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best permanent magnet core materials

best permanent magnet core materials

best permanent magnet core materials

(OP)
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

RE: best permanent magnet core materials

I am not convinced that I fully understand what you are trying to do. A drawing/sketch would help a lot. The question you have to ask (yourself) is what are you trying to achieve with the backing material? Is it really high permeability or is it really high saturation flux density. Any sort of iron is going to be good on relatively permeability. Saturation flux density is another matter.

magnetic saturation

Whilst magnet technology has improved dramatically in recent history, the magnetic irons are still in the dark ages. The best on saturation flux density is Hyperco 50
Hyperco
This stuff (2% vanadium steel) has been around since 1930 (from memory). At 2.4T saturation it is the best stuff around. If you don't need that extra 30% then any low carbon steel will do.

RE: best permanent magnet core materials

Opps. Senior moment. Hyperco is COBALT steel. It is 49% cobalt and that is the key thing. (How come this site doesn't have an EDIT button.)

RE: best permanent magnet core materials

Interesting. Thanks for the sketch. I don't see any stainless steel. Is the 1/16th inch gap all stainless steel, rubbing tightly against one set of magnets?

What you have is like a synchronous motor. Basically almost no starting torque. If the prime mover drives the shaft too hard (fast) at startup the coupling will not be able to accelerate the load and the coupling won't hold. It then will be unable to start. However the stainless steel will act like an induction motor rotor and will give some starting and running torque. It is uncertain if this will be enough to get close enough to the drive shaft speed to lock.

If the total air gap is only 1/16th inch the requirements for the rest of the magnetic circuit are easy. Don't bother with Vanadium Permendur (aka Hyperco). Just use any old iron/steel. (NOT mumetal/radiometal). The iron/steel doesn't even have to be more than a few mm thick.

I bet gluing the magnets in place will be the biggest problem. The pull force could be immense!

RE: best permanent magnet core materials

Quote (me)

I bet gluing the magnets in place will be the biggest problem. The pull force could be immense!
I have changed my mind about this. The magnets will probably prefer to sit in their recesses rather than jump across the gap. However, I think the recesses need to be quite shallow. If the magnets are say 2mm thick, the recesses probably need to be between 0.5mm and 1mm. Otherwise the field will take a shortcut (fringe) rather than jump the gap.

RE: best permanent magnet core materials

(OP)
Hi again logbook,

I see your point about the recesses for the magnets. The fields will choose a shortcut path for sure. Yes ... your assumption is correct ... the "gap" in the sketch is occupied by a stainless steel sleeve. I'm using a stainless steel alloy that produces virtually 'zero' eddy current loss. This coupling only serves to 'lock together' two members (the ring and the hub) that already have a very low relative angular velocity. The hub can be translated axially within the sleeve so it can be re-positioned under any number of rings running on the outside of the stainless sleeve. So, the sleeve isn't in tight contact with either the hub or the rings. The sleeve isolates two environments ... the hub's environment and the rings' environment. The sleeve is mostly just free floating.

As time and testing proceeds, I will be more than happy to share specifics of this beast if you're interested.

I'm uploading a short video showing the "no-loss" stainless steel material I'm using for the sleeve. My YouTube channel is "gmeast".

Thanks for your input ... especially about the magnet capture recesses.

gmeast

RE: best permanent magnet core materials

(OP)
Hi logbook,

Here's the link to the quick video showing the low loss Stainless Steel I'm using for the sleeve. It just shows the simplest of tests for Eddy Currents:
https://www.youtube.com/watch?v=_OY30sA6was

Thanks again for your input,

gmeast

RE: best permanent magnet core materials

Quote (gmeast)

It just shows the simplest of tests for Eddy Currents
Awesome!
I have supermagnets. magnets
I bought squares of aluminium, brass, copper and steel to try that exact test and there was absolutely no eddy current effect.
Then I saw what you had done and put TWO magnets side by side to get an intense field at the join. Suddenly it works! I can't thank you enough.

Brass is pretty poor on this test because of its higher resistivity. HOWEVER, looking at induction motor characteristics, a higher sheet resistance might just change the speed at which the peak torque appears.

Quote (gmeast)

As time and testing proceeds, I will be more than happy to share specifics of this beast if you're interested
Definitely interested.

RE: best permanent magnet core materials

Gmeast,
Interesting project.
I've done about 1/2 of what you want to do: www.sparweb.ca

Magnetic brakes already exist, similar to what you want to build. Basically the proximity of a magnetic disk to a solid steel disk induces such strong eddy currents that a large rotor's spinning kinetic energy can be converted to heat in a very short period of time. Then it has to sit and cool down for an hour, of course.

