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Cable Rotation

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sheafromme

sheafromme

Mechanical
May 1, 2020
26
Hello,
I am designing an electronics package for a camera that pans back and forth 180˚ but never completes a revolution. Currently I have the power cable going down along the pan axis and out through an IP-68 cable gland in the bottom of the enclosure. This works for weather protection but I have concerns about the cable fatiguing over time due to twisting back and forth. Although the camera does not spin around and around, it does pan back and forth a few hundred times a day. The cable is secured and strain relieved at the top, then runs about 12" unsupported until reaching the cable gland. Thus there would be about 15˚ of "twist" per inch. Does this raise any concerns with you all? Can you think of any immediate issues? Or are there resources for cable fatigue in these types of applications? Most of what I've found online is for high-speed motion or CNC, which this is neither.

See attached for quick sketch

Best,
-Shea
 
 https://files.engineering.com/getfile.aspx?folder=86a431f7-914d-4b8d-8b8d-63f46073cbb7&file=wire_sketch.jpeg
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For cable wear/breakage cases like this I am unaware of a way to calculate reliability. We have always had to create life testing rigs and perform testing.

Not sure if you have room in your budget but you could always add a slip ring in there to eliminate any cable twisting. Good suppliers will be able to tell you the life expectations based on their own testing.

I am not endorsing this supplier, this link is just provided as a reference for what a slip ring is: [URL unfurl="true"]https://www.moflon.com/slip-ring/?gclid=EAIaIQobChMI46nP28-O7AIVyEXVCh029QtZEAAYASAAEgIMIfD_BwE[/url]
 
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I'd put a loop or two of cable in the box, to reduce the relative twist/bend per inch. Use the thinnest cable possible, with the slickest (e.g. teflon) wire insulation, and a slick, durable cable jacket.
 
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Work hardening is a definite possibility, so the less stress the better; of course, our system was doing a scan every 1.5 seconds, but we ran into issues with our flex cable, which had copper deposited on Kapton. Oxygen-free copper was our solution

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Thank you all for the responses!

@hendersdc The slip ring definitely makes sense, especially for power. However, the cable we're using is power over ethernet, so would the slip rings have any issues with passage of data? Is there a lot of noise?

@btrueblood and @itsmoked That makes sense, do either of you suggest securing the extra loop of wire or should it be sort of free-floating?

@IRstuff Does the oxygen-free copper just have more resistance to work-hardening? I was hoping to strain-relieve the cable enough so that the twisting was the result of motion alone and not the weight of the cable or other potential forces on it.
 
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sheafromme,

Read up on the deflection of beams. A beam subject to a bending moment will curl up into a radius. If we set that radius to [ρ]...

1/[ρ] = M/EI

That should help you work out the stresses from bending your cable.

--
JHG
 
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You would want the loop to be free. This allows the cable to distribute the bending forces. It will naturally distribute them in a manner that minimizes stress in the cable.

I have to disagree with btrue on the Teflon. Teflon jacket is about the stiffest jacket you will ever come across. That stiffness results in the cable not wanting to bend at all which in-turn causes the cable to bend mostly at the ends where it enters the strain reliefs. With all the bending at just those two points you can see where that will quickly lead. Teflon would be OK in a clean environment dragging (not bending) over a mostly smooth surface and it's da'bomb for chemical resistance.

Keith Cress
kcress -
 
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