NAS Bolts for tension application 4

[noormusa]

Can some one tell me the difference between the different types of NAS hex head bolts such as the 11xx series, the 13xx series and the NAS 501, NAS 464, 564 etc. Also can we use the NAS 13xx series bolts for tension applications or just for shear applications.

 

[tbuelna]

noormusa-

Here is a list of NAS fasteners, including bolts/screws, with a picture and a brief description of their characteristics.

You can use shear bolts for tension applications as long as you take into account their specific design characteristics that reduce their tensile capacity versus a tension bolt of similar size and material. Shear bolts usually have thinner heads, smaller radius under-head fillets, and shorter thread lengths.

Hope that helps.
Terry

 

[noormusa]

Thanks Terry
That helped a lot, however the list did not contain any details on the particular application (shear or tensile) for each bolt. To make my question more specific, can i use a NAS 6604 (1/4" dia) bolt for a tensile+shear application. The 6604 has a dimpled head:

It is rated at 150000 psi UTS.

 

[tbuelna]

Maybe, or maybe not. The dimpled head is typically used for aircraft shear bolts to reduce weight. You will need to do an analysis to determine whether this bolt is acceptable for the combined tension/shear loads of your particular application. The NAS spec should give you all of the information you need for your analysis.

 

[rb1957]

it'll have a shear and a tension allowable ... why not use in a shear + tension application ?

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

 

[stressebookllc]

Yeah, in fact if you look at MMPDS-05 Vol 8, Tables 8.1.5(a) and 8.1.5(b), the allowable tensile and shear loads in those tables are based on the material ultimate tensile and shear strength values.

Just make sure that for a shear application no threads are in shear, and for a heavy duty tension application there is a nice hex or pan head supported with a washer and there are at least 5 threads engaged.

From a production standpoint, a lot of times it just depends on what is in stock.

My 2 cents.

http://www.stressebook.com

Stressing Stresslessly!

 

[tbuelna]

noormusa-

Just to elaborate on stressebookllc's general rule about having at least 5 complete thread pitches engaged for highly loaded tension installations, this assumes the mating component (like a locknut) is made from a material of equal/greater strength.

Also, as I noted previously, aircraft shear bolts have very limited thread lengths and tighter control of the grip length. One reason is because the threaded section of the bolt should never be loaded in shear as stressebookllc pointed out. Another reason is that since these bolts primarily carry shear loads the mating shear nuts have reduced height to save weight, so less thread length is needed. If you're going to use a shear bolt in a tension application you need to be careful about the locknut you use.

Here are links to specs for a typical aircraft hex head shear bolt and a tension locknut. The total thread length listed for a .250-28 bolt is .316". Subtracting the (max) 2 incomplete runout threads and the .018" end chamfer, that leaves around .227" of useable thread. The height shown on the spec for a .250-28 tension locknut is .292". Most aircraft locknut installation procedures require a visual verification of at least one complete thread protruding past the locking feature, which in this case would be the upper edge of the locknut. So that would mean a .328" complete thread length required by the locknut, which is more than the shear bolt has available.

Lastly, with the short thread lengths used on shear bolts you need to pay close attention to the tolerances in your fastener stack-up. You want to avoid a situation where the nut bottoms out on the bolt threads before it clamps tight against the component surfaces.

Best regards,
Terry

 

[WKTaylor]

Tbuelna... good general description of shear VS tension bolts, typical hex-head.

Sorry folks... another long-winded/winding 'Taylor' discussion follows...

To emphasize: head, shank, head-shank-radius, thread style, thread-length, alloy/HT, fabrication methods, etc delineate the male [externally threaded] 1/2 of the threaded fastener equation. The mating female [internally threaded] fastener element [nut, threaded insert, etc] is as critical an element to the equation: Nut depth, thread engagement length, thread style [locking, NOT-Locking, Class, etc], alloy/HT, etc. define the nut capabilities.

NOTE.
Washers are an over-looked/under-appreciated factor in these joints that have multiple functions and their own significant issues.

Here is how I generally try to describe it to engineers.

o A tension-rated hex/flush-Hd/thread-bolt threaded into a tension rated nut/insert, meeting aircraft quality installation standards [not bottomed on thread runout and threads showing past top of nut], produces fastened joint capable of reliable tension-shear [tension and/or shear intended].

NOTE.
One characteristic of a well-designed tension joint [bolt-nut combination] is the presence of a strong washer under the nut [and-sometimes under the bolt head] to mitigate higher torque-tension installation forces and prevent friction-related structural damage; then withstand in-service tension [crushing] loads. SEE CAUTION.

CAUTION.
Bolt and/or nut MUST be torqued to a preload-value consistent with a tension rated joint [high preload].
Added washers MUST be capable of withstanding applied crushing loads from tension forces to be a true tension rated joint. IF the washer fails [deforms] under crushing tensile pre-loads-or-service loads, or torque-tension forces] the joint will fail.

o A tension-rated hex/flush-Hd/thread-bolt threaded into a shear-rated nut/insert [and tension or shear rated washers if needed], meeting aircraft quality installation standards [not bottomed on thread runout and threads showing past top of nut], produces fastened joint capable of reliable shear in with very low-tension capacity [not intended for significant tension other than to stabilize the bolt].

