Screw Loosen Analysis 2

[Lee.Conti]

Hi I wonder if anyone has done any analysis to check if the screw get loosen due to thermal effect?

Designer feedback the screw may get loosen due to thermal cycle... and I wonder if the screw is being screwed into the part and locked, how could it get loosen?

 

[IRstuff]

Conventional lock washers basically don't work, if that's what you're referring to. Sufficient torque will work for thermal effects, so long as the preloading exceeds the differential thermal effects. If the bolt and material being bolted are the same, it's less likely to loosen. Most other ways of increasing friction still depend on sufficient preload.

https://www.boltscience.com/pages/vibloose.htm

https://www.nord-lock.com/insights/knowledge/2018/why-do-bolts-loosen/

http://www.smartbolts.com/insights/loose-bolts-causes-ways-prevent/

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[rb1957]

what they're talking about is thermal strains can cause the preload to relax (whilst at temperature) … dissimilar (why don't we just say "different" ?) materials.

At a previous job I had to analyze bolts with a +120degC to -140degC thermal cycle, including the thermal effect, also including the scatter from preload application; and check that the bolts were always clamped and never yielded.

what is your thermal cycle ? maybe it's not significant in your application ?

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

 

[Compositepro]

"dissimilar (why don't we just say "different" ?) materials."

That is an interesting question. I believe that it is a word choice used by scientists to be very precise in their speech. Different materials can still be similar in the one important relevent characteristic, in this case CTE. Dissimilar has a more precise meaning in this case, materials with different CTE's. I find it reassuring to listen to technical professionals who speak this way, in contrast to engineers who, for example, use the words energy and power interchangeably most of the time.

 

[Tmoose]

Long parts retained by series of screws can really make the screws suffer from "thermal" effects.

The original Audi 5 cylinder engines snapped the exhaust manifold end studs with some regularity. I figured they were forced to bend without sliding.

Even Manifolds with Less severe temperature differentials resorted to clamping methods that tolerate sliding or bending.
Maybe for the gaskets sake sometimes.

http://www.mtfca.com/discus/messages/118802/144917.html?1276305342

https://cdn-0.barnfinds.com/wp-content/uploads/2017/02/51-studebaker-commander-3.jpg

 

[rb1957]

@compositepro … how about mass and weight ?

but if "dissimilar" means "different in s specific, relevant attribute", why can't "different" mean "different in a specific, relevant attribute" ?

why would "different" mean "different in an irrelevant attribute, but similar in the relevant ones" ?

words are just words … their meaning changes with time … particularly true for schedule and SoW !?

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

 

[Compositepro]

Science works because words are specifically defined before they are used. That is why we go to school. In common English, power, force, and energy can be used interchangeably. Not in science, those words have very precise and different definitions.

"words are just words … their meaning changes with time … particularly true for schedule and SoW !?"
I'm not sure what your point is here. This has nothing to do with science. There are many people who intentionally use ambiguous langauge so that they can not be held accountable. English majors love novels with ambiguous meanings. All scientific textbooks start every topic with a definition of terms and reduce concepts to unambiguous mathematical equations.

 

[mfgenggear]

@lee
if allowed may consider a product like Loctite , and look at the different strengths and temperature ratings, there is other products available from different manufactures.
screw will loosen up from vibration and thermal effects.
self locking screws also may be an other option. depending on the requirements and application.
dissimilar metal will cause electrolysis and corrosion. reason for coated threads if allowed and if necessary.

 

[WKTaylor]

OK, L.C.... and all... just catching-up.

A brief definition for precision. had this discussion with another mechanic turned engineer.

Screws to him were any male fastener that could be turned by a screwdriver. Bolts were male fasteners with external drives [hex, 2Xhex, etc] that were engaged into nuts and nutplates [etc].

However, a long-time-ago-and-far-away, a senior engineer clarified the definition of screws and bolts.

