Design Question - Which Fasteners Get Lockwire? 2

[JCorsico]

How do you decide whether or not a given fastener should get lockwire?

Obviously, through-bolts with a nut don't need lock wire, since the nut is considered reliable enough against loosening (except in the most severe of circumstances).

But for bolts threaded into a blind hole, how do you make the decision? I've seen various strategies - will the fastener cause a flight critical fault if it falls out? Is the fastener redundant with at least one other fastener? Are you using a thread locking Helicoil?

What about B-nuts and other nuts securing fluid lines? I know that Parker claims that their Dynatube fittings (aka, beam seal fittings) don't need lockwire because of the inherent design of the joint. But was about regular flared fittings? Sometimes they are lockwired, sometimes not.

Thank you!
Jon

 

[rb1957]

I don't get your train of thought … in my experience through bolts and nuts are just as likely as any other multi-part fastener to be wire-locked.

if the question is "when to wire-lock?", I'd say the answer is "when there's the potential for the fastener to loosen (due to a vibration environment or other) and the consequences of loosening are consider critical enough to want to prevent that from happening".

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

 

[3DDave]

Wiring really doesn't prevent loosening, it's an attempt to prevent loss. I've seen loss of preload in fasteners fixed with locking compound, sometimes identified by the remains of the fastener still in the mating part after fatigue from movement after the pre-load was gone, causing the body to fail.

What wiring does usually prevent the fastener from becoming a projectile, which locking compound does not.

The first line of defense is to ensure that the fastener doesn't lose tension. The second is to ensure there is no sliding. Then comes locking compound vs safety wire.

Though there is one area safety wire rules - if it is only allowed to be done by qualified mechanics it should flag to inspectors that fasteners were installed properly by same and, if missing, indicate they were not so installed. Locking compound doesn't do this, though torque-stripe compound is sometimes used to so indicate. The talent required to correctly safety wire is much higher that to squirt torque-strip material, so there's that.

 

[IRstuff]

cause a flight critical fault if it falls out?

I think the fail occurs when the fastener loses its preload. If that's a concern then you want thread lock or Nordlock. Axle nuts used pinned retainers to keep them from loosening under vibration

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

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[tbuelna]

In aerospace applications, the only time I would use lockwire is when a legacy component/hardware item specifies it. For new design I never use lockwire (or even loctite) since there are much better methods for securing threaded connections. One issue with lockwire is that it can create FOD when being removed/installed.

Here's some good references on the subject-

https://www.faa.gov/airports/resour.../go/document.information/documentNumber/20-71

https://standards.nasa.gov/file/12788/download?token=4aqc1uYN

 

[JCorsico]

Wiring really doesn't prevent loosening, it's an attempt to prevent loss

I've seen lots of people say this, but it doesn't make sense to me, and I've never seen any actual test data to back up this statement. How much force is required to break an 0.032 stainless steel wire? Does the force loosening a bolt (which is really just vibration, causing a temporary relaxation of preload) exceed the breaking force of the wire? Doesn't seem plausible to me. The wire is much much stronger than any tendency of the bolt to just loosen.

Now, obviously, if the wire is installed loose, so that the bolt can rotate before the wire stops further rotation, then a loss of preload is inevitable. But if the wire is properly installed tight, then it should stop bolt rotation.

In aerospace applications, the only time I would use lockwire is when a legacy component/hardware item specifies it.

Well, there are lots of engineers who do not follow this same practice (even though it's logical). Look at a modern GE or Rolls Royce turbine engine. Tons of threaded fasteners and tons of lock wire. How do those engineers decide which fasteners get lock wire and which fasteners don't? More interestingly, how do they decide which fluid connections get lock wire and which ones don't? There must be some objective standard that is applied. It can't be just gut sense of the engineer, as to whether the fastener loosening or falling out would cause a "critical" failure or not.

