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Understanding Magnet Grades and Magnet Tables

July 12, 2016

 

Each day at SM Magnetics and SuperMagnetMan we get asked questions about the differences in magnetic materials.  Neodymium Magnets, Samarium Cobalt (SmCo) Magnets, Ceramic Magnets, Alnico Magnets, Bonded Magnets, and Injection Molded Magnets all have various options, and choosing the correct grade for your application is critical. 

This article will try to simplify how magnets are designated, and define the differences between magnet grades.  We will focus specifically on the strongest rare earth magnets available, neodymium magnets (NdFeB) and samarium cobalt (SmCo) magnets. 

Before You Begin – Your Application
When we assist customers with magnet selection we will undoubtedly ask questions about the application.  This helps us understand the environment and requirements of the magnet.  But, in about 50% of the situations, our customers are not able to make known the application due to Confidentiality and Privacy reasons.  If this is the situation, we will then ask such questions as:

  1. What is the temperature that the magnet will be exposed (maximum operating temperature)?
  2. What other materials (i.e. steel, aluminum, plastic, etc.) will the magnet be next to?
  3. What is the size of the area for the magnet?
  4. What are the tolerances required? Our standard tolerances are 1 mm
  5. Is the magnet in an assembly or operating independently?
  6. Are there specific coating requirements?

While these are only a few of the questions we could ask, this may lead to other questions about operating environments and options.

Neodymium and Samarium Cobalt Magnets are available in many grades.  There are several factors and considerations that go into design, but for this article, the areas discussed will be Magnet Strength, Magnet Coercivity, and the Considerations for Magnet Selection.   


Magnet Strength
The strength of a magnet can be found in the magnet specification known as BHmax, which is the maximum energy density of a magnet.  This is defined in Mega Gauss Oersteds, or MGOe.  On a Magnetic Demagnetization Curve this is the highest point of the magnet’s strength, or the magnet’s maximum energy product.

For Neodymium Magnets the BHmax will range, in general, from 30 MGOe to 55 MGOe.  So, when defining your magnet need, keep in mind that the higher the number, the stronger the magnet.  Neodymium magnets produce the highest MGOe of any permanent magnet material.  The most common grades of Neodymium magnets are N35, N38, N40, N42, N45, N48, N50, N52, and N55. 

For Samarium Cobalt (SmCo) Magnet, the BHmax will range from 16 MGOe to 32 MGOe.  Again, like neodymium magnets, the higher the number, the stronger the magnet.  The common grades for Samarium Cobalt magnets are 16, 18, 20, 22, 24, 26, 28, 30, and 32.  As you can see, the MGOe numbers for neodymium magnets is higher than those of samarium cobalt magnets, which indicates that neodymium magnets are stronger than samarium cobalt magnets.    


Magnet Coercivity
First, let’s define Coercivity, or, the “Hci” of the material.  When you look at a magnet table of available materials there are different letters after some of the grades.  These letters represent the magnet’s ability to withstand demagnetization forces, which can be temperature or other magnetic forces working against the magnet.  In the world of permanent magnets, there are a couple of ways manufacturers or suppliers define the coercive force, but we will concentrate on the most widely used, which is the lettering system.  This letter system uses the following letters after the grade to define the magnets specification to fight demagnetization forces:  M, H, SH, UH, EH, and TH.  

When a letter is used after a magnet grade, it indicates that that specific material has a greater ability to fight demagnetizing forces.  For our purpose, we are going to use heat as the demagnetizing force as it is most common force effecting the magnet.  And, the following examples are “general rules” and not “hard and fast rules”.  We will explain further in the section below, Considerations. 

A Neodymium Magnet with no letter after the grade, i.e. N38, or N45, or N52, would indicate that it has the ability to work in an environment that has a maximum operating temperature no greater than 80C.  A magnet with an “M” (i.e. N35M, N42M, etc.) generally means that a magnet can be used in an operating environment up to 100C.  An “H” material is good up to 120C, “SH” up to 150C, “UH” up to 180C, “EH” up to 200C, and a “TH” up to 220C.  Again, these are general specifications and other factors do play a role in coercivity decisions.    


Considerations 
Strength
So, why not just get the highest strength possible every time?  Simply because the highest strength isn’t always the right choice.  For example, a magnet being used as a sensor magnet may be required to produce specific magnetic field at a specific distance.  This field will be defined in the sensor requirements.  The appropriate magnet may be a samarium cobalt 24, so if a neodymium N52 is used it could cause the sensor to work incorrectly.  Or, another example that we encounter often is with motors.  A motor’s windings may be fine with an N48 material but may overheat with an N55.  Or, in a holding application a stronger magnet may be adequate while a lower grade material may not hold enough.  Defining pull or holding strength, the magnetic field requirements, or the saturation point of surrounding materials will help with magnet choice. 

Cost
Cost considerations are usually part of the discussion.  As a general rule, the higher the grade, the higher the price.  There are other variables, including shape and size, but for this purpose, we will use the general rule of higher grade = higher price.  And, along with this rule, the higher the letter after the grade, the higher the price.  So, for example, an N48H will cost a bit more than an N48.  And an N48SH will cost more than an N48H, and so on.  And finally, even a lower grade material can cost more than a higher grade material if a higher letter is selected.  For example, an N35SH will most likely cost more than an N38 or even an N40.

Application
As mentioned in the first part of this article, we understand that we may not always know the final application.  But there are situations when a magnet can be combined with other materials, or even other magnets, to better influence the field, thus creating a situation where a lesser grade magnet may work fine.  Focused or influenced magnetic fields are a separate topic involving much detail, and we can offer a blog post at another time dealing with this one topic.  But if you need to have a very focused field, contact us and we can work through the details…even if we don’t know the application.

Size & Shape of Magnet
In the above examples we have used “general rules” relating to strength and coercivity.  In any engineering concept, there are factors that in turn influence other factors.  Size & Shape of the magnet also plays its part.  For example, a very thin N45H may be listed as able to handle an operating temperature of 120C, however, if the size of the magnet is extremely thin, or the shape is very tiny, it may be better to choose an N45SH material.  And, if the shape and size do require discussion, we can see if a solution exists to utilize other materials with the magnet to keep the coercivity intact.

Not All Grades Are Created Equal
Use of permanent magnets also means determining the tradeoffs.  As you can see from the above information, you may need to determine if strength or coercivity is more important.  Or, if neodymium magnets or samarium cobalt magnets would be better.  The best situation would be to have everything you need from the magnet, but that is not always possible.  There is a tradeoff between strength and coercivity.  As you get stronger in a magnet grade, the coercive force in a magnet many not be available.   For example, if you need a magnet that can handle 180C temperature, a UH material will be required.  However, only the lower grades are available in a UH material, which means that the magnet strength would need to be an N40 or below.  For a listing of magnet grades available, refer to the magnet table listed on the SuperMagnetMan.com site.  There you can see the options between strength and coercivity.

Selection of the correct magnet is a critical step to make sure your design works correctly and as expected.  We welcome the challenges and opportunity to work with customers for their legacy designs as well as pushing the limits of magnet technology and innovation.  For assistance with your application, you can contact sales@supermagnetman.com, or call 205-989-4353.

 

 

About SM Magnetics:  SM Magnetics is a privately owned company providing assistance from concept to production with magnetic circuit design, engineering support, material selection, and supply of magnetic materials and assemblies.  Inquiries or requests for assistance can be made to sales@supermagnetman.com, or 205.989.4353.


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