Using Neodymium Magnets with N to N poles for a long period of time. 5

[NotSoTypical]

Hello,

I was wondering if there was anyone out there with some experience to guide a decision I am trying to make.

What I am doing is trying to use two disc magnets as a "spring" in a machined sleeve. My concern is that over time the polarity of the magnets will either be weakend or change with respect to one another and that my "spring" or repulsion force would be lost.

Right now I am trying Neodymium Magnets, and they seem to work very well. The manufacture says that the magnets loose 1% of their field strength every 10yrs. I am sure that is true when the magnet is by itself, but what if there were two magnets with simular poles facing eachother?

I need these magnets to retain their repulsive force for at least 10yrs if at all possible, and my "gut" insinct is telling me there could be problems with this long term.

Are there any magnet experts out there who could answer this question, and put my "gut" insecurities to rest?

 

[MagMike]

As long as the magnets are protected from corrosion and excessive heat, they'll last at least 10 years.

 

[MJR2]

As MagMike says corrosion and heat are important.

In addition you need to pick magnets which have sufficiently high coercivity to resist the self demagnetization that will be trying to take place. You must also choose a material to operate at the temperature you expect. Most vendors will pick the neo from a temperature chart and send you away. However most magnetic circuits (those I work with anyway) reduce the permeance of the magnetic circuit such that higher temperature materials are always required.

Plus the overall force drops as the magnets warm.

Mike

 

[cermag]

Magnetically, there is not enough field in either of the magnets to demagnetise or reverse the polarities of themselves. As the guys have allready mentioned, heat and corrosion should be your biggest concearn.

 

[MJR2]

Temperature excursions below that of the rated temperatures for the magnetic material can and do cause demagnetization. It particularly happens if they are opposing such as this ap.

At least it does for me. And I have always thought that physics applied equally to all.

Mike

 

[NotSoTypical]

Thank you to all for the input,

Now a second question...

I am trying to use a reed switch in close proximity to the magnets (for those not familiar with a reed switch, it is simply two small pieces of metal that touch due to a magnetic field to close a circuit). What I am knowticing is that the reed switch only works intermitantly when close to the magnetic field I have described. Can I fix this by purchasing one magnet stronger than the other to have an off-set field so they don't cancel out?

 

[bz2005]

There is always a "dead zone" somewhere as I can imagine, even if one magnet is stronger than the other. The "dead zone" will be shifted farther from the stronger magnet and closer to the weaker in this case. It could work for you depending where you position your reed switch.

 

[MagMike]

The strength of the magnet is not nearly as important as the orientation of the reed switch relative to that magnet. Proper orienation of the reed switch relative to the magnet is critical for it to function properly.

The intermittent on/off behavior is normal/common/expected depending on how you set things up.

 

[cermag]

As MagMike says, the orientation of the reed switch is important. Are you using an "end-on" or "parallel" reed switch?

 

[NotSoTypical]

I am using a parallel reed switch, I did not know that there were end-on versions.

My magnets are polarized axially, do you think that end on or parrallel would be the best?

I will look for some end-on versions to see if I get better results.

 

[daveh55et]

Maybe a Hall Effect sensor would be more reliable.

 

[MJR2]

NotSoTypical,
Up until your last post I thought you had opposing magnets S-N-N-S. Using them charged axially is not so typical. At least in my simple mind. Why did you pick that orientation?

Mike

 

[NotSoTypical]

They are oriented S-N N-S, what I meant by axial polarity is that the line of symetery runs through the center of the rings (ring magnets) , which means they have uniform push against eachother, as compared to diameterically which would have half the ring as N and half as S.

I have these oriented like two doughnuts stacked on top of one another in a sleve, pushing against eachother.

 

[MJR2]

I understood axial as radial I guess now that I think about it. Sorry. Your last explanation is just as I have pictured it. I work with assemblies like that regularly. Just not trying to accomplish what you are.

If you get a B-H curve for your material get one that includes temperatures thru the range of your application. Determine the permeance of your circuit (it will be worst when the magnets are in contact) and plot it on the curve. That will show you the application temperature capability of that material. The temperature ratings are based on a generous permeance assumption for the material.

The B-H curve is often supplied with only a 20C curve. Ask for and get a complete set to the rating for the material. This is what I was trying to say in an earlier post but perhaps not as well.


Mike

 

[NotSoTypical]

Well, I was tinkering with the system and tried a new reed switch and found it to work exactly how I wanted. I guess the other one was cracked or faulty or something

It was a stupid mistake at the wrong time.

Logically I knew it should work, but couldn't get it to work right. Now it works beautifully. The last thing I tried should have been the first.

Thank you all for your good input...