I want to go back to the magnetic coupling idea. Basically you want to feed power into the central shaft, and have it turn the outer cylinder via magnetic coupling of the magnets, right? The outer shaft (in my imagining of the coupling) would drive an output shaft, which allows a load to be driven up to a certain amount of torque. If the load over-torques the coupling, the magnets will slip, thereby preventing mechanical failure.

If that premise is clear and correct, then you want to have the maximum magnetic attraction between poles as possible. Each pole being a magnet face with one polarity (either N or S) facing the opposite pole of the opposing magnet). Lines of flux must cross the gap with no deviation and at a high intensity.
You do not want to embed the magnets into grooves, as you've shown in your diagram.

If you want to simulate some design ideas I would recommend FEMM, http://www.femm.info/wiki/HomePage

Heat dissipation when the coupling is slipping will be a major design issue for you. Once the heat passes the curie point, your magnets will loose their strength and the coupling torque will drop. Wrapping the rotor in the stainless steel band may help shed heat, but you will probably find that tricky to do safely, and maybe not as effective as you hope it to be. The outer rings will need cooling fins, like an induction motor, and the inner rotor may benefit from hollow holes through its core to allow air to pass through, too.

Oh yeah, and I think it's called "Carpenter", a supplier of specialty metals, including some with high magnetic permeability. But plain old electrical iron will do for your first go, and probably way way cheaper, too.

STF

RE: best permanent magnet core materials

(OP)
Hi SparWeb,

As you can see, logbook already made me painfully aware that the magnets must NOT be embedded as deeply as I have shown in my sketch. As you have deduced, what I am exploring is a torsional coupling more than a brake. Also, yes ... I am also planning to take advantage of the 'break-away' characteristics of such a coupling to prevent over stressing other components. In application (except for a 'break-away' scenario) there will never be high relative motion between the hub and whatever ring it is going to couple to.

I put together some NeFeB magnets and some dumb galvanized mild steel strips from HomeDepot as an initial look-see of a linear model of the beast. The mild steel strips are only about 3/32" thick, but I'm astounded at how well that material 'steers' the field of the magnets. There's virtually NO LEAKAGE from the back side!!!. After I post this reply, I will make another short video of my linear iteration an post that also to my Youtube.

I appreciate the input from you guys. I'm glad I stumbled into this forum ... dumb luck!!!

gmeast

RE: best permanent magnet core materials

(OP)
Well I took the advise of those who commented and made a design change to the test fixture which suggested an approach to a near-final design ... or at least 'a step closer'. So I've attached two sketches ... one is just a mod of my initial one and the second is a little more comprehensive scheme for securing the magnets without burying them in recesses. In that design, it is show some non-magnetic clamping blocks that will add some eddy current drag to help an coupling transitions .... I actually want some drag to act kind of like a 'synchro' in a manual transmission. They also act a bearing shoes.\
Thanks for viewing,

gmeast

RE: best permanent magnet core materials

I am not too happy with the wedge design concept. The induction/eddy-current startup concept seems fine. It is the mechanics of the wedges and the tapers on the magnets that bother me. You can buy any size of rectangular Neo magnets without paying tooling charges. But you now have two different types of custom magnets with tapers. Is that expensive?

The wedge is only tight when it has a magnet either side. I guess you have to assemble the whole thing loosely, then go round and round, tightening them up a little here and there until they are all tight. (I am assuming CR SHCS is cold rolled socket head cap screw).

You have to tolerance the whole thing so the wedges are guaranteed to be sub-flush in order not to use up your air-gap tolerance. Because there are so many parts, the tolerance build-up is going to be unpleasant (or you need very tightly toleranced parts.) But the wedge can’t sit on the hub as you have shown. It is over-constrained. You need a definite gap under each wedge in order to get it to grip at the sides.

That’s just my off-the-cuff thoughts on first inspection.

RE: best permanent magnet core materials

(OP)
Hi logbook,

The sketch you're referring to is NOT of the test fixture. What you're looking at and commenting on is a near-final design. In production, both the wedges and the magnets are sintering candidates and the cost would be low. I come from the gas turbine R&D field and always look at certain production considerations early on in any development program.

If you look above immediately after the post you responded to, you'll see the actual test fixture which has the shallow recesses for the magnets. The magnets I've chosen are simple 1/2" X 1/2" X 1/4", magnetized 1/2" square face to 1/2" square face.

