Frequently Asked Questions
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.
Here are the three important properties that characterize magnets for some of the most common magnet materials used today.
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.
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.
A magnet assembly consists of one or more magnets, and other components, such as steel, that generally affect the functioning of the magnet.
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.
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.
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.
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.
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.
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.
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!
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.
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.
The costs of different magnet materials vary significantly from one to the other. Here is an approximate guide as to what magnets cost.
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.
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.
Download a free copy of the current MMPA Standard Specifications for Permanent Magnet Materials here. Adobe Acrobat Reader is required.
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:
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