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NNTP-Posting-Date: Fri, 20 Sep 2024 16:38:11 +0000
From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Magnetic force
Date: Fri, 20 Sep 2024 12:38:03 -0400
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On Fri, 20 Sep 2024 00:11:05 -0700, Don Y
<blockedofcourse@foo.invalid> wrote:

>I have an optically-couple "supervisory port" on my
>devices.  A design requirement is that the outer surfaces
>of the devices must be "wipe clean" -- no ridges or
>grooves.
>
>I mechanically attach the mating "cable" to the port
>magnetically (an idea I stole from designing electric
>power meters -- but, they aren't "wipe clean"!)
>
>As I can't rely on any significant mechanical structures
>to help maintain the attachment's position, that must
>be guaranteed solely by the strength of the magnetic
>bond.

This approach will attract and hold magnetic debris, like rust and
iron filings.  Is that a problem, or is wiping enough?  If the magnets
are strong enough, debris removal may be difficult to do in practice.

Joe Gwinn




>I have a 1/4" dia glass "jewel" that is just barely convex
>to protect the optical port and support light passage.
>The slight bump helps locate the port as well as some small
>assistance in keeping the mated "cable" positioned properly.
>
>In no case can the needs of the port exceed a concentric
>diameter of 1/2".  I plan for the magnet to only be present
>in the mating cable so its adhesion will be determined by
>the metallic ring surrounding the jewel.
>
>I figure I need to specify a magnet strong enough to
>"support" (against the force of gravity exerted on the
>mass of the cable) the cable in a horizontal position
>(i.e., "mated to the ceiling").  And, to support the
>cantilevered weight of the cable when mounted
>vertically.
>
>I may opt to implement the "cable" as a wireless dongle
>if the weight of the cable assembly starts to compromise that
>magnetic adherence.
>
>I guess I also need to consider the magnetic force exerted
>through the friction between the mating surfaces when
>mounted vertically (so the cable doesn't "slide down the wall")
>
>Anything else I should consider?  I realize that the cable end
>now will have a tendency to want to grab onto metallic objects
>so have to factor that into my dongle vs. cable decision
>(it's a lot easier to control the position of a dongle in
>space than the end of a possibly *dangling* cable!)
>
>I suspect the adhering "metal" piece has to be on the skin
>of the device as the magnetic field falls off quickly with
>distance -- diluting the ability of the cable to remain
>EXACTLY where desired.  A "decal" over the surface likely
>would be tolerable...