How are magnets rated?

Magnets are characterized by three main characteristics. These are known as the:

1.      Residual Induction (given the symbol Br, and measured in Gauss). This is an indication of how strong the magnet is capable of being.

2.      Coercive Force (given the symbol Hc, and measured in Oersteds). This is an indication of how difficult it is to demagnetize the magnet.

3.      Maximum Energy Product (given the symbol BHmax, and measured in Gauss-Oersteds). This is an indication of what volume of magnet material is required to project a given level of magnetic flux.

What are the properties of commonly used magnet materials?

Here are the three important properties that characterize magnets for some of the most common magnet materials used today.

 Material Br Hc BHmax Flexible 1,725 1,325 0.6 Ceramic 1 2,200 1,900 1.1 Ceramic 5 3,950 2,400 3.6 SmCo 18 8,600 7,200 18 SmCo 26 10,500 9,200 26 NdFeB 35 12,300 11,300 35 NdFeB 35 13,050 12,500 41

How can I use this information?

Given a magnet size, you can estimate how much magnetic flux different materials will project at a given distance or you can use this information to compare one material to another.

Examples:

For example, How much more flux will a Neo 35 project as compared to a Ceramic 5 of the same dimension at a given distance?

Simply divide the Br of Neo 35 by the Br of Ceramic 5 (12300/3950) to get 3.1. This means that the Neo 35 would give you 3.1 times the flux a Ceramic 5 the same size would at a given distance.

Given a certain flux required at some fixed distance from the magnet, you can use this information to estimate what magnet volume will be required for different magnet materials.

For example, what volume of Ceramic 5 magnet would give the same flux as a Neo 35 magnet at a given distance? Simply divide the BHmax of Neo 35 by the BHmax of Ceramic 5 (35/3.6) to get 9.7. This means that the volume of the Ceramic 5 magnet would have to be 9.7 times that of the Neo 35 magnet to give you the same flux.

What is a magnetic assembly?

A magnet assembly consists of one or more magnets, and other components, such as steel, that generally affect the functioning of the magnet.

How should I assemble magnets to my device?

If a magnet needs to be fastened to a device, you can use either mechanical means, or adhesives to secure the magnet in place.

Adhesives are often used to secure magnets in place. If magnets are being adhered to uneven surfaces, you will need an adhesive with plenty of 'body' so that it will conform to the uneven surface. Hot glues have been found to work well for adhering magnets to ceramics, wood, cloth, and other materials. For magnets being adhered to metal, 'super-glues' can be used very effectively.

We can supply Flexible magnets with an adhesive already attached to the magnet: all you need to do is to peel off the liner and attach to your product.

As with all adhesive applications, it is very important to ensure that all surfaces being bonded are clean and dry before bonding.

What are the maximum recommended operating temperatures for different magnet materials?

The maximum temperature that a magnet may be effectively used at depends greatly on the 'permeance coefficient' - which is a function of the magnetic circuit - the magnet is operating in. The higher the permeance coefficient (the more 'closed' the circuit), the higher temperature at which the magnet may operate at, without becoming severely demagnetized. Shown here are approximate maximum operating temperatures for the various classes of magnet material. At temperatures close to those listed here, special attention may be needed in order to ensure that the magnet will not become demagnetized.

 Material Approx. Maximum Operating Temperatures °C °F NdFeB 140 284 SmCo 300 572 Ferrite 300 572 Alnico 540 1,004 Flexible 100 212

Why is the maximum temperature a magnet can operate at not a set value?

Magnets function at different levels of efficiency given different circuits that they operate in. The more closed the circuit the magnet is operating in, the more stable it is, and the less effect temperature will have on it.

Can I machine magnets?

Magnets can be machined. However, hard magnet materials - as opposed to the flexible or rubber type magnet materials - are extremely difficult to machine. Magnets should be machined using diamond tools or soft grinding wheels, and in the unmagnetized state as far as possible. In general, it is best not to try to machine hard magnet materials unless you are familiar with these specialized machining techniques.

How much does it cost to machine magnets?

The factors which determine cost to machine magnets are:

Quantity - the larger the quantity, the lower the cost since set-up charges must be amortized over the quantity, and special tooling can be created to machine larger quantities;

Material - SmCo materials are more costly to machine since they are very brittle, flexible materials are very inexpensive to machine because of their physical characteristics;

Shape - complex shapes are more expensive than simple shapes; and,

Tolerances - the closer the required tolerances, the more expensive it will be to machine the magnets.

What can I use to block a magnetic field?

Only materials that are attracted to a magnet can 'block' a magnetic field. Depending on how thick the blocking piece is, it will partially or completely block the magnetic field.

Tips on handling and storing magnets

Always take care! Magnets can snap together and injure personnel or damage themselves.

Keep magnets away from magnetic media - such as floppy discs , credit cards and computer monitors.

Store magnets in closed containers, so that they don't attract metal debris.

If several magnets are being stored, they should be stored in attracting positions.

Alnico magnets should be stored with "keepers" (iron or magnetic steel plates that connect the poles of the magnet) since they can easily become demagnetized.

Magnets should be kept away from pacemakers!

What are eddy currents?

These are electrical currents that are induced when a magnetic field moves in relation to an electrical conductor, which is placed within reach of the magnetic field. In turn, these eddy currents create a magnetic field that acts to stop the relative motion of the original magnetic field and electrical conductor.

What are some good magnetic reference books?

Permanent Magnet Design handbook, by Lester Moskowitz, a 385-page book aimed at the technical layperson, price approximately \$150.

Permanent Magnets and their Applications, by Dr. Peter Campbell, a 203-page book aimed at the technical person, price approximately \$40.

The Driving Force, by James Livingston, a 310-page book aimed at the non-technical reader - a very well written and interesting book on the history of magnets and some of their more exotic applications. Price approximately \$20.

What do magnets cost?

The costs of different magnet materials vary significantly from one to the other. Here is an approximate guide as to what magnets cost.

 Material BHmax Relative Cost (\$ / pound) Relative Cost (\$ / BHmax) Flexibible 1 \$1.00 \$0.60 Ceramic 3 \$2.00 \$0.50 Alnico 5 \$20.00 \$4.30 SmCo 20 \$100.00 \$6.00 NdFeB 40 \$50.00 \$1.40

Note: the costs shown here are relative costs based on high volumes of magnet materials that have no special machining or other characteristics.

On a cost-per-pound basis, Neodymium magnets seem very costly. However, on a cost per BHmax basis, they do not seem so costly. Often by using a more powerful magnet, the entire device that the magnet goes into can be miniaturized, yielding cost savings that favor the more powerful magnet materials.

Are there Industry Standards for Magnets?

Yes. Two industry associations produce standards. The Magnetic Materials Producers Association (MMPA) publishes standards for the production of magnetic materials, and the Magnet Distributors and Fabrications Association (MDFA) produces standards on various ways of testing magnets and magnetic devices.

How do I order magnets?

To efficiently order magnets, you need to have a good idea of what you want to accomplish. Here are a few items that you will need to consider:

 General nature of application - Holding, moving, lifting, etc. Shape of magnet desired - Disc, Ring, Rectangle, etc. Size of magnet desired - Diameter, length, width, height, etc Tolerances - what variation in dimensions is allowed. Conditions magnet will be used in - Elevated temperature, humidity, outside, inside, etc. Strength of magnet required - In pounds of holding force, Gauss, etc. Magnet should cost no more than? - This will eliminate certain materials from consideration. Quantities you will need.

 © 2000 Magnet Sales & Manufacturing Company, Inc.