Screws are primarily intended for tensile loads with no value for shear, since the shanks were fully or mostly threaded. Typical use is for lightweight non-structural assembly; or mechanical assembly in-conjunction with precision shear-only-pins or interlocking surfaces. Head style and drive are irrelevant for the function of a screw... internal or external recesses or drives all in-use. Head sizes all tend to be relatively uniform in size. Low strength screws often have cut threads with rolled optional for larger diameters or where strength and reliability are premium. Thread styles tend to be 'lower quality/older style'... but not always... for engagement with common nuts/nutplates, machined or helical-coiled inserts, tap-threaded holes, etc. Often screws are machined from rod-stock... not always.

Bolts are intended for a combination of shear and tensile loads... typically having a relatively shorth thread [1.3-to-2.5-Dia].. with the remaining shank surface precision ground for shear-bearing in holes. Typical use is 'size dependent'... small-Dias for light structural use... larger for heavy structural use. Head style and drive are irrelevant for the function of a bolt... internal or external recesses or drives all in-use. Obviously head style also follows function for low or high tensile use [shear, shear-tensile, primarily tensile, high-tensile-fatigue, etc]. All bolts currently in use have rolled threads... and usually, not-always... cold-worked [roll-squeezed] head-to-shank fillets. Also bolt-thread styles have evolved controls over thread roots/flanks/tips, thread runout to the shank and tips... geometry/surface-finish cold-working. For all current [non-obsolete] bolts... the body shape/head-profile are inevitably made from high-quality/precision forged blanks for desirable grain control and finishing.

OK, OK, OK... male fasteners with long/necked shanks for absorbing high tensile energy, but no shear, are still essentially a special case that is a closer to screws that true bolts. Frankly, as a long-time structures/mechanical guy I have rarely encountered these bolts for any other application that special mechanical assembly.

And YES, bolted joints can be designed for high tensile performance [sustained and cyclic]… often used in pure tensile applications where the holes are simply oversized to the point that the bolts simply cannot 'see' shear loading... to gain maximum tensile performance... and sometimes allow for 'sloppy' tolerance assembly'.

Now having exhausted myself with this discussion... exactly what is Your joint configuration... screws or bolts [PN?] in what tolerance holes with what style nuts/washers [etc]???????????

NOTE. Years ago I worked on a 1960s jet that was intended for a relatively short combat life in VN... ~3500-Hrs. During a routine inspection for service life extension, we had several wings removed and the attaching steel lugs, front/rear spar, were to be inspected at the fatigue-cracking critical spar-cap holes in these joints. During bolt removal, the nuts/washers were mostly found loose... for practical purposes the nuts only had free-running torque [about what the self-locking feature should provide. This phenomena was prevalent for every bolted-hole in the lugs/spar-caps. When lug-sets were removed for further inspection, there was hidden primer/spar-cap rubbing/fretting and noted spar-cap hole wear/elongation. This was a major/important find during a condition inspection...and the BIG question/concern: Where did the torque-tension preload go on all the lugs-to-spar-cap bolts/nuts... and why the hole damage?

Background... Jet typically flew high-weight ordnance delivery missions with +/-G profiles during attack/FAC missions. The assembly drawings revealed the following... Bolt hole were sized for core drills thru the steel-aluminum-steel stack-up... so tolerances varied from 0.001-to-0.004... CLEARANCE... every hole/bolt combo. IF recall correctly... NAS110X series bolts, MS21042 nuts and AN960 steel washers were installed.

Yeah... we figure-out the problem and the recommended solution... which was accomplished by a field-team per TCTO [USAF equivalent to a Service Directive].

So, for conversation-sake... I'll ask...

L.C … Exactly what is Your joint configuration... screws or bolts [PN?] in what tolerance holes with what style nuts/washers [etc]???????????

In MY example... any see 'Where did the torque-tension preload go on all the lugs-to-spar-cap bolts/nuts... and why the hole damage'?
xxxxxxxxxx

FYI, NOTE... IF this is a serious high/wide temp-range bolting situation...

