Help please reading a CoC for a Bespoke Magnet

[Nancy S]

Hi all,
Please bear with me I'm new to the Subject of magnets. I have had a bespoke magnet made and I am having difficulty understanding the CoC.
Please can someone take a look and let me know what the magnetic pull is in Kg.
From what I can understand it looks like it is a 1.4Kg Pull.
Thanks in advance for your help

 

[MagMike]

That's not a C of C, it is a magnetic inspection report.

There is no information in there about pull force.

If you had a specific requirement for 1.4 kilograms (abbreviated kg, not Kg) pull force, there should be a column in the inspection report with pull force values.

 

[EdStainless]

Or you take the model of your system and put those properties into it for the magnet and see what the calculated force is.
A pull force is not a magnetic property, it is the result of the system that you put it into. Measuring it is very inexact. What type of steel? What surface finish? How is the magnet gripped?
The only time that we would measure it is when were building assemblies (with a case and pole pieces) that were intended for holding devices. The assemblies would provide much more holding force than just a magnet and they protected the magnet from impact and the environment.

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[IRstuff]

The number you quote, presumably from the graph, has units of Teslas or kiloGauss.

I don't get how you order 3500 magnets without understanding whether it'll work using your own specifications.

TTFN (ta ta for now)
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[Nancy S]

Thanks MagMike & EdStainless

I have specified 2.4kg vertical pull on the drawing to our supplier. But after testing, using a test rig that simulated it's intended function and using a digital force gauge, it was only reading 1.4kg. my concern is that we are being supplied with a weaker magnets than specified.

I am trying to ascertain what the overall pull value is when i have the following stackup:-
Dia 15 x 1mm thick M52 (1.1kg Vertical Pull) magnet held in an aluminium housing (interference fit) on panel 1
Dia 15 x 2mm thick M52 (2.4kg Vertical Pull) magnet attached to panel 2 using double sided tape

what should my total Vertical pull be? is there a formula i can use to estimate the theoretical value?

again, thanks in advance you any help you send my way.

 

[EdStainless]

IRS, in the magnet world almost nothing is clear and straightforward.

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[Nancy S]

IRStuff

We have done extensive physicals testing of the assemblies and our current stackup is more than adequate for its function. I am trying to work out if the batch we have received is correct to our specification.

 

[MagMike]

Nancy_S, none of us work at your company. 'Panel 1' and 'Panel 2' mean nothing to us.

If it was a critical requirement, it needed to be explicitly mentioned on the drawing or purchase order. As EdStainless mentioned, there is no standard pull test method. More often than not, it is conducted with a thick piece of clean, low carbon steel and the magnet has to make perfect contact with the steel.

You'll need to provide details on 'Panel 1' and 'Panel 2' and details on how you measure pull force to your supplier.

 

[dgallup]

Magnetic pull will always be affected by the complete magnetic circuit, it is not a function of the magnet alone. There are ways to estimate it, the most accurate is finite element but it's possible to do back of the envelope calcs. Far more data is needed including all geometry and the magnetic properties of all the components.

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[Nancy S]

Thank you MagMike

Point taken on the panel 1 & 2 description

All specification are on the drawing from size, Grade, material spec and finish required including vertical Pull force required in kg.

I understand that a standard test is on a "thick piece of clean, low carbon steel". This is for a single magnet. Which we have performed on this batch, and the pull force was weaker than expected.

Part 2 of my inquiry was to see if there is a standard equation to calculate of the overall pull force for a magnet to magnet, of different sizes, situation. I'm guessing I am walking towards the darkside of the Magnetic world with this question.

 

[MagMike]

Nancy_S, there are a few equations for calculating force between simple magnet shapes, but they also assume ideal, perfect conditions that never match real life.

There is some effort to develop a pull test standard within ASTM, but it'll take some time to get all the stakeholders into agreement. Even then, it'll only cover a limited number of test conditions. It sounds like yours is unique. Most customer applications are unique, thus the need for a detailed description of your test procedure.

One other comment: You mentioned 'Vertical' pull force. In my world, this suggests a magnet mounted on a wall, to prevent something from sliding down to the ground, similar to a refrigerator magnet. If 'Vertical' pull force equates to sliding force, then you are really counting on frictional forces, not magnetic forces to hold things in place.

I like your description of this being the darkside. My day job is to help customers stay away from that.

 

[IRstuff]

How much weaker was it? 10%? 90%?


TTFN (ta ta for now)
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[EdStainless]

We used to build small cup assemblies for people. A plated steel cup, a small magnet in the center, a fairly thin pole cap over the magnet, the annular space filled with epoxy, and the pole piece ground flush with the edge of the cup.
If we did it correctly then the lip of the cup was very nearly at saturation and the result was very consistent holding force.
When trying to measure with a bare magnet (usually epoxied to an Al rod) the numbers were everywhere, and any change (size of steel plate, thickness of steel, surface finish of steel) could change the values by >20%. It is super sensitive to alignment so we would use a jig with precision bearings, and still had issues.

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P.E. Metallurgy, consulting work welcomed

 

[MagMike]

When possible, I push for specifying a minimum holding force, de-rated to account for real world effects. It has helped in many instances.

 

[Nancy S]

MagMike
By vertical pull, I mean to pull apart axially. So, the force I am trying to measure is to pull two magnets apart. the sheer / slide force I am not too concerned about.
FYI. apart from the aluminium Cup. with the mating face exposed. there are no other metals in the assembly. the magnet with the a 1mm thin aluminium Cup, sits in a thermoformed composite structure and the mating magnet, with its face exposed, is attached using a double sided adhesive pad, to a polyurethane foam.


IRstuff
40% weaker

I am Beginning to realise that the science of magnetism, if I can refer to it as that, is Like a Jelly on a plate. at first glance, you have a perfectly formed part, but as soon as you poke it, it wobbles all over the place.

Thanks for your help guys, i really appreciate your advice.

 

[MagMike]

If your test conditions haven't changed, then something is likely wrong with your latest batch of magnets. FWIW: The magnetic inspection paperwork you shared looks good, although it doesn't include pull force information.

Did you order the same grade of magnetic material as before? I'd suggest going with a grade that has a higher intrinsic coercivity (Hcj or Hci) value since your magnets are extremely thin.

Also, given the 1mm thickness, physical tolerances will play a huge role in magnetic output. If the tolerance was +/-.05mm, then the magnetic output can vary +/-5% just from dimensional changes. Usually parts come in at nominal to minus tolerance.

The nickel plating makes up a significant volume fraction of the magnet too.

Just poking the Jelly some more...

 

[IRstuff]

I am Beginning to realise that the science of magnetism, if I can refer to it as that, is Like a Jelly on a plate. at first glance, you have a perfectly formed part, but as soon as you poke it, it wobbles all over the place.

It IS a science, demonstrated by Maxwell, Gauss, etal, quite a long time ago. What's jelly-like is people's lack of understanding of what's going on; you don't often see people bitching about kitchen magnets that refuse to hold up 20 sheets of paper, because THAT, they understand.

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

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

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[EdStainless]

Yes it is a science, but most people think of magnets as devices rather than components. In order to design with a magnet you have to be able to analyse the system.
In many cases the simplest approach (using a magnet directly to hold) may be the least efficient and have serious risks (corrosion, damage) associated with it.

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[hacksaw]

If your specification required the supplier to test the pull forces, it would have resolved the issue.

According to your documents only magnetic properties were specified.

 

[MagBen]

Specifying a min pull force is an easy and simple and valid approach, although a thickness of 2mm may be a little challenge. Suppliers normally accept such requirement.