[R-390] Teflon

[Ed Tanton]

To quote a NASA Tech TIP:

Materials Engineering Branch TIP*
No. 009 Teflon Insulated Wiring
Author(s): Ernest Mielke Contact: (301) 286-6882

The cold-flow properties of Teflon are well known to most spacecraft
developers. As every new flight program takes shape, designers and
experimenters are reminded about Teflon cold-flow. And yet, there are still
incidents of electrical shorting caused by the improper use of Teflon
insulated wiring. One of the most publicized examples of a cold-flow event
is the one where Teflon insulated wire comes in direct contact with a metal
component, such as the edge of a chassis feed through hole, where some
stress is involved. As the Teflon insulation cold-flows away from the
contact zone due to the pressure involved, the insulation becomes
sufficiently thin with time to cause metal-to-metal contact thus resulting
in a short.
In a less familiar case, an electrical short due to Teflon cold-flow has
been observed in a bundle of Teflon coated wires wrapped with a lacing cord
or cable ties. It is believed that the force exerted by the lacing cord,
tying the bundle together, was sufficient to cause Teflon cold-flow such
that the wires made contact or that a foreign metallic particle was trapped
within the bundle. In any event, it appears that in spite of all of the
warnings that go out to spacecraft developers, the threat of electrical
failure caused by the cold-flow behavior of Teflon still exists. More effort
needs to be made to insure that the technicians who assemble the hardware
are better instructed in the use of Teflon.

http://code541.gsfc.nasa.gov/Uploads_materials_tips_PDFs/TIP%20009R.pdf 

In practically another life (building prototypes for several companies) I
did not pay enough attention to LOOPING my wire wrap (WW) wires over the pin
field. I made the #30 Kapton WW wires almost tight in their routes from
point A to point B. Made for a neater looking, more easily tested wire wrap
board (when it doesn't appear to go to the right place, you use something
like a knitting needle and tug on a wire to follow where it goes)
wire-wrapped board. TOTALLY WRONG!!! Eventually my Kapton wires would have
cold-flowed enough for the wire to short to the sharply-square gold WW post.
Usually, the life of such things is insufficient to have it show up, but
there WERE folks who actually sent WW things into space. Fortunately, **I**
was not one of them since I was doing strictly prototypes.

Also, I think there were some serious Boeing problems with certain (Kapton I
think?) wiring assemblies on 737s. They were-again: I believe-flexing
against an edge when the flaps were moved. And, THIS NASA tip is the 1st I
had heard about laced wiring bundles being dangerous. Personally, I should
think almost ANY insulation would be subject to wire lacing pressure
problems, therefore any lacing I do (usually with cable ties, not nylon/or,
cotton 'string') will be not so tight as in the past. I have always been
pretty good about using hard-plastic thru-hole grommets. (I never was
impressed with simple rubber grommets.)

I said earlier, and I repeat: I use virtually 100% TFE wire when I build
anything (except Kapton for WW things) and will continue to do so. It's
great stuff. You just have to be careful about the tightness of any
bundling/lacing and not get close to edges. 

Ed Tanton
 
website: http://www.n4xy.com  
 
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-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of rbethman
Sent: Saturday, February 16, 2013 1:40 PM
To: R-390 at mailman.qth.net
Subject: Re: [R-390] Teflon

My perspective comes from aircraft industry and using wiring harnesses that
are prefabricated on a large board that the wires are strung on PRIOR to
applying to an airframe.

It is NOT pulled over an edge.  All openings are turned edges, not leaving a
ragged edge.

Rather the same way that the airfoil shape, (Ribs), are made by holes that
are intentionally radiused.  This is done to reduce fractures and allow for
reduced stresses.

Applies from single engine aircraft to much larger commercial ones.

Wiring harnesses are well planned and laid out far in advance of
installation.

Much Teflon wiring has been used in later production R-390As.  Yet no
rubbing and creating insulation breakage.

Boeing 777 uses this methodology, not to mention newer Cessna 172s.

I'm not a Penguin, I do fly.

YMMV

Bob - N0DGN

On 2/16/2013 1:27 PM, n4xy wrote:
> THE problem with Teflon and Kapton is their cold-flow characteristic. 
> Pull a TFE or Kapton insulated wire against an edge (turning a corner) 
> and-over time-the insulation parts, allowing a short, and probably a 
> spark. Personally, I love TFE, have both mechanical & thermal wire 
> strippers for it, and use it exclusively. 'Course, I really watch out 
> about stretching it against edges, and I DON'T build aircraft!!!  LOL
>

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