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Author Topic: Design life of flex cables  (Read 4189 times)

Lyle Williams

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Re: Design life of flex cables
« Reply #10 on: May 06, 2017, 07:57:21 pm »

The issue is that in this application I need to prove on paper something that is below everyone else's threshold of triviality.

There is zero concern on my part about the durability of the cable.  I just need to find a way past the darn paper-pushers.  :-)

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Stephen Swaffer

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Re: Design life of flex cables
« Reply #11 on: May 06, 2017, 10:01:44 pm »

So, instead of proving an open-ended, "how many times can it be bent?", is there a design life criteria you need to meet?  It might be easier to document that it meets a minimu criteria-especially that a specific cable meets that criteriea.  IME, there is a significant difference in cable quality from one cable to another that are supposedly the same thing.
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Steve Swaffer

Jonathan Johnson

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Re: Design life of flex cables
« Reply #12 on: May 07, 2017, 01:48:38 am »

So, instead of proving an open-ended, "how many times can it be bent?", is there a design life criteria you need to meet?  It might be easier to document that it meets a minimu criteria-especially that a specific cable meets that criteriea.  IME, there is a significant difference in cable quality from one cable to another that are supposedly the same thing.

The factors in "how many times can it be bent" are whether or not it is bent beyond its yield (elastic limit), and how tightly it is bent beyond its yield.

https://en.wikipedia.org/wiki/Yield_(engineering)

If a metal is bent beyond its yield, it deforms plastically and will not return to its original shape. The lesser the bend radius, the greater the deformation. When bent beyond its yield repeatedly, it becomes work hardened and subject to failure from metal fatigue. The tighter the bend, the sooner this happens.

If a metal is NOT bent beyond its yield, it will return to its original shape when released. A metal can be bent within its yield an infinite number of times without failure because no deformation of the intermolecular structure occurs.

I don't know what the yield for copper wire is, but I suspect that it may allow for a tighter radius as the strands get thinner. So, the thinner the strands, the tighter the radius can be before plastic deformation occurs. (Maybe. I don't know this to be true.) If a cable is maintained such that it is never bent tighter than this radius, it should (in theory) never fail from fatigue.

I'm not a mechanical engineer, but I'm related to one. I don't understand the math on that Wikipedia page, but my nephew would.
« Last Edit: May 07, 2017, 11:47:17 am by Jonathan Johnson »
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Stop confusing the issue with facts and logic!

Lyle Williams

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Re: Design life of flex cables
« Reply #13 on: May 07, 2017, 02:33:41 am »

So, instead of proving an open-ended, "how many times can it be bent?", is there a design life criteria you need to meet?  It might be easier to document that it meets a minimu criteria-especially that a specific cable meets that criteriea.  IME, there is a significant difference in cable quality from one cable to another that are supposedly the same thing.

Bent twice a day, say 250 days a year for up to 20 years.  Expected to be five years, but stuff can live on beyond it's expected lifetime.

So 10,000 cycles.  Bend radius no sharper than 200mm.

I have an electric motor/gearbox with 60 rpm output, so I think I will just build something to flex the cable. 86,400 cycles per day X enough days to crush any doubts.

Thanks everyone.

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Tim McCulloch

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Re: Design life of flex cables
« Reply #14 on: May 07, 2017, 03:24:36 pm »

Bent twice a day, say 250 days a year for up to 20 years.  Expected to be five years, but stuff can live on beyond it's expected lifetime.

So 10,000 cycles.  Bend radius no sharper than 200mm.

I have an electric motor/gearbox with 60 rpm output, so I think I will just build something to flex the cable. 86,400 cycles per day X enough days to crush any doubts.

Thanks everyone.

Lyle, I think you need certification from the manufacturer(s) of the specific product(s) being proposed for use.  This would remove any doubt about your testing methods or qualifications and it would put the onus on the manufacturer for any failure to meet the specification.
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Riley Casey

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Re: Design life of flex cables
« Reply #15 on: May 09, 2017, 01:58:47 pm »

In my forty plus years of entirely unscientific experience working in large cities in North America I'm quite sure  that the rubber jacket of portable power cables will fail from exposure to air pollution and UV light long before the conductors will. 

Does anyone know what the design life of flex cable is?

How many times can it be bent?

Stephen Kirby

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Re: Design life of flex cables
« Reply #16 on: May 12, 2017, 05:37:50 pm »

Lyle, what you're talking about is cold working fatigue.  Same thing that happens when you bend a drink pop top back and forth until it breaks.

Drawn and annealed copper is fairly ductile.  That said there is a machine in my industry that is fairly notorious for having cables break every few years.  This is an SMT solder printer where the cables are rolled out and back a couple times a minute when the machine is running.  About a 1.5" radius with multiple cables in a folding track bending back 180 degrees.  So it does happen.

Consumer electronics companies actually do what you proposed.  We have machines that open and close laptops continuously so we can see how long the interconnecting cables or flexible printed circuits last.  Same thing with the flexes inside disk drives when I worked on them.  I've never seen an actual spec though.  Someone comes up with a product lifetime and anticipated cycle count, and the reliability department makes a tester and cycles stuff to death.
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Lyle Williams

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Re: Design life of flex cables
« Reply #17 on: May 28, 2017, 02:29:33 am »

Still ok at a million cycles.  Thanks everyone.
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Re: Design life of flex cables
« Reply #17 on: May 28, 2017, 02:29:33 am »


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