SAE ARP700 High Strength Elevated Temperature Bolting Practice

ASTM A453 Standard Specification for High-Temperature Bolting Materials, with Expansion Coefficients Comparable to Austenitic Stainless Steels

ASTM A193 Standard Specification for ALLOY-STEEL BOLTING MATERIALS FOR HIGH-TEMPERATURE SERVICE

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]

 

[mfgenggear]

the simple definition I have seen is as follows
screws are assembled into an other part with holes with tap threads.
bolts are assembled with nuts and are securing an assembly.
don't remember the exact size but bolts are 1/4 diameter and up
screws are less than 1/4 and can have either nuts or tapped holes. wrong or right.

 

[Lee.Conti]

Hi everyone,

Really appreciate for your thoughts. After I think further and chat with designers... I think there might be some worn happened to the thread due to thermal cycle resulted in loosen ...

 

[Lee.Conti]

Hi rb1957, how do you analyze thermal cycle?

We have thermal cycle from -30 to 85 C.

 

[rb1957]

CTE … this'll change the strains in the bolt and the clamped up flanges.

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

 

[Tmoose]

" CTE … this'll change the strains in the bolt and the clamped up flanges. "

And nearly irresistably cause the flanges to slide, and tear up/fret the faying surfaces, or nibble at the gasket, but a gasketed joint is rarely if ever "structural."

 

[drawoh]

...
However, a long-time-ago-and-far-away, a senior engineer clarified the definition of screws and bolts.
...

That barn ran away from the horse a very long time ago. Those things with number 10.9 on them on your car engine are called "cap[ ]screws". This subject was hammered on very hard in thread1103-224665. Note the very specific original post.

--
JHG

 

[WKTaylor]

L.C.

Thermal cycle -30C to +85C [-22F to 185F] seem pretty 'ho-hum' for Acft I've worked-on.

My old MIL-transport/tanker jet was designed for -65F to +160F [-54C to 71C] but that was when is was polished aluminum sitting alert in the 1950's... now they have dark/flat camouflage and real-world temperatures experienced range from -100F [polar/winter/night] to +190F [Saudi daytime/ramp] [-43.3C to +87.8C]… with +excursions close to +225F on wing trailing edge surfaces and horizontal stabilizers adjacent-to/behind engines... Saudi hot-day, engines exhaust running into a mild-tailwind/recirculating... not to mention high sonic.

So, L.C., I go back to my original question to You... what does Your joint and testing protocol REALLY look like...???

Testing per NASM1312? ...WHICH TESTS!
Protruding head or Flush head fastener.
Single lap-shear
Double-lap-shear
Low or high asymmetry [off-set centroids]
Screw-into a threaded-hole (or) nut/washer (or)nutplate [PNs, please]
Bolt-into a threaded hole, (or) nut/washer (or)nutplate [PNs, please]
Hole-to-bolt-shank clearance or transition or interference [fit]
Materials stack: Al-Al, Al-Stl/SStl, Stl/SStl-Al-Stl/SStl; or metal-composite mix
etc, etc

Hmmm... I see no-one is interested in my war-story 'question' as-to-why my 1960's era jet had so many loose nuts on the on wing-lug/spar-cap attach bolts... found during the [1st] routine 'condition inspection' of high time jets on the spar-joint... that resulted in an urgent TCTO? OH Well...

BTW... DOH... that thread you cited [thread1103-224665: Bolts vs Screws] was NOT one that I participated in... and would not normally have seen/participated-in... back in '08'. DANG.


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]

 

[mfgenggear]

geez Bolt vs Screw should of know not to type something
may be we were taught wrong at a young age when we could turn a wrench
get those 3/8 bolts for the heads, need a sheet metal screw, or that socket head screw.
small hardware fasteners were called screws, large hardware bolts who would of know.
and does it matter really

 

[rb1957]

"does it matter really" … blasphemer ! (in the Monty Python non-religious sense)

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

 

[Tmoose]

https://fivethirtyeight.com/features/how-to-tell-someones-age-when-all-you-know-is-her-name/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3415179/

I submit that it may be possible to judge a man's age by whether he believes this is a bolt or a screw -

https://media.tractorsupply.com/is/image/TractorSupplyCompany/1041302?$456$

 

[rb1957]

"illegal character" in your last link ?

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