magnet in cylinder

[TWJR]

Hello,

I'm designing an assembly in which a .25" neodymium ring magnet is mounted on a shaft. It moves back and forth inside a .281" hole. The shaft is supported on each end, so the magnet is suspended in the .281 ID with no chance of touching the sides (.015 clearance per side). It's radially magnetized, so the poles are on the OD, not the ends. Question: since it seems the magnetic attraction to the sides of the hole (parent part is magnetic steel) is balanced, will the axial movement of the shaft be free, and unaffected by the fact that the magnet is present? I need the parent part to be magnetic steel in order to shield a hall effect sensor from being triggered until the magnet moves over a hole (sensor is below the hole).
Thank you in advance.

 

[ctopher]

I guess I'm ignorant on this type of magnet.
How are the poles on an OD?

Chris, CSWA
SolidWorks 13
ctopher's home
SolidWorks Legion

 

[TWJR]

It's half-and-half, just like a bar magnet, only round.

 

[zeusfaber]

How balanced this arrangement is in real life will depend on how accurately your piston is centred in the bore.

If the sensor hole is big enough to affect the reluctance of the magnetic circuit (and if it isn't, how much field is going to leak out to affect your hall sensor?)then that will have an impact on the axial force.

A.

 

[MikeHalloran]

The magnet, because of physical or magnetic asymmetry, will be attracted to the bore, and will exert a radial load on the support shaft, and in turn on the shaft's bearings.

If you want the magnet and shaft to be magnetically suspended, you probably need a microcontroller and an electromagnet, probably more than one.



Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

It depends on the strength of the magnet and how perfect it and all other parts are and on how free you want the movement to be.

There will be some lack of balance because nothing is perfect, and so there will be a constant, though potentially small, side load. There may also be non-linear attraction that will tend to pull the part harder due to lack of straightness - losing attraction strength on one side at the same time it is gaining on the other. There is also the potential for variable magnetic permativity in the steel such that different areas are more attractive than others or there could be areas that are already slightly magnetic.

To know more you'll need to characterize the particular magnets, the steel tube, and maybe the shaft. You'll then need to go for some FEA code that handles magnetic fields.

Or just build and test it.

Maybe the magnet could roll like a wheel?

 

[TWJR]

Thanks for all the replies. I'm no expert on magnets, hence my question. Sounds like this thing could be problematic. Another thing... with the magnet moving back and forth, I'll probably magnetize the parent part, throwing another variable into the mix! Perhaps I need to rethink this, and find a more reliable solution...

 

[TheTick]

If there is electric connectivity in a way that completes a circuit, there will be impedance.

 

[IRstuff]

Your magnet is in a metastable position, since any deviation from perfect centering will cause it to be more attracted to one side or another. You can think of it as being on a potential energy hill, with very steep sides. The only thing holding it from going down the potential energy hill is your shaft.



TTFN
faq731-376

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[sreid]

Re axial movement: The magnet will try to find the minimum magnetic reluctance position.