What type of magnetic metal to use? 4

[vermonster]

Hello, I am working on a product that uses a cylinder shaped neodymium magnet that is 1/2 inches long and has a diameter of 1/8 inch. It will be held in place by a piece of magnetic metal, with a .05 inch piece of plastic separating the magnet and magnetic metal. My question is what type of material should the 'magnetic metal' piece be made from? Iron, steel, etc? I want the strongest 'pull' factor between the magnet and metal piece, but as the function of this assembly is to measure the amount of force it takes to 'pull' the magnet away from the magnetic metal I need a magnetic metal that will resist getting more magnetized over time because of the close proximity to the magnet, which in turn would change the amount of force required to separate the two, giving different readings over time (I think that will happen, if not let me know). Are there metals like this out there, or is it just the nature of magnetism that the more 'pull' a material has, the more it will get magnetized over time when in proximity to a magnet. Hope that all made sense!

 

[ndsram]

Try using Low carbon Steel like 1010 or 1008 or 1006. Use this as in soft case do not harden it. AS soft as it is, it will be soft magnetic and will not get magnetized over period of time

 

[FiatfLux]

[ol 1]
[li]Any metal that would become magnetized would not become more so over time once it is saturated. This is part of the reason there are permanent magnets. Metals that can maintain some magnetism, such as steel, slowly "bleed off" that magnetism[/li].
[li]Critical: the metal you select must either be able to transport all of the flux generated by the magnet. If it cannot, then any magnets that exceed the metal's flux limit will appear to have the same pull force.[/li].
[li]You don't even need to build this, though with the gap the pull strength will be slightly lower than if the magnet were in contact with the metal. See calculation below:[/li]
[/ol]

Requirements:
substrate does not saturate fully {can transport all flux}

Contact Area: {in this case, 0.01227in[sup]2[/sup]}

B[sub]d[/sub]= in kG, kilogauss, you should be able to acquire this value from the manufacturer. If not, assume ~0.6(Br). Let's assume 5000 Gauss, or 5.0 kGauss

EQN: Holding/Lifting Force, F = 0.577 * (B[sub]d[/sub][sup]2[/sup])*(Contact Area)

For the moment, don't ask where the 0.577 constant comes from. You get what you pay for

F = 0.577 *(5000[sup]2[/sup]) * (0.01227in[sup]2[/sup])

Hypothetical Holding/Lifting force = 0.1770 pounds lifting force. Not a lot.


you shouldn't need a lot of steel to handle the flux for so tiny a magnet. just enough

Presume less than the above with the gap, but your gap is smallish. At least this provides an upper limit.

 

[MagBen]

FiatfLux,
A nice , simplified calculation. I calculate the Bd as the B at a distance of .05'' and double it (due to existing of magnetic Metal): assuming Br=13200G (N42), so B at .05'' distance is 2434G, double it to get Bd=4868G. .6 (Br) assmption is not necessary correct, but it is amusing here t fit the situation.

The material can be any carbon steel, 1010, 1008, 1006 or even high carbon steel would not make much difference as one can see the magnetic field used to magnetize the steel is thounds of Gs or Oe. what matters is the thickness and area of your steel, the volume should not be fully saturated.

 

[vermonster]

Thank you all for the help. It sounds like the soft 1010,1008, or 1006 is what I'm looking for, although much of the detailed explanations are way beyond me! I should have included more information in the first post. The magnet will be a grade N48, 1/16 inch diameter by 1/2 inch thick, magnetized through thickness, with a pull force of 1.24 pounds (from manufacturer's website). The size of the magnetic metal will be either (1/16 diameter x 1/8 length), or (1/8 diameter x 1/8 length), with the diameter face facing the magnet. The gap between the magnet and magnetic metal will be .05 inch.

If possible please clarify the following...
1- Are either of the sizes stated above big enough to handle "transporting all the flux" that FiatfLux and MagBen talks about?
2- Will either of the sizes stated above, assuming I use 1010, 1008, or 1006, exert more pull strength over time?
3- Does anyone know of a company that stocks 1010,1008, or 1006 already cut to the sizes I need (1/16 x 1/8) and (1/8 by 1/8)? I have been scouring the web and not having much luck. Lots of places that do custom cuts, but to save on cost I'm looking for some stock pieces as I would think the sizes I'm looking for would be common. What would you call these parts...disks, cylinders, plugs, pins, dowels, etc? Maybe I'm searching for the wrong search words.
4- FiatfLux, when you say...

"2.Critical: the metal you select must either be able to transport all of the flux generated by the magnet. If it cannot, then any magnets that exceed the metal's flux limit will appear to have the same pull force."

I think I understand what you are saying, please confirm. Once a small amount of magnetic material is subjected to a large enough magnetic field, it will eventually reach a maximum pull force, regardless of increasing the magnetic field. In my case I am more concerned that because of the 'small' magnetic field of the magnet, and the distance between the metal and magnet, the metal will not become fully saturated, but over time the metal will become more saturated, and will increase the 'pull' between the two over time.

