best permanent magnet core materials 1

[gmeast]

Hello,

I'm new here. There are many companies involved in research or outright production of magnetic couplings. I am involved in an engineering project regarding a radial magnetic torsional coupling. There is an internal radial group of PM's in proximity to an external radial group of PM's, separated by a 1/16" austenitic (paramagnetic, nonmagnetic) stainless steel cylinder (gap). The PM's are even-numbered, 1 for 1, N-S-N-S, etc. on the internal and external radial group. There is no oscillating field. I'm regarding the fields as "static". What would be the best "back' material for the PM groups ... ie cast iron, some steel alloy, I don't think anything like MU metal is required though it has a relative permeability of 1,000,000 compared to the next best of 200,000 for pure Hydrogen annealed Iron.

I have tested a linear version of the coupling using NdFeB disc magnets stuck to 'dumb' mild steel strips (3/4" X what looks like 10 ga galv mild steel from Home Depot). Absolutely NO leakage. The back side of the strips can't even pick up a small paper clip! Seems like the PM field is adequately 'steered' from PM to PM as I said N-S-N-S, etc.

Thanks for any input,

gmeast

 

[MagBen]

Interesting projects, guys!

Back to the original question: the best core material in this case is the most economic material. No need a high Bs: as long as no leakage trickly checked by a paper clip. If the core is saturated, you can simply increase the thickness. no need a high permability material since you have a gap and couple of connections at the magnetic circuit, high permeability is a waste.

Eddy current seems not to have much to do with the coupling project, but logbook's reoprt based on a simple phenomenon is impressive. It is understandable more magnets lead to a higher eddy current. The reason why using only one magnet didnot show eddy current effect on aluminium, brass, copper and steel seemed to be more related to the direction of flux (DOM): DOM was perpedicular to the sheet for one magnet, while DOM was within the plan of the sheet when two magnets side by side were used.
P.S. the reason why 304 SS is stick to a magnet is probably due to the 304 was cold worked which led to a slight ferromagnetism. The cutting could only make the magnet sticky to the cutting area (edges)

 

[logbook]

P.S. the reason why 304 SS is stick to a magnet is probably due to the 304 was cold worked which led to a slight ferromagnetism. The cutting could only make the magnet sticky to the cutting area (edges)

Thanks for the confirmation.

 

[gmeast]

Hi MagBen,

I am searching for some published data about the various Stainless Steel Alloys with regards to my coupling project. I work occasionally for a restaurant and we recently upgraded the hood system and I noticed that some of the S.S. used for the hood components is magnetic, and some is not ... and when I say "not", I mean it doesn't even stick a little and when I slide one of my 2" x 2" x 1" thick neo's down it (trying all orientations), the non magnetic S.S. displays minimum-to-no eddy current characteristics. All of the S.S. for the ducting and equipment covers, etc, are cold worked and either rolled, bent or formed in some way and none are in an annealed state ... they're all substantially rigid, yet some display non-magnetic/non-eddy current characteristics.

Did you see the little Eddy Current video I made and posted ... it's in this thread some place. The orientation of the magnet(s) was the same for both the aluminum sheet and the S.S. sheet and the difference was stark.

I'm not confused, I just haven't found the material specs that fit the observations.

Thanks for your input MagBen,

gmeast

 

[MagBen]

Due to lower Ni content, 304 is easier to form martensitic phase under strain than 316. But only when subjected heavy cold working can 304 SS bear slight ferromagnetism. Cold work helps strengthen the material, but not all SS are cold worked.

yes, I watched your Video, very simply desgined, while very effective to show the eddy current effect, I like it. By direction I meant, the magnetic flux flows through the thickness of the sheets when one magnet was used, while flux flows from one magnet to the other through in-plane when two magnets were used, and therefore, the former condition created less eddy current to against the slip. And that was why logbook stated: "I bought squares of aluminium, brass, copper and steel to try that exact test and there was absolutely no eddy current effect."

 

[gmeast]

yes ... thank you MagBen

 

[logbook]

And that was why logbook stated: "I bought squares of aluminium, brass, copper and steel to try that exact test and there was absolutely no eddy current effect."

That is an old comment and I would have deleted it as obsolete/superseded if this site had normal editing facilities.

I was discouraged by my lack of success. But when I went back and did proper experiments I got good results with a single magnet (report page 12). Yes two magnets gave a improved effect (report page 15) but only a 25% reduction in speed.

It is probable that I was using the single magnet long side parallel to the direction of motion, and that gives a relatively weak effect, which is easy to miss by eye.

 

[MagBen]

loglook,
Qualitatively, I donot think there was anything wrong with your old comment. when the magnet is weak and sheet is very thin, it is harder to see the difference between Al, steel and card board. I tried a AlNiCo magnet with very thin sheets, it turned out the difference was subtle.

 

[logbook]

I thought I'd post a link to my recently uploaded "rival" Eddy Current Braking video just for completeness.

YouTube short eddy current braking video

 

[gmeast]

Hi all,

I've received the Ring and Hub for my test fixture and have uploaded a picture of them. Get this: I uploaded the two files.DXF on Tuesday the 9th, they shipped on the 10th and I received them today, the 12th! I'm totally blown away. This kind of service deserves major KUDOS ... $147.00 and that included shipping.

