[R-390] Teflon

Sun Feb 17 12:03:25 EST 2013

Hi

The Navy had some non-trivial issues with Kapton in fighter planes. 

The simple answer with any insulation is not to pull it tight against any abrupt edge. Even if cold flow is not an issue, there likely will be a combination of temperature / time / edge profile that creates issues.

All that said, I'd go with Teflon wire before I'd use just about anything else. With minor care it's very durable stuff. 

Bob

On Feb 17, 2013, at 11:39 AM, "Ed Tanton" <n4xy at comcast.net> wrote:

> 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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