Thanks again for all the help!

 

[prex]

Forget about time dependence, nothing will change over time. And a tiny thickness of plain carbon steel will be sufficient for the flux of your magnet.
However I can't see how you can build such a device without knowing the basics of magnetism. A first point to understand is that the magnetic flux goes from one pole of your magnet to the other one: this means that any piece of ferromagnetic material that would be present in the surroundings of the magnet will influence the pull force.
So you should consider that not only the size of the iron at the base will influence the force (manufacturer's value is likely for a magnet attached to a large piece of metal), but also any metal present in the table or base where the assembly is retained or even in the pliers or whatever you will use to pull the magnet.

prex
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[ndsram]

Based on your dimension, I simulated and found the force to be -0.0643lb

 

[MagBen]

The max force I got was .008 pounds (assuming totally open circuit, i.e. no other magnetic metal around; carbon steel is infinite; Br=14000G for N48), very little. Diameter change from 1/8 to 1/16 ont only decreases the magnetic field at .05'' distance, but decreases the virtual contacting area 4 times.

Ndsram, I believe the higher value you got was due to the assumption of a returning flux path.

If the steel is too thin or has too small contacting area, the whole flux from the magent can not be carried by the steel, the force will be further decreased. The pull force listed by the manufacture normally assumes an infinite plain carbon steel (reltively much larger is sufficient), air gap=0.

 

[vermonster]

Now I’m really confused! I though ndsram, FiatfLux, and MagBen were all agreeing that ‘time’ does matter in this case because they recommended 1010 steel. Am I wrong?

Prex, I will be the first to admit that I am no expert on magnetism, but I do have a patent for the first model of this device and produced over 25,000 units, so I would argue that I have a ‘basic’ understanding of magnetism and the device does work! Sorry I cannot get into more detail about this improved model of my device (still working on 2nd patent). Perhaps I’m not explaining this correctly because everything you said is ‘basic’ stuff except the ‘Forget about time dependence’ part, which is really the heart of my question.

On my previous model I noticed that the 'pull' force to separate the magnet and magnetic metal right after assembly was a ‘little less’ than when measured a week later. Over time the attraction between the two would slightly increase. I assumed that this was due to the magnetic material being in the field of the magnet for a period of time and getting slightly magnetized. Here’s my basic understanding of magnetism, what am I missing?

1. A piece of ferromagnetic material can get slightly magnetized over time when in the magnetic field of a magnet, and as FiatfLux explained this will ‘bleed off’ over time.
2. A piece of magnetized metal will have a stronger ‘pull’ on a magnet than an unmagnetized piece of the same metal. Example: If a hold a magnet x distance from a piece of magnetized and unmagnetized metal, the magnetized piece will ‘jump’ to the magnet first (longer distance) because there is more pull between the two.

If both 1 and 2 are true I can’t see how ‘time’ is not an issue. Please explain.

 

[ndsram]

Just for the moment, we'll keep the time entity away.
So by 1., we will later consider this.
By 2. its true that the magnetic metal will get attracted to the magnet and the pull force will decay as distance increases between magnet and the metal.

Please clarify, if you've a flux return path or do you've any metal piece that will return flux. If not, by having a return flux path, you still can increase the pull force you may require.

Coming to 1.
Please let me know whether your system is under the influence of thermal cycle. If your system is so, then over a period of time, your metal will tend to get slightly magnetized

 

[MagBen]

vermonster,

I think you are making "time" issue complicated. It is hard to imagine the increase of your pulling force (if you really measured the difference) is due to "time" effect. Your steel piece does get magnetized as soon as the magnet is close to it, but the magnetization process is on the order of milli-seconds.

Once the steel piece is magnetized and is seperated from the magnet, it could gain some magnetic remanence, or store some energy which could help to strengthen the force at the very beginning when the magnet is close to the steel piece again. this is because the holding force is proportional to the energy at the air gap. however, the steel piece is magnetically soft, once the magnet is close to it, it will be magnetized to the same level as the first time. The pull force will not be changed.

Actually, the magnet can be weakened over time due to thermal or mechanical effect, and the force will then be decreased, instead of increased.

 

[FiatfLux]

Vermonster, would it help if you provided us with the first patent? I mean, it's already protected by patent, and the details therein may help us help you.

Re: time; believe it or not, we all are saying the same thing, but focus specifically on the last sentence in paragraph two of Magben's post, just above this one.

As for why you are getting a weaker or stronger result, could any part of your test apparatus be experiencing response creep? These seem to be light forces at work, so I can't imagine parts becoming strained to the point they would cause such, but then I have no idea what your test method is.

 

[vermonster]

Thank you all for the help. After rereading Magben's post I think the first part of his paragraph is what was happening...

"Once the steel piece is magnetized and is separated from the magnet, it could gain some magnetic remanence, or store some energy which could help to strengthen the force at the very beginning when the magnet is close to the steel piece again."

I will try some 'soft' metal as suggested. Thanks again for all the help!

 

[FiatfLux]

uh, good luck!