CAUTION.
Bolt and/or nut MUST be torqued to a preload-value consistent with a shear rated joint [low preload].
Added washers MUST be capable of withstanding applied crushing loads from nut torque-tension forces only [usually the only significant tension load on the joint]. Also 'soft' washers [aluminum or low-strength steel, etc] 'by default' de-rate the joint to "shear-only" by being unable to withstand significant tension crushing loads [without deforming].

o A shear-rated hex/flush-Hd/thread-bolt threaded into a shear-rated nut/insert [and thin shear rated washers if needed], meeting aircraft quality installation standards [not bottomed on thread runout and threads showing past top of nut], produces fastened joint capable of reliable shear in with very low-tension capacity [not intended for significant tension other than to stabilize the bolt].

NOTE.
Shear rated male fasteners with short threads are rarely capable of accepting a tension rated nut + washer within normal quality requirements, IE: the nut + washer stack-up is generally too deep for short/shear male threads. At best, a thin shear rated nut, with a thin [0.016, 0.032] washer [low strength OK], stack-up is possible. NOTE in some cases the low torque-tension pre-load on a nut may eliminate the need for any washer: and the joint integrity will rely only on the low-installation shear-nut-torque to prevent structural surface damage.

COMMENTS.
In general lock-bolts [crush-collars] and Hi-Loks [threaded collars] follow-these rules... except washers are generally optional and not necessarily desireable.
Evolution/history of threaded, and collared, solid-shank fasteners adds complexity to simple-strawman picture presented here. I haven’t the time to explain… back-to-work!
I have not even scratched the surface for bolts/nuts intended primarily for high-tension service [limited/no significant shear].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[Sparweb]

WKTaylor,
Never say your sorry for such great info!

About washers in tension joints... I tend to consider the "stiffness" of the washer as a factor in tension joints separating, not just the strength. I've never analyzed the value of this effect, but it just "feels" like aluminum washers under the bolt head in, say, a bathtub fitting, would adversely change the joint behaviour, compared to steel washers. Given the choices of materials are (a) aluminum and (b) steel, I tend to think in "either-or" terms, hence, steel washers for tension, aluminum washers for shear. (Then modify that judgement for corrosion/temperature considerations.)


STF

 

[rb1957]

my 2c ... the washer should be softer than the underlying structure. there will be times when a steel washer is better than Al (from an internal loads perspective) but then there should be an equivalent steel under the washer. you don't want the washer digging into the underlying structure.

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

 

[WKTaylor]

Spar web Concur!

NOTE.
I have seen significant joint failures due to crushed washers... especially in cyclic loading scenarios. The designers got the bolt and nut strength correct... but failed to find a matching washer.

CAUTION.
The actual strength of a washer material can be deceptive: washer specs are hard to interpret and easy to get wrong. I suggest an interesting exercise: Determine the Ftu Fty Fcy of all the alloys listed: some will drive You nuts trying to pin-down from the material specs exactly what the strength condition will be.

WORST EXAMPLE.
I-718 Bolt 220-TKSI + I-718 Nut 220-TKSI** + NAS1587 high temp Washer [1200F] MF 321 or 347 ANNEALED CRES sheet stock, 75-KSI-UTS at RT. Washers always crushed in service loosening the joints accelerating [titanium] hole elongation/wear-out and fretting damaged to the surface. I found out that these washers were selected for materials compatibility, NOT structural compatibility. We locally manufactured washers 'same as per NAS1587', except made them from I-718 barstock [PHT 185-KSI] or CuBe [PHT 175-KSI UTS] to prevent loosening. The strength match was still "low"... but was 'good-enough' to keep washer from crushing/distorting in-service.

** I-718 + I-718 threads require anti-seize compound to prevent galling/seizing when installing the nut on the bolt threads.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[Sparweb]

Thanks guys.
I admit, it's a bit of an oversimplification. But a good starting point so that one thinks about all elements of the joint, not just the bolt that seems to garner all the attention.

Early in my career, I once squeaked out of trouble by knowing just enough about this subject during a post-modification conformity inspection. The inspector (Transport Canada guy with 30+ experience) was examining some altered components in a rotorcraft control system. I'd done all the drawings and done my best to keep up with what the "grown ups" were talking about in terms of loads, range of motion, etc. But none of that was the problem; I was getting snagged on using aluminum washers under the bolt heads, not steel ones. The inspector had the OEM's parts catalog out and comparing my configuration with theirs, and finding the original configuration had steel washers in the stack-up. Taking a breath to gather my thoughts I was able to say directly that "it was selected for better corrosion resistance". His reply was that a steel washer would be "stronger". I was ready with the point that the joint is clearly in shear and the installation torque is also not commensurate with a tension joint. Dodged the bullet. Failing the inspection would have delayed the aircraft a day for disassembly and reassembly of the controls, and a repeat inspection, not to mension the embarrassment of being grounded for a set of 8 washers.



STF

 

[noormusa]

Thanks everyone, your response was very helpful.

 

[tbuelna]

noormusa- Thanks for letting us know you took the time to read thru them. Especially the comprehensive (or "long-winded/winding") posts from wktaylor! Where else except Eng-Tips would you get this level of free technical advice? I'm sure you thought your OP question had a fairly easy answer. But as you can see from the replies above, even a "simple" aircraft fastener installation must be carefully considered.