I wanted to attach both sketches to the same post, but it didn't turn out that way. Following is the sketch belonging to the test fixture:

RE: best permanent magnet core materials

(OP)
Hi logbook,

Thanks for replying,

gmeast

RE: best permanent magnet core materials

(OP)
Hi logbook,

Thanks for the pdf on the eddy current kitchen presentation. Isn't it amazing how easy it can seem to be to "bias" data. The political idiots in every government uses various techniques of 'data biasing' to further their respective agendas. It happens here in the U.S. all the time. Thanks for the post,

Greg

P.S. I sent out for quotes today on my 'Coupling Test Fixture'.

RE: best permanent magnet core materials

Quote (gmeast)

P.S. I sent out for quotes today on my 'Coupling Test Fixture'.
spin

(Coudn't find an emoticon for waiting eagerly. On another site they have an animated popcorn-eating cartoon character.)

RE: best permanent magnet core materials

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)

RE: best permanent magnet core materials

Quote (MagBen)

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.bow

RE: best permanent magnet core materials

(OP)
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

RE: best permanent magnet core materials

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."

RE: best permanent magnet core materials

(OP)
yes ... thank you MagBen

RE: best permanent magnet core materials

Quote (MagBen)

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.

RE: best permanent magnet core materials

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.

RE: best permanent magnet core materials

(OP)
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

RE: best permanent magnet core materials

Thanks thumbsup2

RE: best permanent magnet core materials

Those things looks really difficult to handle now.bomb

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.hammer And I have something like a 6mm air gap in my overall magnetic circuit!

Have you calculated the field strength in the gap?

RE: best permanent magnet core materials

(OP)
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

RE: best permanent magnet core materials

@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.

RE: best permanent magnet core materials

Quote (MagBen)

@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.

RE: best permanent magnet core materials

(OP)
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

RE: best permanent magnet core materials

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.

RE: best permanent magnet core materials

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

gmeast

RE: best permanent magnet core materials

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.

RE: best permanent magnet core materials

Quote (MagBen)

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.)

Quote (MagBen)

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.

RE: best permanent magnet core materials

Of course, the scale measured the force, not the real torque (different unit). By saying that I meant the torque in gmeast's set up where the torque = shear force x radius. since the radius is a constant, so torque is proportional to shear force, so the reltionship holds.

AnSYS Maxwell is a popular one for electromagnetic simulation. I am used to Magneto 2D and Amperes 3D softwares by Integrated Engineering Software. SOMSOL is powerful tool, but maybe too general for such kind of torque simulation.

RE: best permanent magnet core materials

Quote (MagBen)

I meant the torque in gmeast's set up where the torque = shear force x radius
Ahh! Yes. Now we agree.flowerface

RE: best permanent magnet core materials

(OP)
Hi logbook and MagBen,

I'm going to construct a linear test fixture to accurately measure the shear force of two magnets. logbook, your setup has inspired me. MagBen, your analogy is also correct in how the measurement of a single pair of magnets put in shear can be projected onto my coupling investigation. What I need to do is account for the gap between the magnets ... like the one that actually exists in the coupling. This is easy to do. I need only keep the magnets separated by some little brass needles that have a diameter the size of the gap. This might need to be a horizontal setup with a cage for the needles and a support for the cage. I'm uploading a sketch of it.

gmeast

RE: best permanent magnet core materials

GM, without wishing to deflate your “inspired” state, what you would measure with your setup is not representative of your actual task by quite a bit. If you had two magnets on top and two underneath, with iron backing for each pair, then that would be more representative. At the moment you have a huge air gap for the “unused” pole faces of the magnets. The flux needs to get around from one side to the other. When you do that the field will be massively increased.

By the way, my tiny little 2mm thick magnets were 3.5mm apart. If you get your monster magnets 1mm apart the compression force is going to be huge!

Why not just complete the assembly of your rotor and stator and test that? Or are you trying to save a few days before the other parts arrive?

RE: best permanent magnet core materials

(OP)
Hi logbook,

You're right ... don't know what I was thinking. When I build it, I'll do exactly as you have pointed out ... a pair (or maybe 3) above with iron backing and a pair (or maybe 3) below with iron backing. I guess what I was actually trying to show was a practical method for representing the gap with 'frictionless' brass needle bearings so there would be no sliding frictional losses but only rolling losses to impact the results. What I've started is nearing completion as I assemble the frame for testing the coupling ... which consists of a 2" slab of wood, a couple of spacers, the moment arm, an overarm supporting the Hub's axle shaft end ... well that's most of it.

