Thanks for the response Ben!

I think I've got a pretty complete understanding of the regulations now:

A harness wire is considered to have failed if the insulation is no longer adhered to the conductor, as evidenced by buckling on the inside of a bend. Hence conductor make-up, which has a huge effect on number of bend cycles the conductors can withstand, as well as how much work it takes to make it bend, doesn't have a large contribution to lamination failure.

For IPC/WHMA:

Table 14-1 lists a range of bend radii, depending on the cable type. I think Ben nailed it when he suggested that "unshielded wires" (5x allowable radius for larger wires) is multi-conductor cables, whereas "insulated wire" (2X diameter radius) is a single conductor.

For Aerospace:

AS50881 requires 10x wire diameter for wire bend radius, which drops to 3x wire diameter if the wire is fully supported, such as the portion contained entirely within a strain relief. ASTM F2639 applies this latter criteria to fully supported/secured harness service loops.

Waivers can be achieved on these bend radii by running the wire through the same set of torture tests that resulted in the 10x standard, which requires a combination of bake cycles and winding/unwinding the wire on a mandrel followed by visual and electrical testing to detect delamination of the insulation.

Wayne Thayer



-----Original Message-----
From: Gumpert, Ben [mailto:[log in to unmask]] 
Sent: Thursday, August 18, 2016 3:29 AM
To: TechNet E-Mail Forum; Wayne Thayer - EXT
Subject: RE: Harness bend radii

Wayne,

It seems that the 'shielded' wires are assumed to be multiconductor (twisted pair, etc.) and 'insulated' are single wires.

As for the wire construction, I'm not surprised that the spec doesn't get into a high level of detail. I expect the requirements are good for most configurations, and you'll find that with some hardware you can safely take exception to the requirement with agreement from your customer.

Ben

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer - EXT
Sent: Wednesday, August 17, 2016 1:22 PM
To: [log in to unmask]
Subject: EXTERNAL: [TN] Harness bend radii

Hi All-

Finally back "in the saddle" after a tumultuous year. First, I got a fantastic job opportunity to move to the heart of the PNW. Worked my butt off for 7 months, then was let go. Then another sprint to see if I could find ANYTHING in the immediate area, which took another 2 months.

Anyway, here I am at Insitu, maker of premium recon/intelligence UAVs.

So here's the question:

IPC/WHMA-A-620B Table 14-1 lists different bend radii for "unshielded" vs "insulated (non-polyimide)" wires, but there is nothing in the "definitions" section to give me a clue as to how to tell the difference. Anybody know how we're supposed to interpret that? For the "S" version of the document, they pulled out the "insulated" line.

And here's a follow-up question:

Why isn't there anything on wire construction in the spec? The build-up of the wire makes a huge difference to whether a certain bend radius can damage a wire. For example, I'm looking at #12AWG, and I've found (#conductors/gauge each conductor) ranging from 19/25 to 665/40.

Thanks!


Wayne Thayer
Insitu Contract Engineer
Desk: xx-xxx-xxxx
Cell: 443-534-8036
E-mail:  [log in to unmask]<mailto:[log in to unmask]>