Here's a great example. See this video:

https://www.youtube.com/watch?v=eWRnLS_iWb0

In this video, you see a fluid line carrying oil on a GE turbine. There is no lock wire on the b nuts. Why not? There is lock wire holding the sensor in place. But no lock wire on the b-nuts. If one of those nuts fell off in flight, you've have a major issue. So how did GE get comfortable with that risk? B-nuts are known to fall off.

Aerospace engineering is driven by standards and objective testing. There must be something underlying these decisions, better than general guidance about whether a fastener is "critical" or not.

 

[3DDave]

Loosening of a fastener means lowering of the preload. It can come from embedment, shear displacement, or thermal differential expansion, as a few of the causes. It does not usually require rotation of the fastener - that comes later after the preload is lost.

 

[JCorsico]

Loosening of a fastener means lowering of the preload. It can come from embedment, shear displacement, or thermal differential expansion, as a few of the causes. It does not usually require rotation of the fastener - that comes later after the preload is lost.

Okay, but I don't quite follow your point. Based on this logic, there is not a single locking device in existance that could prevent loss of preload. All locking devices do is prevent rotation. And if preload is lost without rotation, a locking device cannot stop that.

But back to the subject! When do you use lockwire? There must be hard standards on this. Not just off-the-cuff advice.

 

[Sparweb]

For a start:
AC 43.13-1B
(Fairly big download)

More background:
FAA-H-8083-31A, Aviation Maintenance Technician Handbook-Airframe Volumes 1 and 2,
and
FAA-H-8083-32A, Aviation Maintenance Technician Handbook-Powerplant Volume 1 and 2
(Even bigger downloads, but worth every Megabyte!)

http://www.sparweb.ca

 

[JCorsico]

For a start:
AC 43.13-1B
(Fairly big download)

More background:
FAA-H-8083-31A, Aviation Maintenance Technician Handbook-Airframe Volumes 1 and 2,

and

FAA-H-8083-32A, Aviation Maintenance Technician Handbook-Powerplant Volume 1 and 2
(Even bigger downloads, but worth every Megabyte!)

I am familiar with all of these documents. They are guides on how to maintain or repair an already existing aircraft. They are not guides on how to design an aircraft. I am looking for a standard that defines when a threaded fastener requires lock wire. GE and Rolls Royce must be following some objective standard in designing their engines, and (for example) deciding which b-nut requires lock wire and which b-nut does not.

 

[rb1957]

"GE and Rolls Royce must be following some objective standard in designing their engines" … why assume/propose this ? I'd bet that GE and RR work to their own design experience.

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

 

[MintJulep]

faq725-1549

 

[drawoh]

Loosening of a fastener means lowering of the preload.

When I was asked to analyse hardware for crash safety in an aircraft, the people in charge refused to consider the possibility of the bolt being preloaded. All of my calculations were for loose bolts. Locking wire does not preserve preload, but it keeps the bolt from rotating completely out of the hole.

The potential projectile was not a bolt. It was a 70kg surveying instrument.

--
JHG

 

[Sparweb]

Are you looking for this?

Subpart D--Design and Construction General
Sec. 25.607 [Fasteners.]

[(a) Each removable bolt, screw, nut, pin, or other removable fastener must incorporate two separate locking devices if--
(1) Its loss could preclude continued flight and landing within the design limitations of the airplane using normal pilot skill and strength; or
(2) Its loss could result in reduction in pitch, yaw, or roll control capability or response below that required by Subpart B of this chapter.
(b) The fasteners specified in paragraph (a) of this section and their locking devices may not be adversely affected by the environmental conditions associated with the particular installation.
(c) No self-locking nut may be used on any bolt subject to rotation in operation unless a nonfriction locking device is used in addition to the self-locking device.]

Amdt. 25-23, Eff. 5/8/70

http://www.sparweb.ca

 

[WKTaylor]

In addition to the great comments by SW, RB59, tbuelna, 3DD...

Hope this makes sense...

