Bolt_Question 3

[Equestion]

Good day everyone

I am wondering if someone can point me to a document of some sort that describes or discuss when a bolt is unfit to reuse. I'm thinking in the line of, type of scratches or markings that can be identified to indicate to one if it is dangerous or unsafe for reuse, for instance, will the scratch mark propagate into something dangerous like a small crack.

Thanks in advance.

E



Life is a DIY project

 

[TimMcK_Aero]

In two cases I can think of the solution is to replace the bolt if there is any doubt:
1) the bolt is for secondary structure, in which case it should <cough> have been designed with standard hardware and it would be cheaper to replace than to do an assessment.
2) the bolt is special and is for primary structure. Failure of the bolt would be catastrophic and the bolt should be replaced.

Obviously there are many bolts that get reused with minor scuffs, nicks of the edge of the bolt heads, coating scuffed off, etc. Generally, we rely on the experience on the maintenance technicians to do the assessment before coming to engineering.

I suspect that something specific has prompted your question. Can you give some more background to your question?

 

[Equestion]

indeed you are correct TimMck_Aero

One of my colleagues came to me with two bolts, received from maintenance. Both came out of a structure where bolts are used as you stated in your first (1) statement.
One of the bolts had some scuffs where the plating has been scratched and damaged.
The other had deeper nicks and scratches on it.
My colleague and I started to wonder when is a bolt safe for reuse. Regarding scrape markings on it. Surely a lot of factors could have contributed to it, incorrect installation, damage during operation etc. etc.… I know of a general rule that exists regarding the yield of a bolt.

I have seen this interesting read:

http://www.boltscience.com/pages/the-re-use-of-threaded-fasteners.pdf

Regards

E

Life is a DIY project

 

[WKTaylor]

Equestion... in addition to comments by TimMcK... and the article You referenced [not bad for such a brief discussion].

I'll wade in with a few broad comments, since this topic has been hard-learned within my experience-base on fasteners.

The mechanic reusing [re-using] any fastener must understand THEY are ultimately responsible for its serviceability, hence airworthiness of the air-vehicle that it is used-on. IF in doubt about serviceability, DO NOT reuse hardware without expert opinion/advice.

Traceability is essential. IF You intend to reuse hardware, maintain strict knowledge as to where [what specific hole or area] it was originally installed-in is ESSENTIAL. Wear patterns, hole-fits, threaded fits, etc are important to an airworthy assy. IF YOU DON'T KNOW WHERE IT CAME FROM, THEN DO NOT REUSE IT.

KNOW YOU PARTS/HARDWARE. Each type/element of every part has critical areas and elements. Some of these elements can be visually inspected [with magnification], some must be physically inspected [such as threads with specific running torque requirements, etc] and some must be non-destructively inspected for cracks pitting, etc that are usually not visually evident.

Before proceeding: is the part worth the effort to validate that it is serviceable/airworthy?

Also consider: how many times has this part already been reused??? Typically 5-reuse is limit of certification tests for threaded parts; however significant wear is noticeable after only a couple of reuses.

IF hardware is deformed during installation, DO NOT reuse it. OBVIOUS: solid rivets, blind fasteners, lock-bolt collars, etc. NOT SO OBVIOUS: small parts that are hard to inspect, soft-washers, self-locking nuts [especially with non-metallic inserts], nutplates [many reasons!!!], shear-pins, any parts that appear to have nicks/dings/scoring due to mallet/hammer impacts or ‘wrench-slip’ rounding of edges, screwdriver-recess distortion/scoring, improper tool-use [pliers, ILO sockets, etc]...

For steel, aluminum or other parts with corrosion evident [with limited exceptions], remember this simple phrase: ‘rusted’ = ‘busted’

IF coatings [platings or organic finishes] are worn, be careful: they perform certain functions that are not obvious... especially for friction-control [torque-drag, insert forces, etc] and protecting the structure that it is installed within from dissimilar metals. IF in doubt about coating serviceability, DO NOT reuse the part without expert opinion/advice.

Consider 100% replacement of small/cheap/commonly available hardware. Inspection, evaluation, segregation may not be worth the effort or aggravation. There is no firm dividing line for ‘small parts’...however my-personal rule of thumb for salvage/reuse is: (a) ~3/8 and larger for high strength high quality parts and oversizes: or (b) ½-inch and larger for lower strength older-style parts [AN, old NAS, etc] with multitudes of potential substitutes. Exceptions, that ‘hurt-my-head’: extremely long grip-lengths, or oversizes that are hard to replace or are very costly MAY be worth the trouble... or not.

