engaged threads.

There may be other valid arguments to support your choices.



Mike Halloran
Pembroke Pines, FL, USA

 

[conrad1]

That´s absolutely correct i think, but i would still choose the stronger nut because the weaker nut could fail during preloading with the wrench.

 

[tbuelna]

In your opinion, the NAS670x is a shear bolt, right? Or is there a difference between the close tolerance bolts (like NAS670x) and shear bolts?

The NAS670X series of bolts have a "close tolerance" body diameter that is slightly larger than the modified UNJF thread major diameter, and tight tolerance control (+/- .010") of the grip length, so they are definitely suitable for shear applications.

I can understand your confusion about this particular bolt. Looking at the current NAS6703 thru NAS6720 bolt standard the title now includes the word "tension", which was not used prior to 1987.

 

[WKTaylor]

Whoooaaaa tigers...

Conrad1 made an initial statement that I’m extremely confused and uncomfortable about, thus...

”I usually choose the combination NAS67xx (bolt) and MS17825 (nut) – and the cotter pin of course. During the stress calculations of the bolted connections,...”

What has me really bothered/confused... is ”NAS67xx (bolt) and MS17825 (nut) – and the cotter pin..”.... why??? This almost sounds like standard practice...

Analysis would NORMALLY lead me to conclude that this is STRICTLY a non-structural mechanical installation, ONLY... since this combination [long-thread bolt, castellated nut, cotter-pin] would be a non-credible departure from established standard structural Assy practices... where a shear-tension rated short thread bolt with a low-height shear-tension self-locking-nut ONLY would be used in typical fastener patterns.

With the one exception of a pin-lug joint where the single bolt connects lug-A to Lug-B and pre-load torque-tension is deliberately held ‘low’ so there is no significant tension loading on the bolt and/or thru the lug stack-up... which is loaded in high/critical shear.... and where lug-bending pre-load would be very bad... and where minute movement in the joint could cause the loosening/loss of a self-locking nut.

Then I got knocked back when You wanted to match a higher-tension/high-temperature rated BARE A286 castellated nut... typically installed in a turbine engine case application... onto a cadmium plated A286 airframe bolt.

CAUTION!
MS9358 [AS9358 procurement spec] is bare A286. A bare A286 nut installed-onto a cadmium plated A286 bolt... similar HT and high-quality/precision threads... torqued together to a high preload limit without any antiseize compound... is almost a guaranteed galling situation [I know all-too-well from experience]. Similar high strength A286 airframe nuts intended for installation on A286 cadmium plated or aluminum coated bolts will either be passivated with a 100% solid film lubricant coating [preferred] or 100% cadmium plating [usually obsolete]... which should be a suitable ant-galling combination... and eliminate the need for high temp antiseize lubricant.

I guess the follow-up question I have is: exactly what kind of installation are You designing/analyzing and why is a high torque-tension preload required and which also needs dual thread locking capability [self-locking + cotter-pin]????

NOTE.
Usually I state the installation process for a castellated nut, thus for [hypothetical mechanical installation]...

Torque nut to 140-to-150-in#, then back-off nut to ‘finger-tight/loose’. Immediately torque-nut to 100-in#... then torque-turn nut [tighter], MINIMUM required to align bolt-thread pin-hole with next nut castellation slot-pair. Install cotter-pin per NASM33540. Ensure cotter pin is fully engaged in castellation slots. May adjust cotter-pin location by adding disassembling/reassembling with [a] [specified] added washer(s) or with a single thinner washer.

NOTE.
Torque-turning a bolt head for this type installation is more complex

FYI NOTE [at risk of sounding like an irritating ‘know-it-all’].
NAS6703-to-6720 [A286] bolt is actually an evolution of the NAS1303-to-1320 [nom Dia] + NAS2903-to-2920 [1/64-OS] + NAS3003-to-3020 [2/64-OS] bolts, as follows...

NAS1303-to-1320 + NAS2903-to-2920 + NAS3003-to-3020 evolved directly to the NAS6603-to-6620 [Cd plated steel... Nom, X, Y Dias]... which also included the A286 version NAS6703-to-6720 [bare, Cd plated, aluminum coated... Nom, X, Y Dias]... and the titanium 6Al-4V NAS6803-to-NAS6820** version [bare or aluminum coated... Nom, X, Y Dias].

**Titanium bolt procurement NEVER allowed diameters up-to 20/16”... only 16/16” [1.000-inch]

FYI NOTE [at risk of sounding like an irritating ‘know-it-all’].
A long thread bolt can always accommodate a wide variety of nuts depths [heights if You prefer], from very shallow [light shear] to very deep [high tension] designated nuts. Hence an LT thread bolt can be used in pure shear or shear-tension applications, simply based on the installed NUT.

