Ring magnet application

[Tuckabag]

Please excuse my complete lack of knowledge re magnets.
I have an application I am looking to solve and am looking for either advise, or a nudge in the right direction as to where I can research the information I need.
I am looking to use a ring magnet, sitting on a hollow S/S shaft, then slide a rod with a round bar magnet on it up the hollow shaft to then latch the ring magnet and allow me to move it up and down along the hollow shaft.
Does this sound feasible? Or am I better trying to get the actuating "rod" to latch onto the OD of the magnet? The only downside I can see to this is that the ring magnet sits inside a S/S tube with 1/8" WT so the mag field will be weakened?
I've attached a (very) basic sketch of what we are trying to achieve.
Appreciate any comments/feedback.
Thanks.

 

[MagMike]

It is definitely feasible and is done in many instances already.

There are two possible approaches:
1. Ring magnet and bar magnet are radially oriented/magnetized
2. Ring magnet and bar magnet are axially magnetized but have steel plates on their poles to re-direct the magnetic flux in an axial direction.

As always, each option has pluses and minuses, a lot depends on how much force is required, how large the magnets can be, temperature, cost, etc. If you can share some more details on size allowance, force requirement and temperature, I or others can confirm feasibility.

 

[Strong]

Another consideration would be how fast the magnet assembly is to be moved (acceleration and velocity).

Walt

 

[EdStainless]

1. The SS tube has to be actually non magnetic, generic 304 might not be good enough.
2. The tube wall needs to be thin, say 0.035" max

These arrangements are used for level indicators that are sealed.
Often they use an axially oriented ring magnet with pole pieces top and bottom.
This is an easy magnet to get, it allows the pole pieces to be used as guides protecting the magnet, and since it focuses the field you can get higher strengths.
Then the inner bar magnet is about the same length as the height of the ring assembly.

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P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

Concentricity and potential for jamming or sticking are also potential issues.

How long will this structure be operated?

Are there other reasons for both the ring and the round to be magnets?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[Tuckabag]

Thanks for the responses so far. I'll try and fill the gaps as best I can.

MagMike

- Size allowance is up to approx 40mm OD and around 11/12mm ID. Thickness can be up to 10mm+/-2mm or so.​

- Operating Temp is normally around 20-40deg C, however we have one client that can subject these to temps up to +100deg C​

- Force requirement is not huge, we need to move the ring magnet (which will be encased in PEEK or similar material) through a liquid to ensure it is a homogenous mix before it is analyzed. Max density of the liquid is not expected to exceed 1000kg/m3. I'd suspect that around 10kg of force would be more than enough, but have not analyzed/calculated this yet.​

Strong

- The rod will be moved by hand back and forth, so I expect that velocity would not exceed 0.5m/sec​

EdStainless

- Both tubes, inner/outer will be 316/316LSS ASTM A312 seamless tube.​

- Noted re the WT, I expected that 0.035" is what we would have been using, or thinner if possible.​

IRStuff

- The "mixer" magnet will be used infrequently during the life of the device, and only in short bursts of say 5 minutes at a time.​

- Concentricity and sticking should be alleviated by the magnet riding on a PEEK (or similar) bush/bearing.​

- Both the ring and round don't necessarily need to be magnets, I just assumed that 2x magnets would have a greater holding force than a single magnet and some form of ferrous material. In fact if the ring wasn't a magnet it would be beneficial to other parts of our design, but I (again) assumed that we would need as great a mass as possible for the magnet for it to be most effective so the ring was a bigger part than the rod so it won.​

Thanks.

 

[Tuckabag]

I just did some quick and dirty drag force calcs based on a 40mm OD flat plate and seems that using water as the fluid and a velocity of 1m/sec I only have a drag force of 0.082kg/f.
Much less than I anticipated. This of course does not take any drag on the bush/bearing into account, nor stiction to get the magnet/mixer assembly moving.

 

[MagMike]

I ran a simple model.
Outer assembly: 40mm OD x 28mm ID x 10mm length
Inner assembly: 24mm diameter x 10mm length
Each assembly consists of a Steel ring/disc (2mm length) - Magnet (6mm length) - Steel ring/disc (2mm length)
Magnet material is SmCo32 and axially oriented
Maximum restoring force is ~66 N when the two assemblies are axially displaced 5mm

 

[EdStainless]

Mike, thanks for that. More than I would have expected.
How near saturation do you think that your outer washers are?

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P.E. Metallurgy, Plymouth Tube

 

[MagMike]

EdStainless: Good question! One small portion of the steel disc on the inner assembly reaches 20,000 Gauss. Other than that, no saturation issues.

 

[MagBen]

Good job Mike! What kind of FEA modeling are you using? I used to have Magneto (2D) and Amperes (3D), but not much anymore.
I understand you left Dexter? are you still with ASTM A06? I quit the Committee since I am now dealing with bar products with a focus on medical, no much on magnetics anymore.

 

[Tuckabag]

Thanks MagMike.
66N "should" be ample for this application.
One quick question for you. Does the addition of the steel discs focus the magnetic fields? Or are they there to improve mechanical strength?
If there is somewhere you can point me to that I can do more research on these instead of just picking your brain it'd be much appreciated.
Thanks.

 

[L.C.Tong]

We are producer of Radial sintered NdFeB ring, maybe we can do something for this.

http://www.radimag.com

 

[MagMike]

Tuckabag: The steel is there to re-direct (or focus) the fields radially.
I don't know of any websites one can use to learn more about this. There are a number of applications that use a magnet arrangement similar to this, but I don't know of anyplace that directly teaches this design.

 

[L.C.Tong]

Hi, Tuckabag, Radial sintered NdFeB ring produce by our patent technology : Method and apparatus for producing radially oriented ring magnet. for any question/information, please let me know