Careful removal/disassy is an essential element of reuse. Abusive practices are guaranteed to limit/exclude reuse. Head, shank, holes/recesses and thread deformation, chipping, nicking and coating loss are common due to mishandling during/after removal/inspection: perfectly good parts can be rendered unserviceable by carelessness.

For unique or very expensive parts attention to details during removal/inspection/reuse process is essential. FOR EMPHASIS: careful removal/disassy/handling/storage is an essential element of reuse. Abusive practices are guaranteed to limit/exclude reuse. Match these fasteners back to their original installation-holes! What wears-together should stay together!

CAUTION. When male parts are removed from holes do so carefully to ensure the HOLES are serviceable: IE in tolerance [diametrical], without removal-induced scoring-scratching-gouging, no evidence of corrosion, etc. Hole damage may mandate repairs to restore serviceability, such as over-sizes or repair bushes, etc... Will the salvage fastener be of any value if the hole has to be oversized or bushed?

Also consider: what is the service history/experience with ‘xxxx reused parts’? Some parts have a dismal record of failure in-service or when reused. Certain very high strength steels [H11!!] and or parts prone to corrosion due to severe environment [exposed to oceanic atmospheres, high temperatures, exhaust-gasses, etc] present excessive risk and should never be reused!!

I have several FAA and USAF maintenance references... and the ‘discussions’ regarding reuse of fasteners in most is limited to: ‘if damaged replace it’.

Perhaps some SRMs have a section on fastener reuse. I suspect that OEMs are squeamish on this subject... and default to replace or ‘mechanic’s judgment/experience or standard shop practices’.

From USAF T.O. 1-1A-8 ENGINEERING MANUAL SERIES - AIRCRAFT AND MISSILE REPAIR - STRUCTURAL HARDWARE, here is a paragraph as to how complex the subject of reusing Nuts can become...

5.9.1.1 Reuse of Self-Locking Nuts. Self-locking nuts, either
all metal construction or non-metallic insert provide a
positive lock in an assembled joint. Many self-locking nuts
are manufactured for reuse. However, with repeated installation
and removals, it is typical for the locking performance
of self-locking nuts to degrade, and corrosion protective coatings
to wear. Reuse cycles of self-locking nuts are not recorded
maintenance actions. While nuts used in critical applications
generally possess redundant positive locking
features, this is not always the case. The potential effects of
degraded locking performance with repeated installation and
removals are: 1) loss of a positive lock with a resulting loosening
of the assembly, 2) loss of joint preload, and/or 3) loss
of joint integrity by either premature fatigue failure or loss of
joining hardware. Unless otherwise specified in maintenance
manuals for the particular type/model/series aircraft or components,
MS21042 and NAS1291 self-locking nuts shall not
be reused in any application, regardless of criticality. If the
maintenance manual does not address the reuse of MS21042
and NAS1291 self-locking nuts, then the requirement above
takes precedence. High strength, cadmium-plated, alloy- steel
self-locking nuts, such as MS21042 and NAS1291, are susceptible
to hydrogen assisted stress cracking (HASC) and hydrogen
environmentally assisted cracking (HEAC). Factors
that affect a nut’s susceptibility include high nut hardness, residual
tensile stresses induced during nut forming, applied installation
loads, and exposure to a corrosive environment.
Both HASC and HEAC are time- delayed fractures, and do
not exhibit any visual indication prior to failure. A nut’s susceptibility
to HASC and HEAC is further increased by worn
or missing cadmium plating on bearing surfaces and threads
that exposes the underlying alloy-steel to the environment,
which promotes active corrosion. Reuse of high-strength,
cadmium-plated, alloy-steel self-locking nuts can wear the
cadmium plating and expose the underlying alloy-steel to the
environment, and increase the potential for environmentally
assisted failures, such as galvanic corrosion, HASC and
HEAC. Eliminating the reuse of high-strength, cadmium plated,
alloy-steel self- locking nuts reduces the potential of
cadmium plating wear and the exposure of alloy-steel to the
environment. Self- locking nuts used in critical applications
or flight critical applications (such as blades, all rotor head
components and rotating controls, flight control actuators and
flight control systems, landing gear systems, wheel/tire assemblies,
external cargo systems, ejection seats, crashworthy
seats, primary load carrying structural attachments) shall not
be reused. This limitation applies to self-locking nuts of all
types of construction. (Navy Only) self-locking nuts in critical
applications or flight critical applications that show evidence
of reuse shall be removed and replaced during maintenance
actions. Eliminating the reuse of self-locking nuts in
critical applications or flight critical applications reduces the
potential for locking performance degradation and loss of
corrosion preventative coatings.

