magnet strength testing- pull test vs gauss meter

[wildi81]

What is the best way to test the strength of a magnet - pull test vs gauss meter?

The magnets are neodymium(probably miss spelled)magnets encased in stainless steel and in the form of bar magnets and plate housing magnets.

The manufacturer believes the pull test is most accurate. our parent company believes a gauss meter is.

I sure would welcome any facts or opinions on this subject.

thanks

Wild i

 

[prex]

The gaussmeter will not be accurate, as it measures a local value (and orientation) of the magnetic induction, and this varies too much from point to point. Even if you measure it in a narrow gap, the values will be somewhat more stable, but still difficult to interpret.
On the contrary a pull test gives you automatically an integral of the magnetic induction (squared), especially if the magnet works in the test the same as in the final application.

prex

http://www.xcalcs.com

Online tools for structural design

 

[MagMike]

The best way to test the strength of a magnet is to test it for what it does in your application.

If it is supposed to hold a load, then a pull test is best. If it is to produce a magnetic field for a sensing application, the a gaussmeter probe (located where the sensor would be) is best.

Application-oriented testing often leads to the least amount of confusion.

Mike

 

[MJR2]

Both of the tests may be quite accurate and meaningless. But they do provide a means of comparison.

As MagMike says application oreinted testing is best.

A vendor of equipment has to offer a standardized set of tests for ease of comparison. A pull test usually satisifes that.

A gauss meter probe is very difficult to use in measuring the field of the rare earth magnets. When using a transverse probe simply turning the probe over results in a different result. I find it necessary to calibrate each new probe against a known standard when it is being used with RE magnets. Manufacturing tolerance often results in over 1000 gauss differnces between probes in high gradient areas in the devices such as you are measuring.

Mike

 

[EdStainless]

Often you can set up a test where the magnet (or assembly) is placed against a steel fixture. Instead of trying to make an accurate and repeatable pull off measurements, the fixture should have a slot cut in the return path. This is where the Gauss meter probe goes. By adjusting the gap you can simulate different conditions.


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[wildi81]

Thanks everyone for your input. It has been every educational.

The application is to remove metal fines, mostly dust from and powdered egg product. Most of this dust is stainless steel, 304 and 316.

Please educate me further, does a higher gauss number mean the magnet is able to pull, for example, metal dust out of the product flow stream the further the metal dust is from the magnet.

thank

Wild i

 

[MagMike]

Hello wildi81,

A higher gauss reading does not necessarily mean it'll do a better job of separation. It helps, but it isn't the only factor.

There are a lot of variables involved, such as arrangement of the magnets, spacing of the poles, etc.

I'm surprised your separator is picking out 304 & 316 stainless steel. Normally those grades are not magnetic (although there are situations that'll change that).

Mike

 

[TStaples]

MagMike,

You are headed down the right path when you stated that there are conditions that make 304 or 316 magnetic. As an example, when 316 stainless wears in a reciprocating motion application, and particles are generated, the particles are sufficiently work hardened that you can capture them using a high field strength magnet.

I designed a system, similar to what wildi81 described, using neodymium magnets to catch metal debris that were generated via component wear inside a hydrogen fuel storage/delivery system. The system I designed was essentially a really high tech "Mag-Vest"(tm) similar to what you wrap on your oil filter canister to trap particles. Funny, our customer didn't really want those microscopic metal particles reaching their fuel cell stack!




-Tony Staples

http://www.tscombustion.com

 

[MJR2]

304 and 316 stainless steels when found as products of abrasion or as chips (like off a mill or lathe) often become sufficiently suseptible that they may be captured.

What actually does your separation is the force not the gauss figure. The force is the gauss times the change in gauss or gradient.

or B*dB/dX

You have two different magnetic devices. One is a tube magnet. It is a thin walled non-magnetic stainless tube with permanent magnets and steel poles inside. When one designs this circuit you try to optimize for wall thickness, magnet length, and pole length. Along with a pretty long list of other things. As you have said there are basically two ways to check the strength. Unless you have a very well calibrated system of measuring the magnetic field with a gauss meter you will have difficulty with your results. The reason for this is the extremely high dB/dX. The pull test is the only reasonable way in the field to check the magnet.

The other device is a plate magnet and for your application it would have limited success. The capture points and gradient are just not competitive with a tube magnet. Plates magnets can pull fines but they are best suited for larger items like staples, nuts, and bolts and such. Although one of my first projects many many years ago was to fix a system of plate magnets which were removing the 'minimum daily requirement' of iron powder from a cereal flour.

Pulling particles of abrasion out of a product stream such as powedered eggs will require contact between the magnet and dust. That is why a multiple tube system with the powder cascading down through it works better than a plate magnet. There is a multitude of capture locations and magnetics can creep around the tube out of the product flow stream.

Ask the vendor to bring an engineer to your plant. They should be able to show you some FEA work on the magnetic circuits you are evaluating. Then you will much better understand all you are being told here. At the very least they should be able to send you a colorful picture of what I'm describing. If not you're dealing with the wrong vendor.

Mike

 

[wildi81]

Thanks every one I really apreciate all the knowledge you have shared.

Thanks again

Wild i

 

[Compositepro]

As others have said there is a lot more to magnetic separation than having a strong magnet. When I was researching the problem years ago, I came across a very sophisticated product that was very interesting. Check out

http://www.sgfrantz.com/

 

[EdStainless]

Buy a magnetic seperator. These are common in food processing lines. There are tow or three major manufacturers.

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Rust never sleeps
Neither should your protection

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