1. Aerospace fastener basics/practices.

Due to cyclic physical/thermal loading, vibration and the general environment EVERY fastener installed in an aircraft MUST be ‘locked in place’... to ensure that it remains in-place to ‘do its job’ reliably for indefinitely.
Solid, blind rivets are deformed.
Blind bolts are deformed or have locking collars
Lock-bolts have swaged collars
EVERY male threaded fastener [bolts, shoulder-bolts, screws, Hi-Lok style threaded pin-fasteners, etc] must have a locking device to secure it under all circumstances....
Self-locking nuts/collars/inserts [metal/plastic, all-metal, distorted, post-installed crimped], castellated nuts [plain or self-locking] with cotter-pins or lockwire. Plain-nuts may be used with locking washers [lock-tab preferred; split locking or deformed-star NOT desirable] or stacked-jam-nuts; or the nuts must have corner-drilled-holes/lockwired; or must have a locking compound/adhesive applied during installation. CAUTION: use of locking compounds is only recommended as a back-up for parts with self-locking features [more later].
Notes: Certain bolts/screws may have self-locking ‘plastic plug-patches’ inserted/embedded-into or molded-onto the threads that effectively lock the threads into plain nuts and inserts.
Pins, with our without heads [solid shank, spiral-or c-spring-pins, etc] have various method of restraint... including cotter-pinning holes, shank-expansion, mechanical capture/restraint], crimped-ends, etc-etc.

2. JC... In-line with Your question.

ANY threaded fastener [duhhh] in any application where it is in a critical with low/no installed torque [to eliminate clamp-up forces]... or that is subject to rotational/torque forces... MUST have an external mechanical locking device for physical restraint and visual inspection. This includes bolts in lug joints and all installations where a pin or bolt is used in a pivoting application, such as moving control shafts, idler/bellcranks, clevis’s, hinges, etc-etc... These mechanical locking devices are traditionally castellated nuts with cotter-pins or lock-wire... or bolts/screws with drilled-holes for lockwire to adjacent structure [hole] or to adjacent bolts/screws in the cluster or pattern... or tab-locking washers... or mechanical crimping-deformation [physically impossible to separate parts]... etc. There are a many other unique/exotic ways to lock these parts together. IF cotter-pins or lockwire are to be used then the locked parts MUST have a hole drilled in the threads [bolt/screw] or castellation [nut]; or thru the heads or corners of the hex’s; or use tabbed washers; etc. REALLY large diameter parts may be locked after torqueing using other methods that are similar to cotter-pinning… but thru drilled-holes.

The selection of fasteners with integral locking... or multi-piece mechanical locking features... is a design decision that is important from the functional perspective of reliability.... and assembly, maintenance, parts procurement, etc.

CAUTION.
Critical permanent structural assembly may only require [1] method of locking... but that method MUST BY DEFINITION... be inspectable and verifiable. Self-locking nuts have a ‘sneaky’ way of loosening undetected under extreme conditions. Sooooo... Critical structural and mechanical fastener installations that rely on locking nuts [or nutplates] should always have marking compound [a brightly colored thickened paint or brittle sealant like material] applied across the exposed fastener elements and onto the structure in one smooth/long bead... after torque verification. An alternate to this method uses wet-sealant [or thread-locking compound] ‘squeeze-out’ that is filleted around the fastener head or nut/washer. IF/when the installation experiences loosening torque [any source], then visual inspection will be able to identify this movement by the ‘sheared/flaked/misaligned-bead or fillet’. NOTE: in my experience, parts always loosen... never tighten [normal service].

NOTE. Hi-Lok collars by definition are 'inspectable' when the collar drive-nut shears-off. I have actually seen installs where they were torqued... buty never broken... gag-me.

NOTE. JC... The USAF recently mandated safetying of all fluid fittings and ‘B’ nuts with lockwire or other devices... where traditionally these parts NEVER had the lockwire holes to do this. Caught us flat footed, since this, in my experience, is a rare to never occurrence. We [our group’s opinion] that the extremely high working pressures of new systems and/or the inexperience of mechanics or some recent mishaps with under/over-torqued parts loosening may be driving this requirement. There could also be something to do with O2 piping system security that I have become aware of... and possible electrical bonding/grounding thru piping systems for high voltage lightning and static dissipation. Uhhhhhh... engineering nightmare for parts substitution on antique jets... and few conforming 'stock parts'.