The linear test fixture mechanism will be used for extrapolating the results of the first Ring and Hub tests. It will be much less expensive than a new Ring and Hub for every iteration. Thanks for pointing out my mess-up.

gmeast

RE: best permanent magnet core materials

Design info for permanent magnets K & J

N42 curve

Taking each magnet assembly as a complete magnetic circuit we have 2 off 1/4" magnets and a 1/16" inch air gap. That makes 1/2" of magnet per 1/16" inch of air. I get a permeance coefficient of 8 to draw on the N42 curve, giving an impressive 1.2T field strength. Given 1/2" square magnets you don't want the iron cross-section in the direction of the flux to be less than half of that or it will be saturating. In other words your backing steel wants to be at least 3/8" thick.

RE: best permanent magnet core materials

ok GM, I'm going to put my money where my mouth is, apart from there being no money involved for me bigsmile

I am going to predict the peak static shear force on your jig as being nominally 8 lbf/magnet pair.
Because of my uncertainty I need limits from 2.4 lbf/magnet pair to 24 lbf/magnet pair.

But I will be disappointed if it is not within the range 4 lbf/magnet pair to 16 lbf/magnet pair.

RE: best permanent magnet core materials

@logbook:
I am curious about the big force range, 2.4-24 lbs, i.e. max/min force ratio = 10. One uncertainty I can think of is the permeance coefficient(Pc), due to the reluctance of the backing steel, and the joined gaps between magnet and steel, Pc must be less than 8. While the min of Pc should be larger than that for an open circuit, which is 1.25. correspondingly B= 0.7T from BH curve of N42.
Assuming the force is proportional to B^2, given the B range of 0.7T to 1.2T, max/min force ratio = 2.9.
Does that mean you have other uncertainties?

RE: best permanent magnet core materials

MagBen, the uncertainty I have put down is x3 in either direction from a very dodgy starting point. I used the K & J magnet calculator to get the pull force between two magnets with steel backings. I have simply said the maximum transverse force will be somewhere near that value. I have made that statement based on using the same K & J calculator on my magnet setup and comparing the transverse force I measured with the normal force from the calculation. The whole process is dodgy, hence my large uncertainty.

If you would like to throw your hat into the ring and predict the peak static shear force I am all ears.bigears

RE: best permanent magnet core materials

It will be complicated to calculate the shear force without FEA/BEA. This force is a function of displacement, as you demonstrated, it will be zero when the pair of magnets are perfectly aligned.

RE: best permanent magnet core materials

(OP)
Hi all,

Well I joined everything together and I was pleased with the test results. I uploaded two pictures of the test setup. One shows the entire test fixture and the other is a (blurry) close-up shot of the gap. The over-all shot shows the moment arm at 12" (1 foot) ... it's labeled on the arm. 40 ft-lbs of torque ... yes ... that builds to 40 lbs at its highest force perpendicular to the arm at that 12" location.

gmeast

RE: best permanent magnet core materials

Check my maths here …

You have 40lbf on a 12 inch lever.
Measuring and scaling from the photo it seems that the magnet gap is at about 2.8 inch radius from the axle.

I therefore get 40*(12/2.8) = 171 lbf maximum shear force for the 18 magnet pairs
= 9.5 lbf/magnet pair.

And I predicted 8lbf/magnet pair

I’m a genius bowleft bow bowright

or just plain lucky atom

RE: best permanent magnet core materials

rockbandlogbook,
I would say you are a genius and in good luck as well.
Transverse (normal, or lateral) force = 0 when pull force is maximized (perfect alignement), the max transverse is less than the max full force, somewhere half of that value when the displacement cause a normal vector, per K & J, NOT near that max pull force.

The pull force between two steel backings (Case 2, still an open circuit) should much smaller that that for the close circuit. Actually the pull force between two magnets without any steel plates (case 3) is even higher (9.91 vs 8.71 lbs). Using 12KG as a good estimation of field at the gap in the setup, the pull force is calculated as F = .577 B2 x S = .577 x 12 x 12 x .5x.5 = 20.77 lbs. so the lateral force per pair = 10.4 lb.

Coincidently, you underestimated pull force by half, but then overestimated lateral force also by half. and then, you got the right answer. Anyway nobody can deny you made a perfect prediction!!

RE: best permanent magnet core materials

(OP)
Hi logbook,

You're a 'lucky genius'. The center of the shear line was designed to be on a 5 inch diameter circle ... which is what it is. The dimensions of the hub and ring (each) were backed off from that circle to create the gap and the recess locations based on the dimensions of the magnets. I'm glad you guys caught me on that 'recess thing' on the original design ... the field lines would probably have shorted out or at best produced lousy results. Thanks, and nice prediction and calcs.

I can now confidently scale for the real thing. Happy Holidays to all,

gmeast

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