Pseudo Off/On Magnetic Coupling Feasibility

Question

Pseudo Off/On Magnetic Coupling Feasibility

aj0206 (Mechanical)

(OP)

29 Feb 08 21:34

Medical application requires two items coupled with neodymium magnets 1/2" apart. One flat/round magnet behind a fabric strong enough to hold it in place, and another on a freely rotating horizontal axis in a stacked formation with the other magnet behind the fabric. If one wanted to attain a 50-60 pound pull force via the stacked magnets, would it be possible to turn, by hand, the magnet on the axis to reverse the polarity, thereby repelling the magnet behind the fabric momentarily...long enough to move the two far enough apart to defeat any attraction. Will the magnet on the freely rotating axis realign itself to pull towards the other magnet when proximity is attained again? How much force, approximately, would be needed to turn, by hand, the one magnet to defeat the attraction on a freely rotating axis with a thumbwheel or key-type grip on one end of the axis, or would this require the use of a force-multiplier, etc.? Not a magnetic engineer, but helpful answers or guidance will greatly simplify and improve device for handicapped individuals. Thank you for your time and concern.

MagMike (Materials) · Answer 1 · 2008-03-01 15:35:52

I would be difficult to design a safe coupling that can generate 50-60 pounds of force across 1/2" air gap. That's huge, as far are magnetics are concerned.

Disregarding the force requirements for a moment: You would achieve the engage/disengagement behavior by using permanent magnets with 2 poles on their flat face. When brought together (and allowed to freely rotate) they will spin together in attraction. If one wants to disengage, rotate one magnet so like poles (north-to-north and south-to-south) come together, producing repulsion.

aj0206 (Mechanical) · Answer 2 · 2008-03-01 20:25:28

Thank you for your reply MagMike. The reason for the 1/2" gap choice was to have room to rotate a flat 1/2" diameter magnet below the other. If another size/style of magnet could be mounted closer together, without actually touching, what would the minimum distace apart be, and what approximate range of pull force could one generate? In theory, there is no reason for the magnets to not touch except for the need to be able to rotate one to repel the other momentarily. Perhaps a mechanism that slides one flat magnet away from the other, so they could be in close proximity to each other at maximum pull force, or perhaps something such as mu-metal slid in between them stacked closer together so the force could be lowered enough to easily sepearte the two parts? Thanks again.

prex (Structural) · Answer 3 · 2008-03-02 03:27:31

It would be better if you explain what your mechanism has to do and how it is arranged.
Achieving a pull of 50 pounds is not a problem, it is just a matter of how many kgs of magnets you use and of how they are arranged. The problem could be that a too high force would be required to disengage, but it is difficult to judge without having a clear setup in mind.

prex
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IRstuff (Aerospace) · Answer 4 · 2008-03-02 12:54:19

Perhaps you should start with the actual problem and not just your proposed solution. While magnets might appear to the a solution, 50-lb pull force implies sizing and weight that might not work with the desired application.

TTFN

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MagMike (Materials) · Answer 5 · 2008-03-03 08:39:05

aj0206: I agree with prex and IRstuff, please describe more about your application.

Mumetal is not the way to reduce the force between the two parts.

MJR2 (Mechanical) · Answer 6 · 2008-03-03 15:48:24

There are permanent magnetic devices used in the metal working industry that may be 'turned on/off' mechanically. Perhaps some version of one of these could satisfy your need. The smallest available off the shelf can fit in your hand.

Do a google search for "permanent lift magnet". Some of the top hits have such a device.

Knowing more would enable a custom device to be suggested.

Mike

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Pseudo Off/On Magnetic Coupling Feasibility