3. Safetying practices... [not including my company’s design or installation standards]... I use/refer to the most...

FAA AC43.13-1 ACCEPTABLE METHODS, TECHNIQUES, AND PRACTICES - AIRCRAFT INSPECTION AND REPAIR
NASM33540 SAFETY WIRING, SAFETY CABLING, COTTER PINNING, GENERAL PRACTICES FOR
SAE AS567 SAFETY CABLE, SAFETY WIRE, KEY WASHERS, AND COTTER PINS FOR PROPULSION SYSTEMS, GENERAL PRACTICES FOR USE OF
SAE AS1043 SAFETY WIRE HOLES, HEX, FITTING, LOCATION OF, DESIGN STANDARD
FAA-H-8083-30 AVIATION MAINTENANCE TECHNICIAN HANDBOOK – GENERAL
FAA-H-8083-31 V1 AVIATION MAINTENANCE TECHNICIAN HANDBOOK — AIRFRAME
NASA RP-1228 FASTENER DESIGN MANUAL
USAF T.O. 1-1A-8 ENGINEERING MANUAL SERIES - AIRCRAFT AND MISSILE REPAIR - STRUCTURAL HARDWARE
USA TM 1-1500-204-23-1 AVIATION UNIT MAINTENANCE (AVUM) AND AVIATION INTERMEDIATE MAINTENANCE (AVIM) MANUAL FOR GENERAL AIRCRAFT MAINTENANCE (GENERAL MAINTENANCE AND PRACTICES)

It is late. I am tired and goofy. Good Night.

Next day edit… there are elements in this discussion that relate to my 'somewhat obscure' response in thread

https://www.eng-tips.com/viewthread.cfm?qid=472092

RE 'screw loosening'

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]

 

[JCorsico]

Wil - exceedingly helpful post. Thank you!

The USAF recently mandated safetying of all fluid fittings and ‘B’ nuts with lockwire or other devices... where traditionally these parts NEVER had the lockwire holes to do this. Caught us flat footed, since this, in my experience, is a rare to never occurrence. We [our group’s opinion] that the extremely high working pressures of new systems and/or the inexperience of mechanics or some recent mishaps with under/over-torqued parts loosening may be driving this requirement. There could also be something to do with O2 piping system security that I have become aware of... and possible electrical bonding/grounding thru piping systems for high voltage lightning and static dissipation. Uhhhhhh... engineering nightmare for parts substitution on antique jets... and few conforming 'stock parts'.

I find this particularly interesting, as I agree that normal historical practice as been to not safety these nuts, except in unusual circumstances. But they do indeed fall off, from time to time. For example, see:

http://aerossurance.com/helicopters/loose-b-nuts/

In this case, it looks like there was a provision for safety wire, and the mechanic just screwed up by not installing it.

 

[tbuelna]

One important part of any aircraft design certification process is a FMECA. It is a standard approach for determining whether the documented design/analysis/manufacturing process/maintenance procedures/operating procedures/etc. comply with all relevant requirements/regulations. A proper FMECA will classify the criticality of your threaded fluid fitting connection based on the hazard level resulting from any credible failure mode involving the fluid fitting connection. The FMECA will ultimately determine whether the type and procedure you specified for securing this threaded fluid connection conforms with all relevant requirements/regulations.

As for lock wiring threaded fluid fitting connections (like B-nuts), there are actually some self-locking proprietary designs that have been qualified for critical aircraft applications, which don't require lock wire.

https://ph.parker.com/us/en/nutlok

Hope that helps.

 

[mfgenggear]

I believe it depends on the product and industry, as a youngster when it was permitted, brother would take me to Miramar while doing rebuilds and maintenance,
lock wire back then was all over the jet engines, has it changed, and yet Will Taylor has noted. Brother use a special tool to twist the lock wire, to the requirements.