Conversely, a short threaded bolt## is usually limited to a very specific range of low depth [height] low/medium tensile strength nuts: usually allowing medium/high-shear with a relatively low [not negligible] tension rating

## NAS6203-to-6220 [steel], NAS6303-to-6320 [A286] and NAS6403-to-6420 [titanium] bolts are short thread versions of the long-thread bolts mentioned above [and also include the Nom, 1/64-OS and the 2/64-OS sizes]. These evolved from [steel] NAS1303-to-1320 [nom Dia] + NAS2903-to-2920 [1/64-OS] + NAS3003-to-3020 [2/64-OS] bolts, as follows...

FYI NOTE [at risk of sounding like an irritating ‘know-it-all’].
The reason for grouping steel, A286 and titanium parts together??? Steel is generally best defined as 'rusted = busted = corroded aluminum or steel or magnesium'.

A286 and Titanium have equivalent strength and fracture performance [with certain exceptions/limitations]... with a low/non-existent potential for [themselves] corroding in-service and affecting the structure. HOWEVER, this all presumes suitable protection for the base alloy structural elements from the peculiarities/dissimilarities of these alloys.

I hope this makes sense... barely makes sense to me... Gotta get back to work.


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]

 

[WKTaylor]

OK... DANG.

TYPO RE evolution of short thread bolts: the NAS6203-xx evolved from NAS464 which led to NAS1103-to-1120 series of bolts [to resolve some strength/fatigue issues]... NOT NAS1303-1320 [which I cut-pasted from prior paragraph].

Also, RE the NAS2903-to-2920 [1/64-OS 'X'] and NAS3003-to-3020 [2/64-OS 'Y'] bolts: they were developed as oversized [OS] replacements for NAS11xx and 13XX hex head bolts. For this purpose, they could be ‘coded’ to have short threads [ST] or long threads [LT]. WHAT A PAIN!

Similar circumstances occurred with evolution of [100-Deg] 'flush-tension-head' [FTH] and flush-shear-head [FSH] bolts with STs or LTs... made from low alloy steel [LAStl], A286 and Titanium [Ti]... but I won't go there unless someone is truly curious.

OH yeah... and most of these bolt have code provisions for threads W/WO cotter-pin holes; and [for protruding head parts, only] for lock-wire holes thru the hex-heads [hex-corners].


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]

 

[tbuelna]

In addition to the detailed posts from WKTaylor regarding material/finish compatibility, there is one other notable difference between the MS17825 and MS9358 castellated nuts. The MS17825 (and similar reduced height MS17826) nut is self-locking and the MS9358 nut is not. For certain applications the nut self-locking feature is required.

Here are a couple good technical references regarding fastener installations:

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_23_607-1.pdf

23.607-1 - Self-Locking Nuts on Bolts Subject to Rotation

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_20-71.pdf

20-71 - Dual Locking Devices on Fasteners

http://standards.sae.org/as567k/

AS567K - Safety Cable, Safety Wire, Key Washers, and Cotter Pins for Propulsion Systems, General Practices for Use of

http://everyspec.com/NASA/NASA-NASA-STD/download.php?spec=NASA-STD-5020.041400.pdf

NASA-STD-5020, NASA TECHNICAL STANDARD: REQUIREMENTS FOR THREADED FASTENING SYSTEMS IN SPACEFLIGHT HARDWARE

Here is a very useful diagram from the NASA std showing some of the basic requirements for an aircraft threaded fastener installation:

 

[conrad1]

Hi guys,

thanks for your replies and recommendations. It took a few days for an update.

I try to answer WKTaylor´s question: "I guess the follow-up question I have is: exactly what kind of installation are You designing/analyzing and why is a high torque-tension preload required and which also needs dual thread locking capability [self-locking + cotter-pin]????"

There are several positions where two plates are clamping a rod end in their middle. The rod end is a ball bearing rod end (MS21151). The intention is on the one hand to fix the rod end in an accurate position and on the other hand to avoid free moving during alternating loads.

More explanation: The high preloaded bolt shall provide enough clamping force (friction) to avoid damaging and deformation of the bore (aluminium plates) over time. The close tolerance section of the bolt shall provide an accurate position of the plates and the rod ends. Furthermore, the bolt shall withstand ultimate load without fracture.

The self locking feature of the bolt is not needed. Also the self locking feature of the MS17825 nut is not needed. Both of them are needles for my application. The cotter pin is my second locking mechanism.


In the meantime i found the NSA5060 castellated nut which is made of cadmium plated steel or passivated a286. I think, during assembling the the bolt and the nut, galling is not an issue when using a proper lubricant. Nevertheless, there is still the indexing issue with the solts of the nut and the pin hole from the bolt.

Thanks for your replies!