NOTE. Experience teaches valuable lessons. I have employed ‘heroic measures’ to salvage fasteners that have proven worthwhile... but have also expended tremendous energy trying to salvage parts that became lost-causes for many reasons. Here are two

I've salvaged/reused Taper-Loks for a jet wing that were removed to facilitate other maintenance. That was scary, since replacement TLs were scarce/non-available; and these bolts are usually carefully matched to their tapered holes; and installing oversizes is ridiculously difficult. In this case the nuts were backed-off the ends of the threads and used as hammer-anvils to assist knocking the TLs loose... which were then removed one-at-a-time and catalogued as to their exact/original hole. NOTE: this process was just the beginning phases of repair work that was mostly successful. BTW: new NUTS were readily available... but NOT cheap... in this case we sacrifice the 'pawn' [nuts] to save the 'Kings' [TL bolts].

Final example of difficult/tricky fastener can be...

A very high strength nut is used to hold wheel-halves together on transport aircraft. Each nut is ~$40/nut X 20-nuts/wheel X 8-wheel/tires/jet. Every time a wheel is disassembled for tire replacement, the nuts had to be inspected. However the use of carbon-disk brakes [which replaced steel-disk brakes] coated the nuts with baked-on carbon that was ridiculously adherent. The NDI shop refused to MPI the nuts due to potential for contamination of the fluorescent inspection fluids. The nuts USUALLY visually inspected ‘OK’... but the high strength nature of the steel mandated MPI... after-all, 'how-many-times had these nuts been reused after they were coated with baked-on-carbon'... 1-or-20???? Sadly, we failed to find an effective cleaner of the baked-on carbon residue; or any other creative solution that worked. The best solution for this problem was a more expensive Inconel 718 Nut of high strength, amazing toughness and virtually impervious to corrosion... and with a small dab of antiseize compound applied to threads during installation to minimize potential for galling with the bolt-threads. These nut no-longer required inspection... and are replaced when damaged or during overhaul, maybe.

Now, Igottagobacktowork.

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]

 

[Equestion]

WKTaylor

Thank you for an AWESOME reply ! I highly appreciate your comments.
Love to learn from these “experience” based comments with actual references. Made my day, especially the true d]story examples at the end !

Kind regards

E


Life is a DIY project

 

[rb1957]

"My colleague and I started to wonder when is a bolt safe for reuse. Regarding scrape markings on it. Surely a lot of factors could have contributed to it, incorrect installation, damage during operation etc. etc.… I know of a general rule that exists regarding the yield of a bolt."

default position ... don't (reuse fasteners).

either fasteners are dirt cheap (so why try to save a penny? the QA documentation would probably out weigh the savings) or they are expensive, in which case probably carefully designed and analyzed and re-using would require much analysis (most of which only the OEM would have).

we've recently accepted reusing High Lok holes if they are within design tolerance (instead of going to the over-size fastener).

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

 

[tbuelna]

If these are aircraft fasteners procured to a standard or specification, a visual/dimensional inspection of a few key characteristics listed in the standard/specification document is a cost-effective way to quickly evaluate them before proceeding with more costly NDI. No use spending money performing NDI on non-conforming fasteners.

What you need to consider is that any structural analysis related to these fasteners usually assumes they conform to the standard or specification, whether new or used. As an example, take a look at this sample bolt standard. With regards to the OP question about acceptable surface damage, in most cases anything exceeding what is specified by the standard should be cause for rejection. In this example, the bolt standard shows a surface roughness limit of 32 microinch for the bolt body, so that would be your acceptance criteria.

With a visual inspection, there are also requirements in the standard that the bolt will not meet after a single installation/removal cycle, such as condition of surface finishes like soft metal electroplating or DFLs, especially on the thread flank surfaces. Not usually a concern unless it causes excessive prevailing torque at re-installation of the fasteners.