A Musing on Modeling Safety Wire - How Much Detail is Necessary?

A question that was recently posed to me was; "How would you model safety wire in an assembly?" 

Safety wire on a fuel
divider on an 
aircraft engine

At first, I thought I'd write a post trying to summarize the standard, and how I've seen it approached in my travels over the years. But no matter how I tried to "summarize" the standard, it ended up too long, and so dry it put me to sleep.  So instead, I'm going to try writing this briefly, and hopefully to the point. 

First of all, Safety (aka lock-wire) is small diameter wire of various sizes that is used to prevent fasteners from loosening and ultimately falling out. It should always pull in a direction that tightens the fastener. It's usually twisted with 6-8 twists per inch. 

Of course there are more details, but they're covered by standards. In my aviation maintenance travels, that standard is AC43.13-1b, issued by the Federal Aviation Administration  (FAA). In my engineering travels, that standard is NASM33540.  I'm sure there are other standards.

That's important. The standards tell the installer how to secure the fasteners with safety wire.

So when it comes time to show safety wire on a model or drawing, is it normal to show the twisted wire?  Is it modeled exactly as shown in the

image to the right?

Heck no! That takes a lot of time and computer resources, which gets expensive fast. And having a standard to reference, there's little to be gained other than bragging rights for the designer. 

Instead, I've seen, and used, on of two alternatives. 

The first, is to use a sweep in the model, showing where the wire should go. This takes a little modeling time and dedication, but it will show up on the model, and propagate to the drawing when its created.

Safety wire shown in the model.
I've colored it in red here to make it stand out. 

The modeled sweep representing the safety wire on the drawing.
A leader references the standards in the notes.

The other, is to use sketch geometry when the drawing is created. It takes a bit of time to sketch in the drawing, but the results do a good job of showing the desired result.

Sketches on the drawing calling out the safety wire
Circles and lines represent the wire's twists.

Which ever method is used, a note can call out the standard to be complied with.  So the wire shown on on the drawing and model show where the wire should go, the note calls out the standard to reference.

If the installer has any doubts, the standard should be readily available for reference. I know in many cases, it's probably even legally required to be available. 

The next logical question for a reader may be, "How does this apply to me?" After all, while safe wire isn't uncommon, there are plenty of users who live long, fulfilling lives without ever touching safety wire.

If you get anything out of this post, ponder if there's anything that can be streamlined by adding more or less detail? How detailed does the model of that purchased part need to be? Are you spending extra time showing model details that are covered by a standards that can just be shown by a note with a leader? 

Perhaps take a few minutes to think it over. You might find you save hours! 

Acknowledgements

Models created by me in Autodesk Fusion 360

McMaster Carr models used:

• Round Head Screws (wire lockable) - P/N 90350A310 
• Flat Washer: P/N 92141A011
• 45 degree elbow (37 degree flare to NPT): P/N 50715K637
• 90 degree elbow (37 degree flare to NPT): P/N 50715K413

FAA Reference for Safetying: AC43.13-1b (See page 7-19)

NASM33540: This document is only available for purchase, so I've added a link to the old standard, MS33540.  It's very similar to NASM33540, as well as AC43-13-1b

Getting Jacked - A Simple and Clever Way to Separate Parts

One thing using 3D CAD programs has taught me, programs like these can make assembling parts together a piece of cake! With just a few clicks, parts can be quickly placed into position.

3D CAD systems have many ways of assembling components

But just because a constraints allow us to easily assemble and disassemble parts, doesn't mean that it will be so easy to do on the assembly line, or during maintenance. 

 

One example is a seal, such as a gasket or packing that locks the two mating parts together. For example, in the image below, the O-ring creating the seal may cause enough friction to prevent the flanges from being easily separated. 

The O-ring sealing the components could be enough to "friction lock"
the assembly, making it difficult to take apart.

One option would be taking a screwdriver or another prying device between the flanges and pry like you're opening a casket of pirate treasure.  But while a tempting option, wedging a prybar between the flanges could result in damage to the flanges.  If the flanges are made of a soft material, such as aluminum, the odds of damaging the parts goes up significantly. 

But the designers of old did come up with a more elegant way of separating these... sticky problems. 

Many times, assemblies such as these will have tapped holes that appear to go nowhere.  They don't have a corresponding hole in any mating part.  They just appear to.... be there.

Why were these holes put there? They do have a purpose!

That's because they aren't there for the purpose of assembling parts.  They're for disassembling parts. 

They're called "jacking holes". By carefully threading screws into these holes, the two flanges can be pushed apart evenly without damaging the parts making up the assembly. 

Using a socket head cap screw to separate the flanges.

It's a simple, and elegant way of solving a challenge. 

So if you should find yourself having to disassemble an assembly similar to what I've shown here, look for those jacking holes and see if it can make your life easier. 

And if you find yourself designing a component that may present a challenge, perhaps adding a couple of jacking holes might make for a design that's easier to disassemble when the need arises! 

Opposing jacking screws help separate the flanges evenly!

About the Author:

Jonathan Landeros is a degreed Mechanical Engineer and certified Aircraft Maintenance Techncian. He designs in Autodesk Inventor, Siemens NX, at work, and Autodesk Fusion 360 for home projects. 

For fun he cycles, snowboards, and turns wrenches on aircraft. 

Standard Parts Used on this Project

A569-343 Viton O-ring - McMaster Carr Part Number 9464K173

Buna-N -343 Backup  Ring - McMaster Carr Part Number 5288T372

1/2-13 x .500 Long Socket Head Cap Screw - McMaster Carr Part Number 92185A712