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[Lyle Williams]

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

How many times can it be bent?

[TJ (Tom) Cornish]

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

How many times can it be bent?

What specific flex cable are you referring to?  Wire gauge or trace thickness makes all the difference.  I presume different products have very different lifespans depending on what they're designed for.

[TJ (Tom) Cornish]

Also bend radius.

[Erik Jerde]

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

How many times can it be bent?

When you say flex in relation to electric I think FMC - flexible metal conduit.  From the way your question is written I think you mean something different.  Please clarify.

[Lyle Williams]

Regular extension cords and power cords.  I think you call these SJ in the US?

Say one of these cords will be rebent very modestly twice a day.  I have a lifetime intuitive understanding that this is OK, but how do I trace this back to a written spec/standard?

[TJ (Tom) Cornish]

Regular extension cords and power cords.  I think you call these SJ in the US?

Say one of these cords will be rebent very modestly twice a day.  I have a lifetime intuitive understanding that this is OK, but how do I trace this back to a written spec/standard?

I can't imagine a cord living long enough for the copper to get damaged by bending. The jacket is typically cut, torn, or otherwise damaged. Sometimes the filler breaks down and the conductors get lumpy, which happens more often with SE cords than with rubber SO cords.

My best suggestion would be to see what the elevator industry says.

[Erik Jerde]

When you say re-bent what exactly do you mean?  A quality stranded wire cable will handle being coiled and uncoiled many times (thousands) without damage.  A hard kink will potentially do damage though filler & jacket along with lubricant in the cable usually goes a long way towards protecting the conductors.  You could check and see if the mfgr has a minimum bend radius spec.

[Lyle Williams]

Clearly a quality stranded copper cable can be coiled and uncoiled many times.

Where do I find a piece of paper that links a cable type to a durability test?  UL std 62 and UL1581 didn't help me because the flex testing sections apply to particular 22AWG cables, or to extreme cold single bends.

...maybe I need to build a mythbusters-style test rig and just run a million reps over two weeks...

[TJ (Tom) Cornish]

Clearly a quality stranded copper cable can be coiled and uncoiled many times.

Where do I find a piece of paper that links a cable type to a durability test?  UL std 62 and UL1581 didn't help me because the flex testing sections apply to particular 22AWG cables, or to extreme cold single bends.

...maybe I need to build a mythbusters-style test rig and just run a million reps over two weeks...

What's your application?  Sometimes data doesn't exist because that particular metric hasn't mattered before.

[Craig Hauber]

Regular extension cords and power cords.  I think you call these SJ in the US?

Say one of these cords will be rebent very modestly twice a day.  I have a lifetime intuitive understanding that this is OK, but how do I trace this back to a written spec/standard?

I've used and serviced many pantograph-type projector and studio light lifts where the power cord is threaded on the mechanism in a way that it gets folded and straightened constantly. 

They come with PVC type SJ, are UL rated and I've never seen an issue with cord damage even though they are pushing the boudary on what I would consider a safe minimum bend radius!

Most of the projector lifts I've installed are similar in design. Along with the power there's video and data cable and that seems to hold up quite well too.
One in particular I helped install in the mid nineties and it's been deployed once daily at a minimum.  Motor has been replaced, some bearings and pins replaced, frequent re-greasings along with signal cable upgrades (SD to HD etc..) but the power cord has survived there since new and I estimate a minimum of 7600 up-down cycles.

So I don't think the regular coiling/uncoiling of a power cord is going to ever wear it out. 
Only abuse is what tends to shorten lifespans (yanking on knots, wrapping around elbow, or like letting any contractor do that macrame style cord wrapping they all seem to love)

[Lyle Williams]

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.  :-)

[Stephen Swaffer]

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.

[Jonathan Johnson]

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.

[Lyle Williams]

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.

[Tim McCulloch]

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.

[Riley Casey]

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]

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.

[Lyle Williams]

Still ok at a million cycles.  Thanks everyone.

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