Anyway, 1/2" square X 1/4" thick magnets live in the recesses you see. That puts about a 0.040" gap when the corners are closest.

Thought I'd share,

gmeast

 

[logbook]

Thanks

 

[gmeast]

Well I received the magnets. Not wanting to permanently mount them quite yet, I'll just temporarily secure them with some Gorilla Tape(tm) somehow, but here's a photo of them stuck to their pockets.

 

[logbook]

Those things looks really difficult to handle now.

I’m guessing you have done the easy part so far. The axles and hubs those things connect to need to be really strong as you only have zero radial force when the two parts are exactly coaxial. If one is able to move slightly the unbalanced pull will be tremendous. The Neo magnets I have been working with are much smaller than yours. When mine are mounted on carriers and you get more iron within a millimetre or so you just can’t hold them apart and they snap together -- hard. And I have something like a 6mm air gap in my overall magnetic circuit!

Have you calculated the field strength in the gap?

 

[gmeast]

Hi logbook,

I have not calculated the field strength. The magnets are grade40 so I will eventually need to make some determinations because I know that grade52 are reliably available. In the proposed application configuration the Hub and Ring are kept in radial register by those bronze shoes that will be running on that sleeve I've alluded to in an earlier post. In the test application, with these parts, the center 3/8" hole will pivot on a 3/8" tooling pin or a dowel in a plate. The ring has a bunch of mounting holes too and will mount to the plate. The whole purpose of the setup is to be able to calibrate the coupling torque of the 'pair'. First static and then I will also establish some dynamic transitional coupling characteristics ... you know relative angular velocities and torques where the pair can achieve complete coupling.

Oh ... on calculations .... I forgot to calculate how many blood blisters I will suffer while handling these magnets. I think I have met my quota.

Thanks,

gmeast

 

[MagBen]

@gmeast: you got the Hub and ring job done in UK just for 3 days? unbelievable! The material looks like cast iron?

@logbook, nice demonstration for eddy current video. I guess the DOM is along the length of the magnet? It may be interested in seeing any effect of DOM which could affect the eddy current patterns, and so, the thickness of substrate may matter (like the laminated structure of Silicon steel). I am thinking to use a cubic magnet, with the DOM perpendicular and parallel to the substrate plane.

 

[logbook]

@logbook, nice demonstration for eddy current video. I guess the DOM is along the length of the magnet?

Thanks.

No, the magnetisation is from one large flat face to the other.

 

[gmeast]

Hi MagBen,

The material is hot rolled steel. Hot rolled steel is usually pretty stress relieved as is ... enough for these tests anyway. I'm in the U.S. I live in Wyoming and the parts were made several states away in Wisconsin. These parts were cut by waterjet using a .DXF file from my CAD. It was the super-good price that mostly blew me away.

The magnetization is radial ... that is to say the poles are across the short dimension of each magnet. The 'coupled state' is with magnet pairs in attraction. The magnet pairs alternate in polarity as you go around the circle. I used Super Glue Gel (tm) to hold them in. You can see one of the magnets is chipped ... located about (7:00 to 7:30 on the clock)... it got away from me on my first attempt at mating them. You can also see I keep them separated from each other by a S.S. band or strap ... the ends are located at about 12:30 on the clock. I uploaded a photo.

Next step is to mount them.

gmeast

 

[logbook]

I was annoyed that I couldn't figure out how to calculate the shear force on the magnetic coupling so I thought I'd have a go at measuring the shear force on magnets that I do have.

Youtube video of lateral separation force on neo magnets

It was actually pretty hard. I'm not happy with the experimental technique, but it's the best I can currently think of.

 

[gmeast]

Thanks logbook for the video showing the measured shear force of permanent neo magnets. Very creative.

gmeast

 

[MagBen]

Very creative indeed.

Although a simple task to simulate the torque, or pull force, using a 3D software, your experiment is good enough to prove the relationship between the shear force and distance, as well as the force differences when pushing across and along the length of the magnet.

The readings from the scale were actually torques, which tended to be a constant at the middle of movement (the pulling force between two magnets should be the largest at the very beginning, then decreased monotonically). The explanation using energy is to the point: the energy density is BgHg/2 and the total energy is W=BgHgSt/2. S is the facing area of magnets and t is the air gap length. so basically the energy is proportional to the moving span of the magnet which in this case are 24 and 9mm, and so the shear force ratio 24/9, approx. 3:1.

 

[logbook]

The readings from the scale were actually torques,

I'm curious why you think that. The force is directed straight down onto the scales so there is no "lever arm" to create a torque. (It is probable that you haven't fully understood the setup just from the pictures, which is easy to understand as they are mostly from one weird angle. It was hard for the camera to see the scales and the ruler at the same time since they are at right angles.)

Although a simple task to simulate the torque, or pull force, using a 3D software

Can you suggest any that would work. I am currently trying to get hold of an evaluation licence for Magsoft Flux 11 and COMSOL multiphysics + AC/DC option. The COMSOL package is up around £12k. The package I was offered from MagNet was up around £20k.